Variation of roughness coefficients for unsubmerged and submerged vegetation

This paper investigates the variation of the vegetative roughness coefficie nt 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 unsu bmerged condition. However, when fully submerged, the vegetative roughness coefficient tends to increase at low depths but then decrease to an asympto tic 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 co efficient into the roughness coefficient. The data of this study are compar ed with those of selected previous laboratory and field tests. The results show a consistent trend of variation for the drag coefficient versus the Re ynolds number. This trend can be represented by a vegetative characteristic number k. Given information such as the bed slope, the height of vegetatio n, and ii, one can apply the proposed model to predict the roughness coeffi cient corresponding to different flow depths.