University of Twente Student Theses


Aggregate roughness modelling: an idealized model study into the effects of patchy vegetation on mean river flow

Noordermeer, Joost (2012) Aggregate roughness modelling: an idealized model study into the effects of patchy vegetation on mean river flow.

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Abstract:Rivers have been a lifeline for civilization throughout human history. It is therefore not surprising that so many cities and villages can be found around these locations. Dikes have been used in the past to protect these cities from floodrisk, however the combination of climate change and land subsidence have created a technological lock-in; dikes can no longer be heightened but it is also impossible to migrate entire cities. As a result the Room for the River policy plan was developed in the Netherlands which aims to increase protection against floods by allowing rivers to reclaim their natural course. A direct result of this is that floodplains will see more frequent inundations as they will be used to temporarily store and transport water. Since floodplains are not inundated at all times, plants and trees can grow, which will alter the transportation capacity of these floodplains because these forms of vegetation induce resistance to the flow. Hydraulic model computations are used to assess the impact of hydraulic measures and use the resistance to flow as an input parameter. These calculations are typically performed for large spatial scales due to computational limitations, which therefore requires an aggregated roughness value for the entire region as small scale processes are not accounted for. There is however little known about the aspects of floodplains and vegetation that influence this aggregated roughness parameter. Several methods, based on fitting WAQUA simulation results, have been developed for this purpose. A new method, based on an analytical rather than a numerical approach, was developed to investigate how spatial scales and various system parameters affect aggregate roughness as induced by vegetation patches. It is based on steady nonlinear depth-averaged shallow water equations while closing turbulence using a spatially constant horizontal eddy viscosity and allowing spatial variations in bed resistance. A weakly non-linear analysis was performed where small changes in resistance and the corresponding response in the flow were approximated up to the second order in a small parameter quantifying these variations. At second order, a spatially invariant contribution to the downstream flow velocity is obtained. This second order spatially invariant contribution is used to calculate the aggregate resistance over a floodplain. Model application is restricted to large spatial scales or small differences in roughness due to the solution method used. The flow response to a variety of roughness patch characteristics was investigated and an increase in flow resistance was always found. Dominant mechanisms were identified in case of parallel roughness variations only (lateral shear), serial variations only (backwater effects) and combined variations (lateral shear, backwater effects and lateral redistribution of longitudinal momentum). Results show that larger spatial scales lead to a reduction in aggregate roughness. Adiv ditionally, the influence of the eddy viscosity on model results is significant but it is difficult to recommend a certain input value as sources disagree. However, higher values of eddy viscosity lead to a greater energy loss to turbulent eddies, resulting in a greater aggregate resistance. Furthermore, it is found that near-diagonally oriented patches minimize the overall flow resistance. The idealized approach allows a quick assessment of the influence of various system parameters on the mean river flow velocity as caused by spatially varying resistance. It also provides insight into the physical mechanisms that lead to a difference between the aggregate resistance and the average resistance of a river section. No explanation has been found yet regarding the unexpected influence of patch orientation on aggregate resistance and this will require further investigation.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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