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 Predicting Gully Locations in Bolin Creek Watershed Carly: Soil Type and Erodibility Query for soil types that have: High K valueHigh T FactorHydrologic 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 valueHigh 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 TypeCount Georgeville-Urban Land Complex 2-10% 1 Georgeville silt loam 2- 6% 10 Georgeville silt loam 6- 10% 19 Herndon silt loam 2-6%7 Herndon 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 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

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 Both impervious cover and slope had a significant influence on stream power. Gullies and areas of concern are located in in places with high imperviousness and on medium slopes 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