University of Twente Student Theses


A new method to predict the aggregate roughness of vegetation patterns on floodplains

Haar, M.B.A. ter (2010) A new method to predict the aggregate roughness of vegetation patterns on floodplains.

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Abstract:Due to computational limitations not all small details that describe the river characteristics can be taken into account in the model, for example the roughness patterns. Therefore weighting methods are used to convert multiple roughness values in one cell to one aggregate roughness value that covers the variation in roughness. Currently, the WA-method is the weighting method that is used in the model WAQUA when more than one roughness value is implemented in one grid cell. This method is based on a small number of WAQUA calculations with different roughness patterns. This method predicts an aggregate value independent of the pattern layout and is therefore not always very accurate in predicting the aggregate roughness. The aim of this research is to investigate whether it is possible to create an improved method that takes pattern characteristics into account. A large series of model calculations with the two dimensional model program WAQUA are carried out to investigate which flow and roughness pattern parameters influence the aggregate roughness of the pattern. WAQUA is a two dimensional model program used for simulation of water movement and transport processes in shallow water and it is based on a vertically averaged approach of the flow field. Different situations are used in the model calculations where the pattern layout, water depth and grid size are varied. The vegetation pattern layout can be subdivided into parallel oriented patterns, serial oriented and a pattern with multiple square patches (2, 4 and 9) spread over the area. These patterns can be distinguished from each other by the streamlining of the pattern. A parallel pattern has a high streamlining in the flow, followed by the patterns with patches and a serial pattern has a very low streamlining. Furthermore different coverings of rough vegetation on the area are used in the investigation. The results of these model runs are the aggregate Chézy values that represent the overall flow field. It turns out that the relative serial or parallel direction has a large influence on the aggregate roughness. This can be explained by the existence of flow adaptation processes due to the smooth to rough vegetation transitions. These processes can be divided into two parts: i) a mixing layer along smooth-rough transitions parallel to the flow and ii) flow adaptation behind a rough patch due to transitions perpendicular to the flow direction. These processes induce an additional roughness on the area, apart from the different roughness of the vegetation. The influences of these mixing layers and adaptation lengths can be expressed as a correction on the aggregate Chézy value. This correction is based on the geometrical lay out of the vegetation pattern. The aggregate roughness resulting from a complete serial pattern can already be adequately predicted by the complete serial function and thus does not need be included in the determination of the new prediction model.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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