The Rational Method David B. Thompson（David b .汤普森的理性方法）.pdf

The Rational Method∗ David B. Thompson Civil Engineering Deptartment Texas Tech University Draft: 20 September 2006 1. Introduction For hydraulic designs on very small watersheds, a complete hydrograph of runoff is not always required. The maximum, or peak, of the hydrograph is sufficient for design of the structure in question. Therefore, a number of methods for estimating a design discharge, that is, the maximum value of the flood runoff hydrograph, have been developed. The rational method is a simple technique for estimating a design discharge from a small watershed. It was developed by Kuichling (1889) for small drainage basins in urban areas. The rational method is the basis for design of many small structures. In particular, the size of the drainage basin is limited to a few tens of acres.1 The method is also described in most standard textbooks.2 ∗ c Copyright 2004–2006 David B. Thompson, all rights reserved. Comments are welcome; please direct comments to david.thompson@. 1Texas Department of Transportation (TxDOT) experts suggest a maximum drainage area of 200 acres for rational method application. While many analysts consider this a “hard” limit, in actuality the limit really depends on watershed complexity. For a complex watershed (such as an urbanized watershed), this limit should probably be much less; for a rural watershed, the limit might be much larger. Therefore, it is the analyst’s responsibility to determine whether the method is applicable or not and justify application of the rational method based on professional judgment. 2 See, for example, Section 15.2 in Viessman, Jr., W. and Lewis, G. L., 1995, Introduction to hydrolog