Beach sediment mixing under drained and undrained conditions

The paper describes a set of experiments undertaken at the Grosser Wellen Kanal (GWK) in Hannover on sediment mixing on a beachface exposed to the action of a buried drainage system. Experiments were performed using a set of drains (up to three working simultaneously), located at variable distance from the shoreline. Wave energy during the tests ranged from medium (0.59 m < Hs < 0.61 m) to high (0.76 m < Hs < 0.83 m). Beach profiles were measured at the end of each test. Further validation came from measurements using a grid of rods that were monitored at the same time as the beach profiles were surveyed. While some beach stabilization was observed for low energy conditions, for medium and high energy conditions the benefit of having the drains operative was not so clear. The comparison between beach profiler and rods showed good agreement regarding relative vertical accretion and erosion events. In any case, it was evident that the sedimentary effect on the beachface was confined to a narrow area above the drains. The drained area showed slightly lower values of sediment mixing under drained conditions, but evidently this factor was not important enough to trigger a widespread beach stabilization. Values of sediment mixing averaged across the whole beachface ranged between 2-4 % and 4-6 % of the breaking wave height respectively for high and medium energy wave conditions. Grain size, beach slope and drain configurations used here are fully comparable to field installations present in the literature