Wave processes and beach responses on a coarse gravel delta (1974)
AuthorsStephen, W. J. M.show all
The coastal environment around the Hapuku Delta is represented as a process-response model. Field data, collected over a period of one year, are expressed in the form of nine wave processes and six beach responses. These are described and inter-related statistically so that sediment gains and losses, foreshore texture, foreshore slope, shoreline position, and degree of cusp development may be predicted from given values of wave height, period, and direction. Additionally, the degree of interdependence among the variables is examined. Some modifications to standard field techniques are developed to overcome the problems presented by the coarse gravel foreshores, and considerable emphasis is laid on the need for excluding a priori assumptions from the functional analysis. Notable among a number of conclusions, is the relative unimportance, of wave steepness as a predictor of foreshore behaviour, and the strong association of well developed cusps with oblique waves on one of the beach sites. In general, the process inter-relationships are spatially and temporally less complicated than either those describing the responses, or those describing the process-response pairs. Seventeen predictor equations are significant at either the .05 or the .01 level for the processes, and fifteen for the responses. Fifty eight equations relating process to response are significant at the .01 level. The Hapuku River is the source of all beach sediment on the delta front. Silt is mainly transported offshore, sand and small pebbles move south onto the beaches, and cobbles and boulders move north. Many of the larger grains however, are permanently lost offshore. The shoreline is retreating near the river mouth, but towards the north it becomes increasingly more stable both because of the higher proportion of large boulders on the foreshore, and because of the presence of an offshore reef which saps the energy of approaching waves. In the extreme south the shoreline is advancing, but at the northern extremity of the area, pronounced shoreline retreat is taking place from the erosion of the glacio-fluvial deposits on the backshore.