Groundwater-level changes in a coastal dune, sea-level fluctuations and shoreline movement on a sandy beach

Abstract

It has been suggested earlier 1 1 Eliot and Clarke (1982). that groundwater conditions may contribute to low-frequency shoreline fluctuations with recurrence intervals of 0.5–10 years. That proposition has been examined by comparison of a 7-year record of groundwater levels from coastal dune lands with mean sea-level fluctuations and shoreline changes from the adjacent coast. In an original analysis of Western Australian records 2 2 Clarke and Eliot (1983a). , the annual cycles of sea level and shoreline change were observed to respond in an inverse manner, such that a 1.0 cm rise in sea level approximately corresponds with a 1.0 m decrease on each profile. In the present analysis, the ratio ranges from 110:1 to 125:1 for oscillations in the annual to triennial band. The ratio is less, approximately 80:1, for the 0.5-year cycle. These results are close to those reported from the original analysis. In other respects, however, the sea-level effect may be in dispute. There is better agreement between time series describing the groundwater and shoreline fluctuations than between sea level and beach change. The annual oscillation of the groundwater table is within 2° of being the inverse of that for the shoreline. In the annual cycle, sea level reaches its maximum in early winter, during May–June, whereas the groundwater peak is highest during September–October, at a time when the beach is in its most depleted state.