Previous field studies have demonstrated that sedimentation is an important factor that can limit oyster reef growth and restoration success. High relief reefs are more productive and resilient than low relief reefs, in part, because increasing reef height reduces sedimentation and enhances oyster growth. In this study, we investigated the relationship between initial reef height and reef development using a simple model. The model contains three coupled differential equations that describe changes in oyster volume, shell volume, and sediment volume per unit area of reef with time. The model was used to investigate how parameters such as flow speed, sediment grain size, and food concentration affect reef survival and final reef height. Whether or not a reef survives depends primarily on the shape of the sediment concentration profile relative to the initial reef height. Over a long time period, three different steady-state reef heights are possible, depending on the environmental parameters and initial reef height: (1) If growth outpaces sedimentation, the reef achieves the maximum possible height, which is independent of sedimentation parameters; (2) if deposition outpaces growth and the shear stress does not exceed the critical shear stress, the reef is buried in sediment and dies; and (3) if deposition outpaces growth and the shear stress exceeds the critical shear stress, a reduced steady-state height is achieved that depends on both growth and sedimentation parameters. The model can be used to assess the ways in which measurable environmental parameters affect reef restoration success.
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.
Storms can have long-term impacts on the groundwater flows and subsurface salinity structure in coastal aquifers. Previous studies have shown that tides, wave driven infiltration, and storm surge elevate the groundwater level within the beach (Nielsen 1999, Cartwright 2004). The resulting bulge of high groundwater propagates inland, and may cause flooding up to several days after a storm has passed (Gallien 2016). In addition, waves, tides, and storm surge force saltwater to infiltrate into the aquifer above the fresher terrestrial groundwater, and storm-driven pulses of salinity may persist for months (Robinson et al. 2014). Here, observations of groundwater heads and salinities collected continuously for three years are used to examine the effects of ocean storms, wind-driven fluctuations in sound water levels, and morphological changes on a barrier island aquifer.
<p>Coastal aquifers supply freshwater to nearly half of the world's population, and their importance for sustainable development in coastal areas is immense. Due to the proximity to the ocean, salinization is typically the biggest risk for coastal groundwater resources. Furthermore, the interactions between groundwater and surface water during coastal flooding often result in surface instabilities arising from elevated groundwater heads. Here, integrated hydrologic modeling is used to examine the effect of groundwater-surface water interactions on the salinity distribution in aquifers and on the stability of beach surfaces. The processes considered include multi-scale fluctuations in sea level (tides, storm surges, and glacial cycles). Results show that modern salt distributions may change even if the current conditions remain stable, when considering short- and long-term cyclical processes that aquifers are likely still responding to. It is also found that during coastal flooding, critical hydraulic gradients may develop, potentially destabilizing the beach surface. The distribution of these critical gradients depends on beach topography, with a non-trivial relationship between surface elevation and the location of critical gradients. These results mean that the interactions between groundwater and surface water likely play a pivotal role in the hydrologic state of coastal systems, with important implications for water resources management and for natural hazard mitigation.</p>
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