Lag Relations for Semi-Arid Regions and Urban Areas

by George V. Sabol, George V. Sabol Consulting Engineers, Inc, Denver, United States,

Document Type: Proceeding Paper

Part of: Engineering Hydrology


The use of synthetic unit hydrographs often requires the estimation of lag or other travel time parameters. Several procedures are available for estimating such unit hydrograph parameters. One method is the lag equation that often is applied to dimensionless S-graphs. The traditional form of the lag equation has been used for about 40 years. That equation is not particularly sensitive to watershed slope. Additionally, the use of that equation requires the estimation of a resistance coefficient. Selection of the resistance coefficient is rather subjective leading to uncertainty and lack of reproducibility when using the method. A new form of the lag equation was developed that is dimensionally correct. Data on measured lag and watershed characteristics was used to develop prediction equations for the lag equation coefficient. These prediction equations do not require subjective selection of resistance coefficients or other uncertain input. Therefore, the new lag procedure provides reproducible results by all users. The lag equation coefficients were developed for use in southwestern desert/rangeland and mountain watersheds. Another lag equation coefficient was developed for use with urban watersheds. The desert/rangeland lag equation was modified for use with agricultural watersheds. These equations are presently used for flood hydrology in Arizona.

Subject Headings: Load and resistance factor design | Arid lands | Urban areas | Watersheds | Travel time | Parameters (statistics) | Floods | Graphic methods | Arizona | United States

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