Vegetation resistance Evaluation of vegetation resistance descriptors for flood management

For the prediction of the behaviour of water levels in rivers, computational river flow models are used. An important parameter of these models is the (hydraulic) flow resistance. The presence of vegetation has a major effect on the flow resistance. Last decades the stimulation of ecological functions around the river became more important, making proper prediction of the resistance caused by vegetation on river flows of vital importance for flood management. For describing the influence of vegetation resistance on river flows several approaches are available. The aim of this research is to identify the practical suitability of different vegetation resistance descriptions, by compiling a data set of flow experiments and to use this data set to evaluate the ranges of applicability of different (existing) vegetation resistance descriptions. Three descriptions were found for emergent rigid vegetation and seven useful descriptions were selected for submerged vegetation. An important description for emergent vegetation is the equation of Petryk and Bosmaijan (1975). The two other descriptions for emergent vegetation and the descriptions for submerged vegetation show resemblance with the equation of Petryk and Bosmaijan (for describing the velocity in the vegetation layer). Therefore, it was concluded that further investigation of descriptions for emergent vegetation were not necessary. For submerged vegetation, most descriptions are based on the two layer theory, which makes a distinction between the velocity in the vegetation layer and in the surface layer. For defining the velocity in the vegetation layer, two different approaches are used. Two descriptors, Klopstra et al. (1997) with three definitions for the turbulent length scale and Huthoff (2007) define the velocity in the vegetation layer by taking the influence of the higher velocities in the surface layer into account. Three other descriptors, Stone and Shen (2002), Van Velzen et al. (2003) and Baptist et al. (2006) assume a constant velocity over the depth in the vegetation layer. Most of these descriptors define the velocity in the surface layer by a logarithmic profile, except the description of Stone and Shen (2002). A theoretical description for flexible vegetation (even in the simplified form without sidebranches and foliage) with input parameters which can be easily measured in the field is still lacking. However, the above mentioned descriptions for rigid vegetation are also used to predict the behavior of flexible vegetation. Therefore, these descriptions are also compared with data of flexible vegetation. An existing data set from 10 different authors was used and extended with 6 new data sets from other literature. One of the main difficulties in deriving a data set from literature is the fact that authors uses different ways to determine the drag coefficient and slope, which makes comparison of different data sets hard. Therefore, a scheme is developed which can be used to correct existing data and to function as a manual for determining the drag coefficient and slope in deriving data from flume experiments with submerged rigid vegetation. The main assumption of the scheme is that the equation of Petryk and Bosmaijan (1975) is reliable enough to use for calculating the velocity, drag coefficient and/or slope in the vegetation layer. Because the new derived data sets performed well in comparison to the calculated velocities (R2 = 95%) no big corrections were needed. Only when values for the drag coefficient were not given, a drag coefficient of 1 was assumed. However, assuming a drag coefficient of 1 for all the data showed no improvement. The total data set consisted of 173 runs from 5 different authors for rigid vegetation and 133 runs from 11 different authors for flexible vegetation. Based on the comparison of the predicted and measured values for both rigid and flexible vegetation, it is concluded that Vegetation resistance Abstract 5 most theoretical descriptions defined for rigid vegetation can also be used for flexible vegetation (without side branches and leaves). However the predictions are less accurate for flexible vegetation. The description of Klopstra et al. (1997) with the turbulent length scale defined by Meijer (1998) and Van Velzen et al. (2003) and the descriptions of and Van Velzen et al. (2003) and Baptist et al. (2006) show good performance for rigid as well as for flexible vegetation. For water levels beneath 1 m these descriptions show an error of the water level smaller than 25 cm. For water levels above 1 m only one dataset was present, which was also used by four descriptors to define a parameter or a relation. Therefore, conclusions for higher water levels are lacking. Besides the performance of the descriptions in predicting the resistance of rigid and flexible vegetation, other criteria are investigated like, easiness to use, theoretical soundness and adaptability to take side branches and leaves into account. Based on this study, the description of Klopstra et al. (1997) with the turbulent length scale defined by Meijer (1998) or Van Velzen (2003) performs best (and equally well) and could be used with the same confidence, although it is not a very simple expression. Care should be taken with all descriptions since none are perfect. Uncertainty in resistance predictions remains an issue to deal with in river modeling.