currituck county final report - Whalehead Property Owners Association

Transcription

FLOODING AND STORMWATER
MANAGEMENT MASTER PLAN
WHALEHEAD SUBDIVISION
COROLLA, NORTH CAROLINA
February 2010
Prepared for:
CURRITUCK COUNTY
FINAL REPORT
Prepared by:
Moffatt & Nichol
1616 E. Millbrook Road, Suite 160
Raleigh, NC 27609
Flooding and Stormwater Management Master Plan for the Whalehead Subdivision Area
Final Report
Currituck County, North Carolina
TABLE OF CONTENTS
LIST OF FIGURES ............................................................................................. II
LIST OF TABLES ............................................................................................... IV
PROJECT BACKGROUND ................................................................................... 1
EXISTING SITE DATA ........................................................................................ 2
EVALUATION OF EXISTING DRAINAGE SYSTEM ............................................ 21
MODELING OF EXISTING DRAINAGE SYSTEM ............................................... 22
FORMULATION OF ALTERNATIVE SOLUTIONS .............................................. 52
EVALUATION OF ALTERNATIVE SOLUTIONS.................................................. 77
SELECTION OF PREFERRED ALTERNATIVE .................................................. 105
PRELIMINARY PROJECT BUDGET ................................................................. 107
CONCLUSION ................................................................................................ 110
APPENDIX A – S&ME Report
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LIST OF FIGURES
Figure 1. Whalehead Subdivision ......................................................................................... 1
Figure 2. Example Flooding Issue ........................................................................................ 1
Figure 3. Typical Island Cross-section .................................................................................. 2
Figure 4. Existing Topography ............................................................................................. 4
Figure 5. Existing Land Use ................................................................................................. 5
Figure 6. Existing Vegetation ............................................................................................... 6
Figure 7. Interpolated Instantaneous Precipitation Rate at Whalehead Subdivision .................. 7
Figure 8. Rainfall Levels at Whalehead Subdivision................................................................ 8
Figure 9. Existing Soils.......................................................................................................10
Figure 10. Boring Well Locations ........................................................................................11
Figure 11. Groundwater Level Measurements from S&ME Study ............................................13
Figure 12. Existing Groundwater Levels...............................................................................15
Figure 13. Measured Tide Information at Duck ....................................................................16
Figure 14. Flood Insurance Rate Map – 100-year ................................................................18
Figure 15. Approximate Wetland Location in the Study Area .................................................20
Figure 16. MIKESHE Graphic Showing the Phases and Interactions of the ..............................22
Figure 17. Topographic Grid of the Study Area ....................................................................24
Figure 18. Vegetation Grid of the Study Area .......................................................................25
Figure 19. Surface Soils of the Unsaturated Zone Grid of the Study Area ...............................27
Figure 20. Horizontal Hydraulic Conductivity Grid of the Study Area.......................................29
Figure 21. Vertical Hydraulic Conductivity of the Study Area .................................................30
Figure 22. Initial Potential Head Grid of the Study Area ........................................................32
Figure 23. Initial Water Depth Grid of the Study Area ...........................................................33
Figure 24. Groundwater Elevation at Boring Well B-1 ...........................................................34
Figure 31. Groundwater Elevation at Boring Well B-11..........................................................37
Figure 35. MIKESHE Model Results for a 2-yr Return Period, 24-hr Rainfall – Existing Conditions
.................................................................................................................................45
.................................................................................................................................46
Figure 37. MIKESHE Model Results for a 10 yr Return Period 24 hrs Rainfall - Existing Conditions
.................................................................................................................................47
Figure 38. MIKESHE Model Results for a 25 yr Return Period 24 hrs Rainfall – Existing
Conditions ..................................................................................................................48
Conditions ..................................................................................................................49
Conditions ..................................................................................................................50
Figure 41. Wetlands Delineated by ESI ...............................................................................54
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Figure 42. Corolla Light Pond .............................................................................................55
Figure 43. Timbuk II Pond .................................................................................................56
Figure 44. Water Sampling Well and Pond Locations ............................................................58
Figure 45. Infiltration Plan and Section (Marlin Street)..........................................................66
Figure 46. Example Dry Detention Basin .............................................................................67
Figure 47. Typical Development Along Whalehead Road .......................................................67
Figure 48. Typical Ocean Outfall Schematic .........................................................................68
Figure 49. Pump to Deepwater Ocean Outfall ......................................................................70
Figure 50. Pump to Backside of Primary Dune .....................................................................72
Figure 51. Top View of Dune Infiltration System ..................................................................73
Figure 52. Storm Chamber Installation ................................................................................73
Figure 53. Pump to Soundside Ponds ..................................................................................74
Figure 54. Northern Pond at Corolla Light Subdivision ..........................................................76
Figure 55. Southern Pond at Timbuk II ...............................................................................76
Figure 56. MIKESHE Model Result for a 2 yr Return Period 24 hrs Rainfall – Existing and
Pumping Conditions ....................................................................................................83
Figure 62. Flood Water Depth above Grade – Marlin Street – 100 yr ......................................90
Figure 63. Flood Water Depth above Grade – South of Marlin Street – 25 yr ..........................90
Figure 64. Tuna Street Pumping - 500 gpm .........................................................................92
Figure 65. Tuna Street - Sheet Pile - 500 gpm .....................................................................93
Figure 66. Tuna Street - Sheet Pile - 250 gpm .....................................................................94
Figure 67. North and South Pond Locations .........................................................................98
Figure 68. North and South Pond Groundwater Extraction Locations ......................................99
Figure 69. Groundwater Elevation Adjacent to the North Pond – ......................................... 100
Figure 70. Groundwater Elevation Adjacent to the South Pond – ......................................... 101
Figure 73. Phased Plan for Soundside Ponds ..................................................................... 106
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LIST OF TABLES
Table 1. Flood Rainfall Depths for Currituck County, NC ........................................................ 8
Table 2. Soil Types Within Whalehead Subdivision ................................................................ 9
Table 3. Summary of Slug Test Hydraulic Conductivity Data..................................................12
Table 4. Summary of February 5-7, 2008 Groundwater Elevations .........................................14
Table 5. Datums at Duck, North Carolina ............................................................................17
Table 6 . Total Volume above Grade for Maximum Rainfall – at the end of the Rainfall – Tuna
Street – Existing Conditions .........................................................................................40
Table 7. Total Volume above Grade for Maximum Rainfall – after 48 hours of the end of the
Rainfall – Tuna Street – Existing Conditions ...................................................................40
Table 8. Total Volume above Grade for Maximum Rainfall – at the end of the Rainfall –
Barracuda Street – Existing Conditions ..........................................................................41
Rainfall – Barracuda Street – Existing Conditions ...........................................................41
Mackerel Street – Existing Conditions ............................................................................42
Rainfall – Mackerel Street – Existing Conditions .............................................................42
Table 12. Total Volume above Grade for Maximum Rainfall – at the end of the Rainfall – Coral
Rainfall – Coral Street – Existing Conditions...................................................................43
Table 14. Total Volume above Grade for Maximum Rainfall – at the end of the Rainfall – Marlin
Rainfall – Marlin Street – Existing Conditions .................................................................44
Table 16 . Total Volume above Grade for Maximum Rainfall – at the end of the Rainfall – Tuna
Street – Pumping Condition .........................................................................................78
Rainfall – Tuna Street – Pumping Condition ...................................................................78
Barracuda Street – Pumping Condition ..........................................................................79
Rainfall – Barracuda Street – Pumping Condition ...........................................................79
Mackerel Street – Pumping Condition ............................................................................80
Rainfall – Mackerel Street – Pumping Condition .............................................................80
Table 22. Total Volume above Grade for Maximum Rainfall – at the end of the Rainfall – Coral
Rainfall – Coral Street – Pumping Condition ...................................................................81
Table 24. Total Volume above Grade for Maximum Rainfall – at the end of the Rainfall – Marlin
Rainfall – Marlin Street – Pumping Condition .................................................................82
Table 26. Percentage of Flood Water Removed....................................................................89
Table 27. Required Time to Pump Surface Flood Volume – Tuna Street Model........................95
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Table
Table
Table
Table
Table
Table
28.
29.
30.
31.
32.
33.
Required Time to Pump Surface Flood Volume – Marlin Street Model ......................95
Pumping Duration until Adjacent Impact is Visible – Tuna Street Model ...................96
Pumping Duration until Adjacent Impact is Visible – Marlin Street Model .................96
Ground Elevation in Groundwater Measuring Locations, meters NAVD88 .................99
Opinion of Probable Cost – Phase 1: Marlin and Coral Streets ............................... 108
Opinion of Probable Cost – Total Cost ................................................................ 109
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PROJECT BACKGROUND
Currituck County communities have been experiencing tremendous growth over the last few
decades. With the retirement of the “baby boomers” and the reputation of North Carolina as a
great place to live, Currituck County watersheds have undergone substantial changes over time
(see Whalehead Subdivision Area in Figure 1). With the current growing trend showing no
signs of slowing, the local watersheds have continued to undergo substantial development and
re-development. The land use changes that come with this development (e.g. increased
impervious surface areas, less vegetation) have significant impacts on flood discharges and
water quality.
Hence, as
watersheds become more
urbanized and populated,
drainage facilities that were
once
adequate
now
experience flooding problems
(see Figure 2).
New
developments
are
being
Figure 1. Whalehead Subdivision
located in areas where there
was little knowledge of previous flooding conditions. In addition to affecting the drainage, the
change in land cover also results in discharge of concentrated pollutants directly into the
ditches, canals, and catch basin systems as point sources. Previously, vegetative buffers served
to protect water quality in these areas.
These watersheds are also unique because the water table is very shallow in many of the floodprone areas. A shallow water table, along with the absence of an existing drainage
infrastructure, exacerbate the flooding
problems, and unfortunately prove to be
limiting factors for many conventional
stormwater treatment options.
During
Ernesto in 2006, an estimated 18” of rainfall
fell on the island in 48 hours (over a 500-yr
event). This very intense and long duration
rainfall caused significant flooding in the
area. Flooding depths of up to 3 feet were
observed and emergency pumps were
operated for weeks to eliminate the flooding
problems. This flooding event and past
storms showed the need for a stormwater
management plan to be developed for the
Whalehead Subdivision area.
Figure 2. Example Flooding Issue
To raise the funds necessary to complete
the study (along with the associated field investigations) and construct a storm management
system, the County (with the approval of the Whalehead Property Owners Association – WPOA)
levied a four cent tax per $100 of property value to be levied on the owners of a newly
developed tax district made up of the Whalehead Subdivision. Moffatt & Nichol was asked to
study the existing drainage system and develop solutions to solve the flooding issues present in
the area. The WPOA elected drainage board has also been instrumental in directing the study.
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EXISTING SITE DATA
Topography
The existing Whalehead Subdivision watershed encompasses approximately 590 acres. As
shown in Figure 4, the existing topographic elevations range from 0 to well over 20 feet (El 14
shown as peak on figure for clarity). A quick inspection of the topography shows that the area
contains a primary coastal dune and subsequent smaller dunes and troughs. A typical section
of this type of system is shown in Figure 3.
From the typical section and the topography, one can begin to understand the main reason why
the Whalehead subdivision area is subject to flooding. In this location, the water table is only
1-3 feet below the ground surface in the trough areas. As it rains, the water infiltrates into the
dunes and troughs and collects at the low points within the trough system. Once the
floodwaters reach the low points of the troughs, the water can only move though groundwater
transmission since there are no significant conveyance systems (swales and/or pipes) within the
subdivision. See for the island’s topography.
Figure 3. Typical Island Cross-section
Land Use and Vegetation
As shown in Figure 5, nearly all of the existing Whalehead subdivision watershed land use is
residential with the exception of a few commercial properties near Albacore Street and public
parking lots within the study area. The watershed is approaching full-build out conditions.
There are few remaining developable lots, and many of those are either slated to be developed,
or are currently under construction, such as the subdivision lots at Herring Street. However,
another factor that should be considered in the future is redevelopment of existing lots. In
some cases, existing lots are being sold and existing homes are being replaced by larger
structures with more impervious surface area.
As shown in Figure 6, there are only a few major vegetation types present within the study
area. Much of the maritime forest that was historically present has been removed such that
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trees, scrub/brush, and grass are now the primary vegetation types. The reduction in
vegetation coverage has likely reduced the evapotranspiration on the island, resulting in
increased runoff.
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Figure 4. Existing Topography
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Figure 5. Existing Land Use
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Figure 6. Existing Vegetation
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Rainfall
Perhaps the most important dataset in the determination of flooding levels is rainfall. Rainfall
patterns along the Outer Banks and Currituck County are highly variable depending on location.
The nearest precipitation data, obtained from the State Climate Office of North Carolina
database (http://www.nc-climate.ncsu.edu), was located at Corolla, North Carolina, 13 miles
north of the study area, and from the Field Research Facility of the US Army Corps of Engineers
located at Duck, North Carolina, 20 miles south of the study area. Daily precipitation was
available for the Corolla station, and hourly precipitation was available for the Duck station. To
estimate the precipitation at the study site, the precipitation of both stations was interpolated
by distance to the study area. This information was transformed to hourly data in mm/hr for the
period of March – April 2008 (Figure 7) to aid in later model calibration. Note that the figure
shows instantaneous rainfall rates. Actual precipitation levels are shown in Figure 8.
Statistically, the average annual precipitation for Currituck County is 48 inches. Table 1 shows
the rainfall statistics for various return period storms for Currituck County. These statistics were
taken from the NOAA Precipitation Frequency Data Server. As can be seen for this data, the
reported rainfall during Hurricane/Tropical Storm Ernesto was over a 500-yr event (18 inches
over 48 hours).
Figure 7. Interpolated Instantaneous Precipitation Rate at Whalehead Subdivision
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Figure 8. Rainfall Levels at Whalehead Subdivision
Table 1. Flood Rainfall Depths for Currituck County, NC
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Soils
The locations of existing soil types are shown in Figure 9 (based on the SCS Soils report).
The soil types are aligned relative to the linear north/south topography of the island. The
majority of the soil types appear in linear striations that parallel the shoreline, representative of
the dune structure found on this type of barrier island. Summaries from the SCS soil survey of
the primary soil types are presented below.
Symbol
Table 2. Soil Types Within Whalehead Subdivision
Predominant
Name
Slope
Soil Type
Permeability
Bn
Beaches-Newhan Complex
Fine Sand
0-25%
Very Rapid
NhC
Newhan-Corolla Complex
Fine sand
0-10%
Very Rapid
NeC
Newhan
Fine sand
0-10%
Very Rapid
Dt
Duckston
Fine sand
Nearly level
Very Rapid
CrB
Corolla-Duckston Complex
Fine sand
0-6%
Rapid
Du
Dune Land
Sand
High
Very Rapid
DwD
Dune Land-Newhan
Complex
Fine sand
2-40%
Very Rapid
OuB
Ousley
Fine sand
0-6%
Rapid
Os
Osier
Fine sand
Nearly level
Rapid
Cu
Currituck
Mucky peat
Nearly level
Moderate
CoB
Corolla
Fine sand
0-6%
Very Rapid
In addition to the soil survey, previous studies have been completed to describe the in-situ soils
of the island. The study completed by Edwin Andrews and Associates in October 2006
determined that a confining layer was present approximately 14 feet below ground and that the
unconfined aquifer had the following characteristics:
Transmissivity = 219 sq. ft./day
Storativity = 2.82 X 10-5
Specific Yield = 2.82 X 10-1
However, given the importance of this project and the variability of subsurface conditions
discussed in previous studies, it was decided that more detailed estimates of the soil conditions
(hydraulic conductivity, etc.) and groundwater conditions were needed. As a part of these
additional field investigations, soil testing was undertaken by S&ME in February of 2008. During
the survey, hydraulic conductivity tests were performed in 19 locations of the Whalehead
Subdivision area in the saturated and unsaturated zone (See Figure 10 for boring locations).
Table 3 presents a summary of the slug test data of hydraulic conductivity.
