modoc county, california

Transcription

MODOC COUNTY,
CALIFORNIA
AND INCORPORATED AREAS
COMMUNITY NAME
COMMUNITY NUMBER
ALTURAS, CITY OF
060193
MODOC COUNTY
(UNINCORPORATED AREAS)
060192
REVISED:
_________________
PRELIMINARY:
FEBRUARY 14, 2013
FLOODINSURANCE
INSURANCESTUDY
STUDYNUMBER
NUMBER
FLOOD
06021CV001A
06049CV000B
NOTICE TO
FLOOD INSURANCE STUDY USERS
Communities participating in the National Flood Insurance Program have
established repositories of flood hazard data for floodplain management and
flood insurance purposes. This Flood Insurance Study (FIS) report may not
contain all data available within the Community Map Repository. Please
contact the Community Map Repository for any additional data.
Selected Flood Insurance Rate Map panels for the community contain
information that was previously shown separately on the corresponding Flood
Boundary and Floodway Map panels (e.g., floodways, cross-sections). Former
flood hazard zone designations have been changed as follows:
Old Zone
New Zone
A1 through A30
AE
B
X
C
X
The Federal Emergency Management Agency (FEMA) may revise and
republish part or all of this FIS report at any time. In addition, FEMA may
revise part of this FIS report by the Letter of Map Revision process, which does
not involve republication or redistribution of the FIS report. Therefore, users
should consult with community officials and check the Community Map
Repository to obtain the most current FIS report components.
This FIS report was revised on __________. Users should refer to Section 10.0,
Revisions Description, for further information. Section 10.0 is intended to
present the most up-to-date information for specific portions of this FIS report.
Therefore, users of this report should be aware that the information presented in
Section 10.0 supersedes information in Sections 1.0 through 9.0 of this FIS
report.
Initial Countywide FIS Effective Date: June 4, 2010
Revised Dates: ____________
TABLE OF CONTENTS
1.0 2.0 3.0 4.0 INTRODUCTION ............................................................................................... 1 1.1 Purpose of Study ...................................................................................... 1 1.2 Authority and Acknowledgments ............................................................ 1 1.3 Coordination ............................................................................................ 2 AREA STUDIED................................................................................................. 3 2.1 Scope of Study ......................................................................................... 3 2.2 Community Description ........................................................................... 4 2.3 Principal Flood Problems ......................................................................... 5 2.4 Flood Protection Measures ...................................................................... 6 ENGINEERING METHODS .............................................................................. 7 3.1 Hydrologic Analyses ................................................................................ 7 3.2 Hydraulic Analyses ................................................................................ 10 3.3 Vertical Datum ....................................................................................... 14 FLOODPLAIN MANAGEMENT APPLICATIONS ....................................... 16 4.1 Floodplain Boundaries ........................................................................... 16 4.2 Floodways .............................................................................................. 17 5.0 INSURANCE APPLICATIONS ....................................................................... 23 6.0 FLOOD INSURANCE RATE MAP ................................................................. 23 7.0 OTHER STUDIES ............................................................................................. 24 8.0 LOCATION OF DATA ..................................................................................... 25 9.0 BIBLIOGRAPHY AND REFERENCES .......................................................... 27 10.0 REVISIONS DESCRIPTION............................................................................ 30 10.1 First Revision ......................................................................................... 30 a. HydrologicAnalysis............................................................................30 b. Hydraulic Analysis.............................................................................31 c. Bibliography and References .................................................................. 32 i
TABLE OF CONTENTS (cont.)
FIGURES
Figure 1 - Floodway Schematic ..................................................................................... 18
TABLES
Table 1 – Initial and Final CCO Meetings ....................................................................... 3 Table 2 – Detailed Studied Streams ................................................................................. 3 Table 3 – Summary of Discharges ................................................................................... 8 Table 4 – Manning’s “n” Value Table ........................................................................... 11 Table 5 – List of Levees Requiring Flood Hazard Revision ......................................... 14 Table 6 – Vertical Datum Conversion ........................................................................... 15 Table 7 – Floodway Data ............................................................................................... 19 Table 8 – Community Map History ............................................................................... 26 Table 9 – Revised Detailed Flooding Sources ............................................................... 30 Table 10 – Summary of Revised Discharges ................................................................. 31 EXHIBITS
Exhibit 1 – Flood Profiles
Ash Creek
Bidwell Creek
Butte Creek
Dry Creek
Pit River
North Fork Pit River
South Fork Pit River
Panel 01P
Panels 02P – 03P
Panel 04P
Panel 05P
Panel 06P
Panels 06P – 08P
Panels 09P – 10P
Exhibit 2 – Flood Insurance Rate Map Index
Flood Insurance Rate Map
ii
FLOOD INSURANCE STUDY
MODOC COUNTY, CALIFORNIA
1.0
INTRODUCTION
1.1
Purpose of Study
This Flood Insurance Study (FIS) revises and supersedes the FIS reports and/or
Flood Insurance Rate Maps (FIRMs) in the geographic area of Modoc County,
California, including the City of Alturas, and the unincorporated areas of Modoc
County (hereinafter referred to collectively as Modoc County), and aids in the
administration of the National Flood Insurance Act of 1968 and the Flood
Disaster Protection Act of 1973. This study has developed flood-risk data for
various areas of the community that will be used to establish actuarial flood
insurance rates. This information will also be used by Modoc County to update
existing floodplain regulations as part of the Regular Phase of the National Flood
Insurance Program (NFIP), and by local and regional planners to further promote
sound land use and floodplain development. Minimum floodplain management
requirements for participation in the NFIP are set forth in the Code of Federal
Regulations at 44 CFR, 60.3.
In some states or communities, floodplain management criteria or regulations may
exist that are more restrictive or comprehensive than the minimum Federal
requirements. In such cases, the more restrictive criteria take precedence and the
State or other jurisdictional agency will be able to explain them.
1.2
Authority and Acknowledgments
The sources of authority for this FIS are the National Flood Insurance Act of 1968
and the Flood Disaster Protection Act of 1973.
This FIS was prepared to include all jurisdictions within Modoc County in a
countywide FIS. The authority and acknowledgments prior to this countywide FIS
were compiled from the previously identified FIS reports for floodprone
jurisdictions within Modoc County and are shown below.
Modoc County
(Unincorporated
Areas)
The hydrologic and hydraulic analyses for the
original study were performed by the U.S. Army
Corps of Engineers (USACE), Sacramento District,
for FEMA, under Interagency Agreement No. EMW89-E-2994, Project Order No. 8. This study was
completed in August 1990.
For the first time countywide FIS, MAP-IX Mainland compiled existing data to
convert the previous Modoc County FIS and FIRM into digital format. MAP-IX
completed this work in May 22, 2009 under Contract No. EMF-2003-CO-0047.
In addition, approximate mapping of “without levee” flooding was also conducted
for some areas to indicate the extent of the “without levee” floodplains per the
Code of Federal Regulations, Title 44, Section 65.10 (44 CFR 65.10), titled
“Mapping of Areas Protected by Levee Systems.” Additional guidance is provided
by FEMA’s “Procedure Memorandum No. 34 – Interim Guidance for Studies
Including Levees”, “Procedure Memorandum No. 43 – Guidelines for Identifying
Provisionally Accredited Levees, and “Procedure Memorandum No. 52 –
Guidance for Mapping Process Associated with Levee Systems.” Mapping
conducted within the City of Alturas was conducted by the FEMA Service Group
(FSG), while other “behind levee” flooding was performed by MAP-IX Mainland.
Base map information shown on the FIRMs was derived from multiple sources.
This information was compiled from the California Spatial Information Library,
1997; U.S. Geological Survey (USGS), 1989, 1999, and 2006; National Geodetic
Survey, 2002; National Atlas, 2003 and 2006; U.S. Census Bureau, 2006; FEMA,
1996 and 1999; and CADWR, 2002 and 2005. Additional Information was
photogrammetrically compiled at a scale of 1:12,000 from U.S. Department of
Agriculture aerial photography dated 2005.
