Onshore‐offshore patterns of sediment exchange in the littoral zone of a sandy beach

Abstract

Measurements of 18 closely spaced beach profiles spanning Warilla Beach, N.S.W., over a 5‐year period, have been examined for the onshore‐offshore sediment exchanges by empirical orthogonal function (EOF) analysis. Each profile was divided into slices of beach corresponding with berm, upper swash, upper and lower intertidal zones. The first EOF mode represents onshore‐offshore movement and contains at least 67.6% of the variance for each profile. The southern half of the beach is backed by a rockwall. Variances on this section range from 81.0% to 95.1%, whereas away from the wall the variances range from 67.6% to 88.4%. The second most important mode is a swash‐function mode representing sediment exchange between the lower‐intertidal zone and the upper beachface. The variance of this mode ranges from 3.9% to 23.2% for the 18 profiles. Other modes were identified, viz., the berm function mode (where sediment is exchanged between the berm and the rest of the beachface), the mean‐sea‐level mode (where exchange is through a pivotal point near mean‐sea‐level), and a secondary‐swash mode. Sediment exchanges occur generally at 24, 12 and 6 month periods. The EOF analysis allows re‐interpretation of a previous alongshore EOF analysis. A regrouping of the alongshore modes further identifies zones of shoreline stability and instability. In the upper swash and berm zones the dominant mode describes areas of instability at profiles 11–12 and 16. Other modes show instability along the rockwall at profile 6 and 9, together with instability at each end profile. The zones of instability are linked to persistent circulation cells in the inshore region.