From ripples to large-scale sand transport : The effects of bedform-related roughness on hydrodynamics and sediment transport patterns in delft3d
Brakenhoff, Laura ; Schrijvershof, Reinier ; Werf, Jebbe van der; Grasmeijer, Bart ; Ruessink, Gerben ; Vegt, Maarten van der - \ 2020
Journal of Marine Science and Engineering 8 (2020)11. - ISSN 2077-1312
Bed roughness - Bedform predictors - Delft3D - Hydrodynamics - Sediment transport
Bedform-related roughness affects both water movement and sediment transport, so it is important that it is represented correctly in numerical morphodynamic models. The main objective of the present study is to quantify for the first time the importance of ripple-and megaripple-related roughness for modelled hydrodynamics and sediment transport on the wave-and tide-dominated Ameland ebb-tidal delta in the north of the Netherlands. To do so, a sensitivity analysis was performed, in which several types of bedform-related roughness predictors were evaluated using a Delft3D model. Also, modelled ripple roughness was compared to data of ripple heights observed in a six-week field campaign on the Ameland ebb-tidal delta. The present study improves our understanding of how choices in model set-up influence model results. By comparing the results of the model scenarios, it was found that the ripple and megaripple-related roughness affect the depth-averaged current velocity, mainly over the shallow areas of the delta. The small-scale ripples are also important for the suspended load sediment transport, both indirectly through the affected flow and directly. While the current magnitude changes by 10–20% through changes in bedform roughness, the sediment transport magnitude changes by more than 100%.
An integral approach to design the Roggenplaat intertidal shoal nourishment
Werf, J.J. van der; Vet, P.L.M. de; Boersema, M.P. ; Bouma, T.J. ; Nolte, A.J. ; Schrijvershof, R.A. ; Soissons, L.M. ; Stronkhorst, J. ; Zanten, E. van; Ysebaert, T. - \ 2019
Ocean & Coastal Management 172 (2019). - ISSN 0964-5691 - p. 30 - 40.
Estuarine management - Intertidal shoal - Morphological modelling - Sediment nourishment design
The Eastern Scheldt, a tidal basin in the southwest of The Netherlands, underwent large physical and ecological changes due to a system-wide human interference. The construction of a storm surge barrier at the seaward side and closure of the upstream branches in the 1980s resulted in intertidal flat erosion. This has far reaching consequences for the ecological functioning of these habitats, especially as foraging ground for many wader species. Therefore, a 1.3 million m3 sand nourishment is foreseen on the Roggenplaat intertidal shoal to mitigate the erosion and preserve suitable foraging habitat for waders for the coming 25 years. This paper presents an integral nourishment design approach. It consists of the following steps: (i) understanding the morphology and ecology, (ii) translation of the nourishment objective into an evaluation framework, (iii) construction of a suitability map indicating potential nourishment locations, (iv) generation of nourishment designs, (v) short-term morphodynamic numerical model simulations, (vi) estimation of the long-term shoal development using a simplified approach, (vii) integral evaluation leading to the preferred design. This integral approach resulted in a design that is expected to fulfill the Roggenplaat nourishment objective, accounting for ecological, morphological, economical and technical aspects. This integrated approach could form a basis for future intertidal shoal nourishment designs worldwide.
The Importance of Combined Tidal and Meteorological Forces for the Flow and Sediment Transport on Intertidal Shoals
de Vet, P.L.M. ; van Prooijen, B.C. ; Schrijvershof, R.A. ; van der Werf, J.J. ; Ysebaert, T. ; Schrijver, M.C. ; Wang, Z.B. - \ 2018
Journal of Geophysical Research: Earth Surface 123 (2018)10. - ISSN 2169-9003 - p. 2464 - 2480.
hydrodynamics - intertidal area - morphology - numerical model - sediment transport - wind
Estuarine intertidal areas are shaped by combined astronomical and meteorological forces. This paper reveals the relative importance of tide, surge, wind, and waves for the flow and sediment transport on large intertidal shoals. Results of an intensive field campaign have been used to validate a numerical model of the Roggenplaat intertidal shoal in the Eastern Scheldt Estuary, the Netherlands, in order to identify and quantify the importance of each of the processes over time and space. We show that its main tidal creeks are not the cause for the dominant direction of the net flow on the shoal. The tidal flow over the shoal is steered by the water level differences between the surrounding channels. Also during wind events, the tidal flow (enhanced by surge) is dominant in the creeks. In contrast, wind speeds of order 40 times the typical tidal flow velocity are sufficient to completely alter the flow direction and magnitude on an intertidal shoal. This has significant consequences for the sediment transport patterns. Apart from this wind-driven flow dominance during these events, the wind also increases the bed shear stress by waves. For the largest intertidal part of the Roggenplaat, only ∼1–10% of the yearly transport results from the 50% least windy tides, even if the shoal is artificially lowered half the tidal range. This dominance of energetic meteorological conditions in the transports matches with field observations, in which the migration of the creeks and high parts of the shoal are in line with the predominant wind direction.