Evaluation of flow formulas for submerged vegetation

Much research has been dedicated to the interaction between flow and vegetation, however, this knowledge is only to a limited extend incorporated in models that are being used for river management practices. This may be partly due to the unknown reliability of the developed flow formulas. This contribution evaluates five different flow formulas derived for submerged vegetation: Klopstra et al. (1997); Stone and Shen (2002); Baptist et al. (2007); Huthoff et al. (2007); and Yang and Choi (2010). Each of these models is based on measurable vegetation characteristics to account for flow resistance by the vegetation. The evaluation of the flow formulas is based on the agreement with experimental data from literature, on their behaviour with respect to submergence ratio and on predicted water levels for different vegetation types. All models showed reasonable correlation to experimental data for rigid and flexible vegetation, however, average relative deviations were quite significant in the range of 24 to 43%. Some models showed unexpected behaviour in the ratio of the flow velocities in the surface and vegetation layer and deduced roughness parameters as function of submergence ratio. Predicted water levels for a given velocity varied up to several meters for some vegetation types. This shows that a particular choice for a model may have huge consequences when being used to predict water levels during flood conditions. The flow formulas proposed by Klopstra et al. (1997) and Yang and Choi (2010) show the best fit to experimental data and also show consistent physical behaviour