Large-scale laboratory investigation into the effect of varying back-barrier lagoon water levels on gravel beach morphology and swash zone sediment transport

Abstract

It is generally accepted that the effect on beach morphology of the interactions between swash motion and the beach groundwater table depends on the elevation of the latter relative to that of the sea. Specifically, a low beach groundwater table is considered to enhance onshore sediment transport and beach accretion, while the reverse is thought to hold for a high beach groundwater table. Here, we use the results of a carefully designed series of tests conducted in the Delta Flume, the Netherlands, as part of the Barrier Dynamics Experiment (BARDEX), to unequivocally demonstrate the validity of this assertion. During the experiment, a 4-m high and 50-m long gravel barrier (D50=11mm) was constructed within the central region of the 250-m long and 7-m deep wave flume, enabling a back-barrier ‘lagoon’ to be located at its landward side. The groundwater table in the beach was manipulated by lowering and raising the lagoon level, and the gravel beach was subjected to variable wave conditions and sea/lagoon levels. Using as the initial beach profile the morphology resulting from 90min of wave forcing with the sea and lagoon at the same level, the identical wave forcing with a low lagoon level consistently resulted in onshore sediment transport in the swash zone and beach accretion, whereas the identical wave forcing with a high lagoon level consistently caused offshore sediment transport in the swash zone and beach erosion. The divergent beach morphological responses for the low and high lagoon levels could not be attributed to changes in the ‘bulk’ swash hydrodynamics (i.e., based on single-point velocity measurements) arising from a difference in the amount of swash infiltration into the unsaturated beach. Instead, application of a sediment transport model that accounts for the effects of in/exfiltration suggested that the onshore sediment transport and beach accretion for the low lagoon tests mainly resulted from enhanced bed shear stresses during the wave uprush due to infiltration into the saturated part of the beachface. Due to the large hydraulic conductivity of gravel (K=O(0.1ms−1)), the effect of altered shear stresses due to in/exfiltration may be an important factor in sediment transport in the swash zone of gravel beaches.