sespe creek hydrology, hydraulics and sedimentation analysis

2

Sespe CreekHydrology, Hydraulics and

Sedimentation Analysis

Presentation to theTechnical Advisory Committee

October 14, 2008

3

Sespe CreekHydrology, Hydraulics and Sedimentation

AnalysisThe Project:

Development of a comprehensive re-evaluation of the Sespe Creek watershed to focus on identifying necessary improvements and maintenance needs to sustain the desired channel capacities of the lower Sespe Creek.

Consultants:RBF Consulting – Engineering/Hydraulics/SedimentationStillwater Sciences – Geomorphology/EnvironmentalAqua Terra Consultants – Hydrology

Timeline:January 2008 – March 2009

4


AnalysisThe Project Scope: Data Collection and Review Field Reconnaissance Hydrology Studies Hydraulic Studies River Morphology and Sedimentation Analysis Flooding, Sedimentation, and Erosion Problem

Identification and Alternative Solution Systems Preliminary Environmental Impact Analysis Progress/Stakeholder Meetings Draft/Final Report and Deliverable

5


Analysis

Purpose of this meeting:

Discuss results of geomorphology and hydrology studies

Review approach for hydraulics/sedimentation analyses

Obtain Stakeholder feedback on studies and key issues in the watershed

6


Analysis

Presentation Topics:

Watershed Assessment of Hillslope and River Geomorphic Processes

Hydrologic Modeling of the Sespe Creek Watershed

Future Hydraulics and Sedimentation Studies

H Y D R A U L I C S A N D S E D I M E N T A T I O N

G E O M O R P H O L O G Y

H Y D R O L O G Y

7

Watershed Assessment of Hillslope and River Geomorphic Processes


8

Scope of Geomorphic Study

Assess the effects of the 2006 Day Fire on sediment yields and downstream channel morphology in the Sespe Creek watershed:

• Research on fire history & sedimentation effects

• Watershed-scale hillslope processes and sediment yields

• River morphology


9

Scope of Geomorphic Study

Specific tasks: literature review, field reconnaissance, and GIS analysis.

• Compile and review existing information relating to fire effects on sediment production in southern California watersheds, and hillslope and channel geomorphic processes

• Characterize hillslope geomorphic processes in the watershed and resulting sediment yields into the mainstem Sespe Creek

• Characterize sediment transport and channel dynamics in the mainstem of Sespe Creek to understand how these processes affect channel morphology, specifically in the lower reach adjacent to the Sespe Creek Levee


10

Presentation Outline

Watershed Characteristics

Fire History

Hillslope Processes• Coarse and Fine Sediment Production and Delivery• GLU Analysis

• Pre-fire conditions• Post-fire conditions

River Morphology• Morphology and Sediment Character• Sediment Discharge• Morphologic Change in Lower Sespe Creek

Conclusions


11




Fire History




Conclusions

12

Topographic Map

Bullet Points


Watershed Area = 674 km2 (260 mi2)Stream length = 97 km (60 mi)Relief = 105-2290 m (350-7500 ft)Unregulated flow and largely undeveloped

2,290 m; 7,510 ft

Sespe Creek Levee

13

Geologic Map

Bullet Points


Eoceneshale

Cretaceoussandstone

Sespesandstone

granitics

Mioceneshale

Eocenesandstone

14

Geologic Map

Bullet Points


36 cm

46 cm

57 cm

70 cm

83 cm

89 cm

Data Sources: Elevation: USGS 10m DEM; Watersheds: Stillwater Sciences; Cities, Road, Rivers, and Counties: ESRI; Precipitation: averages compiled by CDWR and CGS from US Weather Service Data supplemented by county and local agency precipitation. The data was collected over a sixty year period (1900-1960). Minimum mapping unit is 1000+ acres

~2x change

15



Fire History




Conclusions


16

Bullet Points


Day Fire – September 2006

1 per 20 years

1 per 50 years

1 per century

17

Bullet Points


Day Fire – September 2006Day FireDay Fire

PiruPiru

18

Geomorphology Slides

Bullet Points


Day Fire – September 2006

- USFS BAER

19



Fire History




Conclusions


20

6

5

4

3

2

1Sediment Production and Delivery- Tectonic Uplift Rates


Middle Sespe Creek valley

~30 m

modern river

21

San Cayatano faultU

D

Lower Sespe Creek valley:

22

0

200

400

600

800

1,000

1,200

1,400

1,600

0 10 20 30 40 50 60 70

Modern river

Faults

Lower terrace

Upper terraces

Lion Canyon terraces

Munson Creekfault

SantaYnezfault

San Cayatano fault

~1% slope

~3% slope

Kilometers downvalley

Ele

vati

on

(m

eter

s)

