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Predicting morphological storm impact on coastal dunes at Ameland: Simulating storm response on coastal dunes at barrier islands, in the presence of gentle slope beaches, with the numerical model XBeach in a 2DH (depth average) setting

Terlouw, A. (2013) Predicting morphological storm impact on coastal dunes at Ameland: Simulating storm response on coastal dunes at barrier islands, in the presence of gentle slope beaches, with the numerical model XBeach in a 2DH (depth average) setting.

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Abstract:The coastal zone is a focal point in the Dutch economy where a large number of social and economic activities are concentrated. Coastal dunes are the first (and last) line of defense against flooding of the coastal zone from sea. Therefore, the safety of these coastal dunes needs to be assessed whether or not they provide sufficient protection for the region at any given storm event. Current used methods are not sufficient to assess safety of the coastal region in case of complex situations. Complex situations require two-dimensional process-based prediction tools, such as XBeach. However, if XBeach is used for the assessment of safety, it has to be extensively tested and validated. Therefore, XBeach was applied on a complex case. The presented case is one of beaches with a gentle slope, where infragravity waves are expected to be of significant importance. The presented case includes the extraordinary storm, the ‘Allerheiligenstorm’ November 1st, 2006. This storm resulted in overwash at the barrier island Ameland. Due to the fact that pre and post storm LIDAR data of Rijkswaterstaat were available, morphological impact could be identified. The results revealed that the available LIDAR data was of insufficient quality for the required modeling scale. This might be due to different calibration methods, incorrect use of correction factors, visual correction and the collection of the data by different companies. One cannot expect accurate result from when using these data with the model XBeach. This resulted into a qualitative research approach for assessing storm impact. So, criteria were defined based on the storm impact scale of Sallenger (2000) to identify the extent of the locations affected by different impact regimes. Four subsequent levels of storm impact regimes were used: swash, collision, overwash and inundation. Data analyses revealed that the maximum storm impact resulted in overwash for two locations at Ameland. Between these locations, there was a notable difference in extent of the storm impact regime. At the first overwash location (Paal22) only little impact of the overwash was observed. At the second location (DeHon) abundant overwash could be recognized. XBeach simulation showed accurately predicted the occurrence of overwash during the Allerheiligenstorm. The extent of the overwash occurrence showed to be at Paal22 underestimated. At DeHon the extent of overwash occurrence was overestimated. The order of the magnitude showed to be same. Results might be affected by the use the transport limiter and the inaccuracies in the bed level height. Sensitivity analyses showed that impact during collision and swash sensitive to the use of the overwash transport limiter. Overwash occurrence showed to be sensitive to an increase of 20 cm of the bed level height. Infragravity at the dune front reached in the simulations by XBeach a high of about 0.5 m. Recommendations are to add missing physical processes to improve model performance. Short wave run-up and vegetation are not accurately represented in the XBeach model. As a consequence, if the threshold to overwash in XBeach is slightly exceeded, overwash is underestimated due to the lack of short wave run-up. Overestimation of overwash by XBeach might occur due to model inaccurately represents vegetation.
Item Type:Essay (Master)
Clients:
Deltares
Faculty:ET: Engineering Technology
Subject:38 earth sciences
Programme:Civil Engineering and Management MSc (60026)
Link to this item:http://purl.utwente.nl/essays/63082
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