Computer Watersheds

by Paul A. DeBarry, Dir. of Hydraulics and Storm Water Mgmt.; R. K. R. Hess Associates, East Stroudesburg, PA,
James T. Carrington, Supervisory Cartographer; Input/Output Section of the GIS and Remote Sensing Branch, Soil Conservation Service, Ft. Worth, TX,


Serial Information: Civil Engineering—ASCE, 1990, Vol. 60, Issue 7, Pg. 68-70


Document Type: Feature article

Abstract: As development extends further and population densities increase, old methods of stormwater management become obsolete. Runoff from one site can cause flooding elsewhere in a watershed; this makes it imperative that runoff be analyzed on a basin-wide basis. Geographical information systems are an ideal way to do this. After boundaries and characteristics of different land-use, soil group and subbasin boundaries are digitized, GIS can generate composite SCS runoff curve numbers on a subwatershed basis. These can be put directly into a rainfall/runoff program like the Penn State runoff model. The computer creates and displays overlays and generates color or mylar maps at any scale, eliminating the need for conventional mylar overlays. Furthermore, the data can be easily updated, corrected, or changed to reflect proposed development. A watershed recently needing a stormwater management plan was the 172 sq mi Brodhead Creek watershed in northeastern Pennsylvania. The soils data was moved from CADDS4x format and the state plane coordinate system to DLG3 and the universal transverse mercator system, and later to ARC/INFO. Sources for the land-use map were aerial photographs, USGS topographic quadrangles, recorded subdivision plans and tax parcel maps. The boundaries of the 205 subbasins were determined from confluences of streams, bridges, dams and reservoirs. After the soils and subbasin boundaries were digitized, the computer overlaid these with the land-use boundaries to create a 1.6MB drawing. The computer helped develop a future land-use map and curve numbers from zoning maps and plans showing existing conditions. A no-stormwater management case was analyzed using the Penn State runoff model. The model was first calibrated using existing conditions. Predicted streamflow for a stream collecting runoff from 69 subbasins was compared with actual flow records, and coefficients were modified accordingly.

Subject Headings: Geographic information systems | Models | Rainfall-runoff relationships | Runoff | Watersheds

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