Hydrology and watershed report

Burned Area Emergency Response
Sobranes Fire
Los Padres National Forest
Hydrology and Watershed Specialist Report
September 6, 2016
Ocean view from the Joshua Creek watershed
Submitted by:
Luke Rutten, Tahoe National Forest
Kelsha L. Anderson, Angeles National Forest
Fire and site description
This report summarizes the results from the hydrologic assessment of the Sobranes Fire in the
northern portion of the Santa Lucia Mountains in Monterey County, CA, as part of the Burned
Area Emergency Response (BAER). The Fire started on Jun 26, 2016 due to an illegal campfire.
As of the date of this report, the fire has burned over 100,000 acres, and continues to burn to the
south, southwest. This BAER assessment addressed the northwest portion of the fire and covered
75,626 acres. The fire occurred east of State Highway 1 between Carmel Highlands and Big Sur
and south of Carmel Valley and San Carlos, Dormody and Cachagua Roads and west of
Jamesburg. On south facing slopes, the fire burned dense chaparral vegetation. On north facing
slopes, the vegetation is mixed timber dominated by hardwood (including oak and madrone) with
some conifer. In the drainage bottoms is a mix of hardwood and conifer, including redwoods.
The fire affected property across land ownership (Table 1) and watersheds (Figure 1). The final
soil burn severity was 2% High, 51% Moderate, 32% Low, and 6% Very Low/Unburned (Table
2).
Table 1. Acres burned by land ownership
Land owner
Acres burned
U.S. Forest Service
37,927
Bureau of Land Management
655
Other Federal
9
State and Local
11,963
Private
35,073
Total
75,626
Table 2. Acres burned by Soil Burn Severity
Soil Burn Severity
Acres
High
1,795
Moderate
44,687
Low
34,381
Very Low/Unburned
4,763
Total
75,626
The Fire took place in a region that experiences moist winters and dry summers. Precipitation
throughout the burn area ranges from about 23 inches per year near the coast to 48 inches per
year further inland, with the bulk of the precipitation occurring from October through April.
Precipitation is rain dominated by frontal storms which account for nearly all moisture, with
infrequent thunderstorms occurring in summer and fall. Historically, major flooding has
occurred when a weather system dubbed the “Pineapple Express" taps into subtropical moisture
from the latitudes of the Hawaiian islands. These warm and long duration storm events will
cause major deluges and torrential rains leading to catastrophic flooding. Stream channels in the
burn area have the potential to flash flood. Elevation within the burned area perimeter ranges
from 108 ft. to 4770 ft.
This report details a hydrologic analysis conducted for watersheds or various sizes defined by
placing a pour pour at selected values at risk (VARs). These VARs could include homes,
buildings, bridges, roads, culverts, and campgrounds.
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Fire severity assessment
A Burned Area Reflectance Classification (BARC) image was acquired from the Forest Service
Remote Sensing Applications Center. Based on comparisons with archived images, this image
classifies the extent of the burned area into four categories: unburned, low severity burn,
moderate severity burn, and high severity burn. The BAER team modified this image using field
observations to produce a final Soil Burn Severity map of the fire. See figure 2.
Watershed Response and Water Quality
Areas affected by the Fire drain portions of the Carmel River and Santa Lucia Hydrologic Units
(Central Coast Basin Plan 2016). Watersheds directly impacted by the fire are shown in figure 1.
Beneficial uses common across these watersheds include; municipal, domestic and agricultural
supply, groundwater recharge, recreation, wildlife habitat, aquatic habitat and spawning, and
commercial/sport fishing.
Wildfires primarily affect water quality through increased sedimentation. As a result, the primary
water quality constituents or characteristics affected by this fire include color, sediment,
settleable material, suspended material, and turbidity. Floods and debris flows can entrain large
material, which can physically damage infrastructure associated with the beneficial utilization of
water (e.g., water conveyance structures; hydropower structures; transportation networks). The
loss of riparian shading and the sedimentation of channels by floods and debris flows may
increase stream temperature. Fire-induced increases in mass wasting along with extensive tree
mortality can result in increases in floating material – primarily in the form of large woody
debris. Post-fire delivery of organic debris to stream channels can potentially decrease dissolved
oxygen concentrations in streams. Fire-derived ash inputs can increase pH, alkalinity,
conductivity, and nutrient flux (e.g. ammonium, nitrate, phosphate, and potassium), although
these changes are generally short lived. Post-fire increases in runoff and sedimentation within the
urban interface, and burned structures and equipment within the fire perimeter may also lead to
increases in chemical constituents, oil/grease, and pesticides.
