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The applicability of the cellular automata model DUBEVEG on an anthropogenic shore

Dokkum, J.W. van (2019) The applicability of the cellular automata model DUBEVEG on an anthropogenic shore.

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Abstract:A relatively new type of beach nourishment to combat the hydrodynamical erosion is the mega-nourishment where large amounts of sand are placed in a relatively small area. The pilot project called The Sand Motor is a mega-nourishment of 21 Mm3, which was constructed in 2011 at the coast near Ter Heijde in the Netherlands. The objectives of the Sand Motor mega-nourishment are to maintain coastal safety and nature development. This research is focused on modelling dune formation pattern development at this out-of-equilibrium anthropogenic shore like the Sand Motor. Gaining knowledge about the dune growth patterns on this type of nourishment may influence decision making for the shape and elevation of future mega-nourishments projects all over the world increasing coastal safety and nature development. To get understanding which processes are influencing the dune growth at the Sand Motor the cellular model DUBEVEG (DUne BEach VEGetation) is used in this research. Advances of the DUBEVEG model is that complex processes (hydrodynamic erosion, aeolian sediment transport and vegetation development) are partly replaced by stochastic parameters decreasing the computation time significantly compared to computational fluid dynamic (CDF) models. The DUBEVEG is very flexible since many rules in the model can be easily adapted. The DUBEVEG model has not been applied previously for an out-of-equilibrium anthropogenic shore like the Sand Motor and needs adjustments to model this area. To implement the fast-changing coastline, a reference surface elevation map, measured at the Sand Motor, is used to force the changing coastline by hydrodynamical erosion or deposition. Furthermore, a new method for determining the areas that are sheltered from the hydrodynamics is implemented. The model is not able to correctly simulate the observed dune formation patterns at the Sand Motor with the standard model settings. The model results show different dune development, aeolian deposition locations and vegetation locations compared to the LIDAR measurements of actual dune development. The model simulates rows of dunes perpendicular to the wind direction while the LIDAR measurements at the coast do not show these dune patterns. The observed vegetation occurs mainly near the foredune and around the lake, while the model simulates vegetation all over the Sand Motor. A sensitivity analysis is to performed to get insight into the effect of model parameters on dune development. The parameters used in the sensitivity analysis are the aeolian probability of erosion and deposition (PePd), the groundwater depth and the pioneer vegetation expansion rate. The sensitivity analysis for the combined PePd shows that more dunes, but of similar elevation, develop at a higher PePd. The effect of PePd for the number of dunes is relatively small compared to the groundwater level. The sensitivity analysis for the groundwater level shows that increasing the groundwater level results in a decrease in the number and elevation of dunes. The effect on dune elevation is relatively small compared to the pioneer vegetation expansion rate. The sensitivity analysis for the pioneer vegetation expansion rate shows that increasing the pioneer vegetation expansion rate results in an increase in the height of the dunes but the locations of these dunes are similar and not influenced. Furthermore, two model revisions are tested for there influence on the dune development on the Sand Motor. The first model revision is multidirectional wind and the second model revision is beach armouring. In the standard model, unidirectional wind (one aeolian transport direction) is applied. Beach armouring is relevant for this area because large parts of the Sand Motor are elevated above the storm surge level. The lack of hydrodynamic erosion causes the beach armouring to limit the sediment supply available for aeolian erosion. 3 In the first model revision, including multidirectional wind in the DUBEVEG model (two aeolian transport directions) results in different dune patterns and vegetation that is more spread over the Sand Motor area compared to simulation with the unidirectional wind. The dunes, simulated by the model with the multidirectional wind, are smaller and closer together compared to the model results with the unidirectional wind. The results of the simulation with multidirectional wind are more realistic because the dune patterns are closer to observations than the rows of dunes simulated by the model with the unidirectional wind. In the second model revision, including beach armouring in the DUBEVEG model decreases the probability for aeolian erosion for armoured cells. In the model, beach armouring occurs only at the high elevated areas of the Sand Motor resulting in fewer dunes to develop on these highly elevated areas with larger distances between the dunes and fewer vegetated cells. This research shows that it is possible to implement a forced coastline by hydrodynamics, beach armouring and multidirectional wind in the DUBEVEG model, but improvements can be made in future research. The DUBEVEG model in the current form is not applicable for the out-of-equilibrium anthropogenic shores like the Sand Motor because processes are missing. However, Including multidirectional wind (aeolian transport directions) and beach armouring in the model result in a better approach to the observed dunes. Assumed is that improvements in these processes would further increase the approach of the observed dune patterns and increase the applicability of the DUBEVEG model for out-of-equilibrium anthropogenic shores.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
Link to this item:http://purl.utwente.nl/essays/77928
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