The projection used in the preparation of this map was Universal Transverse
Mercator (UTM) Zone 10N. The horizontal datum was North American Datum
of 1983 (NAD83), GRS80 spheroid. Differences in datum, spheroid, projection
or State Plane zones used in the production of FIRMs for adjacent jurisdictions
may result in slight positional differences in map features across jurisdiction
boundaries. These differences do not affect the accuracy of this FIRM.
1.3
Coordination
An initial Consultation Coordination Officer's (CCO) meeting was held with
representatives from FEMA, the community, and the study contractor to explain
the nature and purpose of a FIS, and to identify the streams to be studied by
detailed methods. A final CCO meeting was held with representatives from
FEMA, the community, and the study contractor to review the results of the study.
The dates of the pre-countywide initial and final CCO meetings held for Modoc
County and the incorporated communities within its boundaries are shown in
Table 1, “Initial and Final CCO Meetings.”
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Table 1 – Initial and Final CCO Meetings
COMMUNITY NAME
INITIAL MEETING
FINAL MEETING
*
November 19, 1998
May 9, 1989
July 30, 1992 (1st
Revision)
* (2nd Revision)
March 3, 1992
* (1st Revision)
November 19, 1998 (2nd
Revision)
Alturas, City of
Modoc County
(Unincorporated Areas)
* Data Not Available
For this countywide FIS, an initial CCO meeting was held on December 21, 2006,
and was attended by representatives of the community, the study contractor, and
FEMA. The final CCO meeting was held on July 13, 2009, and was attended by
representatives of the community, the study contractor and FEMA.
2.0
AREA STUDIED
2.1
Scope of Study
This FIS covers the geographic area of Modoc County, California.
All or portions of the flooding sources listed in Table 2, “Detailed Studied
Streams,” were studied by detailed methods. Limits of detailed study are indicated
on the Flood Profiles (Exhibit 1) and on the FIRM (Published Separately).
Table 2 – Detailed Studied Streams
Ash Creek
Bidwell Creek
Butte Creek
Dry Creek
Approximate analyses were used to study those areas having a low development
potential or minimal flooding hazards. The scope and methods of study were
proposed to, and agreed upon, by FEMA and the communities.
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2.2
Community Description
Modoc County is located in the northeastern corner of northern California and is
contiguous to the States of Oregon and Nevada. The County is bordered by
Klamath and Lake Counties to the north; Washoe County to the east; and Lassen,
Shasta, and Siskiyou Counties to the south, southwest, and west, respectively. The
City of Alturas serves as the County seat. Reno is located approximately 180
miles to the south-southeast.
Logging and mining were early endeavors in the area, and the lumber industry
remains the most important present-day focus of the local economy. Ranching
activities have spread out in the Big Valley region adding to the economic base,
and recent impetus has been given to rejuvenating the long-dormant gold-mining
industry. In 2006, the total estimated population of the County was 9,597, of
which 6,802 lived in the unincorporated areas and 2,776 lived in the City of
Alturas (U.S. Census Bureau, 2011).
The climate in the area is characterized as high desert, with cold, wet winters and
warm, dry summers. The average high and low temperatures for January are 40°F
and 16°F, respectively, while for July they are 88°F and 45°F, respectively.
Regional precipitation varies from approximately 8 inches in the low valley areas
to approximately 35 inches in the higher mountain areas.
City of Alturas
The City of Alturas has been the County seat of Modoc County since its
incorporation in 1874. The City is located near the northeast corner of California,
approximately 39 miles south of the Oregon border, 43 miles west of the Nevada
border, and 305 miles northeast of Sacramento, California.
The City of Alturas is serviced by U.S. Highway 395 from the north and south
and State Highway 299 from the west and east. The SPRR runs north-south and
west through the City. The City has a municipal airport serving general aviation
traffic.
The City covers an area of approximately 24 square miles, and is located on the
Modoc Plateau at an elevation of approximately 4,300 feet.
The City of Alturas serves as the government and commercial center for the
surrounding area. The primary economic activities in the area are agriculture,
including livestock, hay and grain, and vegetable crops; outdoor recreation such
as hunting and fishing; lumbering; and some mining of gravel and other minerals.
Modoc County (Unincorporated Areas)
The area is served by State Highways 139 and 299, several County roads, and a
network of Town streets. In addition, bus service, provided by Greyhound Bus
Lines, is available at the City of Alturas; Amtrak connections are available in
Reno, Nevada; and the City of Alturas has a municipal airport.
4
The community of Adin is located in the southwestern quarter of Modoc County.
Just south of the community, State Highways 139 and 299 join from the south and
west, respectively, with State Highway 299 continuing in a northerly direction.
The community of Adin has a public airport. The nearest communities to Adin are
Indian Springs, approximately 7 miles to the north; Lookout and Bieber,
approximately 10 miles to the west; Hayden Hill, approximately 15 miles to the
south; and Madeline, approximately 28 miles to the east. The City of Alturas, the
only incorporated community in Modoc County, is approximately 40 miles to the
northeast.
Adin lies at an elevation of approximately 4,200 feet at the northeastern edge of
an area known as Big Valley. Just to the north and east of the community are
steep, sloped hills separated by a narrow valley that traverses in a northeasterly
direction, through which State Highway 299 travels. From the base of the adjacent
hills, the terrain drains to the west through Big Valley carrying the combined
flows of Ash, Dry, and Butte Creeks.
Ash, Butte, and Dry Creeks originate in the high mountain elevations east and
southeast of Adin. Dry Creek flows into Ash Creek in the northern part of Adin
and Butte Creek joins Ash Creek west of the community. Ash Creek continues
westerly from Adin for approximately 10 miles, flowing into the Pit River.
The community of Fort Bidwell is in the northeastern corner of Modoc County,
approximately 42 miles northeast of the City of Alturas. Fort Bidwell is reached
from the City of Alturas by taking State Highway 299 to Cedarville and traveling
20 miles on Surprise Valley Road. The nearest communities are Lake City,
approximately 15 miles south on Surprise Valley Road, and Willow Creek,
approximately 20 miles west. There is a small airfield located north of Fort
Bidwell.
Fort Bidwell, with a population estimated to be less than 100, is the site of a 19thcentury U.S. Cavalry fort (Hoover, Rensch, Abeloe, and Kyle, 1990). The Fort
Bidwell Reservation is located west of the community and is occupied by
approximately 175 Northern Paiute Indians (Engle, 1992).
Fort Bidwell lies at an elevation of approximately 4,500 feet. The surrounding
area is agricultural, primarily irrigated pasture and hay and grain lands. There are
some mining, hunting, fishing, and other outdoor recreation activities in the
Warner Mountains to the north and west of Fort Bidwell.
2.3
Principal Flood Problems
Flooding in the area can occur any time from fall to spring as a result of the
occurrence of general rainstorms. General rain floods result from prolonged,
heavy rainfall over tributary areas and are characterized by high peak flows and
moderate duration and a large volume of runoff. Flooding is more severe when
antecedent rainfall has resulted in saturated ground conditions, when the ground is
5
frozen and infiltration is minimal, or when rain on snow in the higher elevations
adds snowmelt to rain flood runoff.
No significant flood problems have been experienced in the City of Alturas since
the completion of the USACE channel modification and levee improvement
project in 1972.
2.4
Flood Protection Measures
City of Alturas
Flood protection measures for the area have been constructed by the USACE. The
last major project, which was completed in 1972, included the following
(USACE, April 1972):

Channel excavation from approximately 4,400 feet downstream of the
east-west SPRR spur track to approximately 500 feet upstream of the
SPRR north-south main track. The total project was approximately 1.9
stream miles.