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Figure 9. Existing Soils
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Figure 10. Boring Well Locations
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Table 3. Summary of Slug Test Hydraulic Conductivity Data
Monitoring Well I.D.
Hydraulic Conductivity, Hydraulic Conductivity,
Kh Zone
Kh Zone
Saturated
Unsaturated
(ft/day)
(ft/day)
B-1
24.4
62.0
B-2
22.0
36.1
B-3
27.4
46.4
B-4
15.9
142.0
B-5
23.6
159.0
B-6
18.9
109.0
B-7
26.4
31.2
B-8
17.6
67.5
B-9
15.6
90.6
B-10
5.9
34.3
B-11
39.7
28.6
B-12
16.6
20.8
B-13
15.7
52.4
B-14
17.3
132.0
B-15
24.2
89.2
B-16
13.8
30.8
B-17
13.4
35.1
B-18
13.9
42.3
B-19
63.4
31.3
For the nineteen boring locations, the saturated hydraulic conductivity ranged from 5.9 to 63.4
feet/day, with an average of 21.9 feet/day. Overall, the saturated hydraulic conductivity was
fairly homogenous, with no obvious trends between the boring locations. The unsaturated
hydraulic conductivity ranged from 20.8 to 159.0 feet/day, with an average of 65.3 feet/day.
The wells with the highest unsaturated conductivity rates were located along Herring and
Mackerel Streets. The rest of the boring locations are fairly homogenous. It should be noted
that while fairly homogenous, these values were lower than other areas of the NC coast where
similar studies have been completed. In reviewing the S&ME report, it is apparent that the soils
here have a much higher percentage of fines and fine sands than other areas. This factor
would become important in the design of potential solutions.
Groundwater Levels
As stated previously, an important factor in determining the assimilative capacity of the soils
within barrier island systems is the location of the groundwater table. As part of the S&ME
study, groundwater levels were also recorded at each of the 19 well locations during installation
and long-term monitoring stations were installed at 16 of the well locations (Wells 1, 2, 4-9, 1113, and 15-19). These locations were selected to provide coverage over the majority of the
island as to determine the approximate shape of the water table as well as to determine the
response of the water table during rainfall events. See Figure 11 for time history of
groundwater levels. In most low-lying trough areas, the groundwater levels are only 1 – 3 feet
below the ground surface, even during dry periods.
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In the nine months of measured data it can be noted that the groundwater table elevation
change within each well ranges from 2 to 3 feet. It can also be observed that the water table
reacts to most rainfall events. Around April 21st, a considerable amount of rain affected
groundwater levels within the study area (approximately 2.7 inches of rain in two days). As can
be seen in Figure 11, all the monitoring wells have an increase of the water table of about 1 to
1.5 feet.
The S&ME report also provides information of groundwater elevation levels from February 5th to
7th, which was a period of time preceded by minimal rainfall so it could be considered as an
estimate of average water table elevation. Table 4 provides the average groundwater level
data for those specific dates. Based on the dry period data and the expected domed shape of
the groundwater table (due to freshwater/saltwater density gradients), an estimated long-term
elevation map of groundwater levels was created and can be seen in Figure 12. It is also
interesting to note that along with the expected dome shape, there is also a slight gradient in
groundwater levels from north to south within Whalehead Subdivision.
Figure 11. Groundwater Level Measurements from S&ME Study
The soil characterization, groundwater level, and aquifer data were also used in evaluation of
the existing drainage system and potential solutions. Additional soil information can be found in
Appendix A which includes summaries from the S&ME report.
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Table 4. Summary of February 5-7, 2008 Groundwater Elevations
Monitoring Well I.D.
Ground Water
Levels
(ft - msl)
B-1
4.53
B-2
4.30
B-3
4.50
B-4
4.99
B-5
2.17
B-6
2.69
B-7
5.13
B-8
2.80
B-9
3.54
B-10
5.46
B-11
5.65
B-12
2.87
B-13
5.16
B-14
5.81
B-15
2.12
B-16
2.35
B-17
2.45
B-18
3.03
B-19
3.11
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Figure 12. Existing Groundwater Levels
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Tides
Normal and storm tides are also an important climatic dataset to consider for the flood study.
In areas of low relief, normal tides can create a tailwater condition which will affect
groundwater transmission. Periodic hurricanes and northeasters can also affect proposed
drainage systems with slight to extreme storm surges.
Measured tidal data are available at the Duck Field Research Facility Pier. For an example plot
of tide data at Duck, see Figure 13. Table 5 presents the NOAA (National Oceanic and
Atmospheric Administration) datum information for the Duck, North Carolina tidal monitoring
station (NOAA 8651370).
4
3
Tide, ft NAVD88
2
1
0
-1
-2
-3
08/01/06
08/06/06
08/11/06
08/16/06
08/21/06
08/26/06
08/31/06
09/05/06
Date and Time, EST
Figure 13. Measured Tide Information at Duck
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Table 5. Datums at Duck, North Carolina
Datum Level
Abbreviation
Elevation (ft NAVD)
Mean Higher High Water
MHHW
1.50
Mean High Water
MHW
1.18
North American Vertical Datum
NAVD88
0.00
Mean Sea Level
MSL
-0.42
Mean Lower Low Water
MLLW
-2.19
_
4.73
_
3.32
Highest Recorded Water Level
(8/30/1999)
Highest Recorded Water Level
for Hurricane Ernesto
(9/11/2006)
The Federal Emergency Management Agency (FEMA) developed a Flood Insurance Study (FIS)
for Currituck County, North Carolina (2005). A potential source of flooding in the study area is
storm surge generated in the Atlantic Ocean by tropical storms and hurricanes. The wind
induced by the storms can produce large waves. This wave action associated with the storm
surge can be much more damaging than the higher water level.
Flood elevations for various return periods were established for the project study area. Flood
Insurance Rate Maps (FIRM) provides the 100-year (1% annual chance) return period flood
elevations (in NAVD88), and also designate flood insurance rate zones (See Figure 14). In the
study area, three different types of zone designations were encountered:
•
AE: Flood insurance rate zone that corresponds to the 1% annual chance flood that is
determined in the FIS Report by detailed methods. Whole foot base flood elevations
derived from the detailed hydraulic analyses are shown at selected intervals within this
zone.
•
VE: Flood insurance rate zone that corresponds to the 1% annual chance flood that has
additional hazards associated with storm waves. Whole foot base flood elevations
derived from the detailed hydraulic analyses are shown at selected intervals within this
zone.
•
X: Areas of 500-year (0.02% annual chance) flood; areas of 1% annual chance flood
with average depths of less than 1 foot or with drainage areas protected by levees from
1% annual chance flood.
As seen in Figure 14, the flood elevation at the sound side wetlands (tidal marshes) for the 100year flood is 5 feet (Zone AE), and it ranges from 9 – 12 feet within the Whalehead Subdivision
(Zones AE and VE). In the center of the island the flood depth is less than a foot (Zone X).
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Figure 14. Flood Insurance Rate Map – 100-year
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Environmental Conditions
The Currituck Sound, from its source to the Wright Memorial Bridge, is classified as Class SC
waters (tidal salt waters). SC waters are salt waters acceptable for any usage except primary
recreation or shellfishing for market purposes; usages include aquatic life propagation and
maintenance of biological integrity (including fish and functioning primary nursery areas [PNA]),
wildlife, and secondary recreation. All saltwaters shall be classified to protect these uses at a
minimum. Any source of water pollution which precludes any of these uses, including their
functioning as a PNA, on either a short-term or long-term basis shall be considered to be
violating a water quality standard. This portion of the Currituck Sound west of the Whalehead
Subdivision does not contain PNAs nor does it contain Class SA waters or Outstanding Resource
Waters. This portion of the Currituck Sound is also a Closed Shellfishing Area in its entirety.
The National Wetland Inventory (NWI) mapping depicts several wetland types within the
project area. Generally, estuarine or tidal wetlands are located adjacent to the Currituck Sound
with palustrine or non-tidal wetlands being located inland. No wetland areas are mapped
adjacent to the Atlantic Ocean. The NWI wetlands surrounding the project site are shown in
Figure 15. The following types of wetlands are found within the Whalehead Subdivision.
Wetlands located adjacent to the Currituck Sound are characterized as estuarine, intertidal,
emergent, persistent, irregularly flooded (E2EM1P). These wetland areas are located in the
intertidal zone that extends water-ward to the extreme low tide line where the tidal water
floods the land surface less than once per day. They are characterized by herbaceous
vegetation that persists through the year.
Adjacent to the tidal wetland areas are palustrine, forested, broad-leaved evergreen, and
seasonally flooded (PFO1C) wetlands. These areas are characterized by forest canopy typically
comprised of various pine tree species. Surface water is present for extended periods typically
during the early growing season, but is generally absent by the end of the growing season.
Scattered through the inland area that lies between the palustrine forest and the Atlantic Ocean
are palustrine scrub-shrub and emergent wetlands (Palustrine, scrub-shrub broad leaved
evergreen, seasonally flooded [PSS3C] and palustrine, emergent, persistent, temporarily
flooded, and seasonally flooded [PEM1A/C]). These areas are characterized by either shrubs
and tree saplings or herbaceous vegetation that persists through the year. Surface water is
present for extended periods typically during the early growing season, but is generally absent
by the end of the growing season.
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Figure 15. Approximate Wetland Location in the Study Area
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EVALUATION OF EXISTING DRAINAGE SYSTEM
As stated previously, the existing drainage system consists mainly of a dune-trough
configuration with no outlet. The high dunes drain to the low troughs which then drain
eastward (to the ocean) and westward (to the sound) through groundwater transmission. As
development occurred within the project area, minimal efforts were made to maintain or
enhance the historical system. Low-lying lots within the troughs were developed, and no pipes
or swales were installed near the streets to carry floodwaters away from the site.
During the past years, flooding has occupied portions of the study area. The subdivision
residents decided to impose a tax upon themselves to develop solutions for the existing flooding
problems. During Ernesto, the County expended considerable resources (pumping numerous
areas for weeks) within the study area to minimize the hazards and problems associated with
the flooding problem. Over 100 homes within the Whalehead subdivision were impacted by
Ernesto. Numerous other studies have also been completed by others to develop potential
solutions, but none have been implemented to date.
Due to regulations concerning pumping to the beaches during these flood events, public health
risks are also elevated. The Division of Water Quality will not allow emergency pumping to the
beaches to begin until the floodwater elevation exceeds the clearance of the lowest emergency
vehicle (approximately 14 inches). It usually takes several days for the waters to reach these
elevations for some events since the main outflow is through groundwater transmission and
therefore is a protracted event. This causes water to stand for extended periods inundating
septic leach fields and causing the fecal coliform count in the stagnant water to reach higher
levels. Once the 14-inch flood depth is exceeded, the contaminated waters are then pumped to
the beaches causing the beaches to be posted. The posting of the beaches and prolonged
flooding periods directly affects the community’s tourism rentals and the environment.
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MODELING OF EXISTING DRAINAGE SYSTEM
Model Requirements/Selection
The drainage system in the project area is very complex. Usually, for these types of
hydrologic/hydraulic studies, the Corps of Engineers’ HEC-1 (HMS) and HEC-2 (RAS) models or
the USEPA StormWater Management Model (SWMM) are used. These models do very well in
large scale applications where surface water/groundwater interactions and evapotranspirative
losses are negligible. However, these effects are not negligible in this system; in fact, they are
two of the most important factors in determining system behavior and capacity. Therefore, the
Danish Hydraulic Institute’s (DHI) MIKESHE model was used. This model is one of only a
handful of models in the world which currently link surface water and groundwater behaviors in
the same model. This model was used for a similar project in Emerald Isle which resulted in
the design of a system which has successfully alleviated the existing flooding problems.
The model is unique in the fact that it is actually a series of linked submodels which provide
detailed solutions for each phase of the hydrologic cycle. The phases include:
Groundwater Flows –Saturated Zone (SZ) and Unsaturated Zone (UZ) Submodels,
Surface Water Flows – Overland/Channel Flow (OC) Submodel,
Evapotranspiration (ET) Losses – Evapotranspiration Submodel,
Irrigation (IR) Losses – Irrigation Submodel, and
Snowmelt (SM) Inputs – Snowmelt Submodel
A graphic of all these phases can be seen in Figure 16.
•
•
•
•
ET Interception/Evapotranspiration
– Interception of Rainfall by the Canopy
– Drainage From the Canopy
– Evaporation From the Canopy
Surface
– Evaporation From the Soil Surface
– Uptake of Water by Plant Roots and
Its Transpiration
OC Overland and Channel Flow
– Surface Runoff
– Routing in Rivers
UZ Unsaturated Zone Flow
– Infiltration
– Moisture Distribution
SZ Saturated Zone Flow
– 3D Groundwater Flow
– Exchange With Boundaries
Figure 16. MIKESHE Graphic Showing the Phases and Interactions of the
Hydrologic Cycle
- 22 -
Final Report
Model Setup
The first step in the complex model setup was to establish the period of the simulation by
identifying the available dates of water table elevation data with which to calibrate the model.
First, there were static water levels measured by S&ME at nineteen locations on February 5th to
7th of 2008, and again on March 7, 2008. There were also continuous water level
measurements from March 7th to April 19th of 2008, and from April 22nd to September 15th.
Groundwater measurements during periods of rainfall were also required so that the model’s
groundwater level response to rainfall could be measured. Therefore, the simulation period for
the calibration of the model was set up from March 1st to April 30th of 2008. The simulation
was started one week prior to provide warm up time for the model, and because of computer
run times for the model (5 – 6 days) and the number of runs required to calibrate the model, it
was decided to run the model for a period of two months when multiple rainfall events
occurred.
Different model input grid spacings were used to calibrate the model. A coarse grid (10 x 10
meter cells) was used to simulate the entire study area, while finer grids (5 x 5 meter cells)
were used to simulate the locations where more accurate results were needed to study
proposed alternatives.
A topographic grid file was needed for the model to accurately determine overland flow
directions for the Overland/Channel flow submodel. In order to accurately capture small scale
topographic effects and to allow for accurate modeling of impervious areas, the previous
described grid spacing was utilized. The topographic grid file was developed with LiDAR data
and surveys provided from previous studies. Figure 17 shows the topographic grid file of the
study area.
Land use and cover inputs were also needed in the model to model infiltration,
evapotranspiration, and runoff. Nearly all of the existing Whalehead subdivision watershed
consists of residential land use, with the exception of a few commercial properties near
Albacore Street and public parking lots within the study area. The watershed is also rapidly
approaching full-build out conditions. Among the few remaining developable lots, many are, or
will be, under construction like the subdivision lots at Herring Street.
There are only a few major vegetation types present within the study area. Much of the
maritime forest that was historically present has been removed and altered to accommodate for
structures. The majority of the vegetation is now smaller trees, scrub/brush, and grasses. The
reduction in vegetation coverage has likely reduced the evapotranspiration on the island,
resulting in increased runoff. Figure 18 shows the vegetation grids utilized in the model for the
study area. The vegetation cover of ‘sand’ indicates a lack of vegetation (such as dune areas).
- 23 -
Final Report
[meter]
Topography
4025000
4024500
4024000
4023500
4023000
[meter]
4022500
Above 11
10 - 11
9 - 10
8- 9
7- 8
6- 7
5- 6
4- 5
3- 4
2- 3
1- 2
0- 1
-1 - 0
-2 - -1
-3 - -2
Below -3
Undefined Value
4022000
4021500
4021000
4020500
426000
427000
[meter]
Figure 17. Topographic Grid of the Study Area
- 24 -
Final Report
Brush
Untitled
[meter]
Grass
Untitled
[meter]
4025000
4025000
4024000
4024000
4023000
4023000
4022000
4022000
4021000
4021000
425200
427400
[meter]
425200
Sand
Untitled
[meter]
Trees
Untitled
[meter]
4025000
4025000
4024000
4024000
4023000
4023000
4022000
4022000
4021000
4021000
425200
427400
[meter]
427400
[meter]
425200
Figure 18. Vegetation Grid of the Study Area
- 25 -
427400
[meter]
Final Report
The predominant vegetation types used to model evapotranspiration (ET) effects consisted of
trees, scrub/brush, and grass. MIKESHE requires a vegetation database with a root depth, a
leaf area index (LAI), and other empirical parameters for each type of vegetation specified. The
LAI was calculated from existing aerial topography and the other parameters were estimated
from research conducted at North Carolina State University for the known vegetation types.
