the drainage area and it is the most important watershed characteristic for hydrologic analysis. It reflects the volume of water that can be generated from a rainfall. Thus the drainage area is required as input to models ranging from simple linear prediction equations to complex computer models. Once the watershed has been delineated, its area can be determined, either by approximate map methods or by GIS. 2.
The drainage density, D is the ratio of the total length of streams within a watershed to the total area of the watershed; thus D has units of the reciprocal of length (1/L). A high value of the drainage density would indicate a relatively high density of streams and thus a rapid storm response. Values typically range from 1.5 to 6 mi/mi2. D = Lt/A 3.
Basin shape is not usually used directly in hydrologic design methods; however, parameters that reflect basin shape are used occasionally and have a conceptual basis. Watersheds have an infinite variety of shapes, and the shape supposedly reflects the way that runoff will “bunch up” at the outlet. A circular watershed would result in runoff from various parts of the watershed reaching the outlet at the same time. An elliptical watershed having the outlet at one end of the major axis and having the same area as the circular watershed would cause the runoff to be spread out over time, thus producing a smaller flood peak than that of the circular watershed.
A number of watershed parameters have been developed to reflect basin shape. The following are a few typical parameters:
Length to the center of area (Lca): the distance in miles measured along the main channel from the basin outlet to the point on the main channel opposite the center of area.
Shape Factor (Ll)
Ll = (LLca)0.3
Where L is the length of the watershed in miles
Circularity ratio (Fc):
Fc = P/(4pA)0.5
Where P and A are the perimeter (ft) and...
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