New levee constructed and levee improvements at several locations
throughout the project area, including the construction of a wing levee
upstream of the SPRR north-south main track. The total project was
approximately 1.9 stream miles.

Channel erosion prevention facilities included riprap and sacked concrete
slope protection at numerous locations throughout the project.
This flood control project is maintained by Modoc County under an agreement
with the USACE. The maintenance is performed in accordance with an Operation
and Maintenance Manual prepared by the USACE (USACE, February 1973).
These activities include channel debris and vegetation removal and maintenance
and repair of slope protection and levees. The USACE periodically inspects the
project facilities and provides the County with direction regarding maintenance
and repair. These inspections are to ensure that the project continues to comply
with the USACE standard.
During January 1995, a significant storm event was experienced in the area. No
flooding occurred; however, there was some channel erosion, sediment
deposition, and slope protection damage in the project area. As a result, the
USACE performed a rehabilitation project under Public Law 84-99, which
included sediment removal and the repair of eroded areas and slope protection.
This work was completed in 1996 (USACE, August 1995 and USACE, July
1996).
6
The levees associated with this flood control project are no longer accredited by
FEMA as providing protection against the 1-percent-annual-chance flood.
Tule Lake affords some degree of manmade storage near the Siskiyou County
line. Effects on the 1- and 0.2-percent-annual-chance flood events from this
storage lake cannot be determined though it does reduce discharge volumes
downstream.
There are numerous levee systems in Modoc County; however none of them are
accredited by FEMA as providing protection against the 1-percent-annual-chance
flood.
3.0
ENGINEERING METHODS
For the flooding sources studied by detailed methods in the community, standard
hydrologic and hydraulic study methods were used to determine the flood hazard data
required for this study. Flood events of a magnitude that are expected to be equaled or
exceeded once on the average during any 10-, 50-, 100-, or 500-year period (recurrence
interval) have been selected as having special significance for floodplain management
and for flood insurance rates. These events, commonly termed the 10-, 50-, 100-, and
500-year floods, have a 10-, 2-, 1-, and 0.2-percent-annual-chance, respectively, of being
equaled or exceeded during any year. Although the recurrence interval represents the
long-term, average period between floods of a specific magnitude, rare floods could
occur at short intervals or even within the same year. The risk of experiencing a rare
flood increases when periods greater than 1 year are considered. For example, the risk of
having a flood that equals or exceeds the 1-percent-annual-chance (100-year) flood in any
50-year period is approximately 40 percent (4 in 10); for any 90-year period, the risk
increases to approximately 60 percent (6 in 10). The analyses reported herein reflect
flooding potentials based on conditions existing in the community at the time of
completion of this study. Maps and flood elevations will be amended periodically to
reflect future changes.
3.1
Hydrologic Analyses
Hydrologic analyses were carried out to establish the peak discharge frequency
relationships for floods of the selected recurrence intervals for each flooding
source studied in detail affecting the county.
Peak discharge-drainage area relationships for the 10-, 2-, 1-, and 0.2-percentannual-chance floods for each stream studied by detailed methods are presented in
Table 3, “Summary of Discharges.”
For each community within Modoc County that had a previously printed FIS
report, the hydrologic analyses described in those reports have been compiled and
are summarized below.
7
Table 3 – Summary of Discharges
FLOODING
SOURCE AND
LOCATION
DRAINAGE
AREA
(sq. miles)
PEAK DISCHARGES (cfs)
10%2%1%0.2%ANNUAL- ANNUAL- ANNUAL- ANNUALCHANCE CHANCE CHANCE CHANCE
ASH CREEK
At State Highway 299
Bridge
258.0
*
*
5300
*
BIDWELL CREEK
At Main street (Surprise
Valley Road)
29.2
*
*
715
*
BUTTE CREEK
Bridge
40.3
*
*
1700
*
DRY CREEK
Bridge
8.3
*
*
580
*
* Data not available
Procedures used during these analyses included the unit-hydrograph method and
available streamflow-data analyses and the HEC-1 computer program (USACE,
September 1981).
Flood hydrographs and peak flows for the 1-percent-annyal-chance floods on
Ash, Butte, and Dry Creeks were computed from a 24-hour general rainstorm.
The general rainflood peak flow for Ash Creek was based on an analysis of
available streamflow records and a hydrologic analysis of the drainage basin
using the HEC-1 computer program. There are no streamflow records for Dry and
Butte Creeks; therefore, the peak flows for those two streams were derived from
hydrologic analyses of those basins using the HEC-1 computer program.
The HEC-1 computer program utilizes rainfall, loss rates, unit hydrographs, base
flow, and routing criteria as input.
Rainfall computations were based on precipitation-frequency data and areal
reduction factors derived from the National Oceanic and Atmospheric
Administration (NOAA) Atlas 2 for California (NOAA, 1973). Rainfall for the 18
percent-annyal-chance, 24-hour general rainstorm over the three drainage basins
is: Dry Creek at State Highway 299 culvert - 3.45 inches, Butte Creek at State
Highway 299 bridge-3.35 inches, and Ash Creek at State Highway 299 bridge3.18 inches. Two 24-hour storm distributions were used for this study-one for Dry
and Butte Creeks (approximately 8.3 and 40.3 square miles, respectively) and one
for Ash Creek (258 square miles). The distributions were derived from the 96hour standard-project storm distributions for areas between 1 and 100 square
miles and 101 and 500 square miles.
Loss rates were based on the initial and constant infiltration loss concept and
analyses of soil cover and land use. The drainage basins of all three streams were
considered to have saturated soils at the beginning of the storm. The initial loss
rate used was 0.00 inch and the constant loss rate used was 0.15 inch per hour.
These loss rates are comparable to loss rates used in adjacent watersheds.
Unit hydrographs were synthetically derived and computed based on the
"Modified Los Angeles S-Graph Procedure" described in a November 1989
hydrology study for the community of Adin (USACE, November 1989). The Sgraph (Spanish Springs Valley) and basin characteristics used for this study were
transposed from an area with similar physical characteristics. A 1-hour time
interval was used in the computations.
The Muskingum method was used to route 1-percent-annyal-chance flood
hydrographs through valley areas. The Muskingum routing data were estimated
based on the assumption that the average flood-wave velocity was 6 feet per
second and the flood hydrographs had maximum attenuation in valley areas.
An estimated 1-percent-annyal-chance peak flow of 5,290 cfs was based on a
statistical analysis of the recorded peak flows on Ash Creek at the community of
Adin. The peak flow compares favorably with the peak flow of 5,300 cfs
developed using the HEC-1 computer program.
For the hydrologic analysis for the first revision, the USACE HEC-1 computer
program (USACE, September 1990(a)) computed the synthetic rainfall-runoff
volumes and unit and flood hydrographs for Bidwell Creek. HEC-1 then routed
the flood hydrographs using the modified-Puls technique to tabulate the frequency
peak discharges. The Natural Resources Conservation Service (NRCS) (formerly
the Soil Conservation Service) unit-hydrograph option in the HEC-1 computer
program was used. The storage-discharge relationship was determined using
multiple-discharge hydraulic computations.
Total precipitation was determined from NOAA isohyetal maps (NOAA, 1972).
Rainfall distribution was determined based on statistical data developed by the
CADWR (CADWR, 1976). The distribution was based on the nearest rainfall
gage (Alturas Ranger Station) for which statistics have been developed. A
9
balanced 24-hour nested storm pattern was used with a time increment of 5
minutes through 24 hours.
Infiltration losses were determined based on watershed soil types and NRCS
guidelines (U.S. Department of Agriculture, October 1981). The soils in the
watershed area consist of hydrologic soil groups B and C. The resulting curvenumber values range from 58 to 67.
Times of concentration (Tcs) and basin lag times were determined using NRCS
component methodology (U.S. Department of Agriculture, October 1981). This
method consists of estimating travel times for the sheet, shallow-concentrated,
and channel-flow segments of the runoff. In accordance with NRCS criteria, lag
time was set to equal 0.6 x Tc.
