Transcript AGU 2004 poster - University of Washington

Prediction of Mass Wasting, Erosion, and Sediment Transport With the Distributed
Hydrology-Soil-Vegetation Model
a
Colleen O. Doten , Jordan
Laninia,
b
Laura C. Bowling , and Dennis P. Lettenmaier
a
a. University of Washington, Department of Civil and Environmental Engineering, Box 352700, Seattle, WA 98195
b. Purdue University, Department of Agronomy, Lilly Hall of Life Sciences, 915 West State Street, West Lafayette, IN 47907
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Abstract
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Motivation
Vegetation management, forest road construction
and forest fire impact basin sediment yield, by
increasing the amount of sediment available for
transport and the amount of surface water
available to transport it. Increased sediment
yield can cause:
• sedimentation of water supplies and salmon
habitat,
• damage to hydraulic structures, and
• exceedances of Total Maximum Daily Loads.
Predictive tools would allow land managers to
evaluate management options prior to
implementation.
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Study Area – Hard and Ware Creeks
Hard and Ware Creeks are two headwater
catchments of the Deschutes River. Ware Creek
catchment covers 2.84 km2 and Hard Creek covers
2.31 km2 in southern Puget Sound.
• Topography
•Hard Creek elevation 463-1220 m; slopes
between 60 and 100%.
•Ware Creek elevation 457-1180 m; slopes
between 40 and 60%.
• Precipitation
•average annual precipitation at the lowest
elevations is 2600 mm/year.
• Soils and Geology
• Soils are stony and shallow, averaging 0.6 m
in depth in Hard and 1.0 m in Ware Creek.
• Soils are underlain by resistant and weathered
andesite, basalt and breccia bedrock.
Model Description
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Precipitation
Leaf Drip
Infiltration and
Saturation Excess Runoff
CHANNEL EROSION &
ROUTING
MASS WASTING
SURFACE EROSION
Wildfire Effects
Timber Harvest and Forest Road Effects
Q
The Distributed Hydrology Soil Vegetation Model
(DHSVM) was developed at the University of
Qsed
Washington and Pacific Northwest National
Laboratory for assessment of the hydrologic
consequences of forest management (Wigmosta,
1994). It is a physically-based, distributed model
that simulates the effects of spatially variable
MODEL OUTPUT
topograpy, soil and vegetation on the energy and
water balance at each grid cell at each time step.
The Sediment Transport Component of DHSVM predicts the range and variability of
catchment sediment yield in response to dynamic meteorological and hydrologic
conditions. It builds on existing models such as LISA (Hammond et al., 1992) and
SHETRAN (Burton and Bathurst, 1998 and Wicks and Bathurst, 1996), by including
stochastic mass failure predictions and forest road erosion. It also includes
concepts from the KINEROS (Woolhiser et al., 1990 and Smith et al., 1995) and
EUROSEM (Morgan et al., 1998) models.
DHSVM provides hydrological inputs to the model’s three components:
•mass wasting,
• surface erosion, including forest roads,
•and channel erosion and routing.
Station Name
Basin
Area, km2
Days of
Record
North River, WA
490
365
210
27 Jan 1965
17
Skookumchuck River, WA
290
1,095
376
14 Jan 1970
13
Wynoochee River, WA
107
485
1500
13 Dec 1966
25
Clearwater Creek, WA
85
2,358
14,200
20 Feb 1982
-
218
730
52,600
06 Oct 1981
-
Muddy River, WA
Max,
ppm
Date of Max
Mean,
ppm
Observed sediment concentrations in various Washington rivers. Rainy Creek
concentrations are similar to observed concentrations. Sediment data is from the U.S.
Geological Survey.
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The sediment model was run
on Hard and Ware Creeks with
four different scenarios:
•Pre-harvest scenario, with
roads;
• Post-harvest scenario, with
roads;
• Pre-harvest scenario,
without roads;
• Post-harvest scenario,
without roads.
Channel
Flow
DHSVM was run for six years in the Rainy Creek basin from 1991
through 1997. Mass wasting calculations were performed when at least
20% of the basin soils were 85% saturated. Seven mass wasting events
were identified:
• 8 May 1992
• 18 May 1993
• 30 May 1995
• 29 November 1995
• 08 June 1996
• 17 May 1997
• 15 June 1997
The model was run with the existing road network with a
scenario to evaluate changes in erosion rates due to the effects
of fire.
• The understory was removed from all pixels with an overstory;
• LAI of pixels with an overstory was reduced to 1.0;
• Root cohesion distributions were reduced by 2 kPa, simulating
the loss of understory.
•These modifications were held constant throughout the six-year
simulation.
DHSVM
Soil Moisture
Content
Baseline Run
• Vegetation-The major vegetative type is coniferous forest
influenced by maritime climatic conditions. The basin has a
forest road density of 2.4 km/km2.
