sespe creek hydrology, hydraulics and sedimentation analysis
DESCRIPTION
Sespe Creek Hydrology, Hydraulics and Sedimentation Analysis Presentation to the Technical Advisory Committee October 14, 2008. Sespe Creek Hydrology, Hydraulics and Sedimentation Analysis. The Project: - PowerPoint PPT PresentationTRANSCRIPT
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Sespe CreekHydrology, Hydraulics and
Sedimentation Analysis
Presentation to theTechnical Advisory Committee
October 14, 2008
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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
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Sespe CreekHydrology, Hydraulics and Sedimentation
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
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Sespe CreekHydrology, Hydraulics and Sedimentation
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
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Sespe CreekHydrology, Hydraulics and Sedimentation
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
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Watershed Assessment of Hillslope and River Geomorphic Processes
G E O M O R P H O L O G Y
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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
G E O M O R P H O L O G Y
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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
G E O M O R P H O L O G Y
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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
G E O M O R P H O L O G Y
11
Presentation Outline
G E O M O R P H O L O G Y
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
12
Topographic Map
Bullet Points
G E O M O R P H O L O G Y
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
G E O M O R P H O L O G Y
Eoceneshale
Cretaceoussandstone
Sespesandstone
granitics
Mioceneshale
Eocenesandstone
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Geologic Map
Bullet Points
G E O M O R P H O L O G Y
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
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
G E O M O R P H O L O G Y
16
Bullet Points
G E O M O R P H O L O G Y
Day Fire – September 2006
1 per 20 years
1 per 50 years
1 per century
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Bullet Points
G E O M O R P H O L O G Y
Day Fire – September 2006Day FireDay Fire
PiruPiru
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Geomorphology Slides
Bullet Points
G E O M O R P H O L O G Y
Day Fire – September 2006
- USFS BAER
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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
G E O M O R P H O L O G Y
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6
5
4
3
2
1Sediment Production and Delivery- Tectonic Uplift Rates
G E O M O R P H O L O G Y
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
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Sediment Production and Delivery
G E O M O R P H O L O G Y
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
G E O M O R P H O L O G Y
Estimating sediment production: creation of GLUs
32
Watershed sediment yield—relative values
G E O M O R P H O L O G Y
“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:
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Real Wash Debris Basin
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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
G E O M O R P H O L O G Y
Total sediment production
= 1,760 t km-2 a-1
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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)
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Coarse Sediment Yield
G E O M O R P H O L O G Y
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
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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
G E O M O R P H O L O G Y
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
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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
G E O M O R P H O L O G Y
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
G E O M O R P H O L O G Y
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Observed Fire Impacts Denuded vegetation cover
Widespread rilling
Tributary debris fan deposits
Sand deposition in Lower Gorge pools
No evidence of post-fire landslides
G E O M O R P H O L O G Y
Tributary debris deposit delivery to Sespe Creek
VCWPD rain gage
Sand accumulation
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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
G E O M O R P H O L O G Y
Predicted GLU-predicted fine-sediment production for pre-fire and post-fire (Day and Piru fires)
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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
G E O M O R P H O L O G Y
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
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
G E O M O R P H O L O G Y
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River Morphology
Bullet Points
G E O M O R P H O L O G Y
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
G E O M O R P H O L O G Y
52G E O M O R P H O L O G Y
Sediment Character
Upper Reaches – Rapid visual assessment
Lower Reach – Detailed facies mapping and sediment sampling
53G E O M O R P H O L O G Y
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
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Sediment Discharge
G E O M O R P H O L O G Y
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
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Sediment Discharge
G E O M O R P H O L O G Y
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
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Morphologic Changes in Lower Sespe Creek
(1938 – 2008)
Watershed Characteristics Fire History Hillslope Processes
• Coarse and Fine Sediment Production and Delivery• GLU Analysis
• Pre-fire conditions• Post-fire conditions
• GLU Analysis River Morphology
• Morphology and Sediment Character• Sediment Discharge• Morphologic Change in Lower Sespe Creek
Conclusions
G E O M O R P H O L O G Y
1938 1975 2005
57
Lower Sespe Creek – 1978 Flood Extent (pre-levee)
G E O M O R P H O L O G Y
58
Thalweg Locations in Lower Sespe Creek (1938–2005)
G E O M O R P H O L O G Y
Uplifted Terrace (bedrock)
West Terrace (alluvial)
Terrace (alluvial)East Terrace
(alluvial)
59
Morphologic Changes in Lower Sespe Creek
(1938 – 2008) Watershed Characteristics Fire History Hillslope Processes
• Coarse and Fine Sediment Production and Delivery• GLU Analysis
• Pre-fire conditions• Post-fire conditions
• GLU Analysis River Morphology
• Morphology and Sediment Character• Sediment Discharge• Morphologic Change in Lower Sespe Creek
Conclusions
G E O M O R P H O L O G Y
Active Channel Area in Lower Sespe Creek (1938 – 2005)
West Terrace (Qoa - alluvial)
East Terrace
(Qoa - alluvial)
1938
1970
2005
60
Morphologic Changes in Lower Sespe Creek
(1938 – 2008) Watershed Characteristics Fire History Hillslope Processes
• Coarse and Fine Sediment Production and Delivery• GLU Analysis
• Pre-fire conditions• Post-fire conditions
• GLU Analysis River Morphology
• Morphology and Sediment Character• Sediment Discharge• Morphologic Change in Lower Sespe Creek
Conclusions
G E O M O R P H O L O G Y
Bank revetment
Sespe Creek Levee
Channel constriction at river bend
61
Morphologic Changes in Lower Sespe Creek
(1938 – 2008) Watershed Characteristics Fire History Hillslope Processes
• Coarse and Fine Sediment Production and Delivery• GLU Analysis
• Pre-fire conditions• Post-fire conditions
• GLU Analysis River Morphology
• Morphology and Sediment Character• Sediment Discharge• Morphologic Change in Lower Sespe Creek
Conclusions
G E O M O R P H O L O G Y
Cross-section Analysis(1970s & 2005)
62
Cross-section Analysis - (1970s & 2005)
Watershed Characteristics Fire History Hillslope Processes
• Coarse and Fine Sediment Production and Delivery• GLU Analysis
• Pre-fire conditions• Post-fire conditions
• GLU Analysis River Morphology
• Morphology and Sediment Character• Sediment Discharge• Morphologic Change in Lower Sespe Creek
Conclusions
G E O M O R P H O L O G Y
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
Cross-section Analysis - (1970s & 2005)
63
Cross-section Analysis - (1970s & 2005)
G E O M O R P H O L O G Y
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
64G E O M O R P H O L O G Y
31 Jan 197026 Feb 1969
Ch
ann
eliz
ed w
est
fork
Channel Reconfiguration in Channel Reconfiguration in 19691969
65
Morphologic Changes in Lower Sespe Creek
(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
G E O M O R P H O L O G Y
66
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
G E O M O R P H O L O G Y
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)
G E O M O R P H O L O G Y
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
G E O M O R P H O L O G Y
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
G E O M O R P H O L O G Y
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…
G E O M O R P H O L O G Y
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.
G E O M O R P H O L O G Y
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.
G E O M O R P H O L O G Y
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74
Stakeholder Comments
Discussion&
75
Stakeholder Comments
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
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Subdivision of Lower Sespe Creek
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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
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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
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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
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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
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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
Stakeholder Comments
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
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
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
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
106
Stakeholder Comments
Discussion&
107