Overview

James C. Y. Guo

Department of Civil Engineering
University of Colorado at Denver

The natural condition of a watershed is termed undeveloped condition. Natural streams, creeks, and waterways have been continuously shaped by storm runoff. Development of a watershed results in more paved areas which increase the frequency, magnitude, and volume of storm runoff. Man-made drainage facilities cause stormwater to move faster and to become more concentrated. Urbanization in a watershed is a process that leads to the developed condition. Development of a watershed takes several stages. At each stage, the watershed may have different flooding and stormwater drainage problems. To understand the current drainage problems in the watershed, the existing condition shall be studied. To mitigate a flooding potential or problem, various scenarios can be developed to evaluate the effectiveness and economics of possible remedial alternatives. The impact of a specific development or activity introduced to a watershed can be quantified by the comparison between the pre-development and post-development studies.

The increase of urban storm runoff is closely related to the areal ratio of imperviousness in a watershed (USEPA 1983). Table 8.1 presents a sensitivity study of the areal imperviousness on the 100-yr storm runoff. Let the undeveloped condition with a 5% areal imperviousness be the basis for comparison; it is found that the peak discharge increases 3.84 times and the runoff volume increases 1.61 times when the catchment imperviousness increases to 90%. This example indicates that urbanization process results in...