Another requirement of the ET model was the estimation of potential evapotranspiration (PET).
Most PET models use pan evaporation as a base, but then include wind effects, solar radiation
measurements, etc., to calculate the PET. Due to data availability limitations, a constant value
of 0.05 mm/day was used for the potential evapotranspiration.
Precipitation is the primary water load into the model and hourly data was needed to accurately
model storm events. As previously stated, precipitation for the project area was interpolated
from the State Climate Office of North Carolina station at Corolla (located 13 miles away), and
the Field Research Facility of the US Army Corps of Engineers station (located 20 miles away),
as shown in Figure 7.
The surface soils (unsaturated zone) model included digitized soil maps with like soils grouped
together based on the S&ME field measurements. A separate soil type was defined for
impervious areas, and the hydraulic conductivities for the unsaturated zone model (Table 3)
were defined for each soil type. See Figure 19 for details.
- 26 -
Final Report
Buildings
[meter]
Untitled
Road
[meter]
140Untitled
ft/day
[meter]
Untitled
4025000
4025000
4025000
4024500
4024500
4024500
4024000
4024000
4024000
4023500
4023500
4023500
Road
Road
Road
SoilRoad
40
4023000
Road
Road
Road
SoilRoad
40
4023000
4022500
4022500
4022500
4022000
4022000
4022000
4021500
4021500
4021500
4021000
4021000
4021000
4020500
4020500
4020500
425200
40 ft/day
[meter]
425200
427400
[meter]
Untitled
425200
427400
[meter]
60Untitled
ft/day
[meter]
4025000
4025000
4024500
4024500
4024500
4024000
4024000
4024000
4023500
4023500
4023500
Road
Road
Road
SoilRoad
40
4023000
Road
Road
Road
SoilRoad
40
4023000
4022500
4022500
4022500
4022000
4022000
4022000
4021500
4021500
4021500
4021000
4021000
4021000
4020500
4020500
425200
427400
[meter]
427400
[meter]
80Untitled
ft/day
[meter]
4025000
4023000
Road
Road
Road
SoilRoad
40
4023000
Road
Road
Road
SoilRoad
40
4020500
425200
427400
[meter]
425200
427400
[meter]
Figure 19. Surface Soils of the Unsaturated Zone Grid of the Study Area
The subsurface soils (saturated zone) model required subsurface soil parameters for each soil
layer along with initial groundwater levels and boundary conditions directly affecting the
groundwater table. Because very little water is exchanged between the surficial sand aquifer
and lower layers in the zone of study, only the surficial sand aquifer was included in the
saturated subsurface model.
- 27 -
Final Report
The soil parameters required for input:
Lower Level of the Aquifer
: -2 m NAVD88 (based on Andrews study)
Horizontal Conductivity
: A grid was constructed using The S&ME field
investigation results (Figure 20).
The values of the
Horizontal Conductivity for each well are shown in Table 3.
Vertical Conductivity
: The values of the vertical conductivity were assumed to
be 25% of the values of the horizontal conductivity (based
on literature review and past work at Emerald Isle) (Figure
21).
Specific Yield
: 0.28
Storage Coefficient
: 0.000454685 1/m
As can be seen in Figure 20 and Figure 21, the hydraulic conductivities on the ocean side of the
island are higher than on the sound side of the island. This is due to the predominant soil on
the eastern side of the island being sand, which has a high hydraulic conductivity coefficient
while the sound side has more fines and peat which reduce hydraulic conductivity. The red
zone near the Corolla Bay Pond, indicating the highest hydraulic conductivity in the study area,
is the location of an undeveloped site, one of the few in the study area.
- 28 -
Final Report
[meter]
Horizontal Hydraulic Conductivity
4025000
4024500
4024000
4023500
4023000
[m/s]
Above
0.00021
0.000195 - 0.00021
0.00018 - 0.000195
0.000165 - 0.00018
0.00015 - 0.000165
0.000135 - 0.00015
0.00012 - 0.000135
0.000105 - 0.00012
9e-005 - 0.000105
7.5e-005 - 9e-005
6e-005 - 7.5e-005
4.5e-005 - 6e-005
3e-005 - 4.5e-005
1.5e-005 - 3e-005
0 - 1.5e-005
Below
0
Undefined Value
4022500
4022000
4021500
4021000
4020500
426000
427000
[meter]
Figure 20. Horizontal Hydraulic Conductivity Grid of the Study Area
- 29 -
Final Report
[meter]
Vertical Hydraulic Conductivity
4025000
4024500
4024000
4023500
4023000
[m/s]
Above
5.2e-005
4.8e-005 - 5.2e-005
4.4e-005 - 4.8e-005
4e-005 - 4.4e-005
3.6e-005 - 4e-005
3.2e-005 - 3.6e-005
2.8e-005 - 3.2e-005
2.4e-005 - 2.8e-005
2e-005 - 2.4e-005
1.6e-005 - 2e-005
1.2e-005 - 1.6e-005
8e-006 - 1.2e-005
4e-006 - 8e-006
0 - 4e-006
-4e-006 0
Below
-4e-006
Undefined Value
4022500
4022000
4021500
4021000
4020500
426000
427000
[meter]
Figure 21. Vertical Hydraulic Conductivity of the Study Area
- 30 -
Final Report
The data obtained from the S&ME report was used to estimate groundwater levels and the
initial potential head (initial groundwater level) to be used as a model starting condition. The
report had two sets of groundwater levels, and the data from February 5th to 7th was selected
because it had drier antecedent conditions which translated to a more realistic estimate of the
average water table elevation. The data was then interpolated and extrapolated in the model
domain to estimate the initial potential head (water table elevation) of the model (Figure 22).
It can be seen that the shape of the initial potential head has a classic shape of a barrier island
water table with a peak elevation in the middle of the island caused by the influence of tides
and the density gradients of the freshwater/saltwater interface.
The boundary condition for the saturated zone submodel was tides. The tides work along the
perimeter of the ocean side and force groundwater levels to rise and fall as the tide cycles. The
tide data collected by the USACE at Duck Pier was utilized.
Lastly, a grid with the initial water depth of the entire domain was interpolated to determine the
locations where the water table was above ground level (Figure 23). Existing ponds, lakes or
retention areas were identified, specifically, three ponds are located within the study area; two
on the north/west side (Corolla Light Pond and Corolla Bay Pond) and one in the south/west
side (Timbuk II Pond). In addition, wetlands were identified on the sound side of the Currituck
area.
- 31 -
Final Report
[meter]
Initial Potential Head
4025000
4024500
4024000
4023500
4023000
[meter]
Above 1.8
1.6 - 1.8
1.4 - 1.6
1.2 - 1.4
1 - 1.2
0.8 - 1
0.6 - 0.8
0.4 - 0.6
0.2 - 0.4
0 - 0.2
-0.2 - 0
-0.4 - -0.2
-0.6 - -0.4
-0.8 - -0.6
-1 - -0.8
Below -1
Undefined Value
4022500
4022000
4021500
4021000
4020500
426000
427000
[meter]
Figure 22. Initial Potential Head Grid of the Study Area
- 32 -
Final Report
[meter]
Initial Water Depth
4025000
4024500
4024000
4023500
4023000
[meter]
4022500
Above 1.95
1.8 - 1.95
1.65 - 1.8
1.5 - 1.65
1.35 - 1.5
1.2 - 1.35
1.05 - 1.2
0.9 - 1.05
0.75 - 0.9
0.6 - 0.75
0.45 - 0.6
0.3 - 0.45
0.15 - 0.3
0 - 0.15
-0.15 0
Below -0.15
Undefined Value
4022000
4021500
4021000
4020500
426000
427000
[meter]
Figure 23. Initial Water Depth Grid of the Study Area
- 33 -
Final Report
Model Calibration
As stated previously, the period of March-April 2008 was used for calibration of the MIKESHE
model. Measured rainfall data at Corolla and Duck, NC for the time period above was utilized.
(Recall that the Corolla and Duck stations are located approximately 13 and 20 miles away from
the location of the study area, respectively.) The results of the model were compared with the
measured groundwater levels for wells B-1, B-2, B-4, B-6, B-7, B-8, B-9, B-11, B-12, B-13 and
B-19, as shown in Figure 24 through Figure 34. These wells were selected as representative
given the accuracy and field issues described in the S&ME report. (For reference, see Figure 10
for well locations.)
B1
4.00
3.50
3.00
Elevation (m)
2.50
Model
7-Mar
Measurment
2.00
1.50
1.00
0.50
0.00
2/22/2008
0:00
3/3/2008
0:00
3/13/2008
0:00
3/23/2008
0:00
4/2/2008
0:00
4/12/2008
0:00
4/22/2008
0:00
5/2/2008
0:00
5/12/2008
0:00
Date
Figure 24. Groundwater Elevation at Boring Well B-1
B2
4.00
3.50
3.00
Elevation (m)
2.50
Model
7-Mar
Measurment
2.00
1.50
1.00
0.50
0.00
2/22/2008
0:00
3/3/2008
0:00
3/13/2008
0:00
3/23/2008
0:00
4/2/2008
0:00
4/12/2008
0:00
4/22/2008
0:00
5/2/2008
0:00
5/12/2008
0:00
Date
- 34 -
Final Report
B4
4.00
3.50
3.00
Elevation (m)
2.50
Model
7-Mar
Measurment
2.00
1.50
1.00
0.50
0.00
2/22/2008
0:00
3/3/2008
0:00
3/13/2008
0:00
3/23/2008
0:00
4/2/2008
0:00
4/12/2008
0:00
4/22/2008
0:00
5/2/2008
0:00
5/12/2008
0:00
Date
B6
4.00
3.50
3.00
Elevation (m)
2.50
Model
7-Mar
Measurement
2.00
1.50
1.00
0.50
0.00
2/22/2008
0:00
3/3/2008
0:00
3/13/2008
0:00
3/23/2008
0:00
4/2/2008
0:00
4/12/2008
0:00
4/22/2008
0:00
5/2/2008
0:00
5/12/2008
0:00
Date
- 35 -
Final Report
B7
4.00
3.50
3.00
Elevation (m)
2.50
Model
7-Mar
Measurement
2.00
1.50
1.00
0.50
0.00
2/22/2008
0:00
3/3/2008
0:00
3/13/2008
0:00
3/23/2008
0:00
4/2/2008
0:00
4/12/2008
0:00
4/22/2008
0:00
5/2/2008
0:00
5/12/2008
0:00
Date
B8
4.00
3.50
3.00
Elevation (m)
2.50
Model
7-Mar
Measurement
2.00
1.50
1.00
0.50
0.00
2/22/2008
0:00
3/3/2008
0:00
3/13/2008
0:00
3/23/2008
0:00
4/2/2008
0:00
4/12/2008
0:00
4/22/2008
0:00
5/2/2008
0:00
5/12/2008
0:00
Date
- 36 -
Final Report
B9
4.00
3.50
3.00
Elevation (m)
2.50
Model
7-Mar
Measurement
2.00
1.50
1.00
0.50
0.00
2/22/2008
0:00
3/3/2008
0:00
3/13/2008
0:00
3/23/2008
0:00
4/2/2008
0:00
4/12/2008
0:00
4/22/2008
0:00
5/2/2008
0:00
5/12/2008
0:00
Date
B11
4.00
3.50
3.00
Elevation (m)
2.50
Model
7-Mar
Measurement
2.00
1.50
1.00
0.50
0.00
2/22/2008
0:00
3/3/2008
0:00
3/13/2008
0:00
3/23/2008
0:00
4/2/2008
0:00
4/12/2008
0:00
4/22/2008
0:00
5/2/2008
0:00
5/12/2008
0:00
Date
- 37 -
Final Report
B12
4.00
3.50
3.00
Elevation (m)
2.50
Model
7-Mar
Measurement
2.00
1.50
1.00
0.50
0.00
2/22/2008
0:00
3/3/2008
0:00
3/13/2008
0:00
3/23/2008
0:00
4/2/2008
0:00
4/12/2008
0:00
4/22/2008
0:00
5/2/2008
0:00
5/12/2008
0:00
Date
B13
4.00
3.50
3.00
Elevation (m)
2.50
Model
7-Mar
Measurement
2.00
1.50
1.00
0.50
0.00
2/22/2008
0:00
3/3/2008
0:00
3/13/2008
0:00
3/23/2008
0:00
4/2/2008
0:00
4/12/2008
0:00
4/22/2008
0:00
5/2/2008
0:00
5/12/2008
0:00
Date
- 38 -
Final Report
B19
4.00
3.50
3.00
Elevation (m)
2.50
Model
7-Mar
Measurement
2.00
1.50
1.00
0.50
0.00
2/22/2008
0:00
3/3/2008
0:00
3/13/2008
0:00
3/23/2008
0:00
4/2/2008
0:00
4/12/2008
0:00
4/22/2008
0:00
5/2/2008
0:00
5/12/2008
0:00
Date
Based on the above results, it appears that the model is accurately replicating the behavior of
the groundwater table until April 22nd, at which point the model is overestimating the
groundwater response to rainfall at the well locations. It is assumed that the main issue for this
discrepancy is that the rainfall patterns for the study area may have been quite different during
this event than those measured at the Corolla and Duck stations. In fact, review of both data
sets shows that on some occasions, the daily rainfall is quite different for both stations. Another
reason for this theory is that the model replicated groundwater level response to rainfall quite
well for the event during the first week of April. This discrepancy accounts for the differences
between the model output and the data collected at the boring wells.
The calibrated model was then utilized to run simulations with various return period rainfalls
over a 24-hour time period at the study area to determine how the existing system responds
under various storm events. The model was divided into five sub-areas (to optimize computer
run times) which correspond to the locations of the potential basin collection points (discussed
in further detail in the alternative solutions section). The five sub-areas are Tuna Street,
Barracuda Street, Mackerel Street, Coral Street and Marlin Street. These streets were chosen
because they are owned by the County and were likely candidates for the placement of
potential solutions.
The purpose of these runs is to estimate the surface flooding volume for each event. Table 6
through Table 15 show the total precipitation depth over 24 hours, average surface flooding
depth, and the total surface flood volume for the different sub-areas. The tables also show the
values at the end of the 24-hr rainfall (48 hours of the model simulation) and after 48 hours of
rainfall (96 hrs of the model simulation). This time was selected to show the potential
groundwater movement that affects flooding elevations as well.
Figure 35 through Figure 40 show the flood water depth above grade at the end of the rainfall
and after 48 hours of the end of the rainfall for the complete study area for the 2-yr, 5-yr, 10yr, 25-yr, 50-yr and 100-yr return period of rainfall over 24 hours.