The most noticeable effects on water quality will be increases in sediment and ash from the
burned area into waterbodies in and downstream of the fire area. Flash flooding and debris flows
are natural watershed response for this area. The risk of flash flooding and erosional events will
increase as a result of the fire, creating hazardous conditions within and downstream of the
burned area.
Historical information from prior fires in this area can be used to predict how the landscape will
respond to this fire. Following the Molera Fire in August 1972, rain storms in October and
November having peak rainfall intensities of about 0.7 inches per hour triggered mud flows in
Pheneger Creek (Cleveland 1972). Following the 2008 Basin Fire, a mud/debris flow occurred in
Pfeiffer Redwood Creek (Cooper pers comm). On February 6, 2014, a 2.14 inch rainstorm
generated a mudflow in Sycamore Canyon from the Pfeiffer fire area which burned in December
2013 (Anderson pers comm).
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Figure 1. Pour Point watersheds affected by the Sobranes Fire
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Figure 2. Soil Burn Severity
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Design Flow Runoff Response
The risk or probability (R) that a certain return interval flood (T) will occur over different time
1
periods (n) was calculated by the following equation (Chow et al. 1988):𝑅 = 1 − (1 − 𝑇)𝑛 . A
design flood with a 2 year recurrence interval has a 50% chance of occurring in any given year,
and a 97% chance of occurring in the next five years (recovery period). The 2 year, 30 minute
duration storm anticipated for these watersheds range from 0.589‑0.737 inches (NOAA, 2016).
A 5 year flood has a 20% chance of occurring in any given year, and a 67% chance of occurring
in the next five years. The 5 year, 30 minute duration storms anticipated for these watersheds
range from 0.711‑0.896 inches. A 10 year flood has a 10% chance of occurring in any given
year, and a 41% chance of occurring in the next five years. The 10 year, 30 minute duration
storms anticipated for these watersheds range from 0.805‑1.04 inches. Hydrologic design
information is displayed in Table 3.
Table 3. Hydrologic design factors
A. Estimated Vegetative Recovery Period
3-5 years
B. Design Chance of Success
80 %
C. Equivalent Design Recurrence Interval
2 years
D. Design Storm Duration
30 minute
E. Design Storm Magnitude
0.589‑0.737 in
F. Design Flow
32.5 cfs / mi2
G. Estimated Reduction in Infiltration
20%
H. Adjusted Design Flow
78.4 cfs / mi2
Before an adjusted design flow can be determined, pre-fire design flow must be calculated. This
is the flow expected to occur in pre-fire conditions. This is the flow responsible for forming
present day channel conditions and flows used to estimate proper performance of culverts and
other drainage structures. However due to the lack of real data on that system, and the rapid
nature of this assessment, design flow estimates have been calculated based on the equations in
the document “Methods for Determining Magnitude and Frequency of Floods in California,
Based on Data through Water Year 2006” (USGS, 2012). This is an empirical model based on
gauge data. These estimates assume pre-fire ground infiltration and ground cover conditions.
Adjusted design flow is calculated using the same equations as design flow; however runoff
response is estimated by assuming an increased runoff commensurate with soil burn severity in
terms of recurrence interval. This recurrence interval estimates the response of the newly burnt
landscape to an average annual storm. Areas within the Fire perimeter that experienced moderate
to high soil burn severity are expected to respond to an average rainfall event, an event usually
associated with the 2-year storm, differently than the unburned and low severity burned areas. It
is expected the landscape would respond as if the 2-year storm discharge were associated with a
2-year storm (unburned soil burn severity), 5-year event (low soil burn severity) and 10-year
event (moderate and high soil burn severity), respectively.
The moderate and high soil burn severity in this fire is on high slopes, has little or no soil cover
in the form of duff or expected needle cast, and is prone to flash flood and debris flows under
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pre-fire conditions. These factors increase the probability of increased watershed response from
that area within the burn. As a result, moderate burn severity is assumed to have the same
recurrence interval as high burn severity. The unburned lands within the fire would respond as
the unburned lands outside the fire and would have a discharge associated with the 2-year return
interval. Surface vegetation has burned within the low soil burn severity, however some duff
and roots from the past surface plants remain. Thus, initially the low soil burn severity areas
would be expected to have a higher response than the unburned but would not be as intense and
potentially recover quicker than the moderate and high soil burn severity areas. Consequently the
low soil burn severity area would be expected to have a discharge associated with the 5-year
return interval. The range in return interval is based on the Central Coastal California flood
frequency equations (USGS 2012). Increases in discharge associated with predicted recurrence
intervals are pro-rated across watersheds by soil burn severity to yield post-fire discharge or the
adjusted design flow. The modeling assumes the design storm event.
