Impervious Cover and Erosion:Brushy Creek, Round Rock, Texas

GRG 360-G

Spring 2004

Beau Barnett

Brett Franco

Erosion and Soil Loss Due to the Increase in Impervious Cover

• Increase in impervious cover can have detrimental effects on soil loss and stream erosion.

• Soil Infiltration rate goes down with more impervious cover.

• Runoff increases when there is less soil infiltration.

• With more runoff, the chance for greater soil loss increases

Area of Study

Round Rock, Texas

Williamson County

Brushy Creek

Population Boom of Round Rock

• From 1990 to 2000, population of Round Rock has more than doubled.

• 1994 Dell Computers moved it’s headquarters from Austin to Round Rock, creating over 10,000 jobs for the area.

• With an increase in population, rapid urbanization occurred.

Methods Used for Analysis

• Delineation of Brushy Creek and morphometric analysis using ArcHydro

• Model soil erosion and soil loss using the Revised Universal Soil Loss Equation (RUSLE)

• Digitizing zones of impervious cover and area analysis using the Field Calculator

Sources

Secondary Data:

• 30 meter DEMs from TNRIS

• 2.5 meter DOQQs for Round Rock, Texas

• Annual Rainfall Data for Round Rock, Texas

Techniques• Revised Universal Soil Loss Equation (RUSLE)

A = R * K * LS * C * P

A = estimated average soil loss in tons per acre per year

 R = rainfall-runoff erosivity factor  K = soil erodibility factor L = slope length factor S = slope steepness factor C = cover-management factor P = support practice factor

RUSLE Factors

• R = 94

• Estimated value from tamu.edu

• K = 0.5

• Estimated value due to rocky sandy soils

• LS = Grid derived from slope and flow accumulation

• C = 0.75 for West Brushy Creek, 0.35 for East Brushy Creek

• Value based on percentage of impervious cover in basin

• P = 1

•Value constant due to size of study area

Sources of Error• More recent DOQQs

• Imprecise soil data for K factor

• C factor is not constant on a landscape

• RUSLE equation is a model, not an exact science!

Future Research• Better defined factors for RUSLE equation

• More widely available data

• Study amount of soil deposition downstream

• Development of environmentally sensitive construction practices

• Use of newly developed pervious concrete

Conclusions

• With the relocation of Dell causing a population boom in Round Rock the amount of impervious cover increased greatly.

• This in turn increased runoff and soil erosion in the area and measures need to be taken to lessen the impacts of this urban sprawl.

Works Cited

• http://www.tnris.state.tx.us

• http://www.iwr.msu.edu/rusle/

• http://www.dell.com/

• http://www.ci.round-rock.tx.us

• http://www.utexas.edu/depts/grg/hudson/grg360g/EGIS/E_grg360g.htm