Sespe Creek long profile

23

Santa Paula Creek watershed

Santa Clara River watershed

Sespe Creek watershed

1-22-4 5

1

0.5

0.5

0.3

1-104

2

SOURCES:

Rockwell 1988 Yeats 1988 Çemen, I. 1989 Lajoie et al. 1991 Huftile and Yeats 1995 Orme 1998 Trecker et al. 1998 Blythe et al. 2000

Geologically determined uplift ratesGeologically determined uplift rates

~3 (west) to ~5 (east) ~3 (west) to ~5 (east) mm/yr upliftmm/yr uplift

24

Sediment Production and Delivery


Example of the delivery of coarse sediment blocks into the channel network from the weathering of a single sandstone interbed.

Sandstone bedding-plane surface of the Sespe Formation, east of the Sespe Creek gorge near the Dough Flat trailhead.

Typical exposure of thin-bedded shale of the Cozy Dell Formation

Large volumes of fine sediment (silts and clays) are derived from highly erodible silt- and mudstonesCoarse sediment (sand – boulders) are derived by rockfall from harder sandstones and granitic rocks in the Middle and Gorge subwatersheds

Slope categories

Controls on sediment production:

•SLOPE

•GEOLOGY

•LAND COVER

(USGS 10-m DEM)(USGS 10-m DEM)

Geology categories(Mapping by Dibblee, (Mapping by Dibblee, 1970-1990)1970-1990)

Land cover categories

(Landsat 30-m data)(Landsat 30-m data)

LOW sediment delivery

MODERATE sediment delivery

HIGH sediment delivery

31

Geomorphology Slides

Bullet Points


Estimating sediment production: creation of GLUs

32

Watershed sediment yield—relative values


“High”

“Medium”

“Low”

33

Waring Debris Basin

Watershed sediment yield—quantified values

34

Name

Contrib. area

(km2, from GIS)

Annual Average Sediment

Yield (yd3 a-1)*

Sediment Yield per Unit Area (t km-2 a-1)

Years evaluated*

Location

Real Wash 0.6 7,423 18,929 1969–2005 12 km east of Sespe Creek

Warring Canyon Debris Basin 2.8 12,039 6,578 1969–1998

0.4 km east of Real Wash

Jepson Wash Debris Basin 3.5 9,174 4,010 1969–2005 Southwest edge, Sespe Creek watershed

Fagan Canyon 7.5 12,500 2,550 1994–2005 2 km west of Santa Paula Creek

Adams Barranca Debris Basin 21.8 27,362 1,920 1998–2005

2 km west of Fagan Canyon

Debris basin data from Ventura County used to quantify rates of sediment delivery

“Low” ≈ 300 t km-2 a-1“High” ≈ 20,000 t km-2 a-1“Medium” ≈ 2000 t km-2 a-1

Used for calculations:

35

Real Wash Debris Basin

36

Five catchments with measured sediment-removal rates, showing GLU-derived levels (H, M or L) of sediment production.

= location of monitored debris basin

Sespe CreekSanta Paula

Creek

37

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

AB (21.4) FC (7.8) JW (3.4) WC (2.7) RW (0.6)

Measured

Calculated

H

M

L

“Measured” values from Ventura County; “Calculated” values use presumed unit-area sediment-delivery factors (previous slide) (LH scale).

Colored bars “H”, “M”, and “L” show proportional area (RH scale) in each category in the contributing watershed.

Total watershed area (in km2) in parentheses.

38-yr records;

small basins

<15-yr records;

large basins

Sed

imen

t yi

eld

(104

tonn

es k

m-2 a

-1)

100%

80

60

40

20

0

Measured and predicted debris basin sediment yields

38

Watershed Sediment Yield


Total sediment production

= 1,760 t km-2 a-1

39

Sesp

e Creek n

r. mo

uth

LowerSespeCreek

40

0

2,000,000

4,000,000

6,000,000

8,000,000

10,000,000

12,000,000

14,000,000

16,000,000

18,000,000

192

8

193

2

193

6

194

0

194

4

194

8

195

2

195

6

196

0

196

4

196

8

197

2

197

6

198

0

198

4

199

3

199

7

200

1

200

5

Year

An

nu

al

sed

imen

t yi

eld

(to

nn

es)

Total sediment

Coarse (>0.0625 mm) sediment

Calculated sediment load for Sespe Creek at Fillmore [USGS gage 11113000]