- 39 -
Final Report
Table 6 . Total Volume above Grade for Maximum Rainfall – at the end of the
Rainfall – Tuna Street – Existing Conditions
Precipitation Depth
Area of
Average Depth
Surface Flood
Surface Flood
Flood Volume
Flood Volume
24 hours
Flooding
Above Grade*
Volume
Volume
per Acre
per Acre
[in]
[acre]
[in]
[cu ft]
[gal]
[cu ft / acre]
[gal / acre]
2 YR
3.93
1.0
3.39
12,452
93,146
231
1,725
5 YR
5.08
1.6
3.58
20,888
156,257
387
2,894
10 YR
6.04
1.9
3.78
25,426
190,202
471
3,522
25 YR
7.46
2.7
3.94
38,493
287,945
713
5,332
50 YR
8.69
6.0
4.37
95,646
715,478
1,771
13,250
100 YR
10.04
8.2
6.06
180,555
1,350,648
3,344
25,012
Return Period
* Accounting only for model cells with surface flooding
Table 7. Total Volume above Grade for Maximum Rainfall – after 48 hours of the end
of the Rainfall – Tuna Street – Existing Conditions
Precipitation Depth
Area of
Average Depth
Surface Flood
Surface Flood
Flood Volume
Flood Volume
24 hours
Flooding
Above Grade*
Volume
Volume
per Acre
per Acre
[in]
[acre]
[in]
[cu ft]
[gal]
[cu ft / acre]
[gal / acre]
2 YR
3.93
0.8
3.78
11,527
86,225
213
1,597
5 YR
5.08
1.6
3.94
22,248
166,427
412
3,082
10 YR
6.04
1.5
4.29
22,711
169,888
421
3,146
25 YR
7.46
2.1
4.37
33,319
249,244
617
4,616
50 YR
8.69
5.8
4.84
102,511
766,833
1,898
14,201
100 YR
10.04
7.9
7.17
206,314
1,543,334
3,821
28,580
Return Period
- 40 -
Final Report
Table 8. Total Volume above Grade for Maximum Rainfall – at the end of the Rainfall
– Barracuda Street – Existing Conditions
Precipitation Depth
Area of
Average Depth
Surface Flood
Surface Flood
Flood Volume
Flood Volume
24 hours
Flooding
Above Grade*
Volume
Volume
per Acre
per Acre
[in]
[acre]
[in]
[cu ft]
[gal]
[cu ft / acre]
[gal / acre]
2 YR
3.93
1.0
3.54
13,349
99,856
247
1,849
5 YR
5.08
1.7
3.94
23,661
176,994
438
3,278
10 YR
6.04
2.1
4.53
34,520
258,226
639
4,782
25 YR
7.46
3.5
4.37
55,663
416,385
1,031
7,711
50 YR
8.69
7.5
4.88
132,245
989,264
2,449
18,320
100 YR
10.04
13.3
5.63
272,698
2,039,921
5,050
37,776
Return Period
Table 9. Total Volume above Grade for Maximum Rainfall – after 48 hours of the end
of the Rainfall – Barracuda Street – Existing Conditions
Precipitation Depth
Area of
Average Depth
Surface Flood
Surface Flood
Flood Volume
Flood Volume
24 hours
Flooding
Above Grade*
Volume
Volume
per Acre
per Acre
[in]
[acre]
[in]
[cu ft]
[gal]
[cu ft / acre]
[gal / acre]
2 YR
3.93
0.9
3.82
12,332
92,248
228
1,708
5 YR
5.08
1.4
4.21
21,916
163,944
406
3,036
10 YR
6.04
2.1
4.33
33,796
252,811
626
4,682
25 YR
7.46
3.7
4.41
59,328
443,806
1,099
8,219
50 YR
8.69
6.5
5.47
130,081
973,070
2,409
18,020
100 YR
10.04
11.5
6.26
260,535
1,948,941
4,825
36,091
Return Period
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Table 10. Total Volume above Grade for Maximum Rainfall – at the end of the
Rainfall – Mackerel Street – Existing Conditions
Precipitation Depth
Area of
Average Depth
Surface Flood
Surface Flood
Flood Volume
Flood Volume
24 hours
Flooding
Above Grade*
Volume
Volume
per Acre
per Acre
[in]
[acre]
[in]
[cu ft]
[gal]
[cu ft / acre]
[gal / acre]
2 YR
3.93
0.9
3.31
10,382
77,666
192
1,438
5 YR
5.08
1.1
3.74
15,097
112,933
280
2,091
10 YR
6.04
1.6
3.78
22,036
164,842
408
3,053
25 YR
7.46
2.3
4.09
34,891
261,000
646
4,833
50 YR
8.69
5.1
4.29
80,065
598,926
1,483
11,091
100 YR
10.04
8.6
5.55
174,277
1,303,679
3,227
24,142
Return Period
Table 11. Total Volume above Grade for Maximum Rainfall – after 48 hours of the
end of the Rainfall – Mackerel Street – Existing Conditions
Precipitation Depth
Area of
Average Depth
Surface Flood
Surface Flood
Flood Volume
Flood Volume
24 hours
Flooding
Above Grade*
Volume
Volume
per Acre
per Acre
[in]
[acre]
[in]
[cu ft]
[gal]
[cu ft / acre]
[gal / acre]
2 YR
3.93
0.7
3.31
8,899
66,571
165
1,233
5 YR
5.08
1.2
3.46
14,917
111,585
276
2,066
10 YR
6.04
1.7
3.70
22,905
171,341
424
3,173
25 YR
7.46
1.6
3.98
23,184
173,428
429
3,212
50 YR
8.69
3.6
5.51
71,688
536,265
1,328
9,931
100 YR
10.04
8.5
6.73
207,130
1,549,436
3,836
28,693
Return Period
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Final Report
Rainfall – Coral Street – Existing Conditions
Precipitation Depth
Area of
Average Depth
Surface Flood
Surface Flood
Flood Volume
Flood Volume
24 hours
Flooding
Above Grade*
Volume
Volume
per Acre
per Acre
[in]
[acre]
[in]
[cu ft]
[gal]
[cu ft / acre]
[gal / acre]
2 YR
3.93
1.1
3.19
12,586
94,150
233
1,744
5 YR
5.08
1.7
3.90
23,774
177,839
440
3,293
10 YR
6.04
2.1
4.09
31,218
233,526
578
4,325
25 YR
7.46
3.6
4.72
62,295
465,996
1,154
8,630
50 YR
8.69
6.4
5.67
131,709
985,248
2,439
18,245
100 YR
10.04
10.1
5.87
215,209
1,609,878
3,985
29,813
Return Period
end of the Rainfall – Coral Street – Existing Conditions
Precipitation Depth
Area of
Average Depth
Surface Flood
Surface Flood
Flood Volume
Flood Volume
24 hours
Flooding
Above Grade*
Volume
Volume
per Acre
per Acre
[in]
[acre]
[in]
[cu ft]
[gal]
[cu ft / acre]
[gal / acre]
2 YR
3.93
1.3
3.31
15,129
113,170
280
2,096
5 YR
5.08
1.5
4.09
22,403
167,589
415
3,104
10 YR
6.04
1.9
4.33
29,134
217,940
540
4,036
25 YR
7.46
3.9
4.88
68,750
514,286
1,273
9,524
50 YR
8.69
5.9
6.30
135,607
1,014,413
2,511
18,785
100 YR
10.04
9.4
6.69
228,731
1,711,029
4,236
31,686
Return Period
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Final Report
Rainfall – Marlin Street – Existing Conditions
Precipitation Depth
Area of
Average Depth
Surface Flood
Surface Flood
Flood Volume
Flood Volume
24 hours
Flooding
Above Grade*
Volume
Volume
per Acre
per Acre
[in]
[acre]
[in]
[cu ft]
[gal]
[cu ft / acre]
[gal / acre]
2 YR
3.93
0.8
3.07
9,365
70,058
173
1,297
5 YR
5.08
1.8
3.07
19,833
148,358
367
2,747
10 YR
6.04
2.1
3.54
26,698
199,713
494
3,698
25 YR
7.46
3.4
3.78
47,124
352,508
873
6,528
50 YR
8.69
6.7
5.43
132,556
991,589
2,455
18,363
100 YR
10.04
10.9
6.38
252,865
1,891,563
4,683
35,029
Return Period
end of the Rainfall – Marlin Street – Existing Conditions
Precipitation Depth
Area of
Average Depth
Surface Flood
Surface Flood
Flood Volume
Flood Volume
24 hours
Flooding
Above Grade*
Volume
Volume
per Acre
per Acre
[in]
[acre]
[in]
[cu ft]
[gal]
[cu ft / acre]
[gal / acre]
2 YR
3.93
0.8
2.95
8,740
65,382
162
1,211
5 YR
5.08
1.4
3.43
16,898
126,405
313
2,341
10 YR
6.04
2.1
3.46
25,794
192,950
478
3,573
25 YR
7.46
3.2
4.65
54,172
405,237
1,003
7,504
50 YR
8.69
6.5
6.26
148,797
1,113,080
2,756
20,613
100 YR
10.04
11.3
7.83
322,565
2,412,955
5,973
44,684
Return Period
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Existing Conditions at the end of the Rainfall – 48 hrs of Modeling
Existing Conditions after 48 hrs from the end of the Rainfall – 96 hrs of Modeling
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Existing Conditions after 48 hrs from the end of the Rainfall – 96 hrs of Modeling
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Existing Conditions after 48 hrs of the end of the Rainfall – 96 hrs of Modeling
Figure 37. MIKESHE Model Results for a 10 yr Return Period 24 hrs Rainfall - Existing Conditions
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Figure 38. MIKESHE Model Results for a 25 yr Return Period 24 hrs Rainfall – Existing Conditions
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It is interesting to note that the average flooding volume per acre for each sub-model is within
the same order of magnitude for the same return periods. Also, the average depth above
grade has similar patterns in all the sub-areas.
Another interesting note is that the Barracuda Street and Marlin Street areas appear to be more
susceptible to flooding based on the modeling. After comparing the north and south extremes
of the study area, this difference is likely due to the lower topography of both areas (being low
spots) and the less permeable soils present in the Marlin Street area. For example, the soil
hydraulic conductivity in the Tuna Street area ranges from 12 to 24 ft/day, while in the Marlin
Street area, it ranges from 3.2 to 8 ft/day.
The flooding volume is larger two days after the rainfall event has ended for the majority of the
cases. This is because the maximum flood elevation is not reached during the rainfall period,
but after all runoff has collected, infiltrated, and moved by groundwater transmission several
days later. Additionally, the shape of the water table, higher at the center and decreasing in
elevation towards the shoreline, will add to flooding problems. After a rainfall event, the water
table slope directs infiltrated waters towards the shoreline in an attempt to reach an equilibrium
level. Delayed flooding is a result of this movement. There are also some localized pronounced
gradients longitudinally along the island which direct infiltrated waters northward and
southward to low spots. This event can be observed in the figures above; after 48 hours of the
end of the rain, there are flooded areas towards the sides of the island that are not present
immediately after the rain stops.
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Final Report
FORMULATION OF ALTERNATIVE SOLUTIONS
Several viable solutions exist to prevent flooding within the existing physical constraints. The
existing topography is one of the greatest constraints. The study area is located on a barrier
island that has a series of interior dunes and troughs which direct all runoff to a few low-lying
areas where the only way for floodwaters to move is through groundwater transmission. To
further complicate matters, the area is heavily developed and the natural trough system has
been interrupted by the development. Therefore, the island location and the hydraulic
complexities involved on a barrier island, along with the physical infrastructure constraints, will
have a direct effect on solutions that are viable.
Permitting issues also play a large role in developing feasible solutions. There are many options
that may work from a flooding or hydraulic perspective but they are not permitted. Therefore,
the existing permitting issues were identified.
Project team members visited numerous locations throughout the study area to assess the
existing natural resources, including wetland areas. The areas visited included several locations
along a natural swale that occurs behind the second row of houses. This inter-dune swale
consists of both uplands and wetlands. Another area that was visited includes a property that is
owned by Currituck County and lies directly adjacent to Currituck Sound east of Herring Street.
This property also consists of both uplands and wetlands.
USACE Jurisdictional Wetlands
Section 404 of the Clean Water Act (CWA) requires regulation of discharges into "Waters of the
United States." Although the principal administrative agency of the CWA is the U.S.
Environmental Protection Agency (EPA), the United States Army Corps of Engineers (USACE)
has major responsibility for implementation, permitting, and enforcement of the provisions of
the Act. The USACE regulatory program is defined in 33 CFR 320-330. Water bodies such as
rivers, lakes, and streams are subject to jurisdictional consideration under the Section 404
program. However, by regulation, wetlands are also considered "Waters of the United States."
Wetlands have been described as:
“Those areas that are inundated or saturated by surface or ground water at a frequency and
duration sufficient to support, and that under normal circumstances do support, a prevalence of
vegetation typically adapted for life in saturated soil conditions. Wetlands generally include
swamps, marshes, bogs, and similar areas.”
According to the 1987 Corps of Engineers Wetland Delineation Manual, areas must exhibit three
distinct characteristics to be considered jurisdictional wetlands: 1) display a prevalence of
hydrophytic (water tolerant) plants, 2) area dominated by hydric soils, and 3) possess sufficient
wetland hydrology. Vegetation, soils, and hydrology data were collected by ESI during the field
surveys in order to determine whether the three criteria were satisfied within each potential
wetland area.
Wetlands subject to the jurisdiction of USACE under Section 404 were identified during the site
investigations (see Figure 41). The inter-dune swale that runs parallel with the beach contains
pockets of non-tidal freshwater wetlands. The location of the wetlands occurring within the
inter-dune swale are dictated primarily by site topography and it is important to note that some
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Final Report
portions of this inter-dune swale have been affected by previous construction activities.
Dominant wetland vegetation occurring within the inter-dune swale wetlands include such
species as wax myrtle (Myrica cerifera), black willow (Salix nigra), groundsel tree (Baccharis
halimifolia), plume grass (Erianthus giganteus), soft rush (Juncus effusus), cattail (Typha
latifolia), beakrush (Rhynchospora sp.), pennywort (Hydrocotyle sp.), knotgrass (Paspalum
distichum), and switchgrass (Panicum virgatum). Hydrology in these wetlands appears to be
driven primarily by precipitation, surface runoff and groundwater. Hydric soils are present in
those areas that are considered jurisdictional. Given the presence of these wetland areas,
significant mitigation costs would be realized if these areas were disturbed to construct a swale
or some other type of stormwater BMP.
Wetlands were also observed on the County-owned property located adjacent to Currituck
Sound. The wetlands on this particular parcel consist of a man-made pond adjacent to the
sound, the shoreline along the sound, and a small depression located beside the main road
(Ocean Trail). The shoreline along the sound was not investigated. However, the small
depression along Ocean Trail was vegetated with red maple (Acer rubrum), wax myrtle, and
royal fern (Osmunda regalis). Hydrology was evidenced by buttressed tree trunks and water
marks on the trees. The depression also contained hydric soils.
On July 23, 2009, a site visit was conducted to determine the presence/absence of jurisdictional
wetland areas within the corridor for the proposed outfall to the Corolla Light pond. The project
area encompassing the proposed infiltration basins was delineated by ESI; however, no
delineation of the ponds (Corolla Light and Timbuk II) or surrounding areas was conducted
since it had been determined that the ponds were not considered CAMA Area of Environmental
Concerns (AECs).
Mowed/maintained lawn areas surround the majority of the Corolla Light pond area with the
exception of the southwest portion, which is bordered by a wetland area. There is a large
wetland characterized as high marsh dominated by narrow-leaved cattail (Typha angustifolia),
common reed (Phragmites australis), and silverling (Baccharis halimifolia) located on the sound
side of the Corolla Light pond. The western boundary of the marsh is the Currituck Sound and
the eastern boundary of the marsh is located from 30 to 75 ft west of the pond in the northern
portion and abuts the southern portion of the pond. Based on visual inspection and aerial
photo interpretation, the marsh appears to extend both to the south and north of the Corolla
Light pond.
Mowed/maintained lawn areas were characterized primarily by Bermuda grass (Cynodon
dactylon), as well as scattered rush, sedge, and disturbance related weed species. Soil was
characterized by an A horizon of loamy sand with no redoximorphic features to 4”. The B
horizon was characterized by loamy sand with no redoximorphic features to 14”. The soil was
moist but not saturated at 12”. Therefore, the area would meet the criteria for hydrophytic
vegetation, but not for hydric soils or wetland hydrology and would not be considered
jurisdictional wetland. Figure 42 shows photos of the area surrounding the Corolla Light pond.
The southern pond located near the Timbuk II development has a healthy wetland system
located between the pond and the Currituck Sound. There is an upland area on the northwest
side of the pond that forces the drainage through the wetland system. There is no direct
connection of the pond to the Currituck Sound, so the pond would not be considered
jurisdictional wetland. Figure 43 shows photos of the area surrounding the Timbuk II pond.