Watershed areas were determined using a watershed boundary map that was
developed from USGS quadrangle mapping (USGS, n.d. (a)). The watershed was
divided into nine subbasins. Soil type and permeability rates were determined
from NRCS soil-survey maps (U.S. Department of Agriculture, April 1974).
3.2
Hydraulic Analyses
Analyses of the hydraulic characteristics of flooding from the sources studied
were carried out to provide estimates of the elevations of floods of the selected
recurrence intervals. Users should be aware that flood elevations shown on the
FIRM represent rounded whole-foot elevations and may not exactly reflect the
elevations shown on the Flood Profiles or in the Floodway Data tables in the FIS
report. Flood elevations shown on the FIRM are primarily intended for flood
insurance rating purposes. For construction and/or floodplain management
purposes, users are cautioned to use the flood elevation data presented in this FIS
in conjunction with the data shown on the FIRM.
For each community within Modoc County that had a previously printed FIS
report, the hydraulic analyses described in those reports have been compiled and
are summarized below.
Locations of selected cross sections used in the hydraulic analyses are shown on
the Flood Profiles (Exhibit 1). For stream segments for which a floodway was
computed (Section 4.2), selected cross-section locations are also shown on the
FIRM (Exhibit 2).
The hydraulic analyses for this study were based on unobstructed flow. The flood
elevations shown on the Flood Profiles (Exhibit 1) are thus considered valid only
if hydraulic structures remain unobstructed, operate properly, and do not fail.
Channel and overbank roughness factors (Manning's "n") used in the hydraulic
computations were established by USACE and USGS guidelines (Chow, 1959
and USGS, 1987), determined by engineering judgment, and based on field
10
observations of the streams and floodplain areas. Table 4, “Manning’s “n”
Values”, shows the channel and overbank “n” values for the streams studied by
detailed methods.
Table 4 – Manning’s “n” Value Table
STREAM
Ash Creek
Bidwell Creek
Butte Creek
Dry Creek
CHANNEL
0.040 – 0.050
0.020 – 0.100
0.040 – 0.050
0.040 – 0.050
OVERBANK
0.040 – 0.050
0.050 – 0.085
0.040 – 0.050
0.040 – 0.050
For the original study and the first revision, analyses of the hydraulic
characteristics of flooding from Ash, Bidwell, Butte, and Dry Creeks were carried
out to provide estimates of the elevations of the base (1-percent-annual-chance)
flood. Water-surface elevations were computed through the use of the USACE
HEC-2 computer program (USACE, September 1982 and USACE, September
1990(b)). Starting water-surface elevations for Ash, Bidwell, and Butte Creeks
were developed by the slope-area method. For Dry Creek, the 1-percent-annualchance stage in Ash Creek was used.
Cross sections for backwater analyses were located at close intervals upstream
and downstream from bridges, culverts, and other hydraulically significant
features in order to establish the backwater effect of such structures in areas
presently urbanized or subject to development. Additional cross sections were
located at other representative locations in the study area. Cross-section data were
derived from field surveys. HEC-2 normal and special bridge routines were used
to model existing road crossings.
Structural data for bridges and culverts were obtained during a field
reconnaissance of the study area.
Supercritical flow conditions can occur in some channel reaches. In accordance
with FEMA guidelines (FEMA January 1995), subcritical analyses were
conducted to determine base flood elevations (BFEs) for all stream reaches.
For the first revision, split-flow routines were used to determine discharges for
overbank flow paths that are hydraulically separated from the main channel. The
split was based on the weir option with a coefficient of 2.6. Separate HEC-2
analyses were conducted for two overbank areas along Bidwell Creek where flow
splits off of the main channel and floods additional areas during the 1-percentannual-chance storm event. In areas subject to flooding from split flow, the
average depths were determined to be less than 1 foot. HEC-2 special culvert
routines were used to model the existing road crossings, with a weir coefficient of
2.6 used to determine road overtopping.
11
For the first revision, contraction and expansion coefficients of 0.1 to 0.3 were
used for open- channel sections in accordance with USACE guidelines.
Contraction coefficients at culverts ranged from 0.3 to 0.5, depending on
configuration. Channel and overbank cross sections were determined from
surveyed channel cross sections and topographic mapping obtained for this study
(Geonex, Inc., June 1993 (a and b)).
For the first revision, floodway limits between Cross Sections A through F on
Bidwell Creek could not be defined.
For the October 20, 1999 study of the North Fork Pit River, Water-surface
elevations were computed through the use of the USACE HEC-2 computer
program (USACE, September 1990(b)). Channel and overbank cross sections
were determined from the USACE as-built cross section for the channel reach
where sediment was removed due to the 1995 storm event (Snow, 1986).
Overbank sections were digitized or interpolated from topographic mapping at a
scale of 1:2,400, with a contour interval of 2 feet (Geonex, Inc., January 1996).
Digitized channel cross sections were supplied and verified by field surveys.
HEC-2 split flow analyses were performed using either the normal depth or rating
curve options. The grade line slopes used in the normal depth option were
estimated based on the topographic maps. The rating curve used for the split flow
that will pass through the Parker Creek Road (County Road 56) culvert, south of
the stream upstream of the north-south SPRR tracks, was determined based on
manual culvert hydraulic calculations.
This Countywide Study
In 2008, the CADWR completed Awareness Floodplain Mapping studies for all
California counties. The intent of the project was to identify flood hazard areas for
all regions for mapped under FEMA’s NFIP. The awareness maps identify the 1percent-annual-chance flood hazard areas using approximate assessment
procedures. These floodplains are shown as floodprone areas without specific
depths and other flood hazard data (CADWR, 2009).
Some flood hazard information presented in prior FIRMs and in prior FIS reports
for Modoc County and its incorporated communities was based on flood
protection provided by levees. Based on the information available and the
mapping standards of the NFIP at the time that the prior FISs and FIRMs were
prepared, FEMA accredited the levees as providing protection from the flood that
has a 1-percent chance of being equaled or exceeded in any given year. For
FEMA to continue to accredit the identified levees with providing protection from
the base flood, the levees must meet the criteria of the Code of Federal
Regulations, Title 44, Section 65.10 (44 CFR 65.10), titled “Mapping of Areas
Protected by Levee Systems.”
On August 22, 2005, FEMA issued Procedure Memorandum No. 34 - Interim
Guidance for Studies Including Levees. The purpose of the memorandum was to
help clarify the responsibility of community officials or other parties seeking
recognition of a levee by providing information identified during a study/mapping
project. Often, documentation regarding levee design, accreditation, and the
12
impacts on flood hazard mapping is outdated or missing altogether. To remedy
this, Procedure Memorandum No. 34 provides interim guidance on procedures to
minimize delays in near-term studies/mapping projects, to help our mapping
partners properly assess how to handle levee mapping issues.
While 44 CFR Section 65.10 documentation is being compiled, the release of
more up-to-date FIRM panels for other parts of a community or county may be
delayed. To minimize the impact of the levee recognition and certification
process, FEMA issued Procedure Memorandum No. 43 - Guidelines for
Identifying Provisionally Accredited Levees on March 16, 2007. These guidelines
will allow issuance of preliminary and effective versions of FIRMs while the
levee owners or communities are compiling the full documentation required to
show compliance with 44 CFR Section 65.10. The guidelines also explain that
preliminary FIRMs can be issued while providing the communities and levee
owners with a specified timeframe to correct any maintenance deficiencies
associated with a levee and to show compliance with 44 CFR Section 65.10.
FEMA contacted the communities within Modoc County to obtain data required
under 44 CFR 65.10 to continue to show the levees as providing protection from
the flood that has a 1-percent chance of being equaled or exceeded in any given
year.
