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Identifying gullies in bolin creek watershed. Christina Hurley Alyssa James Carly Buch Nicole Meyer. Introduction to Gullies. What is a Gully? Where do Gullies occur?. Flow Chart. Soils. Factors that affect soil erodibility. Particle Size Organic Matter Content - PowerPoint PPT Presentation
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IDENTIFYING GULLIES IN BOLIN CREEK WATERSHEDChristina HurleyAlyssa JamesCarly BuchNicole Meyer
Introduction to Gullies• What is a Gully?• Where do Gullies occur?
Flow Chart
Predicting Gully Locations in Bolin Creek Watershed
Carly: Soil Type and Erodibility
Query for soil types that have:
High K value
High T Factor
Hydrologic Group D
Nicole and Alyssa: Calculate Upstream Drainage Area for
Outlets
Digitize all outlets in Bolin Creek Watershed
Use digitized outlets to calculate
watersheds for each outlet
Christina: Finding Stream Power for Outlet Drainage
Areas
Calculate flow accumulation weighted by impervious
Calculate slope for Bolin Creek DEM
Calculate stream power: As*Slope
Remove streams from stream power
raster
Filter new stream power raster
Multiply filtered raster by outlet watersheds
Soils
Carly: Soil Type and Erodibility
Query for soil types that
have:
High K value
High T Factor
Hydrologic Group D
Factors that affect soil erodibility• Particle Size• Organic Matter Content• Plant/vegetation and animal life• Climate• Land Management• Slope and length of slope• Precipitation
Mapped Gully Soil Types
K Value as an indicator
T Factor as an Indicator
Outlets that fall in high K values and high T factors
Soil Type Count
Georgeville-Urban Land Complex 2-10%
1
Georgeville silt loam 2-6%
10
Georgeville silt loam 6-10%
19
Herndon silt loam 2-6% 7Herndon silt loam 6-10%
6
Considering other factors• Slope >10• Loamy soils • High Runoff• Poor Drainage Class
Outlet Wastersheds
Nicole and Alyssa: Calculate Upstream Drainage Area for
Outlets
Digitize all outlets in Bolin Creek Watershed
Use digitized outlets to calculate
watersheds for each outlet
Digitizing Outlets and Calculating Watersheds
Outlet Watersheds
Outlet Watersheds
Outlet Watersheds
Outlet Watersheds
Outlet Watersheds Near Gullies
Stream Power
Christina: Finding Stream Power for Outlet Drainage
Areas
Calculate flow accumulation weighted by impervious
Calculate slope for Bolin Creek DEM
Calculate stream power: As*Slope
Remove streams from stream power
raster
Filter new stream power raster
Multiply filtered raster by outlet
watersheds
Stream Power and Outlets
Impervious Cover has Larger Influence than Slope
Flow accumulation weighted by impervious cover
Slope
Stream powers at outlets and existing gullies
1406
416
Stream powers at outlets
1393
Stream Powers and Outlets
307
Conclusions• Existing gullies were not by outlets with large watersheds,
or with necessarily high stream powers• Means that it is difficult to predict where gullies will form• A larger sample size of existing gullies might yield more conclusive
results• Impervious cover had a more significant influence on
stream power that slope overall • Outlets with a higher stream power going into the outlet
were more likely to have gullies (there or below the outlet)• Soil T-factor has strong influence on the formation of
gullies
Future Considerations• In order for Orange County to prevent further gullies from forming they
should continue to watch:• Present soil mass movement and rate• Parent material • Cemented, compacted or impermeable subsoil layers• Area impervious upstream• Size of watershed• Stream power• Root distribution• Vegetation type and distribution• Saturated hydraulic conductivity• Pore water pressure• Precipitation occurrence an distribution• Temperature fluctuations • Degree of weathering• Dips parallel to slopes along streams
• Summarized from Brooks, 1997