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Final Report
Figure 41. Wetlands Delineated by ESI
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Final Report
Figure 42. Corolla Light Pond
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Final Report
Figure 43. Timbuk II Pond
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Final Report
Groundwater and Surface Water Quality
Section 401 of the Clean Water Act, as regulated by NCDWQ, requires that existing uses of
surface waters (based on their current Best Usage Classifications) are not removed or degraded
by discharges into the resource. Section 401 review by DWQ can triggered whenever a federal
action is requested (i.e. wetland permit, NPDES, etc.). DWQ, along with other regulatory
agencies, will be concerned about the potential for detrimental affects to the water quality and
biological integrity of the Currituck Sound from any potential outflow from the chosen pond.
The Currituck Sound, from its source to the Wright Memorial Bridge, is classified as Class SC
waters (tidal salt waters). SC waters are salt waters acceptable for any usage except primary
recreation or shellfishing for market purposes; usages include aquatic life propagation and
maintenance of biological integrity (including fish and functioning primary nursery areas [PNA]),
wildlife, and secondary recreation. All saltwaters shall be classified to protect these uses at a
minimum. Any source of water pollution which precludes any of these uses, including their
functioning as a PNA, on either a short-term or long-term basis shall be considered to be
violating a water quality standard. This portion of the Currituck Sound west of the Whalehead
Subdivision does not contain PNAs nor does it contain Class SA waters or Outstanding Resource
Waters. This portion of the Currituck Sound is also a Closed Shellfishing Area in its entirety.
ESI Project team members completed a sampling event on October 2nd, 2008 in Corolla,
Currituck County, North Carolina. Two sets of groundwater samples were collected from
existing, shallow monitoring wells (MW-1, MW-3, MW-7, MW-13 and MW-14) and two sets of
surface water samples from the north and south ponds (Figure 44). The samples collected
were submitted for laboratory analyses for fecal coliform, nitrate/nitrite, total nitrogen,
phosphorus, total suspended solids, copper and zinc. Table 15 shows the laboratory results of
the analysis for the groundwater sampling and Table 16 shows the laboratory results of the
surface water sampling.
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Final Report
Figure 44. Water Sampling Well and Pond Locations
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Final Report
The results of the sampling conducted by ESI indicate that three parameters analyzed from
both groundwater and surface water samples exceed state surface water quality standards
regulated under 15 NCAC 2B .0300 Classifications and Water Quality Standards Applicable to
Surface Waters and Wetlands. These parameters exceeding surface water quality standards
applicable to Class SC waters include fecal coliform, copper and zinc.
FECAL COLIFORM
State surface water quality standards applicable to Class SC waters limit fecal coliform
concentrations to 200 CFO/100ml. Results from groundwater monitoring well 7 (MW-7)
exceeds surface water quality standards for fecal coliform pursuant to 15 NCAC 2B .0300. The
levels reported from the two replicates at MW-7 were 277 and 233 CFU/100ml, respectively.
This result from MW-7 could be the results of a failing septic system, but that cannot be
reported with any degree of certainty at this time.
The northern pond located at Corolla Bay also exceeded surface water quality standards for
fecal coliform in both replicates (250 and 220 CFU/100ml). Note that the fecal coliform NCAC
2B standard is based on the geometric mean of 5 consecutive sampling events during a 30 day
period. Therefore, the results presented in the report represent only one point in time and
cannot be used to make an accurate determination of the overall water quality of the Corolla
Bay pond.
COPPER
State surface water quality standards applicable to Class SC waters limit copper concentrations
to 3 µ/L. Copper levels recorded from monitoring wells 1 and 7 (MW-1 and MW-7) both
exceeded the state surface water quality standard for copper pursuant to 15 NCAC 2B. MW-1
showed copper levels of 50.4 and 73.2 µ/L. MW-7 showed copper levels of 28.4 and 39.6 µ/L.
The amount of copper occurring in both replicates at each of these monitoring well locations
could be a natural occurrence or it could be the result of an anthropogenic source. Copper is
generally not bio-accumulative and has variable toxicity to aquatic life.
NITRATE/NITRITE
State surface water quality standards applicable to Class SC waters limit nitrate/nitrite
concentrations to 10 mg/L. None of the monitoring wells exceeded the surface water quality
standards pursuant to 15 NCAC 2B .0300. The two ponds also did not exceed surface water
quality standards for nitrate/nitrite. Nitrogen input into surface is discouraged due to its
association with algal blooms.
ZINC
State surface water quality standards applicable to Class SC waters limit zinc concentrations to
86 mg/L. Zinc levels from monitoring wells 1, 3, 7, and 14 all exceeded the state surface water
standards pursuant to 15 NCAC 2B .0300. Zinc is generally not bio-accumulative. And has
variable toxicity to aquatic life.
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Final Report
Table 15. Summary of Groundwater Analytical Data
1) TSS = Total suspended solids; 2) NC 2B = 15A NCAC 2B .0300 Classifications and Water Quality
Standards. Applicable to Surface Waters and Wetlands of NC; 3) ND = Not Detected; 4) NS = No Standard
listed in NC 2L or NC 2B standards; 5) * Higher detection limit due to higher content of sediment in the
sample; 6) b = The analyte was detected in the associated method blank; 7) Highlighted concentration
exceeds the applicable standard; 8) (µ/L) = micrograms per liter; 9) (mg/L) = milligrams per liter.
Table 16. Summary of Surface Water Analytical Data
1) NP = North Pond; 2) SP = South Pond; 3) TSS = Total suspended solids; 4) NC 2B = 15A NCAC 2B .0300
Classifications and Water Quality Standards. Applicable to Surface Waters and Wetlands of NC; 5) AL Aquatic Life; 6) ND = Not Detected; 7) NS = No Standard listed in NC 2L or NC 2B standards; 8) b = The
analyte was detected in the associated method blank; 9) Highlighted concentration exceeds the applicable
standard; 10) µ/L = micrograms per liter; 11) mg/L = milligrams per liter; 12) Fecal Coliform NC 2B
standard based on the geometric mean of 5 consecutive sampling events during a 30 day period.
NCDCM Areas of Environmental Concern
The North Carolina Division of Coastal Management (NCDCM) and the Coastal Resources
Commission (CRC) oversee the Coastal Area Management Act (CAMA), which affords additional
protection to certain areas located within any of the twenty coastal counties. Activities that
impact certain areas that are under the jurisdiction of CAMA, also known as Areas of
Environmental Concern (AEC), typically require CAMA approval as granted through the NCDCM.
Pre-determined areas within the project study area will qualify as AECs because they meet one
or more of the following criteria defining CAMA’s Estuarine and Ocean System AECs: 1) public
trust waters; 2) estuarine waters; 3) coastal shorelines; 4) coastal wetlands; and 5) ocean
hazard system. Public trust waters are the coastal waters and submerged lands that every
North Carolinian has the right to use. These areas often overlap with estuarine waters, but also
include many “inland” fishing waters as defined by the North Carolina Marine Fisheries
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Final Report
Commission. Estuarine waters are the state’s oceans, sounds, tidal rivers and their
tributaries, which stretch across coastal North Carolina and link to the other parts of the
estuarine system: public trust areas, coastal wetlands and coastal shorelines. Coastal
shorelines include all lands within 75 feet of the normal high water level of estuarine waters.
Coastal wetlands include any marsh in the 20 coastal counties that regularly or occasionally
flood by lunar or wind tides, and includes one or more of the ten-listed CAMA plant species.
Ocean hazard systems cover North Carolina’s beaches and other oceanfront lands that are
subject to long-term erosion and significant shoreline changes. The seaward boundary of this
AEC is the mean low water line. The landward limit of this AEC is measured from the first line
of stable vegetation and is determined by adding a distance equal to 60 times the long-term,
average erosion rate for that shoreline to the distance of erosion expected during a major
storm.
Protected Species in Currituck County
The United States Fish & Wildlife Service (USFWS) lists eight federally protected species with
ranges considered to extend into Currituck County based on the most recent list (1-31-08)
available at http://www.fws.gov/nc-es/es/countyfr.html. A brief description of each species’
habitat requirements follows, along with the Biological Conclusion rendered based on a detailed
habitat evaluation and survey results in the study area. Habitat requirements for each species
are based on the current best available information as per referenced literature and USFWS
correspondence.
Table 17. Federally Protected Species Listed for Currituck County
Scientific Name
Common Name
Federal
Status1
Haliaeetus
leucocephalus
Dermochelys coriacea
Bald eagle
Leatherback sea
turtle
Loggerhead sea
turtle
Piping plover
Red-cockaded
woodpecker
Shortnose sturgeon
Caretta caretta
Charadrius melodus
Picoides borealis
Acipenser
brevirostrum
Trichechus manatus
Amaranthus pumilus
West Indian manatee
Seabeach amaranth
BGPA
Potential
Habitat
Present
Yes
Biological
Conclusion
Not Required
E
Yes
NLAA2
T
Yes
NLAA
T
E
No
No
No Effect
No Effect
E
Yes
NLAA
E
T
Yes
No
NLAA
No Effect
1) BGPA –Bald and Golden Eagle Protection Act, E – Endangered, T –Threatened;
2) Not Likely to Adversely Affect
BALD EAGLE
Effective August 9, 2007, the bald eagle was delisted from the Endangered Species Act. A
biological conclusion is no longer necessary for this species. The bald eagle is protected under
the Bald and Golden Eagle Protection Act and the Migratory Bird Treaty Act.
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Final Report
USFWS optimal survey window: year round.
Habitat Description: Habitat for the bald eagle consists primarily of mature forest in proximity to
large bodies of open water for foraging. Large dominate trees are utilized for nesting sites,
typically within 1.0 mile of open water. The National Bald Eagle Management Guidelines restrict
disturbance activities within a primary zone extending 330 to 660 ft outward from a nest tree,
which is considered critical for maintaining acceptable conditions for bald eagles.
Biological Conclusion: Not required.
The bald eagle is listed as having a range that extends into Currituck County. The Currituck
Sound provides foraging habitat and the forested shoreline offers trees that may be suitable for
nesting. No evidence of bald eagle nests or foraging activity has been observed by ESI staff
during the several site visits conducted during 2008.
LEATHERBACK SEA TURTLE
USFWS optimal survey window: April – August.
Habitat Description: Leatherbacks are distributed world-wide in tropical waters of the Atlantic,
Pacific, and Indian oceans. They are generally an open ocean species, and may be common off
the North Carolina coast during certain times of the year. However, in northern waters
leatherbacks are reported to enter into bays, estuaries, and other inland bodies of water. Major
nesting areas occur mainly in tropical regions. In the United States, primary nesting areas are
in Florida. However, nests are known from Georgia, South Carolina, and North Carolina as well.
Nesting occurs from April to August. Leatherbacks need sandy beaches backed with vegetation
in the proximity of deep water and generally with rough seas. Beaches with a relatively steep
slope are usually preferred.
Biological Conclusion: Not Likely to Adversely Affect.
The proposed project will not affect beach habitat that could be utilized by the leatherback sea
turtle and should not adversely affect this species.
LOGGERHEAD TURTLE
USFWS optimal survey window: April – August.
Habitat Description: The loggerhead is widely distributed within its range, and is found in three
distinct habitats during their lives. These turtles may be found hundreds of miles out in the
open ocean, in neritic (nearshore) areas, or on coastal beaches. In North Carolina, this species
has been observed in every coastal county. Loggerheads occasionally nest on North Carolina
beaches, and are the most common of all the sea turtles that visit the North Carolina coast.
They nest nocturnally, at two to three year intervals, between May and September, on isolated
beaches that are characterized by fine-grained sediments. In nearshore areas, loggerheads
have been observed in bays, lagoons, salt marshes, creeks, ship channels, and the mouths of
large rivers. Coral reefs, rocky places, and shipwrecks are often used as foraging areas.
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Final Report
The proposed project will not affect beach habitat that could be utilized by the loggerhead sea
turtle. Although the loggerhead has been observed in inshore waters, this project should not
adversely affect this species.
PIPING PLOVER
Habitat Description: The piping plover breeds along the entire eastern coast of the United
States. North Carolina is uniquely positioned in the species’ range, being the only state where
the piping plover’s breeding and wintering ranges overlap and the birds are present year-round.
They nest most commonly where there is little or no vegetation, but some may nest in stands
of beach grass. The nest is a shallow depression in the sand that is usually lined with shell
fragments and light colored pebbles.
Biological Conclusion: No Effect.
Habitat for the piping plover does not occur within the proposed project area. The proposed
project should have no effect on the piping plover.
RED-COCKADED WOODPECKER
USFWS optimal survey window: year round; November-early March (optimal).
Habitat Description: The red-cockaded woodpecker (RCW) typically occupies open, mature
stands of southern pines, particularly longleaf pine (Pinus palustris), for foraging and
nesting/roosting habitat. The RCW excavates cavities for nesting and roosting in living pine
trees, aged 60 years or older, and which are contiguous with pine stands at least 30 years of
age to provide foraging habitat. The foraging range of the RCW is normally no more than 0.5
miles.
Habitat for the RCW does not occur within the project area. This project should have no effect
on the RCW.
SHORTNOSE STURGEON
USFWS optimal survey window: surveys not required; assume presence in appropriate waters.
Habitat Description: Shortnose sturgeon occurs in most major river systems along the eastern
seaboard of the United States. The species prefers the near shore marine, estuarine, and
riverine habitat of large river systems. It is an anadromous species that migrates to fastermoving freshwater areas to spawn in the spring, but spends most of its life within close
proximity of the river’s mouth. Large freshwater rivers that are unobstructed by dams or
pollutants are imperative to successful reproduction. Distribution information by river/waterbody
is lacking for the rivers of North Carolina; however, records are known from most coastal
counties.
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Final Report
Suitable habitat for this species does exist within the Currituck Sound. The project is not
expected to adversely affect this species due to the lack of direct impacts to the sound.
WEST INDIAN MANATEE
Habitat Description: Manatees have been observed in all the North Carolina coastal counties.
Manatees are found in canals, sluggish rivers, estuarine habitats, salt water bays, and as far off
shore as 3.7 miles. They utilize freshwater and marine habitats at shallow depths of 5 to 20
feet. In the winter, between October and April, manatees concentrate in areas with warm
water. During other times of the year, habitats appropriate for the manatee are those with
sufficient water depth, an adequate food supply, and in close proximity to freshwater.
Manatees require a source of freshwater to drink. Manatees are primarily herbivorous, feeding
on any aquatic vegetation present, but they may occasionally feed on fish.
Suitable habitat for this species does exist within the Currituck Sound. The project is not
expected to adversely affect this species due to the lack of direct impacts to the sound.
SEABEACH AMARANTH
USFWS optimal survey window: July-October.
Habitat Description: Seabeach amaranth occurs on barrier island beaches where its primary
habitat consists of overwash flats at accreting ends of islands, lower foredunes, and upper
strands of noneroding beaches (landward of the wrack line). In rare situations, this annual is
found on sand spits 160 feet or more from the base of the nearest foredune. It occasionally
establishes small temporary populations in other habitats, including sound-side beaches,
blowouts in foredunes, interdunal areas, and on sand and shell material deposited for beach
replenishment or as dredge spoil. The plant’s habitat is sparsely vegetated with annual herbs
and, less commonly, perennial herbs (mostly grasses) and scattered shrubs. It is, however,
intolerant of vegetative competition and does not occur on well-vegetated sites. The species
usually is found growing on a nearly pure silica sand substrate, occasionally with shell
fragments mixed in. Seabeach amaranth appears to require extensive areas of barrier island
beaches and inlets that function in a relatively natural and dynamic manner.
These
characteristics allow it to move around in the landscape, occupying suitable habitat as it
becomes available.
Suitable habitat for this species does not exist within the study area. The proposed project will
not impact any beach habitat and the sound side habitat is unsuitable for this species. This
project should have no effect on seabeach amaranth.