• Major debris slides occurred in Rainy Creek in 1990 and
1995/1996 during flood events
• Vegetation- Both basins lie entirely within Weyerhaeuser Company’s Vail Tree Farm and
have seen extensive harvesting and road construction. Logging began in 1974 and
continues to the present. Relative vegetation age in Hard and Ware Creeks is shown above.
The road density in Ware Creek is 3.8 km/km2, 5.0 km/km2 in Hard Creek catchment.
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Rainy Creek is a 44 km2 tributary to the Little
Wenatchee River located in the Wenatchee
National Forest on the eastern side of the
Cascade Mountains.
• Topography-Elevation ranges from
630 to 2100 meters, with 300 to 600
meters of relief.
• Precipitation
• snow-melt dominated basin;
• mean annual precipitation ranges
from 125-400 cm
• Soils and Geology• Upper slopes are steep (30 to 45
degrees) with excessive rock
outcrop.
• Lower slope gradients range from
10 to 30 degrees and are mantled
with glacial drift and colluvium.
• bedrock is largely igneous and
metamorphic.
Erosion and sediment transport in a temperate forested watershed are predicted with a new sediment module linked to the Distributed Hydrology-Soil-Vegetation
Model (DHSVM). The DHSVM sediment module represents the main sources of sediment generation in forested environments: mass wasting, hillslope erosion and
road surface erosion. It produces failures based on a factor-of-safety analysis with the infinite slope model through use of stochastically generated soil and
vegetation parameters. Failed material is routed downslope with a rule-based scheme that determines sediment delivery to streams. Sediment from hillslopes
and road surfaces is also transported to the channel network. Basin sediment yield is predicted with a simple channel sediment routing scheme. The model was
applied to the Rainy Creek catchment, a tributary of the Wenatchee River which drains the east slopes of the Cascade Mountains, and Hard and Ware Creeks on
the west slopes of the Cascades. In these initial applications, the model produced plausible sediment yield and ratios of landsliding and surface erosion , when
compared to published rates for similar catchments in the Pacific Northwest. We have also used the model to examine the implications of fires and logging road
removal on sediment generation in the Rainy Creek catchment. Generally, in absolute value, the predicted changes (increased sediment generation) following
fires, which are primarily associated with increased slope failures, are much larger than the modest changes (reductions in sediment yield) associated with road
obliteration, although the small sensitivity to forest road obliteration may be due in part to the relatively low road density in the Rainy Creek catchment, and to
mechanisms, such as culvert failure, that are not represented in the model.
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Study Area-Rainy Creek
Forest Roads Effects
Two road decommissioning
scenarios were run for the
Rainy Creek basin:
• Partial decommissioning removal of road segments
particularly susceptible to
erosion, as determined by the
US Forest Service.
• Total decommissioning –
All roads removed.
• Vegetation for the postharvest scenarios was as it
existed in 1996, while the preharvest scenarios assumed
homogeneous vegetation
similar to the unharvested
sites in 1996.
The Rainy Creek basin road network is shown with road segments in
black and red. Red segments were removed for the partial
decommissioning scenario.
• Maps show probability of
failure on February 9, 1996,
during a large rain-on-snow
event that caused debris flows
in adjacent catchments.
Road Network
Fire-altered vegetation model output is shown in red; baseline
runs are shown in blue.
Hydrographs show the
large February 9th storm.
Both basins showed
much higher predicted
probability of failure with
harvested vegetation.
Likewise, predicted
sediment concentrations
at the outlets of Hard and
Ware Creeks were much
greater.
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Sediment Landslide
Yield,
Rate,
Mg/ha
kg/ha
Hillslope
Erosion
Rate,
kg/ha
Road Erosion Rate
kg/ha
(kg/ha road
surface)
kg/km
of road
Existing
1.05
9519
1380
35-43
(6,717–8,205)
334-408
Partially
Decommissioned
1.03
9535
1380
5-6
(1,266-1,609)
46-59
None
1.00
9564
1386
0
0
Concluding Remarks
A test application to the Rainy Creek catchment shows that the model produces plausible sediment yields in comparison with literature
values for similar catchments. Likewise, ratios of landsliding and surface erosion rates are plausible when compared to published rates for
various watersheds in the Pacific Northwest. The model was applied to compare the effects of reducing road densities on erosion and
sediment transport in the Rainy Creek drainage. This scenario showed only small changes in mass wasting rates and sediment yield, and
some spatial changes in mass wasting locations. Also, as road density decreased the road erosion rate/road area decreased. Larger
changes were not realized, either due to the limited hydrologic changes caused by the roads, the construction of roads at low elevation
along the main channel, or because road characteristics that contribute to road-related mass wasting (i.e. blocked culverts) are not
represented in the model. A second scenario, representing a forest fire, showed an increase in all erosion components due to decreases in
root cohesion and increases in surface runoff and thus transport capacity. Timber harvest scenarios in Hard and Ware Creeks also showed
increases in mass wasting rates.