The effect of waves on benthic exchange: measurement and estimation over a broad range of spatial and temporal scales
Date
2018
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Publisher
University of Delaware
Abstract
The loads and forms of nutrients, metals, and carbon in coastal seas have impacts on nearshore marine ecosystems. These solutes can undergo biogeochemical transformations in the shallow seabed aquifer, which is controlled by chemistry in surface water and groundwater and the benthic exchange that drives this mixing. This dissertation focuses on quantifying the effect of waves on ambient groundwater discharge, understanding how wave pumping compares to other driving mechanisms, and understanding how wave-induced benthic exchange varies on second-to-decadal temporal scales and spatial scales ranging from meters to the globe. ☐ A laboratory study on the influence of waves on seepage meter measurements confirmed the utility of and quantified potential uncertainties associated with seepage meters. Tank tests showed seepage meters were efficient in measurement of both ambient discharge and recharge. Results of wave tank tests indicated that although waves did induce some net discharge through the seepage meter, measured fluxes were well below the theoretical rates expected for waves traveling over a flat seabed. ☐ A field and modeling study investigated how wave, current/bedform, and tidally-induced benthic exchange compare and how these fluxes vary over time at a shallow estuarine location. Field measurements showed wave-induced exchange greatly exceeded exchange driven by currents or tides over the study duration. Darcy-calculation of fluxes from high-resolution pressure measurements were used to validate an analytical solution with field measurements of wave pumping for the first time. Results of the numerical models showed the effect of aquifer properties on benthic exchange—seabed aquifers with low hydraulic diffusivity had higher fluxes, shallower exchange, and shorter residence times. ☐ A global modeling study quantified the rates and temporal and spatial variability of wave pumping over the Earth’s oceans. Global wave pumping between 1979 and 2010 averaged 1.8x10^5 km^3/yr, which is equivalent to an average of 6.1 m/yr over the entire global shelf area. Seasonally, winter wave pumping rates exceed summer values by about one-third except in the circumpolar oceans where ice-cover drastically reduces wave pumping during winter months. Results of a regional analysis showed that extreme (top 10%) of wave pumping events drive about one-fifth of annual wave pumping. ☐ These studies provide better estimates of wave pumping rates on a variety of scales and quantify the uncertainties associated with measurement of benthic exchange in the presence of waves. Thus, the findings have important implications for understanding chemical cycling in the seabed aquifer and, thus, for management of coastal ecosystems, and preservation of the recreational and economic resources they provide.
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Keywords
Applied sciences, Earth sciences, Benthic exchange, Coastal hydrology, Groundwater, Hydrogeology, Submarine groundwater discharge, Waves