Variations of Time of Concentration Estimates Using NRCS Velocity Method
Publication: Journal of Irrigation and Drainage Engineering
Volume 133, Issue 4
Abstract
Time of concentration is the time required for runoff to travel from the hydraulically most distant point to the outlet of a watershed. The Natural Resources Conservation Service (NRCS) velocity method commonly is used to estimate for hydrologic analysis and design. The NRCS velocity method applies the physical concept that travel time is a function of runoff flow length and flow velocity. Time of concentration for 96 Texas watersheds is independently estimated by three research teams using the NRCS velocity method. Drainage areas of the 96 watersheds considered in the study are approximately . Digital elevation models having a grid size of were used to derive watershed physical characteristics using ArcGIS or HEC-GeoHMS. Average channel width was estimated from or digital orthoimagery quarter quadrangle or aerial photography. Each team made independent decisions to estimate parameters needed for different flow segments for the NRCS velocity method. Estimates of time of concentration made by three research teams are compared, and both graphic comparison and statistical summary demonstrate that time of concentration estimated using the NRCS velocity method is subject to large variation, dependent on the analyst-derived parameters used to estimate flow velocity. Because of the propensity for different analysts to arrive at different results, caution is required in application of the NRCS velocity method to estimate .
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Acknowledgments
The writers would like to thank for their guidance and assistance: TxDOT program coordinator David Stolpa, P.E., project director Jaime E. Villena, P.E., and project monitoring advisor George (Rudy) Herrmann, P.E. Thanks for support and permission for publication from researchers Meghan C. Roussel (Civil Engineer), Dr. Williams Asquith (Research Hydrologist), Amanda C. Garcia (Civil Engineer), at the U.S. Geological Survey, Austin, Texas, as team members of the research group. This study was partially supported by TxDOT Research Project Nos. 0-4193, 0-4194, and 0-4696. The contents of this manuscript reflect the views of the writers who are responsible for the facts and accuracy of the data presented. The contents do not necessarily reflect the official view and policies of the Texas Department of Transportation. This manuscript does not constitute a standard, specification, or regulation.
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Information & Authors
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Published In
Journal of Irrigation and Drainage Engineering
Volume 133 • Issue 4 • August 2007
Pages: 314 - 322
Copyright
© 2007 ASCE.
History
Received: Jul 24, 2006
Accepted: Mar 16, 2007
Published online: Aug 1, 2007
Published in print: Aug 2007
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