HGS RESEARCH HIGHLIGHT – Coastal Topography and Hydrogeology Control Critical Groundwater Gradients and Potential Beach Surface Instability During Storm Surges
Paldor, A., Stark, N., Florence, M., Raubenheimer, B., Elgar, S., Housego, R., Frederiks, R. S., & Michael, H. A. (2022). Coastal topography and hydrogeology control critical groundwater gradients and potential beach surface instability during storm surges. In Hydrology and Earth System Sciences (Vol. 26, Issue 23, pp. 5987–6002). Copernicus GmbH. https://doi.org/10.5194/hess-26-5987-2022
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This week we’re highlighting another article by our friends at the University of Delaware (Holly Michael’s hydrogeology research group) who continue to use HydroGeoSphere for coastal hydrology modelling projects.
Figure 1: HGS model domain, boundary conditions (red and blue boxes), and the surge height evolution curve (inset). The blue curve is the terrestrial freshwater recharge boundary, the red rectangle is where a fixed seawater head and concentration are applied to the subsurface domain, and the red dashed line is where the sea level height boundary condition (h0.t/) is applied to the surface domain.
The current study investigates the impact that oceanic storm surges can have on the stability of coastal soils. Coastal storm surges alter the hydrogeological regime drastically and abruptly, and have the potential to induce critical hydraulic gradients that could liquefy the beach soil. Predicting the spatio-temporal distribution of these surge-induced critical gradients is highly non-trivial, as they depend on both the surface and the subsurface flow domains, as well as the interaction between them.
This work harnessed HydroGeoSphere to simulate storm surges and their geotechnical impact on coastal environments. The explicit coupling between the surface and subsurface domain revealed a non-trivial correlation between alongshore topography and the distribution of surge-induced critical gradients. It was found that, in contrast with traditional approaches, the lowest areas along a coastline are not necessarily the ones that are most vulnerable. Rather, critical gradients develop preferentially under intermediate topographic elements, at least under certain hydrogeologic conditions. These findings have important implications for mitigating natural hazards in coastal environments, and for coastal geomorphology.
In this study a 10 km^2 HydroGeoSphere model domain is used (Figure 1), with topographic characteristics which are representative of the US Atlantic and Gulf coastal systems averaged over large cross-shore distances. The results of the model are used to determine the seepage-liquefaction factor of coastal soils (a geotechnical stability metric which relates vertical hydraulic gradients to the theoretical quicksand potential of a soil), to determine the erosive and “quicksand” (i.e. sediment mobility) potential of coastal storm surges (Figure 2).
Figure 2: Surface flooding and vertical hydraulic gradients at (a) 0.5, (b) 4.3, (c) 6.2, and (d) 8.4 h after the simulated surge begins. In each panel, the surface domain is shown on top, the subsurface three-dimensional domain and vertical gradients are shown below, and the two cross sections through the subsurface are shown: shore-parallel (left in each panel) and shore-perpendicular (right).
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Abstract:
Ocean surges pose a global threat for coastal stability. These hazardous events alter flow conditions and pore pressures in flooded beach areas during both inundation and subsequent retreat stages, which can mobilize beach material, potentially enhancing erosion significantly. In this study, the evolution of surge-induced pore-pressure gradients is studied through numerical hydrologic simulations of storm surges. The spatiotemporal variability of critically high gradients is analyzed in three dimensions. The analysis is based on a threshold value obtained for quicksand formation of beach materials under groundwater seepage. Simulations of surge events show that, during the run-up stage, head gradients can rise to the calculated critical level landward of the advancing inundation line. During the receding stage, critical gradients were simulated seaward of the retreating inundation line. These gradients reach maximum magnitudes just as sea level returns to pre-surge levels and are most accentuated beneath the still-water shoreline, where the model surface changes slope. The gradients vary along the shore owing to variable beach morphology, with the largest gradients seaward of intermediate-scale (1–3 m elevation) topographic elements (dunes) in the flood zone. These findings suggest that the common practices in monitoring and mitigating surge-induced failures and erosion, which typically focus on the flattest areas of beaches, might need to be revised to include other topographic features.
If you are interested in this research be sure to check out earlier HGS research highlights by the same authors (third link is a webinar featuring three related studies):
Webinar - Thermal Energy Transport and Saltwater Intrusion Modelling with HydroGeoSphere
Modeled Salinities In Coastal Aquifers Depend On High- And Low-Frequency Fluctuations In Sea Level, Controlled By Storage Properties
Vulnerability To Sea-Level Rise and Storm-Surge Salinization Differs For Topography-Limited And Recharge-Limited Systems: Insights From Groundwater Flow and Transport Modeling at Assateague Island, MD
Impact Of Ocean Surge Profiles On Overwash-Driven Salinization In Coastal Aquifers