FEMA understood that it may take time to acquire and/or assemble the
documentation necessary to fully comply with 44 CFR 65.10. Therefore, FEMA
put forth a process to provide the communities with additional time to submit all
the necessary documentation. For a community to avail itself of the additional
time, it had to sign an agreement with FEMA. Levees for which such agreements
were signed are shown on the final effective FIRM as providing protection from
the flood that has a 1-percent chance of being equaled or exceeded in any given
year and labeled as a Provisionally Accredited Levee (PAL). Communities have
two years from the date of FEMA’s initial coordination to submit to FEMA final
accreditation data for all PALs. Following receipt of final accreditation data,
FEMA will revise the FIS and FIRM as warranted.
FEMA coordinated with the USACE, the local communities, and other
organizations to compile a list of levees that exist within Modoc County. Table 5
lists all levees previously shown on an effective FIRM, to include PALs, for
which corresponding flood hazard revisions were made.
On April 24, 2009 FEMA issued Procedure Memorandum No. 52 – Guidance for
Mapping Processes Associated with Levee Systems. This memorandum provides
guidelines for mapping landward of levee systems. Approximate analyses of
“without levee” flooding were conducted for all the levees in Table 5 to indicate
the extent of the “without levee” floodplains in accordance with Procedure
Memorandum No 52. The methodology used in these analyses is discussed below.
13
Table 5 – List of Levees Requiring Flood Hazard Revision
Community
Flood Source
Levee Inventory
Identification
Number
USACE
Levee
City of Alturas
311
Yes
City of Alturas
312
Yes
City of Alturas
313E
Yes
City of Alturas
313W
Yes
Tule Lake
1911
No
Tule Lake
2141
No
Tule Lake
2142
No
South Fork Pit River
2264
No
Fairchild Swamp
826
No
Fairchild Swamp
1532
No
Fairchild Swamp
2518
No
Modoc County
Modoc County
Modoc County
Modoc County
Modoc County
Modoc County
Modoc County
The approximate levee analysis for Modoc County (Unincorporated Areas) was
conducted by MAP-IX using information from existing flood maps and USGS
topographic data. The extent of the 1-percent-annual-chance flood for the
‘without levee’ condition was determined using this data. The 1-percent-annualchance floodplain was traced along the contour lines representing each base flood
elevation as determined by portions of the floodplain not constrained by levees.
The approximate floodplain boundary limits were refined using orthophotography
and topographic features such as high ground.
In the area around Tule Lake, levee 2142 on the north side of the lake was treated
as a dam rather than a levee. No behind levee analysis was conducted. This is
consistent with how this levee is treated in Siskiyou County (FEMA, Unpublished
(c)).
3.3
Vertical Datum
All FIS reports and FIRMs are referenced to a specific vertical datum. The
vertical datum provides a starting point against which flood, ground, and structure
elevations can be referenced and compared. Until recently, the standard vertical
datum used for newly created or revised FIS reports and FIRMs was the National
14
Geodetic Vertical Datum of 1929 (NGVD29). With the completion of the North
American Vertical Datum of 1988 (NAVD88), many FIS reports and FIRMs are
now prepared using NAVD88 as the referenced vertical datum.
Prior versions of the FIS report and FIRM were referenced to NGVD29. When
datum conversion is effected for an FIS report and FIRM, the flood profiles, and
BFEs, reflect the new datum values. To compare structure and ground elevations
to 1-percent-annual-chance (100-year) flood elevations shown in the FIS and on
the FIRM, the subject structure and ground elevations must be referenced to the
new datum values.
The conversion from NGVD29 to NAVD88 ranged between +3.39’ and +4.06’
for Modoc County. Accordingly, due to the statistically significant range in
conversion factors, an average conversion factor could not be established for the
entire county. The elevations shown in the FIS report and on the FIRM were,
therefore, converted to NAVD88 using a stream-by-stream approach. In this
method, an average conversion was established for each flooding source and
applied accordingly. The conversion factor for each flooding source in the county
may be found in Table 6, “Vertical Datum Conversions.”
Table 6 – Vertical Datum Conversion
Stream Name
Ash Creek
Bidwell Creek
Butte Creek
Dry Creek
Conversion
Factor
+3.39’
+4.06’
+3.39’
+3.39’
+3.58’
The BFEs shown on the FIRM represent whole-foot rounded values. For example,
a BFE of 102.4 will appear as 102 on the FIRM and 102.6 will appear as 103.
Therefore, users that wish to convert the elevations in this FIS to NGVD29 should
apply the stated conversion factor(s) to elevations shown on the Flood Profiles
and supporting data tables in the FIS report, which are shown at a minimum to the
nearest 0.1 foot.
For additional information regarding conversion between the NGVD29 and
NAVD88,
visit
the
National
Geodetic
Survey
website
at
http://www.ngs.noaa.gov, or contact the National Geodetic Survey at the
following address:
15
NGS Information Services
NOAA, N/NGS12
National Geodetic Survey, SSMC-3, #9202
1315 East-West Highway
Silver Spring, Maryland 20910-3282
(301) 713-3242
Temporary vertical monuments are often established during the preparation of a
flood hazard analysis for the purpose of establishing local vertical control.
Although these monuments are not shown on the FIRM, they may be found in the
Technical Support Data Notebook associated with the FIS report and FIRM for
this community. Interested individuals may contact FEMA to access these data.
4.0
FLOODPLAIN MANAGEMENT APPLICATIONS
The NFIP encourages State and local governments to adopt sound floodplain
management programs. Therefore, each FIS provides 1-percent-annual-chance (100-year)
flood elevations and delineations of the 1- and 0.2-percent-annual-chance (500-year)
floodplain boundaries and 1-percent-annual-chance floodway to assist communities in
developing floodplain management measures. This information is presented on the FIRM
and in many components of the FIS report, including Flood Profiles and Floodway Data
Table. Users should reference the data presented in the FIS report as well as additional
information that may be available at the local map repository before making flood
elevation and/or floodplain boundary determinations.
4.1
Floodplain Boundaries
To provide a national standard without regional discrimination, the 1-percentannual-chance flood has been adopted by FEMA as the base flood for floodplain
management purposes. The 0.2-percent-annual-chance flood is employed to
indicate additional areas of flood risk in the community. For each stream studied
by detailed methods, the 1- and 0.2-percent-annual-chance floodplain boundaries
have been delineated using the flood elevations determined at each cross section.
The 1- and 0.2-percent-annual-chance floodplain boundaries are shown on the
FIRM (Exhibit 2). On this map, the 1-percent-annual-chance floodplain boundary
corresponds to the boundary of the areas of special flood hazards (Zones A, and
AE), and the 0.2-percent-annual-chance floodplain boundary corresponds to the
boundary of areas of moderate flood hazards. In cases where the 1- and 0.2percent-annual-chance floodplain boundaries are close together, only the
1-percent-annual-chance floodplain boundary has been shown. Small areas
within the floodplain boundaries may lie above the flood elevations but cannot be
shown due to limitations of the map scale and/or lack of detailed topographic
data.
16
For streams studied by approximate methods, only the 1-percent annual-chance
floodplain boundary is shown on the FIRM.
Between cross sections, the boundaries were interpolated from the study
contractor's field observations and topographic workmaps.
The approximate 1-percent annual-chance floodplain boundaries in some portions
of the study area were taken directly from the FIRM for Modoc County (FEMA,
October 1999 (a-b)). These areas were adjusted as necessary to match the
background aerial photography.
4.2
Floodways
Encroachment on floodplains, such as structures and fill, reduces flood-carrying
capacity, increases flood heights and velocities, and increases flood hazards in
areas beyond the encroachment itself. One aspect of floodplain management
involves balancing the economic gain from floodplain development against the
resulting increase in flood hazard. For purposes of the NFIP, a floodway is used
as a tool to assist local communities in this aspect of floodplain management.
