Improving XBeach non-hydrostatic model predictions of the swash morphodynamics of intermediate-reflective beaches

Improving XBeach non-hydrostatic model predictions of the swash morphodynamics of intermediate-reflective beaches

Jongedijk, Cleo (TU Delft Civil Engineering and Geosciences; TU Delft Hydraulic Engineering)

McCall, Robert (mentor)
Reniers, Ad (mentor)
de Schipper, Matthieu (graduation committee)
van der Werf, JJ (graduation committee)

Delft University of Technology

Comidas

2017-10-12

A very common observation is the episodic erosion of beaches during storms and the slow recovery (accretion) afterwards (Yates et al. 2009). Morphodynamic models parameterize physical processes in order to relate the fluid motions (hydrodynamics) to the bed level changes (morphodynamics) over a wide range of spatial and temporal scales. Despite recovery of the beach profile being a slow process, accretion mechanisms in the swash zone are complex to represent by a numerical model due to the shallow, rapidly varying flows and high concentration gradients (Brocchini & Baldock 2008). The swash zone on most beaches is readily accessible, but this accessibility does not translate into a broad knowledge of the underlying physical processes (Chardón-Maldonado et al. 2015). This research focused on assessing the representation of physical processes in the swash zone of intermediate-reflective beaches during erosive and accretive conditions in the non-hydrostatic version of the XBeach model (XBeach). This is a depth-averaged phase resolving model, mainly used for calculations of storm impact (hours-days) on sandy and gravel beaches. Based on conclusions in literature, relevant physical processes that contribute to accretion were determined. Using a simple planar beach bathymetry, the sediment transport formulations in XBeach and the individual influence of groundwater effects, bed slope effects, sediment response time and wave breaking induced turbulence were assessed. The results showed that for both accretive and erosive wave conditions, XBeach predicts erosion in the swash zone. Groundwater infiltration and wave breaking induced turbulence are likely to enhance onshore transport significantly Reniers et al. (2013), Turner &Masselink (1998). To verify the findings of the planar beach modeling approach, in the second part of this thesis the morphodynamical predictions of XBeach were compared to the dataset collected during the Bardex II experiment. Bardex II was performed in 2012 in the Delta Flume in the Netherlands and the dataset contains observations of a series of experiments focusing on the effect of varying wave, sea level and beach groundwater conditions on a sandy beach (D50=0.42 mm)(Masselink et al. 2013). Stored data and published resultswere used for comparison with the morphodynamical prediction performance of XBeach. Two timescales have been analyzed, the total morphological response of the beach on a timescale of 100 minutes and the intra-swash sediment transport processes on a timescale of 10 seconds. Two experiments from the series have been reproduced. The first, experiment A4, has an almost stable, slightly erosivemorphological response throughout the swash zone. The second, experiment A8, shows accretion in the upper swash and a stable profile in the rest of the swash zone. XBeach erroneously (over)predicted erosion above the mean sea level (MSL) for both A4 and A8 conditions. This conclusion was related to the results of the intra-swash sediment transport assessment. The modeled velocity in both uprush and backwash were higher than the Bardex II measured velocity and XBeach extremely underpredicted uprush sediment concentrations suggesting that turbulence induced by the bore is not enough taken into account. Over-predicted backwash sediment concentrations for both accretive and erosive conditions suggested that groundwater infiltration was not strong enough. However, enhancing wave breaking induced turbulence and groundwater infiltration did not lead to an improvement of the predictions of sediment concentrations in the swash. The sediment transport formulations of XBeachwere developed using a long wave resolving (short wave averaging) model, andwidely validated and calibrated on field and experimental data (Soulsby 1997, van Rijn et al. 2007, Van Thiel De Vries 2009). Therefore, in the last part of this thesis two possible improvements of the two sediment formulations of XBeach (van Thiel-van Rijn and Soulsby-van Rijn) when applying them in a short wave resolving model are assessed using a 1D sediment transport model. The decomposition of the velocity signal in amean and a fluctuating part improved mainly the predictions using Soulsby-van Rijn where the separate calibration of turbulent kinetic energy resulted in better predictions for both transport formulations. This analysis was performed only at one point on the cross-shore domain (slightly above MSL). Although the results for this position were promising, comparison of modeled sediment transport with Bardex II observations at different positions throughout the upper and lower swash zone is needed to give a full validation of the proposed adaptations to the transport formulations.

Swash
XBeach
Morphodynamics
numerical modeling
Bardex II
Delta Flume
sediment transport modelling
sand
Nearshore morphology
Morphology

http://resolver.tudelft.nl/uuid:822dc768-b9a5-485d-bfd6-d102c5af1924

Student theses

master thesis

© 2017 Cleo Jongedijk