Yashar Rafati and Tian-Jian Hsu

In the coastal zones the beach response to storm events. Our ability to predict beach profile evolution from before to after the storm is of the greatest concerns in coastal management. During extreme storm events, significant erosion occurs as a result of waves, storm surge and flooding. Recent studies by Deltares researchers have indicated that our capability to predict dune erosion and breaching processes also depended on the model’s capability to predict onshore sediment transport. This is likely because onshore transport can slowdown the eroded sand masses moving offshore and preserves them in the inner surf zone. To better evaluate existing morphodynamic model’s capability in predicting onshore and offshore transport in a simpler setting, this study adopts a widely used nearshore morphodynamic model XBeach. Through an ongoing project supported by Office of Naval Research, we collaborate with Ap van Dongeren and Ellen Quataert from Deltares to evaluate XBeach’s capability to predict onshore/offshore sandbar migration events in the surf zone observed at Duck, NC during Duck94 field experiment.

Surf zone is the transition zone from the ocean to the coastline, where waves evolve the shape to sharper crest and steeper wave front, and consequently experience breaking (Figure 1). Surf zone sandbars are important morphological features protecting beaches against storms and extreme events. During storms the intense wave breaking in the nearshore causes the generation of significant offshore directed current known as undertow. The strong undertow drives the sediment offshore which has been found to be the main mechanism of the offshore bar migration. On the other hand, the surface waves drive the sediment onshore through several complex processes near the bed and leads to onshore sandbar migration. Because wave-driven onshore transport requires more detailed intra-wave analysis, it has to be parameterized into wave-averaged morphodynamic models. The capability of these onshore transport parameterizations has not been systematically investigated.

Process-based modeling tools XBeach is adopted in this study as it has demonstrated a very good performance in predicting dune erosion, overwash, and breaching processes. We perform a more extensive evaluation of XBeach to concurrently simulate onshore and offshore sandbar migration events by tuning a minimum set of model parameters. In Duck94 field experiment, significant onshore and offshore sandbar migration occurred during high-energy waves (Significant wave height Hs > 2m, October 10-14) and low-energy waves (Hs ~ 1m, September 21-26), respectively. Based on an improving XBeach parameterization of calculating undertow and wave-driven onshore sediment transport, we are able to demonstrate a good prediction of the onshore and offshore migration of a sandbar (Figure 2). From these simulations, we extract local onshore transport rate predicted by XBeach parameterization and compare with small-scale intra-wave two-phase model to further evaluate a more physically-based onshore transport parameterization for XBeach.

Figure 1: Schematic sketch of surf zone processes







Figure 2: Observed and computed bed profiles corresponding to Duck94 experiment. (a) An onshore sandbar migration event during low-energy wave condition, September 21-26. (b) An offshore migration event during high-energy waves condition, October 10-14, Computed profiles correspond to the improved parameterization of XBeach.