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Final Report
This proposed project should have no effect on piping plover, red-cockaded woodpecker and
seabeach amaranth. The project is not likely to adversely affect (NLAA) the leatherback sea
turtle, loggerhead sea turtle, shortnose sturgeon, or West Indian manatee.
USFWS
concurrence may be needed for the species receiving a NLAA determination. No evidence of
bald eagle activity was observed although the sound side shoreline does represent optimal
habitat for the bald eagle.
State and Federal Permitting Agencies
Dredge and fill activities in “Waters of the United States” must be authorized by USACE
pursuant to Section 404 of the CWA. Waters considered navigable by the USACE are also
subject to permitting of obstructions to navigation under Section 10 of the Rivers and Harbors
Act. Activities authorized by USACE are subject to further requirements of Section 401 of the
CWA. The North Carolina Division of Water Quality (NCDWQ) administers the Section 401
Water Quality Certification process in North Carolina. NCDCM also must issue authorization for
any impacts to any AEC’s under the jurisdiction of CAMA through either a General Permit or a
Major Permit, depending on the amount and type of impact.
Possible Solutions
From discussions with environmental agencies and the direction given by the County and
Drainage Board, the optimum solution would be one that could meet the following expectations:
1) Improve water quality.
2) Be installed with a minimum of disruption to the community.
3) Be permitted by environmental agencies.
4) Have minimal adverse affects on the beaches or sound.
Therefore, only the solutions that meet these requirements would be considered. The possible
solutions include ocean outfalls, dune infiltration by pumping into backside of primary dune and
pumping water into the existing soundside ponds. All the alternatives use an infiltration dry
basin collection system and pumping to convey the runoff to the ocean, dunes, or ponds.
The system will consists of five (with capacity of up to seven) infiltration basins that will pump
groundwater and infiltrated surface waters to either the ocean, dunes, or ponds. It is important
to note that the five infiltration basins could only be located along County-owned east/west
streets. The infiltration basins will be dry and the lift stations will be outfitted with infiltration
piping six-eight feet below ground along the basins to act as a wet well for the pump to
minimize pump short-cycling (see Figure 45). The pumps will tie into force mains installed four
to five feet below ground (to minimize utility conflicts) by directional drilling which ultimately
will join a major trunk line and ultimately discharge to the various locations. The infiltration
basins will include indigenous plantings to help maintain the basin hydrology and groundwater
levels (see Figure 46). The pumps themselves will be outfitted with automatic float systems to
maintain local groundwater levels (as to not cause excessive drawdown) during normal events
but will also contain a manual override so that the groundwater levels can be lowered in the
case of an impending hurricane or significant event. Similar systems have been installed at
Emerald Isle and operating for 3-4 years with great success. Figure 47 shows typical
development in the area where the dry basins could be installed.
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Final Report
Figure 45. Infiltration Plan and Section (Marlin Street)
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Final Report
Figure 46. Example Dry Detention Basin
Figure 47. Typical Development Along Whalehead Road
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Final Report
The following sections discuss each system and provide the pros and cons for each alternative:
OCEAN OUTFALL
This option would eliminate the need for constructing a large treatment impoundment and
would cause little disruption to the existing infrastructure if several outfalls were installed
adjacent to the flood areas. However, it is more likely that a single outfall would be required to
limit the number of discharge points due to regulations, overall costs and construction
efficiency, and therefore the collection system would impact infrastructure the same as other
alternatives. Refer to Figure 48 below for a typical outfall. A single outlet would also make it
easier to monitor the outfall for any environmental effects (see Figure 49).
This particular option had several major obstacles to overcome. The most notable is that North
Carolina law and regulations do not currently allow ocean outfalls. Acquiring environmental
permits would be an extensive and prolonged process with no guarantee of a permit being
granted. The permitting would require direct coordination with NCDCM. This scenario will incur
impacts to the Ocean Hazard System AEC. Although discharging onto the beach or in an outfall
may be the most practical and efficient option, constructing new oceanfront stormwater outfall
structures would likely meet considerable regulatory resistance from NCDCM, NCDWQ, local
health departments, and potentially citizens due to perceived environmental and health risks.
From an ecological perspective, it would be most beneficial to outfall the stormwater a
minimum of 1,000 feet offshore due to the potential issues with fecal coliform and other
pollutants; however, offshore discharges are likely cost-prohibitive.
Diffuser
Figure 48. Typical Ocean Outfall Schematic
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Final Report
Permitting for oceanfront outfalls would require a Major Permit from NCDCM as well as
concurrence from both USACE and NCDWQ. A project that proposes discharge into the surf
zone can be expected to receive considerable comments from both the regulatory community
and the general public. USACE can authorize certain outfall structures under Nationwide Permit
(NWP) #7 if the proposed effluent is regulated under the National Pollutant Discharge
Elimination System (NPDES) program. However, due to the proposed location of the proposed
oceanfront outfall structures, NCDCM will likely remain the lead regulatory agency and will
oversee the regulatory requirements.
This outfall system may also require the posting of the beaches during discharge periods
depending upon the distance from the shore to the discharge. This system also requires a
collection system and a pumping station to move the stormwater to the discharge. Since a
nearshore discharge would likely be required due to economic reasons, the probability of the
diffuser head being damaged during a hurricane is almost certain. The time and cost required
making repairs could be substantial and the project area would be without protection during
these periods.
In discussions held with the various agencies, significant concerns and opposition to this option
were raised. An Environmental Impact Statement (EIS) would be required and likely cost the
County $2 - $3 million with no guarantee of approval. Also, the marine construction costs alone
would likely reach over $5 million based on recently completed projects in Myrtle Beach. The
State of North Carolina does not currently allow ocean outfalls and a recently released Ocean
Policy report completed by the State recommends upholding the ban. These factors and the
extreme costs for the EIS study required (with no guarantee of approval) removed this option
from further consideration.
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Final Report
Figure 49. Pump to Deepwater Ocean Outfall
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Final Report
DUNE INFILTRATION INTO BACKSIDE OF PRIMARY DUNE
This alternative consists of pumping water from the groundwater located under selected eastwest street areas using infiltration pipes and discharging to the back side of the primary dune
(Figure 50).
There is some experience in North Carolina using dune infiltration systems. The NC Department
of Transportation and the Town of Kure Beach wanted to reduce the amount of stormwater
from nearby US 421 and other residential and commercial sites from entering ocean
recreational areas. Two stormwater Dune Infiltration Systems (DIS) were designed to divert a
portion of the flow into the beach dunes. This option was investigated since sand filters have
historically been successful in bacterial removal. The infiltration systems were constructed
using commercially available open-bottomed infiltration chambers. Due to limited land area, the
systems were designed to infiltrate 0.5-inch storms, which comprise approximately 80% of the
rainfall events at the site. The watersheds of both sites were small (4.5 ac and 8.1 ac) and of
mixed urban and residential land use. Water table measurements indicated a tidal influence,
but approximately 7 ft of sand was available for infiltration in the vertical direction. Figure 51
provides a plan view of the Dune Infiltration System used at Kure Beach, and Figure 52 shows
the storm chamber installation.
At both sites, the Dune Infiltration Systems reduced runoff volume and peak flow discharging
directly onto the beach. Overall, the two systems captured 97% of runoff from the two
watersheds during the study period. Routing the stormwater runoff through the sand beneath
the dune and into groundwater below did not cause significant increase fluctuations in the
groundwater.
The Town of Emerald Isle has also utilized the primary dune as a receiving area for floodwaters
in the past. The floodwaters were pumped to these areas and pump operations were
modulated as to not overwhelm the dunes. These systems have worked well based on Town
observations.
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Final Report
Figure 50. Pump to Backside of Primary Dune
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Final Report
Figure 51. Top View of Dune Infiltration System
Figure 52. Storm Chamber Installation
PUMPING WATER INTO SOUNDSIDE PONDS
This alternative consists of pumping groundwater from under selected east-west street basins
using infiltration pipes to the south and north ponds located close to the sound. The water will
overflow through the wetlands before spilling into the sound (Figure 53).
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Final Report
Figure 53. Pump to Soundside Ponds
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Final Report
The use of existing ponds is a recognized method for treatment of stormwater in North
Carolina, and therefore, the use of existing ponds for floodwater / stormwater management is
the option with the best chance of being permitted. This type of treatment system allows for
many of the contaminants in stormwater to be effectively removed by natural means. The
vegetation along the pond edge helps to remove phosphorous and nitrogen while the
vegetation also slows the flow to allow sediments to settle. Downward percolation through the
soils also removes additional contaminants and enteric bacteria.
Discharging the stormwater into Currituck Sound would only require a CAMA permit if a physical
connection to the sound is observed, and approval from USACE and NCDWQ would also be
needed. The construction of a direct sound-side discharge has the potential to impact Coastal
Wetlands, Coastal Shorelines and Estuarine Waters, all three of which are categories of CAMA
AECs. Additionally, surveys for submerged aquatic vegetation and shellfish beds may be
necessary to document potential impacts.
The northern pond located at the Corolla Bay development has high water levels and almost
overflows its banks on the southern end. A channel averaging 4 feet wide and 2 feet deep
connects the pond to the sound. It was determined that the channel is navigable, which places
the pond within CAMA jurisdiction.
However, the other northern pond located at the Corolla Light Subdivision was evaluated as
well and it was determined that there is not a navigable connection between this pond and the
Currituck Sound (See Figure 54).
The south pond is located within the adjacent lot to the north of the Timbuck II shopping area.
There is no direct connection between the pond and the Currituck Sound, and therefore the
pond does not fall under the CAMA jurisdiction. See Figure 55 for a view of the pond.
It is also important to note that this option would likely have the minimum opposition from the
Division of Water Quality and the Division of Shellfisheries (based on prior communication especially if no sound-side direct point discharge is proposed). The Division of Water Quality
has approved use of a similar system at Emerald Isle.
The system would require a costly array of pumps and piping to direct water from the basins to
the ponds. The pumps would be located in manholes below grade and would not be
susceptible to damage during hurricane events. However, due to power outages experienced
during hurricanes, an emergency generator is recommended to provide power for the pump
systems. Since the generators are located above grade, they would be susceptible to damage.
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Final Report
Figure 54. Northern Pond at Corolla Light Subdivision
Figure 55. Southern Pond at Timbuk II
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Final Report
EVALUATION OF ALTERNATIVE SOLUTIONS
Since all the alternatives have the proposed infiltration pipe system located in the east-west
streets basin areas, the groundwater hydraulic behavior around the potential flooded areas are
similar for all 3 proposed alternatives. For this reason, several pumping stations at each of the
five selected sub-areas (Tuna, Barracuda, Mackerel, Coral and Marlin streets) were placed in the
existing conditions model to create the pumping condition model. Each sub-area extracts 500
gpm of water for a combined total of 2500 gpm for the complete study area. The pumps were
set to start 12 hours prior to the beginning of the rainfall and then work nonstop for 8 days.
The alternative models were run for the same return periods and settings as the existing
conditions model.
Like the existing conditions model runs, the purpose of these simulations is to estimate the
surface flooding volume for each event. Table 16 through Table 25 show the total precipitation
depth over 24 hours, average surface flooding depth, and the total surface flood volume for the
different sub-areas. The tables show the results at the end of the 24 hours rainfall (48 hours of
the model simulation) and after 48 hours of the end of the rainfall (96 hours of the model
simulation).
Figure 56 through Figure 61 show the flood water depth above grade for the 2-yr, 5-yr, 10-yr,
25-yr, 50-yr and 100-yr return periods of rainfall over 24 hrs. Each figure shows the depth
above grade at the end of the rainfall and after 48 hours of the end of the rainfall for the
existing and pumping condition. Note that the flood depth and volume is reduced in every
case.