The fire has been analyzed by watersheds. Watersheds are various sizes and shapes and are
dependent on the analysis of the desired outlet or pour point above a value at risk or area of
concern. Watersheds are defined as the area above a point in which all surface water will flow.
These sites may be within or downstream of the burned area. Size of watershed is dependent on
the local flow patterns in addition to the need to evaluate a basin for values at risk. Figures 3 and
4 display the response of pour point watersheds for a 2 year flood. The results show the response
ranged from 1.07 to 3.34 times the unburned pre-fire condition.
Post-fire flood peaks for the 5-year and 10-year floods were also modelled using the process
described above. For the 5-year flood, low burn was calculated at the 10-year flow while
moderate/high burn was calculated as the average between the 10- and 25-year flows. For the 10year peak, low burn was calculated as the average between the 10- and 25-year flows and the
moderate/high burn was calculated as the 25-year flood.
Results for the post-fire 5-year flood were 1.04 to 1.78 times the pre-fire values. The post-fire
10-year flood results were 1.03 to 1.40 times pre-fire. This result is to be expected as the
influence of the fire effects lessens for larger fires. The increase in flow is mainly due to the loss
of infiltration. This is a smaller portion of the total runoff of larger flows.
The areas that have the highest potential for flooding include:
 Pfeiffer Redwood Creek
 Juan Higuera Creek
 Palo Colorado Upper Road Crossing
 Doud Creek
 Sobranes Creek
Watersheds along the coast are expected to have the highest post fire watershed response. State
and private property and state, county and private roads within and downstream of these areas
are identified as values at Risk.
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Figure 3. Watershed response for 2 year flood.
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The increase in peak flows is most applicable during the first year of recovery, as hydrologic
response will decrease in subsequent years. Predicted post-fire peak flows show an increase of
about one to three times pre-fire values. The peak flow values highlight the post-fire effects on
the Fire, with the most increase reflected in watersheds where burn severity is moderate and high
and where the greatest susceptible soils are affected. The early precipitation events fill in
available slope detention storage and create the rill and gully networks that are necessary to fully
induce the expected increase in flood response from rainstorms.
As previously mentioned, the post-fire flows could lead to plugged culverts, flow over road
surfaces, rill and gully erosion of cut and fill slopes, erosion and deposition along road surfaces
and relief ditches, loss of long-term soil productivity, and threats to human safety. Some
sedimentation of the ephemeral channels is likely to occur at an accelerated rate until vegetation
establishes itself and provides ground cover.
Implications of post-fire runoff
The objective of this analysis is to predict post-fire runoff with the goal of mitigating risk to life,
property, and natural and cultural resources. After identifying potential VARs, the magnitude of
this risk was systematically evaluated. The risk matrix shown in Table 4 was utilized to identify
values in need of mitigation efforts.
Table 4. Risk assessment matrix
Magnitude of Consequences
Probability of Damage or Loss
Major
Moderate
Minor
Risk
Very likely
Very High
Very High
Low
Likely
Very High
High
Low
Possible
High
Intermediate
Low
Unlikely
Intermediate
Low
Very Low
The probability of damage or loss within one to three years is classified into four categories:
unlikely occurrence (<10%); possible occurrence (>10% to <50%); likely occurrence (>50% to
<90%); and very likely or nearly certain occurrence (>90%). This information is combined with
an assessment of the magnitude of the consequences. These are classified as major, with
implications for loss of life or injury to humans, substantial property damage, irreversible
damage to critical natural or cultural resources; moderate, indicating injury or illness to humans,
moderate property damage, damage to critical natural or cultural resources resulting in
considerable or long term effects; or minor, with property damage limited in economic value
and/or too few investments, damage to natural or cultural resources resulting in minimal,
recoverable or localized effects.
Due to the increase in post-fire peak flows, values at risk were evaluated for potential damage.
Values at risk for hydrology include:
Forest Service, County, and State of Califorinia roads
Increased post-fire runoff may overwhelm existing road crossing structures, causing washouts,
and stream diversion down the road. This can result in a threat to public safety, damage to
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infrastructure, and increased sediment delivery to downstream channels. The Forest Service road
to the Boy Scout Camp is in need of treatment, which is detailed in the roads report.
County, State, and private roads affected by the fire may also have a need for treatment. Sharing
this assessment with those stakeholders will aid in their risk and treatment determination for
those features.
Private Property
Property within and downstream of the fire may also have a need for treatment. Sharing this
assessment with those stakeholders will aid in their risk and treatment determination for those
features.
Water quality for Domestic, and Irrigation Uses
Turbid water from the burned area will impact the quality of domestic and irrigation water within
and downstream of the fire. This impact will be short-term and only occur during and shortly
following storm events. Sharing this assessment with those stakeholders will aid in their risk and
treatment determination for those features.
