University of Twente Student Theses
The influence of vegetation on the hydraulic roughness : determining the influence of vegetation on the hydraulic roughness and the variability therein in two streams of Waterschap Rijn & IJssel
Janssen, L. (2021) The influence of vegetation on the hydraulic roughness : determining the influence of vegetation on the hydraulic roughness and the variability therein in two streams of Waterschap Rijn & IJssel.
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| Abstract: | Regional water authorities like Waterschap Rijn & IJssel have the task to regulate water levels but also to improve the water quality, water availability, and biodiversity in and around the surface water. The awareness of the ecological values of watercourses has grown and the regional water authorities now aim for a more natural environment in and around the water. The amount of vegetation plays an important role in this: it adds to habitat variation but it also increases the hydraulic roughness and thus the water level. When the discharge is too high, this can cause flooding. Removing vegetation, also called mowing, can help to reduce this. Finding the balance between the amount of vegetation removal and flood risk reduction is challenging and requires more knowledge on the development of vegetation over time. Vegetation growth increases the hydraulic roughness of the river bed. The hydraulic roughness is hard to estimate and causes a lot of uncertainty in hydrodynamic models. This thesis is a first step in gaining more insight in the seasonal variation of the hydraulic roughness and the influence of vegetation on the hydraulic roughness. For this research, the hydraulic roughness was calculated for two watercourses in the area of Waterschap Rijn & IJssel: the Baakse Beek and Zwarte Beek. Using a SOBEK model and a Python optimization script, the Manning roughness coefficient was determined for each day of the year from 2013 up until 2020. Next, the influence of vegetation on the hydraulic roughness was determined for both streams using the Beekruwheidsmodel. Even though the values for the vegetation parameter for the Baakse Beek and Zwarte Beek are relatively low, a seasonal pattern is noticeable. An increase in the vegetation parameter can be observed from April until August for most years. This increase is probably caused by the vegetation that starts growing during this period. For some years, also a sudden drop in the vegetation parameter and Manning coefficient can be seen. The beekruwheidsmodel does not predict this drop based on the discharge which makes it likely that it is caused by human intervention like mowing of the stream. In this report, an extensive analysis of the results is done including a sensitivity analysis and a comparison to the Leijgraaf, a stream in Noord-Brabant. An earlier study in this stream showed that the Beekruwheidsmodel is capable of modelling the influence of vegetation on the hydraulic roughness. The comparison showed that a possible cause for the low value of vegetation parameter, is the low discharge in the Baakse Beek and Zwarte Beek. This thesis finalizes with the conclusion that for both the Baakse Beek and Zwarte Beek, the vegetation parameter is relatively low and that vegetation thus does not influence the hydraulic roughness much according to the Beekruwheidsmodel. Several suggestions and recommendations are given for further research: continue monitoring the water levels and discharges in the streams and add more measurement locations to streams that have a continuous discharge and a relatively high flow velocity. |
| Item Type: | Essay (Bachelor) |
| Faculty: | ET: Engineering Technology |
| Programme: | Civil Engineering BSc (56952) |
| Link to this item: | https://purl.utwente.nl/essays/87651 |
| Export this item as: | BibTeX EndNote HTML Citation Reference Manager |
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