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Final Report
Table 16 . Total Volume above Grade for Maximum Rainfall – at the end of the
Rainfall – Tuna Street – Pumping Condition
Precipitation Depth
Area of
Average Depth
Surface Flood
Surface Flood
Flood Volume
Flood Volume
24 hours
Flooding
Above Grade*
Volume
Volume
per Acre
per Acre
[in]
[acre]
[in]
[cu ft]
[gal]
[cu ft / acre]
[gal / acre]
2 YR
3.93
0.5
2.68
4,563
34,131
84
632
5 YR
5.08
0.9
3.27
10,259
76,741
190
1,421
10 YR
6.04
1.4
3.54
17,481
130,764
324
2,422
25 YR
7.46
2.0
3.82
27,404
204,996
507
3,796
50 YR
8.69
2.8
4.45
45,093
337,319
835
6,247
100 YR
10.04
6.3
5.08
115,256
862,172
2,134
15,966
Return Period
end of the Rainfall – Tuna Street – Pumping Condition
Precipitation Depth
Area of
Average Depth
Surface Flood
Surface Flood
Flood Volume
Flood Volume
24 hours
Flooding
Above Grade*
Volume
Volume
per Acre
per Acre
[in]
[acre]
[in]
[cu ft]
[gal]
[cu ft / acre]
[gal / acre]
2 YR
3.93
0.6
2.68
6,244
46,705
116
865
5 YR
5.08
0.9
3.35
10,806
80,836
200
1,497
10 YR
6.04
1.1
3.43
14,133
105,721
262
1,958
25 YR
7.46
1.1
4.09
16,894
126,379
313
2,340
50 YR
8.69
2.4
4.76
42,303
316,449
783
5,860
100 YR
10.04
3.8
6.38
88,102
659,051
1,632
12,205
Return Period
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Final Report
Rainfall – Barracuda Street – Pumping Condition
Precipitation Depth
Area of
Average Depth
Surface Flood
Surface Flood
Flood Volume
Flood Volume
24 hours
Flooding
Above Grade*
Volume
Volume
per Acre
per Acre
[in]
[acre]
[in]
[cu ft]
[gal]
[cu ft / acre]
[gal / acre]
2 YR
3.93
0.7
2.95
7,416
55,476
137
1,027
5 YR
5.08
1.2
3.39
15,185
113,593
281
2,104
10 YR
6.04
1.3
4.21
19,649
146,984
364
2,722
25 YR
7.46
2.7
3.70
36,515
273,152
676
5,058
50 YR
8.69
6.6
4.13
98,633
737,827
1,827
13,663
100 YR
10.04
10.6
5.08
194,978
1,458,535
3,611
27,010
Return Period
end of the Rainfall – Barracuda Street – Pumping Condition
Precipitation Depth
Area of
Average Depth
Surface Flood
Surface Flood
Flood Volume
Flood Volume
24 hours
Flooding
Above Grade*
Volume
Volume
per Acre
per Acre
[in]
[acre]
[in]
[cu ft]
[gal]
[cu ft / acre]
[gal / acre]
2 YR
3.93
0.5
3.35
6,304
47,154
117
873
5 YR
5.08
1.2
3.46
15,538
116,235
288
2,152
10 YR
6.04
1.3
3.94
19,070
142,652
353
2,642
25 YR
7.46
2.7
3.66
36,455
272,703
675
5,050
50 YR
8.69
5.6
4.57
92,580
692,548
1,714
12,825
100 YR
10.04
7.8
5.87
166,274
1,243,818
3,079
23,034
Return Period
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Final Report
Rainfall – Mackerel Street – Pumping Condition
Precipitation Depth
Area of
Average Depth
Surface Flood
Surface Flood
Flood Volume
Flood Volume
24 hours
Flooding
Above Grade*
Volume
Volume
per Acre
per Acre
[in]
[acre]
[in]
[cu ft]
[gal]
[cu ft / acre]
[gal / acre]
2 YR
3.93
0.3
2.99
3,757
28,108
70
521
5 YR
5.08
0.7
3.23
8,398
62,820
156
1,163
10 YR
6.04
0.9
3.62
12,021
89,923
223
1,665
25 YR
7.46
1.1
4.09
16,527
123,632
306
2,289
50 YR
8.69
3.1
4.17
46,792
350,025
867
6,482
100 YR
10.04
5.9
5.00
106,293
795,125
1,968
14,725
Return Period
end of the Rainfall – Mackerel Street – Pumping Condition
Precipitation Depth
Area of
Average Depth
Surface Flood
Surface Flood
Flood Volume
Flood Volume
24 hours
Flooding
Above Grade*
Volume
Volume
per Acre
per Acre
[in]
[acre]
[in]
[cu ft]
[gal]
[cu ft / acre]
[gal / acre]
2 YR
3.93
0.3
3.11
3,627
27,130
67
502
5 YR
5.08
0.9
3.19
10,012
74,892
185
1,387
10 YR
6.04
1.0
3.54
12,395
92,724
230
1,717
25 YR
7.46
1.3
3.74
17,110
127,990
317
2,370
50 YR
8.69
2.1
5.31
41,000
306,701
759
5,680
100 YR
10.04
4.9
5.79
103,824
776,660
1,923
14,383
Return Period
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Final Report
Rainfall – Coral Street – Pumping Condition
Precipitation Depth
Area of
Average Depth
Surface Flood
Surface Flood
Flood Volume
Flood Volume
24 hours
Flooding
Above Grade*
Volume
Volume
per Acre
per Acre
[in]
[acre]
[in]
[cu ft]
[gal]
[cu ft / acre]
[gal / acre]
2 YR
3.93
0.6
2.60
6,060
45,332
112
839
5 YR
5.08
1.0
3.46
13,052
97,637
242
1,808
10 YR
6.04
1.6
3.46
20,511
153,430
380
2,841
25 YR
7.46
2.8
3.98
41,018
306,833
760
5,682
50 YR
8.69
4.7
5.08
86,556
647,481
1,603
11,990
100 YR
10.04
8.8
6.02
193,431
1,446,965
3,582
26,796
Return Period
end of the Rainfall – Coral Street – Pumping Condition
Precipitation Depth
Area of
Average Depth
Surface Flood
Surface Flood
Flood Volume
Flood Volume
24 hours
Flooding
Above Grade*
Volume
Volume
per Acre
per Acre
[in]
[acre]
[in]
[cu ft]
[gal]
[cu ft / acre]
[gal / acre]
2 YR
3.93
0.4
2.56
3,673
27,474
68
509
5 YR
5.08
0.9
3.54
12,078
90,346
224
1,673
10 YR
6.04
1.7
3.46
21,132
158,079
391
2,927
25 YR
7.46
2.6
4.25
40,047
299,569
742
5,548
50 YR
8.69
4.7
5.67
96,620
722,769
1,789
13,385
100 YR
10.04
6.5
6.73
159,423
1,192,569
2,952
22,085
Return Period
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Final Report
Rainfall – Marlin Street – Pumping Condition
Precipitation Depth
Area of
Average Depth
Surface Flood
Surface Flood
Flood Volume
Flood Volume
24 hours
Flooding
Above Grade*
Volume
Volume
per Acre
per Acre
[in]
[acre]
[in]
[cu ft]
[gal]
[cu ft / acre]
[gal / acre]
2 YR
3.93
0.2
2.72
2,437
18,228
45
338
5 YR
5.08
0.9
2.87
9,538
71,352
177
1,321
10 YR
6.04
1.5
3.07
16,803
125,692
311
2,328
25 YR
7.46
2.6
3.54
33,054
247,263
612
4,579
50 YR
8.69
5.5
4.65
92,510
692,020
1,713
12,815
100 YR
10.04
8.6
5.83
181,361
1,356,671
3,359
25,124
Return Period
end of the Rainfall – Marlin Street – Pumping Condition
Precipitation Depth
Area of
Average Depth
Surface Flood
Surface Flood
Flood Volume
Flood Volume
24 hours
Flooding
Above Grade*
Volume
Volume
per Acre
per Acre
[in]
[acre]
[in]
[cu ft]
[gal]
[cu ft / acre]
[gal / acre]
2 YR
3.93
0.3
2.72
2,924
21,873
54
405
5 YR
5.08
1.0
2.83
10,171
76,081
188
1,409
10 YR
6.04
1.2
3.15
13,561
101,441
251
1,879
25 YR
7.46
1.9
3.98
26,751
200,109
495
3,706
50 YR
8.69
4.0
5.51
79,104
591,741
1,465
10,958
100 YR
10.04
9.1
5.79
192,075
1,436,821
3,557
26,608
Return Period
- 82 -
Final Report
Existing Condition – 48 hrs of Modeling
Pumping Condition – 48 hrs of Modeling
Figure 56. MIKESHE Model Result for a 2 yr Return Period 24 hrs Rainfall – Existing and Pumping Conditions
- 83 -
Final Report
- 84 -
Final Report
- 85 -
Final Report
- 86 -
Final Report
- 87 -
Final Report
- 88 -
Final Report
Model results clearly show that the pumps help in reducing the overland flooding volume. Table
26 summarizes the percentage of flood water removed 48 hours after the end of the rainfall, as
compared to the existing conditions. It is very important to note that while the duration of
flooding should be considerably lessened with the system in place, the affect on peak flooding
levels will be less pronounced.
Table 26. Percentage of Flood Water Removed
Tuna
Barracuda
Mackerel
Coral
Marlin
Average
2 YR
46%
49%
59%
76%
67%
59%
5 YR
51%
29%
33%
46%
40%
40%
10 YR
38%
44%
46%
27%
47%
40%
25 YR
49%
39%
26%
42%
51%
41%
50 YR
59%
29%
43%
29%
47%
41%
100 YR
57%
36%
50%
30%
40%
43%
Return Period
Figure 62 shows that for the 100 year return period event, the flood water depth peak reduces
approximately 50% (in a single location along Marlin Street), and the flood duration lasts for
only 3 days instead for over a week. Figure 63 shows that for the 25 year storm (in another
location along Marlin Street), the peak of the flood water depth does not reduce in a
considerable percentage, but the duration of the flood does decrease over a substantial amount
of time. Analyzing different locations in the study area and recurrence intervals, it can be
observed that the peak flood elevation reduction ranges from 50% to negligible, and the flood
duration reduction varies from weeks to days. Therefore, the system impacts on flood volumes
and duration is much more pronounced than on peak flood elevations.
Given that pumping floodwater into the back side of the primary dune and into the soundside
ponds options had the potential to affect the neighboring properties, they were analyzed further
to determine which option would be preferred.
- 89 -
Final Report
Water Depth Over the Grade - Marlin Street - 100 yr
1.2
1
Elevation (ft)
0.8
Existing Condition
Pumping Condition
0.6
0.4
0.2
0
3/1/2008
0:00
3/2/2008
0:00
3/3/2008
0:00
3/4/2008
0:00
3/5/2008
0:00
3/6/2008
0:00
3/7/2008
0:00
3/8/2008
0:00
3/9/2008
0:00
3/10/2008
0:00
Date
Figure 62. Flood Water Depth above Grade – Marlin Street – 100 yr
Water Depth Over the Grade - South of Marlin Street - 25 yr
1.2
1
Elevation (ft)
0.8
Existing Condition
Pumping Condition
0.6
0.4
0.2
0
3/1/2008
0:00
3/2/2008
0:00
3/3/2008
0:00
3/4/2008
0:00
3/5/2008
0:00
3/6/2008
0:00
3/7/2008
0:00
3/8/2008
0:00
3/9/2008
0:00
3/10/2008
0:00
Date
Figure 63. Flood Water Depth above Grade – South of Marlin Street – 25 yr
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Final Report
DUNE INFILTRATION INTO BACKSIDE OF PRIMARY DUNE
Multiple cases were tested in the MIKESHE model. These models were run without precipitation
and included only the pumping wells and discharge points. This was done to allow a more
accurate determination of the impacts of pumping these waters on adjacent property owners
without precipitation effects.
Both the Tuna and Marlin Street sub-areas were assumed to be representative of the study area
(low and high flood volume per acre). Only the Tuna model results are shown in the following
figures. The pumping flow rate injected into the dune ranged from 200 to 500 gpm. Figure 64a
and Figure 64b show snapshots of groundwater table levels and flood water depth above grade
after one day of pumping water into the dune (at 500 gpm) for the Tuna Street area. Figure
64c and Figure 64d show snapshots of groundwater table levels and flood water depth above
grade after two days of pumping water (500 gpm) into the dune.
- 91 -
Final Report
a)
b)
c)
d)
Figure 64. Tuna Street Pumping - 500 gpm
a) Groundwater Table Levels after One Day of Injecting Water into the Backside of the Dune, b) Flood Water
Depth Above Grade After One Day of Injecting Water into the Backside of the Dune, c) Groundwater Table
Levels after Two Days of Injecting Water into the Backside of the Dune, d) Flood Water Depth Above Grade
After Two Days of Injecting Water into the Backside of the Dune
Note in Figure 64a that at the Tuna Street location, the water table has a lower depth than the
surrounding areas due to the extraction of water at that location. It can also be seen that at the
location of the injection in the back side of the dune, there is an increase in the water table
level. Figure 64b shows a higher elevation on the exact location of the injection of the pumped
water, and water flowing south along the backside of the dune. (Note that this is surface
flooding.) In Figure 64c, the depression of the groundwater table at the extraction area at
Tuna Street is more pronounced than in Figure 64a. The groundwater table at the injection
location has also increased in area. In Figure 64d, it can be observed that there are flooded
areas along the backside of the dune with heights up to 0.3 to 0.5 m (1 – 2 ft).
- 92 -
Final Report
Based on these model results it appeared that the dunes are not able to infiltrate the amount of
water that is being pumped into it without containment. Consequently, the idea of surrounding
the injection site with sheet piling was tested to see if the results could be improved. Figure 65a
to Figure 65d show snapshots of the groundwater table head and flood water depth above
grade for the Tuna Street area.
a)
b)
c)
d)
Figure 65. Tuna Street - Sheet Pile - 500 gpm
a) Groundwater Table Levels after One Day of Injecting Water into the Backside of the Dune b) Flood Water
Depth Above Grade After One Day of Injecting Water into the Backside of the Dune c) Groundwater Table
Levels after Two Days of Injecting Water into the Backside of the Dune d) Flood Water Depth Above Grade
In Figure 65a, the Tuna Street location water table is at a lower depth than the surrounding
areas due to the extraction of water at that location. Also, like in Figure 65a, it can be seen
that there is an increase in the water table level at the backside of the dune. The difference is
that in this case, the elevation is confined in a rectangle set by the limits of the sheet piling.
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Final Report
Figure 65b shows a higher elevation on the exact location of the injection of the pumped water
and water flowing south along the backside of the dune. In Figure 65c, the depression of the
groundwater table at the extraction area at Tuna Street is more pronounced than in Figure 65a.
The groundwater table at the injection location has maintained its characteristics. In Figure 65d
it can be observed that there are flooded areas in the backside of the dune similar to the
alternative without the sheet piling.
Considering the results from Figure 64a through Figure 65d, similar scenarios for the sheet
piling cases were tested, varying the pumping rate into the dune. The additional pumping rates
simulated were: 200 gpm, 250 gpm, 300 gpm and 400 gpm. Figures 54a through 54d show
snapshots of groundwater table levels and flood water depths above grade after one and two
days of injecting water into the dune at a pumping rate of 250 gpm for the Tuna Street area.
a)
b)
c)
d)
Figure 66. Tuna Street - Sheet Pile - 250 gpm
a) Groundwater Table Levels after One Day of Injecting Water into the Backside of the Dune b) Flood Water
Depth Above Grade After One Day of Injecting Water into the Backside of the Dune c) Groundwater Table
Levels after Two Days of Injecting Water into the Backside of the Dune d) Flood Water Depth Above Grade
- 94 -
Final Report
At the Tuna Street location, the water table has a lower depth than the surrounding areas due
to the extraction of water at that location. Also, compared with Figure 65a, water is slightly
higher due to the lower 250 gpm pumping rate. Figure 66b shows that after 24 hours of
injecting water at a rate of 250 gpm, the adjacent properties have not been impacted. In Figure
66c, the depression of the groundwater table at the extraction area at Tuna Street is more
pronounced than in Figure 66a. The groundwater table at the injection location has maintained
its characteristics. Figure 66d shows a higher elevation on the exact location of the injection of
the pumped water and water is just beginning to flow south along the backside of the dune.
Table 27 and Table 28 show the required time needed to pump the surface flood volume for the
different pumping rates for the Tuna and Marlin Street areas.
Table 27. Required Time to Pump Surface Flood Volume – Tuna Street Model
Hours / Days Required to Pump Surface Flood Volume
Surface Flood
Return Period
Volume
200 [gpm]
250 [gpm]
300 [gpm]
400 [gpm]
500 [gpm]
[gal]
[hrs]
[days]
[hrs]
[days]
[hrs]
[days]
[hrs]
[days]
[hrs]
[days]
2 YR
93,146
8
0.3
6
0.3
5
0.2
4
0.2
3
0.1
5 YR
156,257
13
0.5
10
0.4
9
0.4
7
0.3
5
0.2
10 YR
190,202
16
0.7
13
0.5
11
0.4
8
0.3
6
0.3
25 YR
287,945
24
1.0
19
0.8
16
0.7
12
0.5
10
0.4
50 YR
715,478
60
2.5
48
2.0
40
1.7
30
1.2
24
1.0
100 YR
1,350,648
113
4.7
90
3.8
75
3.1
56
2.3
45
1.9
Table 28. Required Time to Pump Surface Flood Volume – Marlin Street Model
Hours / Days Required to Pump Surface Flood Volume
Surface Flood
Return Period
Volume
200 [gpm]
250 [gpm]
300 [gpm]
400 [gpm]
500 [gpm]
[gal]
[hrs]
[days]
[hrs]
[days]
[hrs]
[days]
[hrs]
[days]
[hrs]
[days]
2 YR
70,058
6
0.2
5
0.2
4
0.2
3
0.1
2
0.1
5 YR
148,358
12
0.5
10
0.4
8
0.3
6
0.3
5
0.2
10 YR
199,713
17
0.7
13
0.6
11
0.5
8
0.3
7
0.3
25 YR
352,508
29
1.2
24
1.0
20
0.8
15
0.6
12
0.5
50 YR
991,589
83
3.4
66
2.8
55
2.3
41
1.7
33
1.4
100 YR
1,891,563
158
6.6
126
5.3
105
4.4
79
3.3
63
2.6
Table 29 and Table 30 show the pumping duration until impacts to the adjacent areas are
visible for the different pumping rates (numbers estimated by observing the simulation). With
this, an estimate can be made of how many hours the pumps can pump water without
impacting the adjacent properties.
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Final Report
Table 29. Pumping Duration until Adjacent Impact is Visible – Tuna Street Model
Pumping Duration Until Adjacent
Impact Visible
Pumping Rate
[hrs]
[days]
200 [gpm]
54
2.3
250 [gpm]
45
1.9
300 [gpm]
35
1.5
400 [gpm]
27
1.1
500 [gpm]
18
0.8
Table 30. Pumping Duration until Adjacent Impact is Visible – Marlin Street Model
Pumping Rate
Pumping Duration Until Adjacent
Impact Visible
[hrs]
[days]
200 [gpm]
6
0.3
250 [gpm]
6
0.3
300 [gpm]
5
0.2
400 [gpm]
4
0.2
500 [gpm]
4
0.2
By comparing Table 27 and Table 29, it can be seen that at a 300 gpm rate (nearly the 50 year
flood) could be pumped before visible impacts to adjacent properties would be seen for the
Tuna Street model (given normal groundwater level conditions). This is in an idealized case
that does not account for rainfall during the model simulation period.
By comparing Table 28 and Table 30, a 250 gpm rate (nearly the 2 year flood) could be
pumped before visible impacts to adjacent properties would be seen for the Marlin Street
model. The decreased infiltration capacity at Marlin is mostly due to the smaller hydraulic
conductivity coefficients. At that location, the conductivity coefficient is 4 to 5 times smaller
than the conductivity coefficient at the pump locations in the Tuna Street model. This,
combined with the lower topography of the back side of the dune of the Marlin model, causes
- 96 -
Final Report
the lower values in Table 30 and rends this option to be not cost-effective across the entire
study areas (even with containment sheet piling). Further discussions with the environmental
agencies also discouraged this option since sheet piling would be required and the State has
restrictions against fixed structures within the dune system.
