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The mutual relation between vegetation and inundation characteristics in floodplains : a case study of the Duursche Waarden

Graaf, Jeroen de (2018) The mutual relation between vegetation and inundation characteristics in floodplains : a case study of the Duursche Waarden.

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Abstract:This report describes the investigation of the mutual relation between vegetation and environmental conditions (flood duration, frequency, dry periods, flow velocity). In the Netherlands water management is an ongoing challenge, of which the idea behind the solution has changed several times. This lead in recent times to giving the rivers more room to flow during high water (“Ruimte voor de rivier”), however the effect on the vegetation growth in the floodplains has not been studied very well. It is known that vegetation growth leads to an increase in the flood risks for that area, as the vegetation increases hydraulic roughness and therefore causes increased water levels (project “Stroomlijn”). It is therefore important to know how vegetation evolves in an area, to be able to manage it better and maintain water safety. In this report we will look at the link between environmental conditions and vegetation of the Duursche Waarden, which is a floodplain along the IJssel. Understanding this relation will help water managers to better manage the floodplains and possibly predict where vegetation might lead to problems for the water safety. To investigate the relation between vegetation and environmental conditions, firstly the environmental conditions are mapped for the Duursche Waarden. For this, a model (WAQUA) is used to simulate flood waves to determine a Q-h relation and calculate the flow velocities in the Duursche Waarden. The Q-h relation is used to calculate inundations frequency, maximum inundation duration and a maximum dry period. These mapped environmental conditions are linked to vegetation classes of the ecotope map (grass, shrubs and forest) which remain the same over the years of 1998, 2005 and 2012. Areas which experience changes in the vegetation classes from those ecotope maps will also be linked to the environmental conditions, to see if this can be explained by environmental conditions. Comparing the different vegetation classes and under which environmental conditions these vegetation classes are found most, it can be concluded that there is a difference between grass and shrubs and under what environmental conditions they are present. Interestingly, the areas where the vegetation changes between they years show to be contained to areas which experience environmental conditions more similar to what vegetation they change into. Secondly, the relation of vegetation on the flood characteristics is investigated. To do that, the Duursche Waarden is simulated as completely covered by paved surface, grass, shrubs or forest. The results from the simulations show how much different types of vegetation affect the flood characteristics and with that the water safety. The resulting information is compared to the current situation, which leads to the conclusion that not all vegetation has necessarily a bad influence on a flood. While a large forest does slow down the water flow and increase the water levels in an area, there are still locations within the Duursche Waarden which do not seem little to not affected. To conclude, vegetation affects floods and floods affect vegetation. Linking vegetation to environmental characteristics can help predict where certain vegetation is likely to change into another type of vegetation. Furthermore, not all vegetation forms a risk for the water safety. Depending on the amount and where it is located the effect could almost be neglectable. Knowing this mutual relation between vegetation and environmental conditions could help water managers manage floodplains, while still reserving space for nature.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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