Under this concept, the area of the 1-percent-annual-chance floodplain is divided
into a floodway and a floodway fringe. The floodway is the channel of a stream,
plus any adjacent floodplain areas, that must be kept free of encroachment so that
the 1-percent-annual-chance flood can be carried without substantial increases in
flood heights. Minimum Federal standards limit such increases to 1 foot, provided
that hazardous velocities are not produced. The floodways in this study are
presented to local agencies as minimum standards that can be adopted directly or
that can be used as a basis for additional floodway studies.
The floodways presented in this FIS report and on the FIRM were computed for
certain stream segments on the basis of equal-conveyance reduction from each
side of the floodplain. Floodway widths were computed at cross sections.
Between cross sections, the floodway boundaries were interpolated. The results of
the floodway computations have been tabulated for selected cross sections (Table
7, “Floodway Data”). The computed floodways are shown on the FIRM. In cases
where the floodway and 1-percent-annual-chance floodplain boundaries are either
close together or collinear, only the floodway boundary has been shown.
The area between the floodway and 1-percent-annual-chance floodplain
boundaries is termed the floodway fringe. The floodway fringe encompasses the
portion of the floodplain that could be completely obstructed without increasing
the water-surface elevation of the 1-percent-annual-chance flood more than 1 foot
at any point. Typical relationships between the floodway and the floodway fringe
and their significance to floodplain development are shown in Figure 1.
Encroachment into areas subject to inundation by floodwaters having hazardous
velocities aggravates the risk of flood damage and heightens potential flood
hazards by further increasing velocities. A listing of stream velocities at selected
17
cross sections is provided in Table 7. To reduce the risk of property damage in
areas where the stream velocities are high, the community may wish to restrict
development in areas outside the floodway.
Figure 1 - Floodway Schematic
18
FLOODING SOURCE
BASE FLOOD WATER-SURFACE ELEVATION
(FEET NAVD)
FLOODWAY
DISTANCE
WIDTH
(FEET)
SECTION AREA
(SQUARE FEET)
MEAN VELOCITY
(FEET PER SECOND)
REGULATORY
WITHOUT
FLOODWAY
WITH FLOODWAY
INCREASE
A
14851
62
693
7.6
4,200.9
4,200.9
4,201.7
0.8
1
CROSS SECTION
ASH CREEK
B
1835
65
780
6.8
4,201.7
4,201.7
4,202.5
0.8
C
22351
307
2,246
2.4
4,202.7
4,202.7
4,203.6
0.9
D
25751
100
984
5.4
4,202.7
4,202.7
4,203.7
1.0
E
28651
96
620
8.5
4,203.8
4,203.8
4,204.3
0.5
F
30201
62
707
7.5
4,204.6
4,204.6
4,205.3
0.7
G
H
1
36401
75
81
759
729
7.0
7.3
4,205.5
4,206.1
4,205.5
4,206.1
4,206.2
4,206.7
0.7
0.6
G
13120 3
45
147
4.9
4,536.7
4,536.7
4,537.7
1.0
H
13670 3
35
144
5.0
4,544.4
4,544.4
4,545.0
0.6
I
14080
3
42
186
3.8
4,549.1
4,549.1
4,550.0
0.9
J
14207 3
83
256
2.8
4,552.5
4,552.5
4,552.5
0.0
K
14867
3
22
76
9.5
4,558.2
4,558.2
4,558.4
0.2
L
15457 3
24
161
4.4
4,569.6
4,569.6
4,570.4
0.8
M
15628 3
46
221
3.2
4,573.6
4,573.6
4,573.6
0.0
3
3385
BIDWELL CREEK
A-F 2
1
N
15948
75
105
6.8
4,576.0
4,576.0
4,576.0
0.0
O
16548 3
136
155
4.6
4,586.3
4,586.3
4,587.2
0.9
P
Q
16888 3
21
72
104
166
6.9
4.3
4,590.1
4,599.1
4,590.1
4,599.1
4,591.1
4,599.2
1.0
0.1
17358 3
Feet Above Confluence of Butte Creek
2
Floodway Not Determined
3
Feet Above Confluence With Upper Lake
TABLE 7
FEDERAL EMERGENCY MANAGEMENT AGENCY
FLOODWAY DATA
MODOC COUNTY, CA
ASH CREEK / BIDWELL CREEK
FLOODING SOURCE
BASE FLOOD WATER-SURFACE ELEVATION
(FEET NAVD)
FLOODWAY
DISTANCE1
WIDTH
(FEET)
SECTION AREA
(SQUARE FEET)
MEAN VELOCITY
(FEET PER SECOND)
REGULATORY
WITHOUT
FLOODWAY
WITH FLOODWAY
INCREASE
A
1,020
200
400
4.3
4,202.9
4,202.9
4,203.6
0.7
B
1,450
176
463
3.7
4,204.8
4,204.8
4,205.1
0.3
C
1,795
120
399
4.3
4,205.3
4,205.3
4,205.9
0.6
D
2,220
92
464
3.7
4,206.4
4,206.4
4,207.0
0.6
E
2,620
83
285
6.0
4,208.1
4,208.1
4,208.2
0.1
F
G
2,950
3,230
112
183
436
753
3.9
2.3
4,208.9
4,209.3
4,208.9
4,209.3
4,209.6
4,210.1
0.7
0.8
A
55
165
866
0.7
4,202.4
4,202.4 2
4,203.3
0.9
B
225
68
328
1.8
4,202.4
4,202.4 2
4,203.3
0.9
C
370
48
205
2.8
4,203.1
4,203.1
4,204.0
0.9
D
596
121
215
2.7
4,204.2
4,204.2
4,205.2
1.0
E
730
34
176
3.3
4,204.5
4,204.5
4,205.2
0.7
CROSS SECTION
BUTTE CREEK
DRY CREEK
F
865
36
153
3.8
4,204.8
4,204.8
4,205.4
0.6
G
935
74
265
2.2
4,205.0
4,205.0
4,205.7
0.7
H
1,260
42
138
4.2
4,206.2
4,206.2
4,207.0
0.8
I
1,495
27
99
5.9
4,207.5
4,207.5
4,208.3
0.8
1
Feet Above Confluence With Ash Creek
2
Elevation computed without consideration of backwater effects from Ash Creek
TABLE 7
FLOODWAY DATA
MODOC COUNTY, CA
BUTTE CREEK / DRY CREEK
Along streams where floodways have not been computed, the community must
ensure that the cumulative effect of development in the floodplains will not cause
more than a 1.0-foot increase in the BFEs at any point within the county.
No floodways were computed for the North Fork Pit River.
5.0
INSURANCE APPLICATIONS
For flood insurance rating purposes, flood insurance zone designations are assigned to a
community based on the results of the engineering analyses. The zones are as follows:
Zone A
Zone A is the flood insurance rate zone that corresponds to the 1-percent-annual-chance
floodplains that are determined in the FIS report by approximate methods. Because
detailed hydraulic analyses are not performed for such areas, no BFEs or depths are
shown within this zone.
Zone AE
Zone AE is the flood insurance rate zone that corresponds to the 1-percent-annual-chance
floodplains that are determined in the FIS report by detailed methods. Whole-foot BFEs
derived from the detailed hydraulic analyses are shown at selected intervals within this
zone.
Zone AH
Zone AH is the flood insurance risk zone that corresponds to the areas of 1-percentannual-chance shallow flooding (usually areas of ponding) where average depths are
between 1 and 3 feet. Whole-foot BFEs derived from the detailed hydraulic analyses are
shown at selected intervals within this zone.
Zone AO
Zone AO is the flood insurance risk zone that corresponds to the areas of 1-percentannual-chance shallow flooding (usually sheet flow on sloping terrain) where average
depths are between 1 and 3 feet. Average whole-foot base flood depths derived from the
detailed hydraulic analyses are shown within this zone.
