Variation of Roughness Coefficients for Unsubmerged and Submerged Vegetation
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Volume 125, Issue 9
Abstract
This paper investigates the variation of the vegetative roughness coefficient with the depth of flow. A horsehair mattress is used in the experimental study to simulate the vegetation on the watercourses. Test results reveal that the roughness coefficient reduces with increasing depth under the unsubmerged condition. However, when fully submerged, the vegetative roughness coefficient tends to increase at low depths but then decrease to an asymptotic constant as the water level continues to rise. A simplified model based on force equilibrium is developed to evaluate the drag coefficient of the vegetal element; Manning's equation is then employed to convert the drag coefficient into the roughness coefficient. The data of this study are compared with those of selected previous laboratory and field tests. The results show a consistent trend of variation for the drag coefficient versus the Reynolds number. This trend can be represented by a vegetative characteristic number k. Given information such as the bed slope, the height of vegetation, and k, one can apply the proposed model to predict the roughness coefficient corresponding to different flow depths.
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Information & Authors
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Published In
Journal of Hydraulic Engineering
Volume 125 • Issue 9 • September 1999
Pages: 934 - 942
History
Received: Sep 29, 1998
Published online: Sep 1, 1999
Published in print: Sep 1999
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