Roughness Coefficients of Large Floods

by Douglas J. Trieste, US Bureau of Reclamation, Denver, CO, USA,
Robert D. Jarrett, US Bureau of Reclamation, Denver, CO, USA,

Document Type: Proceeding Paper

Part of: Irrigation Systems for the 21st Century


Most hydraulic calculations of flow in channels and overbank areas require an evaluation of flow resistance generally expressed as Manning's roughness coefficient, n. Literature is available for estimating n for discharges that generally are within the main channel. However, little information and guidance are available for estimating n for large out-of-bank floods including dam-break floods. This paper presents factors affecting n for large floods and presents information in support of n typically being underestimated. An approach is presented for estimating n, based on the Froude number. The basis of this approach is that Froude numbers are less than 1, because supercritical flow, except for very short distances, generally does not occur in the natural environment.

Subject Headings: Channel flow | Hydraulic roughness | Floods | Froude number | Supercritical flow | Fluid flow | Dam failures | Flow resistance

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