1645 t/km2/yr calculated from rating curve

1760 t/km2/yr predicted from GLU analysis

Average yield = 1,109,000 t/yr (1645 t/km2/yr)

41

Coarse Sediment Yield


31%

34%

23%

11%2%

26%

36%

26%

2%

11%

Sespe SSSespe

SS

Cold-water

SS

Matilija SS

Matilija SS

Granitics Granitics

Fraction of coarse-grained litohologies, watershed-wide

Fraction of coarse-grained litohologies on lower Sespe floodplain

Cold-water

SS

Sespe Gorge

Coarse sediment production – no connectivity impediments

42

Wildfire Effects on Sediment Production

Affects hydrologic and geomorphic processes of sediment production & delivery

Vegetation and runoff: burn-off increases dry ravel (loss of organic ‘check dams’), overland flow, rainsplash on bare surfaces

Soil structure: more friable, less cohesive, more water repellent (depends on fire severity, organic litter loss)

Rock weathering: fire decreases rock strength spalling (fragments) and fracture, especially in igneous rocks


Conceptualization of sediment yield and associated vegetation and litter recovery during the fire-induced ‘window of disturbance’ (based on Shakesby and Doerr 2006).

vegetation cover

litter cover

fire-induced sediment

yield

‘background’sediment yield

window of disturbance

F I R

E

T I M E

S E

D I M

E N

T Y

I E

L D

Increasing influence of

erosion-limiting factors

vegetation cover

litter cover

fire-induced sediment

yield

‘background’sediment yield

window of disturbance

F I R

E

T I M E

S E

D I M

E N

T Y

I E

L D

Increasing influence of

erosion-limiting factors

~10 years~10 years

43

Wildfire Effects on Sediment Production

USFS BAER (Burned Area Emergency Response) method: based on Rowe et al. 1949 peak flows & debris basins yields in LA County

Scott and Williams 1978: storm-induced sediment yields, regression based on 1969 storm event, including “fire factor”

Geomorphic Landscape Units: GIS-based DEM analysis based on attributing yields to geology, slope, and land cover combinations


Log Sy = 1.244 + 0.828(log A) + 1.382(log ER) + 0.375(log SF) + 0.251(log FF) and 0.840(log K)

Sy = Sediment Yield; A = area; ER = elongation ratio; SF = Area of soil failures; FF = fire factor [unveg * fire area]; K = storm factor

(areas < 2.7 km2)

source: Lave & Burbank 2003, based on LACFCD 1959

Alternative approaches:

44

Observed Fire Impacts Denuded vegetation cover

Dry ravel, rilling, and gullying


45

Observed Fire Impacts Denuded vegetation cover

Widespread rilling

Tributary debris fan deposits

Sand deposition in Lower Gorge pools

No evidence of post-fire landslides


Tributary debris deposit delivery to Sespe Creek

VCWPD rain gage

Sand accumulation

46

Wildfire Sediment Production

Watershed-wide predictions:

• BAER (USFS 2006)- 6.1-fold increase (1,663 t km-2 a-1 to 10,188 t km-2 a-1)

• GLU (this study)- 4.7-fold increase (1,760 t km-2 a-1 to 8,200 t km-2 a-1)

• Scott and Williams (1978)- 3-fold increase based on max. possible increase in Fire Factor


Predicted GLU-predicted fine-sediment production for pre-fire and post-fire (Day and Piru fires)

47

Transport dynamics of large sediment pulses

pulses evolve, mostly by dispersionTime 1

Time 2

(a)

Time 1

Time 2

(b)

DISPERSAL – Where the pulse sediments are of similar size to downstream bed material: coarse sand and gravel

TRANSLATION & ATTENUATION – Where the pulse sediments are finer than the downstream bed material: sand and finer (<2 mm)

See: Lisle et al 2001; Cui et al 2003a, b; Cui and Parker 2005


48G E O M O R P H O L O G Y

Sediment Delivery - field evidence

Upper half of the Sespe Gorge (2-5 m

deep pools filled with sands in early 2008)

Date

Gauge Height Change Aggradatio

n / Incisionm ft

7 May 2003

11 Apr 2005 +1.69 +5.54 Aggradation

3 Mar 2008 -1.67 -5.47 Incision

49



Fire History




Conclusions


50

River Morphology

Bullet Points


51

Gorge subwatershed

Middle subwatershed Upper subwatershedL

ow

er

Su

bw

ate

rsh

ed

HeadwaterWashUpper GorgeUpper Terrace

0.8%

2.4%

1.0%

1.7%

1.3%

1.6%

0.9%

4.1%

West Fork Sespe Creek Confluence

Granitics Middle Terrace



Sediment Character

Upper Reaches – Rapid visual assessment

Lower Reach – Detailed facies mapping and sediment sampling


Subwatershed Type Reaches Length

(km) Channel

gradient a

Average channel width b

(m)

