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Coastal gravel barrier hydrology — Observations from a prototype-scale laboratory experiment (BARDEX)

Turner, Ian L. / Masselink, Gerd

Abstract A prototype-scale laboratory study (BARDEX — Barrier Dynamics Experiment) was conducted within the 250-m long and 7-m deep Delta wave flume, The Netherlands. A 4-m high and 50-m long gravel barrier (D 50 =11mm) was constructed within the central region of the flume, enabling a back-barrier ‘lagoon’ to be located at its landward side. By the installation of a multi-stage pump system the water levels at the ‘sea’ and ‘lagoon’ sides of the barrier could be manipulated independently. A series of experiments were undertaken to examine the groundwater fluctuations and flow within the barrier system subject to varying lagoon levels and wave conditions. In accordance with previous sand experiments, the wave-induced watertable over-height at the beachface was confirmed to be independent of sediment characteristics. Varying back-barrier lagoon levels were observed to have significant effect on both the maximum watertable over-height within the barrier and shoreline set-up (vertical position of the point of intersection between mean water surface through the swash zone and beachface), as well as the direction and magnitude of though-barrier groundwater flux. The effective hydraulic conductivity of the D 50 =11mm gravel barrier was determined to be K =0.16ms⁻¹. At higher Reynolds numbers (Re >~40) associated with local watertable gradients exceeding 0.025 and resulting turbulent losses, the transition from laminar to turbulent through-barrier flow was indicated by a reduction in local K. However, for the parameter range encountered this effect was modest only (<10%), and for many practical purposes it is proposed that the application of Darcy's Law to estimate the exchange of ground-ocean water through coastal gravel barriers is well within the range to which the necessary field parameters can be realistically estimated.

Highlights ►Groundwater fluctuations and flow within a gravel barrier have been studied. ►Wave-induced watertable over-height is independent of sediment characteristics. ►Varying water levels had a significant effect on the maximum watertable over-height. ►The effective hydraulic conductivity, K, of the D 50 =11mm gravel barrier was 0.16ms⁻¹. ►At higher Reynolds local K values were around 10% lower. ►For practical applications Darcy's Law is appropriate.