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