Facies distribution (% of reach

channel area) d

Dominant Facies Type distribution (% of reach

channel area) d

Upper Alluvial / Confined

Headwater Wash

Upper Gorge Upper Terrace

42.7 1.7% 31 CG (>50%)

CSG (<50%) G (>50%)

Middle Alluvial Middle Terrace

Granitics 27.0 1.0% 55

CSG (33%) S (15%) GS (6%)

G (58%) S (27%) C (15%)

Gorge Confined Lower Gorge 19.1 2.4% 45 BGC (>50%) SGC (<50%)

C (>50%)

Lower Alluvial Valley

Fillmore 8.3 0.8% 224 CSG (>50%) G (>50%)

Sespe Creek channel characteristics by reaches

Upper Middle Gorge Lower

54

Sediment Discharge


0.001

0.01

0.1

1

10

100

1000

10000

100000

0.0001 0.001 0.01 0.1 1 10 100 1000 10000

Daily mean flow (m3s-1)

Flo

w f

req

uen

cy

0.0000001

0.000001

0.00001

0.0001

0.001

0.01

0.1

1

10

100

1000

10000

100000

1000000

10000000

100000000S

edim

ent lo

adS

edim

ent yield

flow frequency (days)

flow frequency, fitted

coarse sediment load (kg/sec)

coarse sediment load, fitted

total coarse sediment yield (tonnes)

total coarse sediment yield, fitted

Flow frequency and coarse (>0.0625 mm) sediment load for long-term daily mean flow record for Sespe Creek at Fillmore [USGS gage 11113000].

Large flows are rare

Large flows are the most powerful

Large flows, over time, move the most sediment

55

Sediment Discharge


0.001

0.01

0.1

1

10

100

1000

10000

100000

0.0001 0.001 0.01 0.1 1 10 100 1000 10000

Daily mean flow (m3s-1)

Flo

w f

req

uen

cy

0.0000001

0.000001

0.00001

0.0001

0.001

0.01

0.1

1

10

100

1000

10000

100000

1000000

10000000

100000000S

edim

ent lo

adS

edim

ent yield

flow frequency (days)

flow frequency, fitted

coarse sediment load (kg/sec)

coarse sediment load, fitted

total coarse sediment yield (tonnes)

total coarse sediment yield, fitted

Flow frequency and coarse (>0.0625 mm) sediment load for long-term daily mean flow record for Sespe Creek at Fillmore [USGS gage 11113000].

•Majority of sediment is transported during very brief intervals

•The “dominant discharge” (i.e., the single flow that performs the most work, over the long term) is the highest flow on record (to date, 2005)

•Trend is similar for results from other Santa Clara River watershed locations (i.e., semi-arid environments)

•Sespe Creek is prone to abrupt changes during episodically high flows

56

Morphologic Changes in Lower Sespe Creek

(1938 – 2008)

Watershed Characteristics Fire History Hillslope Processes

• Coarse and Fine Sediment Production and Delivery• GLU Analysis


• GLU Analysis River Morphology

• Morphology and Sediment Character• Sediment Discharge• Morphologic Change in Lower Sespe Creek

Conclusions


1938 1975 2005

57

Lower Sespe Creek – 1978 Flood Extent (pre-levee)


58

Thalweg Locations in Lower Sespe Creek (1938–2005)


Uplifted Terrace (bedrock)

West Terrace (alluvial)

Terrace (alluvial)East Terrace

(alluvial)

59


(1938 – 2008) Watershed Characteristics Fire History Hillslope Processes





Conclusions


Active Channel Area in Lower Sespe Creek (1938 – 2005)

West Terrace (Qoa - alluvial)

East Terrace

(Qoa - alluvial)

1938

1970

2005

60







Conclusions


Bank revetment

Sespe Creek Levee

Channel constriction at river bend

61







Conclusions


Cross-section Analysis(1970s & 2005)

62

Cross-section Analysis - (1970s & 2005)

Watershed Characteristics Fire History Hillslope Processes





Conclusions


435

440

445

450

455

460

465

470

475

0 500 1,000 1,500 2,000

Hor. Dist. (ft)

Ele

vatio

n (

ft)

East Fork (overflow) West Fork (mainstem)

Sespe Creek Levee (1981)

Elev

ation

(ft)

25x

verti

cal e

xagg

erati

on

Horizontal Distance (ft)

