Interaction of tides, groundwater levels and surface moisture on a sandy beach
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Wind-driven (aeolian) sand transport from the beach is of prime importance to coastal dune formation and growth, as well as dune recovery after a severe storm. The amount of aeolian sand transport is not only determined by wind speed, but also by beach surface characteristics, predominantly moisture content. The surface moisture content is determined by processes in the atmosphere (evaporation and precipitation) and in the groundwater (via capillary transport). The groundwater level below the beach is governed mostly by the tide. However, the exact importance of the tide in determining surface moisture content is still unknown. Therefore, the present study aims to find this relation, in order to better understand and predict aeolian sediment transport patterns in the future. A six-week field campaign was performed at the Sand Motor in fall 2014. Sea surface elevation was measured with a pressure transducer below the low-water line, while 8 dipwells were deployed to measure groundwater levels in a 80-m long cross-shore array. A Delta T Theta probe was used to measure surface moisture content along the same array. It was found that the tide and the bed profile control the spatial and temporal patterns in surface moisture content. The tidal oscillations in the sea surface propagate into the beach, which makes the water table below the beach oscillate with tidal frequency. The inland water table of the Sand Motor is located 1m above NAP. The slope of the water table is relatively constant, and it is independent of the slope of the beach surface. Therefore, a low beach slope results in an intersection of the water table and the bed surface. This gives a high surface moisture content. The water table affects the surface moisture content at the Sand Motor when it is within 0.3m below the bed surface. In this case the potential for aeolian sand transport can be reduced. A spring-neap tidal cycle was found in the amount of time throughout the day that this influence was present. During neap tide, the groundwater level was further below the surface, so the capillary fringe could reach the surface for a smaller amount of time. Therefore, the potential for aeolian sand transport will be highest during neap tide.