Comparison of vegetation roughness descriptions

Vegetation roughness is an important parameter in describing flow through river systems. Vegetation impedes the flow, which affects the stage-discharge curve and may increase flood risks. Roughness is often used as a calibration parameter in river models, however when vegetation is allowed to develop naturally in river restoration projects, it is important to have suitable predictions of the increased resistance caused by the vegetation. There are many formulas available for describing vegetation roughness in one or two dimensional flow, ranging from simple wall roughness approximations to (semi-) empirical or theoretically derived roughness descriptions that are a function of flow and plant characteristics. In this paper a number of roughness descriptions are compared. All descriptions give a reasonable fit to experimental flume data. However, the models show significant deviation when extrapolating to large water depths typical for extreme discharge conditions. Therefore, to identify the suitability of existing methods for flood modeling, more data at large water depths are necessary. At large submergence ratios vegetation roughness can be approximated by a constant Manning coefficient.