1970s2005

View looking downstream


63



Example of retreat (51 m [167 ft]) of right bank upstream of channel constriction and river bend near the rock-revetted left bank at XS 6B

475

480

485

490

495

500

505

510

515

0 200 400 600 800 1,000

Hor. Dist. (ft)

Ele

vatio

n (

ft)

Horizontal Distance (ft)

19772005

View looking downstream

Elev

ation

(ft)

10x

verti

cal e

xagg

erati

on

Bank retreatBar growth


31 Jan 197026 Feb 1969

Ch

ann

eliz

ed w

est

fork

Channel Reconfiguration in Channel Reconfiguration in 19691969

65


(1938 – 2008) - Summary Since 1938, the creek has followed a similar course, while terrace scarps indicate historic channel locations closer to the valley walls

Thalweg positions have moved frequently and have reset after each flood event

Both the west (mainstem) and east (overflow) forks have been continuously active

Since 1971, the east (overflow) fork has become the dominant stream course (wider and deeper) while the west (mainstem) fork has aggraded slightly

Active channel area has contracted since 1938, with some expansion: Bed elevation changes since the 1970s:

• Upstream end of reach: Bed lowering up to 1.5 m (4.9 ft) (XS 9 and XS 10)

• Middle portion of reach: Bed aggradation up to 1.2 m (3.9 ft) (XS 5A to XS 8)

• Upstream of Old Telegraph Rd Bridge: • West (mainstem) fork has aggraded by up to 1.2 m (3.9 ft) • East (overflow) fork has incised by up to 1.2 m (3.9 ft)

• Downstream of bridge:• West fork has aggraded by up to 1.8 m (5.9 ft) • East fork has incised by up to 0.6 m (2 ft)

• Downstream of Hwy 126 towards mouth: Slight bed aggradation in both forks


66



Fire History




Conclusions


67

Conclusions

Sespe Creek watershed has a long history of wildfires due to dominance of chaparral vegetation in a semi-arid environment• 73% of the watershed has burned at least twice in the last century

Long-term sediment production (pre-fire):• Rates of tectonic uplift are rapid (3-5 mm a-1)

• Large volumes of fine sediment (silts and clays) are derived from highly erodible silt- and mudstones

• Coarse sediment (sand to boulders) are derived by rockfall from harder sandstones and granitic rocks in the Middle and Gorge subwatersheds

• Vast majority of watershed has moderate sediment-production rates—no one “critical” area

• Annual background rate of sediment yield from Sespe Creek is ~1,700 t km-2 yr-1 (1.1 million tons/yr)


68

Wildfire effects on sediment production:• Large increases in fine sediment yield from hillslopes• Wildfire impacts wane after 5-10 years as vegetation recovers• Comparison of post-fire sediment yield estimates:

• GLU analysis predicts a 10-fold increase in sediment production in burned areas (Day and Piru fires) 5-fold increase from entire watershed

• USFS BAER method predicted a 6-fold increase in total sediment yield

• Field observations in 2008 indicated variable impacts throughout watershed:

• In areas burned by the 2002 Wolf Fire, new vegetation growth and few sediment accumulations

• Day Fire areas: – Widespread rilling on hillslopes, fine-grained (silt to fine gravel)

debris deposits at tributary mouths—dispersal should dominate– No landslides on hillsides– Potential storage along much of the mainstem and floodplain for

post-fire sediment delivered by tributaries and adjacent hillslopes


Conclusions (cont.)

69

Channel morphology of the Lower Reach:• Sediment discharge:

• Delivery to lower reach is sporadic, occurring during short-duration, high-intensity storm events

• Four El Nino years (WY 1969, 1978, 1995, and 2005) account for over half the total sediment yield since 1928

• The “dominant discharge” is the largest flow on record (2005)• Historical changes to channel morphology:

• Since 1938, Sespe Creek has occupied a largely similar course through its alluvial fan

• Location of the channel thalweg(s) has re-set after each flood event

• Prior to 1938 through post-1975, the west (mainstem) fork carried more discharge than the east (overflow) fork

• Some time after 1975 and continuing to the present, the east (overflow) fork has widened and deepened becoming the dominant channel


Conclusions (cont.)

70

Worst-case scenario for post-fire bed aggradation: relatively small flood events that move the sediment load only incrementally

The risk is likely to be more a function of local erosion/deposition than to widespread, reach-scale aggradation

Modeling would help…


Hypotheses

71

Implications for Management

Sespe Creek is a largely unmanaged (“pristine”) watershed with minimal human controls on sediment production and channel morphology• Geomorphic conditions in the lowest (Fillmore) reach are

largely those imposed by progressive environmental fluctuation rather than recent human modifications.

