Modeling Impact of Intertidal Foreshore Evolution on Gravel Barrier Erosion and Wave Runup with XBeach-X

Benjamin T Phillips,Jennifer M Brown,Andrew J Plater

doi:10.3390/jmse8110914

Benjamin T Phillips, Jennifer M Brown + Show 1 more

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https://doi.org/10.3390/jmse8110914

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Abstract

This paper provides a sensitivity analysis around how characterizing sandy, intertidal foreshore evolution in XBeach-X impacts on wave runup and morphological change of a vulnerable, composite gravel beach. The study is motivated by a need for confidence in storm-impact modeling outputs to inform coastal management policy for composite beaches worldwide. First, the model is run with the sandy settings applied to capture changes in the intertidal foreshore, with the gravel barrier assigned as a non-erodible surface. Model runs were then repeated with the gravel settings applied to obtain wave runup and erosion of the barrier crest, updating the intertidal foreshore from the previous model outputs every 5, 10 and 15 min, and comparing this with a temporally static foreshore. Results show that the scenario with no foreshore evolution led to the highest wave runup and barrier erosion. The applied foreshore evolution setting update is shown to be a large control on the distribution of freeboard values indicative of overwash hazard and barrier erosion by causing an increase (with 5 min foreshore updates applied) or a decrease (with no applied foreshore updating) in the Iribarren number. Therefore, the sandy, intertidal component should not be neglected in gravel barrier modeling applications given the risk of over- or under-predicting the wave runup and barrier erosion.

Highlights

Gravel barrier coasts, found worldwide on high-latitude, previously glaciated coasts (Northern Europe, Japan, U.S.A.) can experience erosion and overtopping during high-energy storm events, resulting in financial and societal losses and fatalities [1]
This section explores the results of the XBeach-X modeling of the Hs and Tp percentile combinations under each of the foreshore evolution settings described in Section 3.4 (S1 to S5)
The following proxies are used to show the influence of the foreshore evolution setting on wave runup and morphological response of the gravel barrier: 1. Freeboard: Calculated as the difference in elevation between the barrier crest and R2% at 0.5 s intervals when the water level exceeds the toe of the barrier (1.92 m above Ordnance Datum Newlyn (m ODN))

Summary

Introduction

Found worldwide on high-latitude, previously glaciated coasts (Northern Europe, Japan, U.S.A.) can experience erosion and overtopping during high-energy storm events, resulting in financial and societal losses and fatalities [1] These coastlines are becoming increasingly vulnerable as wave climates become modified by changing storm tracks [2] and as sea-level rise acts to shorten the return period of a given extreme water level and increase the frequency of coastal flooding [3]. In the short and medium term, barriers are affected by the local wave climate and episodically when wave runup exceeding the barrier crest allows the mobilization of sediments onto the barrier crest and back barrier (overwash), inundation and in extreme scenarios, barrier breaching [5] These events are likely to pose a hazard to hinterland communities, which will face an amplified risk of barrier breaching and overwash from future sea-level rise [3]. Insights into the long-term evolution of barriers provide critical information on the underlying drivers of coastal barrier response

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