Beach morphology and sediments of the Canterbury Bight.
Degree GrantorUniversity of Canterbury
Degree NameMaster of Arts
The 84 miles of mixed sand and shingle beach between Banks Peninsula and Dashing Rocks, Timaru, is a high energy shoreline exposed to vigorous wave action emanating from the south Pacific Ocean. Much of the coastline is actively retrograding. However, short term and seasonal variations in beach profiles are small. This is unusual in relation to previous studies of shingle beaches. Analysis of wave processes and of beach slopes and materials indicates that beach morphology is in short term erosional equilibrium with the prevailing south-easterly swell and with southerly storm waves. Long term changes in beach profiles indicate that, over much of the Canterbury Bight, the narrow profile envelopes are retreating landward. There is an excess of wave energy over the supply of materials and so the profiles are becoming wider and flatter. This condition is termed sub-equilibrium. In plan a similar situation is distinguished, the beach being most stable in the north. Over the last century erosion has been most vigorous in the central section and slower in the south near Timaru. Thus, near equilibrium conditions exist in the north. By comparison with the theoretical stable shape the shoreline curve is too flat in the central area where present erosion is most vigorous. These results are not consistent with a high order of net longshore transport to the north under present conditions. Previous works have suggested that this has occurred in the past but the maximum appears to have been reached and passed. Sediment appears to be moved offshore rather than transported along it in large amounts of angular sand to the littoral zone but, surprisingly, there is little indication of a significant supply of pebbles and larger sizes under present conditions. Because of the intensity of coastal erosion the beach deposit reflects the alluvial origins of much of the material. The deposit is texturally sub-rnature. A medium and coarse sand fraction and a pebble fraction are combined by surf action to produce characteristic size-sorting relationships. Erosion of the coast allows little time for the production of changes in grain shape and roundness, so that there are only small differences in these properties between samples taken from the beach and from the coastal cliffs and the present river channels. The shapes of beach pebbles reflect the breakdown of the parent greywackes and sorting for shape and roundness are poorly developed on the beach. There is little abrasion of sand indicated. A case study of the mixed sand-shingle beach is made using accepted principles of beach study drawn from the literature on both sand and shingle beaches. The study beach has many of the morphological features of the shingle beach but few of the sand beach. This is partly due to the larger grain size of the beach and to the prevailing plunging surf. The sand-shingle profile is almost entirely swash dominated since there is little tidal translation of the breakpoint of the waves. Characteristic sorting processes include the movement of sands in different directions under differing wave conditions. Sand is moved onshore under storm conditions and is winnowed from the gravels and moved alongshore under swell conditions. Pebbles appear to undergo a net offshore motion during storms but are more stable during swell conditions when they characteristically adopt pronounced imbrications. Cobbles are moved to the higher berms by swash since backwash does not usually have the power to move them owing to loss of head by percolation into the beach. These processes result in size-frequency distributions which are characteristically positively skewed-1eptokurtic; reflecting a dominance of coarse bed-load material with a significant proportion of infiltrated fines. Analysis of offshore bottom transport potentials confirms the observed mobility of sands and demonstrates that pebbles moved seaward of the breaker zone are unlikely to be returned to the foreshore. It also indicates that there may be small net movements of fine sands into the area from south of Timaru, and out of the area toward Banks Peninsula in the north.