Zone X
Zone X is the flood insurance rate zone that corresponds to areas outside the
0.2-percent-annual-chance floodplain, areas within the 0.2-percent-annual-chance
floodplain, areas of 1-percent-annual-chance flooding where average depths are less than
1 foot, areas of 1-percent-annual-chance flooding where the contributing drainage area is
less than 1 square mile (sq. mi.), and areas protected from the base flood by levees. No
BFEs or depths are shown within this zone.
Zone D
Zone D is the flood insurance rate zone that corresponds to unstudied areas where flood
hazards are undetermined, but possible.
6.0
FLOOD INSURANCE RATE MAP
The FIRM is designed for flood insurance and floodplain management applications.
23
For flood insurance applications, the map designates flood insurance risk zones as
described in Section 5.0 and, in the 1-percent-annual-chance floodplains that were
studied by detailed methods, shows selected whole-foot BFEs or average depths.
Insurance agents use the zones and BFEs in conjunction with information on structures
and their contents to assign premium rates for flood insurance policies.
For floodplain management applications, the map shows by tints, screens, and symbols,
the 1- and 0.2-percent-annual-chance floodplains, floodways, and the locations of
selected cross sections used in the hydraulic analyses and floodway computations.
The countywide FIRM presents flooding information for the geographic area of Modoc
County. Previously, FIRMs were prepared for each incorporated community of the
County identified as flood-prone. This countywide FIRM also includes flood hazard
information that was presented separately on Flood Boundary and Floodway Maps
(FBFMs), where applicable. Historical data relating to the maps prepared for each
community are presented in Table 8, “Community Map History.”
7.0
OTHER STUDIES
This FIS report either supersedes or is compatible with all previous studies published on
streams studied in this report and should be considered authoritative for the purposes of
the NFIP.
Hydrologic studies of the North Fork Pit River were performed by the USACE in 1967
(USACE, July 1967) for the planning and design of the Alturas Flood Control Project. In
1989, a hydrologic flood insurance study was performed by the USACE that was not
adopted by FEMA.
Hydrologic and hydraulic analyses for the October 20, 1999 study of the City of Alturas
were performed by Ensign & Buckley, for the Federal Emergency Management Agency
(FEMA), under Contract No. EMW-90-C-4151. This study was completed in April 1998.
(FEMA, October 1999(a)).
This study was revised to incorporate detailed flood-hazard information along the North
Fork Pit River from the east-west Southern Pacific Railroad (SPRR) to approximately 1.3
stream miles upstream of the north-south SPRR tracks. The hydrologic and hydraulic
analyses for the North Fork Pit River were performed by Ensign & Buckley, for FEMA,
under Contract No. EMW-90-C-4151. This work was completed in April 1998. (FEMA,
October 1999(b)).
As mentioned previously some data from CADWR Awareness Floodplain Mapping
Studies was incorporated into this study. The Awareness Floodplain Maps are not FEMA
regulatory floodplain maps; however, at the request of the community FEMA will include
the data on the county’s maps (CADWR, 2009).
Flood data presented are in agreement with the FIS currently in production for Lassen,
Shasta, and Siskiyou, California (FEMA, Unpublished (a-c)). Flood data is also
compatible with Washoe, Nevada (FEMA, March 2009) Klamath, Oregon (FEMA, June
1984).
24
8.0
LOCATION OF DATA
Information concerning the pertinent data used in the preparation of this FIS can be
obtained by contacting:
FEMA, Federal Insurance and Mitigation Division
1111 Broadway, Suite 1200
Oakland, California 94607-4052.
25
COMMUNITY NAME
Alturas, City of
Modoc County
INITIAL
IDENTIFICATION
FLOOD HAZARD
BOUNDARY MAP
REVISION DATE (S)
FLOOD INSURANCE
RATE MAP
EFFECTIVE DATE
FLOOD INSURANCE
RATE MAP
REVISION DATE (S)
May 31, 1974
January 2, 1976
September 24, 1984
October 20, 1999
January 24, 1975
July 25, 1978
September 24, 1984
September 30, 1992
September 17, 1997
October 20, 1999
TABLE 8
MODOC COUNTY, CA
COMMUNITY MAP HISTORY
9.0
BIBLIOGRAPHY AND REFERENCES
California Department of Water Resources, Peak Discharge Gate Data, North Fork Pit
River at Alturas – 1991-1995, Ash Creek at Adin – 1958-1988, n.d.
California Department of Water Resources, Short-Duration Precipitation Frequency Data,
1976.
California Department of Water Resources, Awareness Floodplain Maps, 2009,
http://www.water.ca.gov/floodmgmt/lrafmo/fmb/fes/awareness_floodplain_maps/.
Chow, V.T., McGraw-Hill Book Company, Open Channel Hydraulics, 1959.
Engle, Dolan H., The Earth Is Our Mother, a Guide to the Indians of California, Their
Locales and Historic Sites, Trees Company Press, 1992.
Federal Emergency Management Agency, Flood Insurance Study and Flood Insurance
Rate Map, Klamath County, Oregon (Unincorporated Areas), June 18, 1984.
Federal Emergency Management Agency, Guidelines and Specifications for Study
Contractors, July 1992.
Federal Emergency Management Agency, Flood Insurance Study Guidelines and
Specifications for Study Contractors, Appendix 6, January 1995.
Rate Map, City of Alturas, Modoc County, California, Revised October 20, 1999(a).
Rate Map, Modoc County, California (Unincorporated Areas), Revised October 20,
1999(b).
Rate Map, Washoe County and Incorporated Areas, Nevada, March 16, 2009.
Rate Map, Lassen County and Incorporated Areas, California, Unpublished (a).
Rate Map, Shasta County and Incorporated Areas, California, Unpublished (b).
Rate Map, Siskiyou County and Incorporated Areas, California, Unpublished (c).
Geonex, Inc., Fort Bidwell – Topographic Map, Scale 1:4,800, Contour Intervals 1 and 2
feet, June 1993(a).
Geonex, Inc., Aerial Photographs – Fort Bidwell, Scale 1:9,600, June 1993(b).
27
Geonex, Inc., Topographic Map, Scale 1” = 200’, Contour Interval 2 feet, January 13,
1996.
Hoover, Rensch, Abeloe, and Kyle, Historic Spots in California, Stanford University
Press, 1990.
Snow, Williams G., Land Surveyor, North Fork Pit River, Alturas, Cross Sections, 1986.
Thomas, W.O., Jr., Michael Baker Jr., Inc., Flood Frequency Analysis for the North Fork
Pit River Near Alturas, May 23, 1997.
U.S. Census Bureau, State and County Quick Facts,
http://quickfacts.census.gov/qfd/states/06/06049.html, accessed November 10, 2008.
U.S. Department of Agriculture, Soil Conservation Service, Soil Survey of Modoc
County, April 1974.
U.S. Department of Agriculture, Soil Conservation Service, National Engineering
Handbook, Section 4, Hydrology, October 1981.
U.S. Department of the Army, Corps of Engineers, Detailed Project Report – North Fork
Pit River at Alturas, July 1967.
U.S. Department of the Army, Corps of Engineers, North Fork Pit River Project – AsConstructed Plans, April 3, 1972.
U.S. Department of the Army, Corps of Engineers, Operation and Maintenance Manual
for North Fork Pit River at Alturas, February 5, 1973.
U.S. Department of the Army, Corps of Engineers, Hydrologic Engineering Center,
Generalized Computer Program HEC-1, Flood Hydrograph Package, Davis California,
September 1981.
U.S. Department of the Army, Corps of Engineers, HEC-WRC, Flood Flow Frequency
Analysis, February 1982.
Generalized Computer Program HEC-2, Water-Surface Profiles, Davis, California,
September 1982.
U.S. Department of the Army, Corps of Engineers, Sacramento District, Hydrologic
Study: Ash Creek, Dry Creek, and Butte Creek at Adin, Modoc County, November 1989.
Generalized Computer Program HEC-1, Flood Hydrograph Package, Davis California,
September 1990(a).