PUMPING WATER INTO SOUNDSIDE POND
Four cases were tested in the MIKESHE model: pumping 3500 gpm for 10 days into the south
(Timbuk II) and north pond (Corolla Light), and pumping 6250 and 9000 gpm into the south
pond to account for other uses.
The 3500 gpm scenario simulates the project functioning with seven east-west basins (full
build-out with two additional pumps going to each pond). This conservative case was run for
each pond to estimate the possible impacts of all the design flow being placed in each individual
pond. It is expected that the actual flowrates will be 1000 - 1500 gpm to the north pond and
1500 - 2000 gpm to the south pond. The other two pumping scenarios (6250 and 9000 gpm)
to the south pond, simulates the project functioning with a future expansion and other planned
uses for the Timbuk II pond.
Following the previous models, this model was run without precipitation and including only the
discharge points. This would allow a more accurate determination of the impacts of pumping
these waters on adjacent property owners. Figure 67 shows the locations of the ponds.
The model shows that the maximum expected impact to the Corolla Light pond is expected to
be 1.5 feet (pumping 3500 gpm for 10 days). For the Timbuk II pond, the model shows the
maximum expected impact to be 1.6 feet (pumping 3500 gpm for 10 days). The model also
took into account the Monteray Shores wastewater treatment facility’s use of the pond as a
green area, which would add an additional 5500 gpm. The expected impact to the pond for this
scenario is 2.3 feet (pumping 9000 gpm for 10 days). The large surface area of the pond as
well as the low-lying area between the pond and the sound allow for most of the pumped
waters from both ponds to sheet flow toward the sound.
Figure 68 shows the sites where the groundwater elevations were measured within the model.
The ground elevations for the groundwater measurements are shown in Table 31. Figure 69 to
Figure 72 show how the groundwater table elevation reacted in locations next to the ponds
(properties adjacent to the ponds) and locations in the nearby wetlands for the three modeled
cases.
- 97 -
Final Report
North Pond
South Pond
Figure 67. North and South Pond Locations
- 98 -
Final Report
South Pond
North Pond
4
12 3
5
1
2
3
4
6
5
Figure 68. North and South Pond Groundwater Extraction Locations
Table 31. Ground Elevation in Groundwater Measuring Locations, meters NAVD88
Location
North Pond
South Pond
1
2
3
4
5
6
1
2
3
4
5
- 99 -
Ground
Elevation
1.50
2.06
2.13
0.61
0.79
0.60
1.37
1.60
1.67
0.30
0.30
Final Report
Ground Water Elevation - North Pond - 3500 gpm
1.60
1.40
1.20
Height (m)
1.00
Elevation - Location 1
Pond Elevation
0.80
0.60
0.40
0.20
0.00
2/27/2008 0:00
3/3/2008 0:00
3/8/2008 0:00
3/13/2008 0:00
3/18/2008 0:00
3/23/2008 0:00
Time
Ground Water Elevation - North Wetlands - 3500 gpm
0.70
0.60
Height (m)
0.50
0.40
0.30
0.20
0.10
0.00
2/27/2008 0:00
3/3/2008 0:00
3/8/2008 0:00
3/13/2008 0:00
3/18/2008 0:00
3/23/2008 0:00
Time
Figure 69. Groundwater Elevation Adjacent to the North Pond –
Pumping at a 3500 gpm rate
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Final Report
Ground Water Elevation - South Pond - 3500 gpm
1.60
1.40
1.20
Height (m)
1.00
Pond Elevation
0.80
0.60
0.40
0.20
0.00
2/27/2008 0:00
3/3/2008 0:00
3/8/2008 0:00
3/13/2008 0:00
3/18/2008 0:00
3/23/2008 0:00
Time
Ground Water Elevation - South Wetlands - 3500 gpm
0.70
0.60
Height (m)
0.50
0.40
0.30
0.20
0.10
0.00
2/27/2008 0:00
3/3/2008 0:00
3/8/2008 0:00
3/13/2008 0:00
3/18/2008 0:00
3/23/2008 0:00
Time
Figure 70. Groundwater Elevation Adjacent to the South Pond –
Pumping at a 3500 gpm Rate
- 101 -
Final Report
2
1.8
1.6
1.4
Height (m)
1.2
Pond Elevation
1
0.8
0.6
0.4
0.2
0
2/27/2008 0:00
3/3/2008 0:00
3/8/2008 0:00
3/13/2008 0:00
3/18/2008 0:00
3/23/2008 0:00
Time
0.9
0.8
0.7
Height (m)
0.6
0.5
0.4
0.3
0.2
0.1
0
2/27/2008 0:00
3/3/2008 0:00
3/8/2008 0:00
3/13/2008 0:00
3/18/2008 0:00
3/23/2008 0:00
Time
- 102 -
Final Report
2.00
1.80
1.60
1.40
Height (m)
1.20
Pond Elevation
1.00
0.80
0.60
0.40
0.20
0.00
2/27/2008 0:00
3/3/2008 0:00
3/8/2008 0:00
3/13/2008 0:00
3/18/2008 0:00
3/23/2008 0:00
Time
0.90
0.80
0.70
Height (m)
0.60
0.50
0.40
0.30
0.20
0.10
0.00
2/27/2008 0:00
3/3/2008 0:00
3/8/2008 0:00
3/13/2008 0:00
3/18/2008 0:00
3/23/2008 0:00
Time
- 103 -
Final Report
Figure 69 through Figure 72 indicate that the water level does not reach the ground level in the
adjacent properties east of the south pond for either case. However, in Location 1 of the north
pond, the groundwater level reaches the ground elevation. The maximum increase in water
flooding level is approximately 0.05 meters (0.15 ft) and will last for approximately 2 days. It is
also important to note that with both pumps operational, the expected flowrate to the north
pond should only be 1000 – 1500 gpm.
The minimal groundwater elevation difference given by the different pumping rates is because
the water pumped into the pond generates sheet flow over the wetlands adjacent to the sound.
This flow pattern does not allow the water surface elevation of the pond to increase
considerably, having little impact over the adjacent properties regarding the pumping rate. The
impact of this laminar flow can be observed looking at the groundwater elevation in Locations 4
to 6 in the north pond area, and 4 and 5 in the south pond area. The water level in the
surrounding wetlands increases in manageable levels. In Location 6 of the north pond, and 4
and 5 of the south pond, the laminar flow pattern can be observed.
Given that the actual flowrates to each pond are expected to be considerably less than 3500
gpm, these results should be conservative and the actual pond impacts should be much less.
This option and the capacity of the existing ponds would also allow the addition of other
infiltration pipe systems along the street right-of-way in other flood-prone areas not included
within the County-owned east/west streets if needed.
- 104 -
Final Report
SELECTION OF PREFERRED ALTERNATIVE
After extensive analysis of the results obtained by the different modeled options, the preferred
alternative selected was pumping water into the soundside ponds. This alternative was chosen
for being the most feasible, while achieving the desired results.
The ocean outfall option was not chosen due to several reasons. Environmentally, this
alternative was not desirable because an environmental impact statement ($2 - $3 million
effort) was needed for this alternative with no guarantee of approval. The overall budget for
this type of project would also be cost prohibitive ($7 - $10 million). For this alternative to be
environmentally viable, the stormwater collected by the east/west basins may need to be
treated to reduce (or eliminate) the contaminants present in the water. In conclusion, this
alternative was not environmentally/cost effective.
Infiltrating the water into the backside of the primary dune would reduce the contaminants of
the water that reaches the ocean, but in North Carolina there may be permitting issues for this
alternative due to the need for sheet piling. In addition, the total water pumped overwhelmed
the receiving capacity of the dune, and the adjacent neighboring properties would have likely
experienced significant impacts. The finer sands present in the project location (compared to
other areas of the coast) likely caused this behavior. For this reason, this alternative was not
selected.
On the other hand, the use of existing ponds/infiltration basins is a recognized method for
treatment of stormwater in North Carolina, and therefore, the use of soundside ponds is the
option with the best chance of being permitted. The capacity of the existing ponds would allow
for future collection systems to be added in flood-prone areas.
This alternative can be constructed in three phases (as shown in Figure 73). The first phase
would include the infiltration basins located at Coral and Marlin Streets, the force main to the
south (Timbuk II) pond, and the outfall to the north (Corolla Light) pond. The second phase
would include the infiltration basins located at Tuna and Barracuda Streets and the connection
of the force main to the north pond outfall. The final phase would include the infiltration basin
located at Mackerel Street and the connection of the two force main systems together. This
alternative will also allow for additional infiltration pipe collection systems in other areas along
the street right-of-way if necessary.
- 105 -
Final Report
Figure 73. Phased Plan for Soundside Ponds
- 106 -
Final Report
PRELIMINARY PROJECT BUDGET
Schematic level opinions of probable cost for the selected alternative are presented below.
Table 32 shows the budget needed for Phase 1 – Marlin and Coral Streets. Table 33 shows the
total cost for all three phases. The estimated cost for all three phases is approximately $5.36
million.
- 107 -
Final Report
Table 32. Opinion of Probable Cost – Phase 1: Marlin and Coral Streets
Coral Street - E
Item Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Description
Mobilization
Clearing and Grubbing
Excavation
4" Concrete Sidewalk
6" Concrete Driveway
Boardwalk
Remove and Dispose of Road
24" HDPE Pipe for Infiltration Trench
14" HDPE
Class B Riprap
Lift Station
Generator
Electrical
Generator Landscaping
6" Force Main (HDPE)
Remove and Dispose of Driveways
Erosion Control
Sod
Infiltration Trench
Dewatering
Plantings
Quantity
Unit
1
0.5
530
910
40
6130
1540
500
125
2
1
1
1
1
530
960
1
14700
500
1
1
LS
AC
CY
SF
CY
SF
SY
LF
LF
Ton
EA
EA
LS
LS
LF
SF
LS
SF
LF
EA
LS
Unit Price
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
20,000.00
5,600.00
16.00
2.45
337.00
6.00
5.93
26.00
18.00
40.00
75,000.00
50,000.00
30,000.00
2,500.00
55.00
25.00
15,000.00
2.00
130.00
40,000.00
20,000.00
Subtotal
Contingency (15%)
Total
Say
Total
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
20,000.00
2,800.00
8,480.00
2,229.50
13,480.00
36,780.00
9,132.20
13,000.00
2,250.00
80.00
75,000.00
50,000.00
30,000.00
2,500.00
29,150.00
24,000.00
15,000.00
29,400.00
65,000.00
40,000.00
20,000.00
488,281.70
73,242.26
561,523.96
561,500.00
Marlin Street - D
Item Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Force Main - Coral to Marlin
Item Number
1
Force Main - Marlin to Dolphin
Item Number
1
Description
Description
Force Main - Dolphin to Albacore to South Pond
Item Number
Description
1
Quantity
Unit
2606
LF
Quantity
Unit
1264
LF
Quantity
Unit
2730
LF
Unit Price
$
90.00
Subtotal
Contingency (15%)
Total
Say
Mobilization
Clearing and Grubbing
Excavation
4" Concrete Sidewalk
6" Concrete Driveway
Boardwalk
Remove and Dispose of Road
24" HDPE Pipe for Infiltration Trench
14" HDPE
Class B Riprap
Lift Station
Generator
Generator Landscaping
Electrical
Remove and Dispose of Driveways
Erosion Control
Sod
Infiltration Trench
Dewatering
Plantings
Quantity
Unit
1
0.5
760
1160
140
5375
1580
520
121
430
2
1
1
1
1
1685
1
15105
520
1
1
LS
AC
CY
SF
CY
SF
SY
LF
LF
LF
Ton
EA
EA
LS
LS
SF
LS
SF
LF
EA
LS
Unit Price
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
20,000.00
5,600.00
16.00
2.45
337.00
6.00
5.93
26.00
18.00
55.00
40.00
75,000.00
50,000.00
2,500.00
30,000.00
25.00
15,000.00
2.00
130.00
40,000.00
20,000.00
Subtotal
Contingency (15%)
Total
Say
Total
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
20,000.00
2,800.00
12,160.00
2,842.00
47,180.00
32,250.00
9,369.40
13,520.00
2,178.00
23,650.00
80.00
75,000.00
50,000.00
2,500.00
30,000.00
42,125.00
15,000.00
30,210.00
67,600.00
40,000.00
20,000.00
538,464.40
80,769.66
619,234.06
619,200.00
Force Main - Corolla Drive to North Pond
Item Number
Description
1
OPTIONAL ITEMS
Convert to Three-Phase Power
SCADA Control System
Trellix Walkways
Additional Pump
Total Phase 1 Cost
- 108 -
$
$
$
$
$
Unit Price
$
110.00
Subtotal
Contingency (15%)
Total
Say
$
110.00
Subtotal
Contingency (15%)
Total
Say
234,540.00
234,540.00
35,181.00
269,721.00
269,700.00
Total
$
$
$
$
$
Unit Price
Phase 1 Subtotal
Description
Total
139,040.00
139,040.00
20,856.00
159,896.00
159,900.00
Total
$
$
$
$
$
300,300.00
300,300.00
45,045.00
345,345.00
345,300.00
$
1,955,600.00
Quantity
Unit
Unit Price
Total
1449
LF
$
80.00
Subtotal
Contingency (15%)
Total
Say
$
$
$
$
$
115,920.00
115,920.00
17,388.00
133,308.00
133,300.00
2
1
11505
1
LS
LS
SF
EA
$
$
$
$
45,000.00
28,400.00
3.00
6,000.00
Subtotal
Contingency (15%)
Total
Say
$
$
$
$
$
$
$
$
90,000.00
28,400.00
34,515.00
6,000.00
158,915.00
23,837.25
182,752.25
182,800.00
$
2,271,652.25
$
2,271,700.00
Final Report
Item Number
Table 33. Opinion of Probable Cost – Total Cost
Description
Total Cost
1
Phase 1 Cost
$2,271,700
2
Phase 2 Cost
$1,591,700
3
Phase 3 Cost
$1,497,300
Total Cost
$5,360,700
- 109 -
Final Report
CONCLUSION
Flooding has become a more prevalent problem at the Whalehead subdivision in Currituck
County, NC. Construction has decreased the amount of permeable land and increased the
amount of runoff occurring during rain events. An economic, permitable, efficient solution was
needed to reduce the effects of stormwater in the area.
A MIKESHE model was created for the Whalehead Subdivision. The model was built in order to
study the feasibility of developing a flooding and stormwater management plan for the
subdivision. Simulations were conducted to calibrate the model and then conduct testing of
different alternatives to address the flooding issues in the region.
Three possible alternatives were studied to alleviate the flooding problems of the Whalehead
Subdivision: ocean outfalls, dune infiltration into the backside of the primary dune system, and
pumping water into soundside ponds. All the alternatives use an infiltration pipe collection
system to carry the runoff to the ocean, dunes, or ponds.
Discharging the runoff to the ocean using an outfall was much more expensive than the other
alternatives, plus current North Carolina laws and regulations do not currently address ocean
outfalls. Acquiring environmental permits would be an extensive and prolonged process with no
guarantee of a permit being granted.
Infiltrating the water into the primary dune is not very effective in receiving all the pumped
water for a large event, given the finer sands present.
Pumping the water into the soundside ponds allows plenty of storage room for large rainfall
events and the chances of obtaining a permit are very favorable. Also, analyzing the MIKESHE
model results, it can be observed that the option of pumping water into the soundside ponds is
more feasible than pumping the water into the backside dune due to the negative effect in
neighboring parcels and pump rates.
In summary, due to permitting regulations, water removal/receiving capacity and cost, the
alternative of pumping water into the soundside ponds was selected as the most feasible
alternative. It is expected that the preferred alternative will be constructed in three phases,
and the system has been designed to accommodate up to two or more pump stations if needed
in the future.
- 110 -
APPENDIX A
S&ME REPORT

currituck county final report - Whalehead Property Owners Association

Transcription

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