• Shorter-term morphological changes likely occur as a consequence of climate oscillations that affect vegetation cover, the natural frequency of wildfires, and the frequency of large flood events.

• Therefore, the morphology of the lower reach (i.e., bed-level and planform position) should be expected to oscillate over time.


72

Implications for Management (cont.)

The Lower reach is a naturally dynamic environment subject to “re-setting” by very large floods rather than progressive alteration by intermediate floods.

The entire alluvial fan extent of Sespe Creek is potentially part of the active channel bed.• Modifying fluvial processes (i.e., channelization, dredging,

bridge construction, or levees) will likely result in expected but largely unpredictable responses by the river morphology during large flood events.

• Although prediction of such changes is not possible, modeling potential fluctuation in bed levels (using the predicted range of sediment yields delivered from the upper watershed) would quantify the possible risk to those residing on the adjacent floodplain areas.


74

Stakeholder Comments

Discussion&

75


Discussion&

76

Sespe Creek Hydrology Study

Sespe Creek HSPF Model Sub-component of SCR HSPF Model Baseline & Natural Conditions Post-Day Fire Conditions 100-year event storm Multipliers for other events Draft Report, 9/18/08

H Y D R O L O G Y

77

HSPF: : Hydrologic Simulation Program - FORTRAN

Continuous simulation modelContinuous simulation model

Natural and developed watersheds and water Natural and developed watersheds and water systemssystems

Land surface and subsurface hydrology and quality Land surface and subsurface hydrology and quality processesprocesses

Stream/lake hydraulics and water quality processesStream/lake hydraulics and water quality processes

Core watershed model in EPA BASINS and Army Core watershed model in EPA BASINS and Army Corps WMSCorps WMS

Development and maintenance activities sponsored Development and maintenance activities sponsored by U.S. EPA and U.S. Geological Surveyby U.S. EPA and U.S. Geological Survey

FEMA-accepted model for NFIPFEMA-accepted model for NFIP

H Y D R O L O G Y

78

Sespe Creek HSPF Model

Subcomponent of SCR HSPF Model

Calibrated/validated as part of SCR effort; details available in SCR Draft Report

Calibration: WY97 – WY05

Validation: WY87 – WY95

Lower Sespe Study Area further subdivided for current study

H Y D R O L O G Y

79

Sespe Creek HSPF Model Segmentation

20 stream reaches4 Precip gages

80

Subdivision of Lower Sespe Creek

81

Sespe Creek HSPF ModelModel Land Uses:

Forest/Woodland – 11 %

Shrub/Scrub – 82%

Open/Grass – 3%

Agriculture (irrigated) – 2%

Low Density Residential (irrigated) – <1%

Medium Density Residential (irrigated) – <1%

High Density Residential (irrigated) – <1%

Commercial/Industrial (irrigated) – <1%

Effective Impervious Area – <1%

H Y D R O L O G Y

83

“Weight-of-Evidence” Approach for Hydrology

Mean runoff volume for simulation period (inches)

Annual and monthly runoff volume (inches)

Daily flow timeseries (cfs)• observed and simulated daily flow• scatter plots

Flow frequency (flow duration) curves (cfs)

Storm hydrographs, hourly or less, (cfs)

H Y D R O L O G Y

84

Calibration/Validation Summary Results

Sim. Obs. R R2 R R2

Sespe at Wheeler Springs (RCH704)

711 10/1/86-9/30/96 7.2 7.3 -1.2 0.91 0.82 0.98 0.96 3.8

Sespe at Fillmore (RCH713) 710A 10/1/93- 9/30/96 10.5 9.8 7.4 0.92 0.84 0.97 0.95 9.3

Gage Name Time Period % Vol Error

DailyGage ID

Flow (in) Monthly

Peaks % Diff.

Calibration

Validation

Sim. Obs. R R2 R R2

Sespe at Wheeler Springs (RCH704)

711 10/1/02-9/30/05 9.7 8.9 9.0 0.95 0.91 0.98 0.97 4.6

Sespe at Fillmore (RCH713) 710A 10/1/96-9/30/05 10.6 10.9 -3.3 0.97 0.94 0.99 0.98 -3.6

Gage Name Time Period % Vol Error

DailyGage ID Flow (in) MonthlyPeaks % Diff.

