Storm Water Modeling using the Curve Number Runoff Method

Town Creek Watershed - Salisbury NC

Presented by: Chris Spry

12.17.10

Context of Problem

• Land use change alters the hydrology of an area and has the potential to create an increase in storm water runoff and flooding hazards.

• Increasing impermeable surfaces such as parking lots, roads, and buildings has an additive effect.

• Decreasing natural areas such as forests, open grassy areas, and misuse of flood plains also increase risk

• The Town Creek Watershed (part of the Yadkin River basin), located in Salisbury, NC is a prime example of these changes.

Project Objectives

• Model land use change between the years of 1978 and 2010 within the study boundaries.

• Determine the makeup of land uses (forest, open space, and urban space) in this area within these time periods.

• Calculate the change in land uses and storm water runoff over this time period.

Methods (Data Collection)

• DEM (Digital Elevation Model) obtained from City of Salisbury GIS Division

• Soil Data obtained from Rowan County• Aerial Imagery from 1978 obtained from City of

Salisbury GIS Division• Satellite Imagery from 2009 obtained from City of

Salisbury GIS Division

The following data were obtained before the project began:

1978 Aerial vs. 2009 Satellite Imagery

Zoomed

Methods (Incorporate data into GIS)

• DEM was used to create data involving flow direction, slope, and create stream channels

• 1978 and 2009 Imagery were digitized and clipped to create two land cover maps

• Soil Data was clipped for the study area and grouped in to Hydro groups (A-D) (A having a low runoff potential and D having a high runoff potential.)

• Hypothetical 4 inch Rainfall map was created for the area

• The Curve Number Runoff (CRN) Equation was used to show runoff totals for both time periods and is explained on the following slide.

Methods: (CRN) Equation

Q = (P - Ia )2

P - Ia + SQ = runoffP = precipitationS = potential maximum soil moisture retention after runoff beginsIa = initial abstraction, basically the amount of water before

runoff, such as infiltration, or interceptance by vegetation.Ia is the percentage of S: Ia = 0.2S• Q is then:• Q = (P - 0.2S)2

(P + 0.8S)The Curve Runoff Number, CN is then related:S = 1000 - 10 CN

ResultsLand Use Statistics for

the year (1978)Forest = 53%

Open Space = 40%Urban = 7%

Flow Accumulation at Innes St. point from a 4 inch

rainfall: 78,592Land Use Statistics for

the year (2009)Forest = 50 %

Open Space = 35%Urban = 15%

Flow Accumulation at Innes St. point from a 4 inch

rainfall: 92,131Land Use Change

between 1978 and 2009Forest = -3%

Open Space = - 5%Urban = +8%

Project Significance and Encountered Issues

• This project is significant because it reinforces what is already known about the impact of urbanization on storm water runoff.

• Knowledge has been used in the development of storm water retention ponds.

• Downstream development which is near the floodplain has been removed and discouraged.

• Issues: CNR Method would work better for areas with greater change in elevation and less urbanized environments.

Thanks for your attention!Questions?