U.S. GEOLOGICAL SURVEY
Jeff V. Phillips and Saeid Tadayon
This report describes and presents the results of a study by the U.S. Geological Survey, done in cooperation with the Flood Control District of Maricopa County, to (1) describe procedures used in selection of Manning's n values coefficient, n, for stream channels located in semi-arid to arid environments, (2) describe a method for determining impact of flow on channel-vegetation conditions, and (3) describe and evaluate a new method for management of vegetation in stream channels. Verified and estimated n values for natural and constructed stream channels in Arizona have been presented in several published documents. Much of this information is in the form of guidelines, tables, figures, and examples. This information has been compiled into this comprehensive document to be used as a guideline and tool for estimating n values for hydraulic computation of flow in open channels.
Proper estimation of n values for vegetated channels in arid to semi-arid environments can present difficulties in estimating the channel's resistance to flow. For example, vegetation in ephemeral and intermittent streams may change considerably over a period of time or during a flood event. Because vegetation can be a constantly changing factor, estimating n values for the vegetation component can be difficult. Semi-empirical relations and guidelines developed to estimate impact of flow on channel-vegetation conditions are presented.
Developed engineering-based guidelines presented herein are intended to aid in optimizing the preservation of a riparian habitat and aesthetics of multi-use areas, while mitigating damage from floodflows along natural and constructed channels. The guidelines primarily are based on the vegetation component of n values that should be maintained in a waterway to allow adequate freeboard, which is an additional amount of conveyance area intended to mitigate risk by providing a factor of safety. Vegetation maintenance plan guidelines are presented, including example cases that illustrate their use.
Description of Study Area
Values of Manning's n For Natural and Constructed Main Channels and Overbank Areas
Vegetation Maintenance Plan Guidlines
Summary and Conclusions
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For more information about USGS activities in Arizona, visit the USGS Arizona Water Science Center home page
Send questions or comments about this report to Saeid Tadayon.