86

Sespe Calib/Valid. Flow Duration Results

Calibration

Calibration

Validation

Validation

Fillmore

Wheeler Springs

87

Sespe Model Daily Flows, 2005 and 1995Calibration - 2005

Validation - 1995

88

Storm SimulationsCalibration: March 4-5, 2001

Validation: January 9-11, 2005

89

Sespe Creek HSPF Model Performance

Weight-of-Evidence Summarymean range mean range

Runoff Volume, % Δ 2.8 -3.3 /9.0 3.1 -1.2 / 7.4 Very Good

Correlation Coefficient, R:

- Daily R 0.96 0.95 / 0.97 0.92 0.91 / 0.92 Very Good- Monthly R 0.99 0.98 / 0.99 0.98 0.97 / 0.98 Very Good

Coefficient of

Determination, R2:

- Daily R2 0.93 0.91 / 0.94 0.83 0.82 / 0.84 Good / Very Good

- Monthly R2 0.98 0.97 / 0.98 0.96 0.95 / 0.96 Very Good

Flow-Duration Good/ Very Good

Water Balance

Storm Events:- Daily Storm Peak, % D 0.5 -3.6 / 4.6 6.6 3.8 / 9.3 Very Good

Good / Very Good

Overall Model Performance

Good / Very Good

Good / Very Good

Good

Good / Very Good

Calibration Validation*

Sespe Model is robust and acceptable for this effort

90

Scenario Analyses

Baseline Conditions: Calibration period & 2001 SCAG landuse

Simulation Period: WY1960 – WY2005

Natural Conditions

Post Day Fire Conditions

100-year event hydrograph

H Y D R O L O G Y

91

Model Changes for Natural Conditions

Remove all irrigation applications

Convert all impervious areas to pervious

Convert all developed (urban and agriculture) to forest, shrub, and open/grass lands

Eliminate all point source discharges and diversions

H Y D R O L O G Y

92

Model Changes for Post Day Fire Conditions

Landscape impacts of fire• Loss of vegetation and litter• Reduction in ET• Reduction in soil moisture capacity• Hydrophobic (water repellant) layer formation• Reduction in infiltration

Changes to Model Parameters• Reduce interception by 90%• Reduce infiltration by 35% (LAC DPW Method)• Reduce upper layer/interflow by 50%• Reduce soil ET parameter by 70%

Changes to Sespe Model Setup• Overlay Day Fire boundaries on model segments• Define burned regions and adjust parameters

H Y D R O L O G Y

93

Day Fire Impacts on Daily Flow Duration Curve

Fillmore

94

Day Fire Impacts on Annual Peak Flows

Scenario BASE NATURAL BURN Percent BASE NATURAL BURN Percent Location Fillmore Fillmore Fillmore Change SCR SCR SCR Change

cfs cfs cfs Base to BurnConfluenceConfluence Confluence Base to Burncfs cfs cfs

Mean 25,392 25,350 31,440 919% 24,002 23,766 29,642 363%Max 88,983 88,923 96,553 6006% 82,611 82,086 85,797 3087%Min 28 21 879 4% 146 12 833 2%

Percent Change, Mean Annual Peak 24% 23%Percent Change, Maximum Peak 9% 4%Percent Change, Minimum Peak 3092% 470%

SELECTED YEARS

9/30/1969 77,628 77,615 80,936 4% 82,611 82,086 84,528 2%9/30/1974 29,330 29,276 39,293 34% 26,104 25,599 34,993 34%9/30/1982 15,672 15,617 23,238 48% 15,426 15,251 22,531 46%9/30/1983 88,983 88,923 96,553 9% 77,933 77,553 83,383 7%9/30/1994 1,736 1,696 8,062 364% 1,702 1,410 7,933 366%9/30/1995 70,148 70,119 77,722 11% 70,452 69,883 79,925 13%9/30/1998 47,948 47,923 50,789 6% 48,810 48,677 51,110 5%9/30/2004 28,386 28,311 42,536 50% 28,255 28,034 42,395 50%9/30/2005 77,964 77,928 82,796 6% 70,094 70,155 74,266 6%

95

Day Fire Impacts on Annual Peak Flows

Annual Peaks 919% (4% - 6006%) 363% (2% - 3087%)

Mean Annual Peak 24% 23%

Maximum Peak 9% (1983) 2% - 4% (1969, 1978)

Minimum Peak 3092% (1987) 1168% (1961)

Flow Ranges:>30,000 cfs 10% 9%10,000 - 30,000 cfs 27% 30%1,000 - 10,000 cfs 424% 490%<1,000 cfs 3798% 1176%

Fillmore SCR Confluence

96

Day Fire Impacts on Annual Runoff (inches)

