Wave-Driven Porewater-Seawater Exchange in Sandy Coastal Sediments

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ABSTRACTOCE-0327332Shallow-water surface waves traveling above permeable sandy sediments (i.e., those with grains coarser than ~80 um) can induce greatly enhanced mixing across the sediment-water interface and rapid transport of water and particles within the sediment. Thus, hydrodynamic transport in sandy sediments in nearshore and continental shelf waters can be expected to (1) substantially increase the rates of biochemical processes occurring in sediments by increasing the supply of oxygen and particulate matter to the sediment, and (2) significantly increase the rate of return of degradation products such as nutrients to the water column, thereby increasing water-column and benthic primary production. The rapid transport of porewater and particles in sandy sediments renders the calculation of benthic fluxes using conventional diffusion-based models invalid and thus confounds the interpretation of chemical profiles collected in coastal areas and on the continental shelf.In this project, researchers at the University of Hawaii will conduct studies to describe and quantify the transport of solutes and particulate matter in sandy sediments. Using field investigations, they will confirm or refute the validity of available theoretical models of hydraulic transport through sediment pores and evaluate the importance and timescales of the hydrodynamic pumping mechanisms. They propose to (1) measure wave-induced porewater movement/dispersion under a range of wave conditions; (2) evaluate current theories of wave-enhanced porewater movement/dispersion and exchange using the results of these measurements; (3) establish an array of physical sensors at the field site, and collect synoptic data over a range of wave conditions; (4) measure porewater concentrations of biogeochemically important constituents across the array; (5) compute porewater-seawater fluxes as functions of wave conditions; (6) compare these porewater-seawater fluxes with those calculated using measured 222Rn distributions and eddy correlation techniques; (7) measure wave-induced particle transport rates into and within sandy sediments, as a function of particle size and wave conditions; and (8) investigate the effect of bedforms on porewater and particle transport. This research will be conducted in sandy sediment off the south shore of Oahu, Hawaii, a site that is subject to a wide and predictable range of wave conditions.There is presently a lack of suitable tools for quantifying porewater and particle motion, including sediment-water fluxes, in sandy sediments. The proposed research aims to provide practical methodologies to overcome this limitation and to develop a generic process understanding that will allow the estimation of interstitial transport rates in biogeochemical models used to represent diagenesis in sandy sediments.