Print Email Facebook Twitter Modelling sediment sorting near the large scale nourishment 'The Sand Motor': Understanding cause and impact of sediment sorting processes Title Modelling sediment sorting near the large scale nourishment 'The Sand Motor': Understanding cause and impact of sediment sorting processes Author Van der Zwaag, J. Contributor Stive, M.J.F. (mentor) Huisman, B.J.A. (mentor) Luijendijk, A.J.L. (mentor) Van Prooijen, B.C. (mentor) Faculty Civil Engineering and Geosciences Department Hydraulic Engineering Date 2014-08-21 Abstract The sediment composition of the sea bed is of relevance for various coastal properties such as bed forms, beach slopes and marine ecology. Furthermore it may significantly influence the morphological evolution of the coast. Knowledge about the processes forcing the spatial distribution of a certain grain size is therefore of great importance. Examining these processes requires good quality data sets of bed sediment composition with high temporal and spatial coverage and for many research questions the aid of numerical modelling. However, these aspects have been lacking in many studies and need additional research This research investigates the sorting of sediment at a a large coastline disturbance: the Sand Motor. For this purpose (1) five field campaigns are analysed to examine the evolution of the bed composition near the Sand Motor, (2) the existing Delft3D 2DH hindcast model of the Sand Motor is extended with five sediment fractions to hindcast the sediment sorting with respect to the data and (3) single processes are assessed with a 3D extension of the 2DH model, to be able to account for undertow processes. The hindcast of the sediment composition proved that Delft3D is well capable of hindcasting characteristics such as fining and coarsening of areas: the Brier skill score increased from 0 to a value of about 0.45. However, precise values of the sediment composition appeared to be inaccurate, as an average error of approx. 50 mu and local errors up to 180 mu between field- and model data remained. In addition, alongshore gradients were underestimated with respect to the field data. It is noted that quite a bit of the mentioned errors may be related to small shifts in the predicted and measured spatial distribution (double penalty effect). Model results including maximum gradients, the size of fine patches and the location of the minimum/maximum D50 showed that storm events cause rapid coarsening of the swash zone, while forcing finer sediments both offshore and towards less energetic areas located north and south of the Sand Motor. Furthermore, a spatial pattern of the bed sediment distribution will develop. The time scale of this distribution differs per processes and per location: from an almost instant response (O(day)) at the swash zone to a period of weeks for the offshore tidal influence. The spatial characteristics appeared to be in a dynamic equilibrium after a few storm events: only more severe hydrodynamic conditions than before, or from a non-westerly (e.g. non-dominant) direction could cause major shifts in the spatial distribution. Since the mentioned features are persistent in space and time, a coupling between the spatial distribution of grain sizes and the morphological active zone is possible, enabling an estimate of the morphological characteristics with a single spatial D50 measurement. The modelling consequences of applying a multifraction model compared to a single fraction model include a significantly faster morphological response: eroded volumes were 1.8 times that of the data and 2.4 times that of the single fraction model. Scaling of the cumulative erosion curve with the mentioned factor 1.8 provides a perfect fit on the data, indicating that the relative impact of conditions in time is in agreement with measured bathymetry changes. Beach profiles in the multifraction model steepened between 8 and 4 meters depth, while becoming slightly more gentle between at depths between 4 and 0 meter. Although including sediment fractions is theoretically closer to reality, it also includes more assumptions. In general it will depend on the aim of the applied model, required data and available time if modelling with a multifraction sediment composition is beneficial. Subject Sand MotorSediment SortingDelft3DMorphologyCoastal Engineering To reference this document use: http://resolver.tudelft.nl/uuid:328250df-6038-4582-97d8-e7ba743e0761 Embargo date 2015-02-21 Coordinates 52.052285, 4.184252 Part of collection Student theses Document type master thesis Rights (c) 2014 Van der Zwaag, J. Files PDF Graduation_thesis_J._van_ ... _Zwaag.pdf 19 MB Close viewer /islandora/object/uuid:328250df-6038-4582-97d8-e7ba743e0761/datastream/OBJ/view