Scenario BASE NATURAL BURN Percent BASE NATURAL BURN Percent Location Fillmore Fillmore Fillmore Change SCR SCR SCR Change

inches inches inches Base to Burn Confluence Confluence Confluence Base to Burninches inches inches

Mean 8.35 8.32 10.42 91.0% 7.75 7.58 9.56 102.1%Max 35.42 35.37 38.65 404.0% 33.72 33.35 36.67 499.0%Min 0.26 0.24 0.89 9.0% 0.15 0.11 0.56 8.6%

Percent Change, Mean Annual Runoff 25% 23%Percent Change, Maximum Annual Runoff 9% 9%Percent Change, Minimum Annual Runoff 249% 273%

SELECTED YEARS

9/30/1962 11.91 11.88 14.20 19.2% 11.35 11.10 13.38 17.9%9/30/1965 1.26 1.23 3.47 175.3% 0.95 0.83 2.84 200.4%9/30/1969 26.81 26.78 29.51 10.0% 25.75 25.49 28.19 9.5%9/30/1972 2.09 2.06 3.75 79.3% 1.81 1.66 3.28 81.4%9/30/1973 11.71 11.67 14.09 20.4% 10.86 10.64 13.00 19.7%9/30/1978 32.16 32.12 35.04 9.0% 30.60 30.33 33.22 8.6%9/30/1983 27.83 27.79 31.44 13.0% 26.35 26.06 29.61 12.4%9/30/1984 2.37 2.35 4.37 84.1% 1.87 1.78 3.65 95.4%9/30/1986 11.52 11.49 14.22 23.4% 10.78 10.55 13.20 22.5%9/30/1994 1.07 1.05 2.72 153.8% 0.76 0.65 2.13 180.1%9/30/1995 28.00 27.96 31.06 10.9% 26.66 26.34 29.43 10.4%9/30/1998 28.25 28.22 31.34 10.9% 26.70 26.43 29.54 10.7%9/30/2001 9.73 9.69 12.13 24.8% 9.19 8.90 11.36 23.6%9/30/2005 35.42 35.37 38.65 9.1% 33.72 33.35 36.67 8.7%

97

100-Year Event Hydrograph Development

Obtain 100-year event hyetograph from VCWPD for all rain gages used in the Sespe model

With VCWPD, select January 9-11, 2005 event for starting conditions

Run Sespe Creek model through midnight 1/9/05 and extract final storage values, to be initial conditions

Run HSPF model with 100-year rainfall from hyetograph, with AR factors applied, starting on 1/10/05

Output/analyze resulting 100-year storm hydrograph

Re-do analyses for Base, Natural, and Burn conditions

H Y D R O L O G Y

98

100-Year Event Hydrograph @ Fillmore

143,000 cfs - Burn

136,000 cfs – Base & Natural

100

Summary Conclusions Current Sespe Creek HSPF model is a robust representation

of the watershed response, and an appropriate tool for this study

The greatest uncertainties in the hydrology are related to the rainfall inputs, spatial and temporal

There are uncertainties in the Burn impacts on model parameters, and these should be addressed through sensitivity/uncertainty analyses

Wildfires appear to have a greater relative impact on the more frequent events, i.e. the 5 – 20 year events

The standard LP3 analysis appears to over-estimate extreme event peaks, and should be further investigated and/or replaced

H Y D R O L O G Y

103


Discussion&

104

Hydraulic Study Approach

Flood conveyance capacity evaluation• Baseline (pre-Day Fire) conditions

• HEC-RAS model based on FEMA 2008 SCR FIS• Peak flows from calibrated HSPF

• Post-Day Fire conditions• Hydraulic roughness adjustments• Peak flow adjustments

• Pre-European conditions• Qualitative assessment of flood extents• Backwards projection

• Flood damage assessment (USACE 1110-2-1619)• Baseline (pre-Day Fire) conditions• Post-Day Fire conditions• 10-, 50-, 100-, and 500-year flood events


105

Sedimentation Study Approach

Evaluation of sediment load, and erosion and deposition rates• Sediment yield estimates

• Stillwater Sciences• Scott’s Method

• Sediment Transport Analysis (HEC-6T)• Calibration

– Continuous simulation– Historical flood record/Calibrated HSPF– Topography: Fillmore aerial (7.26.2004) calibrated to

County LIDAR (3.10.2005)• Predict long-term adjustment

– Continuous simulation– Estimated effective discharge


106


Discussion&

sespe creek hydrology, hydraulics and sedimentation analysis

Documents

mainstem of sespe creek

sedimentation analysisflooding

sedimentation analysispurpose

fine sediment production

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