28
Generalized Computer Program HEC-2, Water-Surface Profiles, Davis, California,
September 1990(b).
U.S. Department of the Army, Corps of Engineers, PL84-99 – Project Information Report
– North Fork of the Pit River at Alturas, August 17, 1995.
U.S. Department of the Army, Corps of Engineers, PL84-99 – Emergency Gabion Repair
and Channel Clearing – North Fork Pit River – Alturas, July 23, 1996.
U.S. Department of Commerce, National Oceanic and Atmospheric Administration,
National Weather Service, Precipitation Frequency Maps of California, 1972.
U.S. Department of Commerce, National Oceanic and Atmospheric Administration,
National Weather Service, Precipitation-Frequency Atlas 2 of the Western United States,
Volume XI, California, 1973.
U.S. Department of the Interior, Geologic Survey, 7.5-Minute Series Topographic Maps,
Scale 1:24,000, Contour Interval 20 feet, Fort Bidwell, Mount Bidwell, California, n.d.
(a).
U.S. Department of the Interior, Geologic Survey, Annual Peak Flow Frequency
Analyses, North Fork Pit River at Alturas – 1930-1990, Pit River at Canby – 1904-1994,
South Fork Pit River at Likely – 1930-1994, n.d. (b).
U.S. Department of the Interior, Geologic Survey, A Comparison of Four Stream Flow
Record Extension Techniques, R.M. Hirsch, August 1982.
U.S. Department of the Interior, Geologic Survey, Roughness Characteristics of Natural
Channels, 1987.
29
10.0
REVISIONS DESCRIPTION
This section has been added to provide information regarding significant revisions made
since the original Flood Insurance Study was printed. Future revisions may be made that
do not result in the republishing of the Flood Insurance Study report. To assure that any
user is aware of all revisions, it is advisable to contact the community repository.
10.1
First Revision
This study was revised on ___________________, to include a detailed hydrologic
and hydraulic study for the Pit River, North Fork Pit River, and South Fork Pit
River near the City of Alturas. This study was performed by BakerAECOM (the
study contractor) under Contract No. HSFEHQ-09-D-0368. The study was
completed in January 2013.
A detailed list of the flooding sources and reaches studied is given in Table 9,
“Revised Detailed Flooding Sources”.
Table 9 – Revised Detailed Flooding Sources
FLOODING
SOURCE
REACH
STREAM
MILES
PIT RIVER
At Alturas
From mouth of North Fork Pit River to
approximately 400 feet downstream of mouth.
0.1
NORTH FORK PIT
RIVER
At Mouth
From mouth to approximately 6,600 feet
upstream of Lake County Railroad Bridge
2.8
SOUTH FORK PIT
RIVER
At Mouth
From mouth to approximately 3,800 feet
upstream of US Highway 395
1.8
a. Hydrologic Analysis
BakerAECOM determined the 1- and 0.2-percent-annual-chance peak discharges
for the North Fork Pit River, South Fork Pit River, and the Pit River at the City of
Alturas using the HEC-HMS Version 3.5 computer program (USACE, 2010).
The Synder unit hydrograph method was utilized and watershed parameters were
developed from the USDA/NRCS National Land Cover Dataset (USDA 2001),
four USDA Soil Surveys for California (USDA, 2009), and the NOAA Atlas 14
precipitation depth estimates (NOAA, 2004). The 24-hour rainfall distribution is
centered around the midpoint of the storm and the time intervals of 5 minutes, 15
minutes, 1 hour, 2 hours, 3 hours, 6 hours, 12 hours, and 24 hours are input. The
30
many reservoirs and lakes within the Pit River basin, including Dorris Reservoir,
Payne Reservoir, Dannhauser Reservoir, West Valley Reservoir, Moon Lake and
several others, were not accounted for in the rainfall runoff routing in HEC-HMS.
Lastly, baseflow and impervious areas were not considered for any of the subbasins.
A of revised discharges and associated flooding sources is given in Table 10,
“Summary of Revised Discharges”.
Table 10 – Summary of Revised Discharges
FLOODING
SOURCE AND
LOCATION
DRAINAGE
AREA
(sq. miles)
PEAK DISCHARGES (cfs)
10%2%1%0.2%ANNUAL- ANNUAL- ANNUAL- ANNUALCHANCE CHANCE CHANCE CHANCE
PIT RIVER
At Alturas
226.8
*
*
6,665
11,420
NORTH FORK PIT
RIVER
At Mouth
625.3
*
*
5,332
9,410
865.1
*
*
9,718
16,776
SOUTH FORK PIT
RIVER
At Mouth
b. Hydraulic Analysis
Terrain data used for hydraulic modeling and mapping was based on Digital
Elevation Models (DEM) and contours developed by BakerAECOM. The terrain
data is based on Light Detection and Ranging (LiDAR) data collected by Digital
Mapping, Inc. on May 4, 2011. Field survey data for channel cross sections and
structures was collected by Harned Surveying and Engineering, Inc. in April
2011. Also used was the MAP-IX survey data of the North Fork Pit River
collected by URS Corporation in November 2009. The topographic data are
referenced to the North American Vertical Datum of 1988 (NAVD88).
The Pit River, North Fork Pit River, South Fork Pit River and their surrounding
floodplain were modeled using the two-dimensional hydraulic model FLO-2D
Version 2009.06 (FLO-2D, 2012). The modeled grid system, covering
approximately 10.0 square miles, was comprised of cells of 50 feet. Manning’s
“n” values ranged from 0.04 for channels and Lakes to 0.2 for urban landscape
and crop land. Precipitation and precipitation losses were not considered in the
31
hydraulic model. Flood hydrographs for the 1- and 0.2-percent-annual-chance
flood events were assigned to respective inflow cells of the North Fork Pit River
and South Fork Pit River. Due to the short studied reach length of the Pit River,
no user defined inflow was assigned. Model simulation duration was extended to
capture peak discharge at all outflow cells.
Study reaches were modeled using the channel segments. Both natural channel
and trapezoidal channels were modeled for each reach, dependent on the extent of
available filed survey data. The North Fork Pit River and South Fork Pit River
were initially modeled using HEC-RAS Version 4.1 (USACE 2010) to develop
rating tables of in-channel hydraulic structures for use in the FLO-2D model.
Manning’s “n” values for modeled channels ranged from 0.04 to 0.05.
Levees were modeled using field survey data collected in April 2011. Width
reduction factors were applied to levee grid cells to accurately represent weir flow
top width should the levee be overtopped. Two model conditions were performed;
“with levee” and “without levee”. The “with levee” condition considered crest
survey data at representative cells. The “without levee” condition did not consider
survey data of those levees located along the North Fork Pit River or the Southern
Pacific Railroad embankment west of the North Fork Pit River. Grid element
elevations that were impacted by levees and the failed embankment that were
captured in the DEM were adjusted to adopt a landward toe-of-levee elevation
from adjacent model grid cells.
c. Bibliography and References
FLO-2D Software Inc., FLO-2D Flood Routing Model, Version 2009.06, Build
No. 09-12.06.09. June 2012.
U.S. Department of the Army, Corps of Engineers, Hydrologic Engineering
Center, HEC-RAS, River Analysis Software, Version 4.1, Davis County,
California, January 2010.
U.S. Department of the Army, Corps of Engineers, Hydrologic Engineering
Center, HEC-HMS, Hydrologic Modeling System, Version 3.5, Davis County,
California, August 2010.
U.S. Department of Commerce, National Oceanic and Atmospheric
Administration, NOAA Atlas 14 Precipitation-Frequency Atlas of the Western
United States Volume I, Version 4.0 – California, Silver Spring, MD, 2004.
U.S. Department of Agriculture, Natural Resources Conversation District,
National Land Cover Dataset, 2001.
U.S. Department of Agriculture, Natural Resources Conversation District, Soil
Survey Geographic Database for California, Fort Worth, Texas, Accessed 2009.
32

modoc county, california

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