Conclusions
As a result of the Sobranes Fire, significant increases in runoff are expected in some pourpoint
watersheds. These increases are the result of the large percentages of high or moderate soil burn
severity within these watersheds. The pourpoint watersheds show increases in 2-year peak flow
from 1.07 (San Clemente/Dormody Rd) to 3.34 (Palo Colorado upper road crossing) times
greater than pre-fire conditions. This increase is for the 2-year storm or a discharge that has a
50% chance of occurring over the course of a year. Depending on location of the values at risk
from the Sobranes Fire; these values may be affected as a result of the burn under design storm
conditions. Details on values at risk and treatments may be found in the 2500-8 BAER Report
for the Sobranes Fire.
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References
Anderson, Kelsha. Angeles National Forest. Personal communication. September 1, 2016.
Central Coast Regional Water Quality Control Board. 2016. Water Quality Control Plan for the
Central Coastal Basin, March 2016 Edition. California Environmental Protection
Agency.
Chow, V.T., Maidment D. R., and Mays L.W. 1988. Applied Hydrology. McGraw-Hill, New
York.
Cleveland, George B. 1972. Fire + Rain = Mudflows. California Geology, Volume 26 Number 6.
Cooper, Kevin. Los Padres National Forest. Personal communication, August 29, 2016.
USGS. Department of the Interior, United States Geologic Survey. 2012. Methods for
Determining Magnitude and Frequency of Floods in California, Based on Data through
Water Year 2006. Scientific Investigations Report 2012-5113.
NOAA. Hydrometerological Design Studies Center. http://hdsc.nws.noaa.gov/hdsc/pfds/.
Accessed 13 August 2016.
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Appendix A.
Results of runoff modelling.
Post-Fire Peak
Pre-fire Peak
Watershed
Acres
2-year
(cfs)
5-year
(cfs)
10-year
(cfs)
2-year
(cfs)
5-year
(cfs)
10-year
(cfs)
522
70
122
168
40
97
150
1,165
273
355
431
90
209
318
545
136
179
217
43
103
158
4. Little Sur River
25,607
3936
5163
6231
1495
3193
4817
5. Sierra Creek
1,357
121
197
272
46
128
214
6. Bixby Creek
5,539
1063
1390
1667
376
835
1262
7. Rocky Creek
2,225
657
811
949
239
498
721
8. Palo Coloradow Lower Canyon
1,195
99
170
241
43
119
199
442
112
146
178
34
82
126
10. Garrapatos RD
2,734
804
972
1131
276
577
839
11. Garrapata Creek at Trout Farm
6,696
1280
1633
1965
429
956
1452
12. Doud Creek
1,740
336
445
545
105
254
395
13. Unnamed Tributary to Pacific Ocean 01
120
7
15
24
4
12
21
14.Granite Canyon
992
168
243
306
61
151
236
15. Unnamed Tributary to Pacific Ocean 02
128
9
17
26
4
12
21
16. Unnamed Tributary to Pacific Ocean 03
212
12
25
38
6
19
33
1,929
248
353
446
78
206
336
Watershed
1. Phenegan Creek
2. Juan Higuera Creek
3. Pfeiffer Redwood Creek
9. Palo Colorado Upper RD crossing
17. Soberanes Creek
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18. Malpaso Creek
2,109
189
307
415
72
196
326
19. Carmel River
39,871
3301
5277
7095
1718
3859
6003
20. San Clemente Creek/San Clemente Dam
10,666
827
1403
1939
421
1028
1645
21. San Jose Creek
9,041
378
764
1159
214
596
1015
22. Las Gazas Creek at Rancho San Carlos RD
2,852
388
606
787
187
433
662
23. Danish Creek/Rattlesnake Creek
29,364
2669
4346
5886
1535
3343
5101
24. Pine Creek
5,074
870
1174
1416
305
699
1073
25. Middle Little Sur
11,685
2757
3428
4014
1109
2193
3153
26. White Rock Lake
2,164
477
661
805
210
448
657
27. San Clemente Creek/Dormody RD
3,697
156
392
628
147
377
611
28. Black Rock Creek
5,233
845
1167
1432
316
723
1110
29. Turner Creek on Palo Colorado
1,630
572
702
809
226
449
633
30. Bixby Creek on Coast Road
7,057
1058
1446
1786
377
876
1358
31. Little Sur River on Coast Road
17,033
3261
4121
4873
1216
2526
3739
32. South Fork Little Sur River on Coast Road
7,297
1189
1617
1976
447
1003
1529
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