Study of coastline recession down drift of Chewton Bunny, Christchurch Bay

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Description/Abstract

Throughout the study of coastline downdrift of Chewton Bunny two principle objectives are covered.

(i) The observation of how the Naish Farm site has developed historically, the rate of coast line recession and the change of geomorphology in the cliff degradation sequence.

(ii) The usability and accuracy of a Geographical Information System (GIS) to analyse the rate of coastal recession and identify geomorphological features.

To understand the development of the site the process of erosion downdrift of a strong point needs to be appreciated. In this circumstance the strong point acts as a littoral drift barrier, trapping beach sediment as it travels longshore. In addition to this, the barrier causes diffraction of the waves.

To achieve these objectives the study area was observed using two main sources of data, these being historical aerial photographs and maps. In conjunction with these a number of site visits allowed an up-to-date appraisal of the morphology.

The aerial photos were digitized and assigned OS coordinates through modem GIS techniques. This allowed accurate measurements of the coastline positions. Similar
techniques were applied to the historical maps. This produced a comprehensive database of coastline positions going back over 100 years. From this rates of recession could be determined.

The aerial photographs going back 35 years allowed a detailed geomorphological study to be carried out, observing the change in the bench widths and the cyclic occurrence of a number of chff features.

In addition to the GIS data site visits allowed the verification of a number of geomorphological features observed in aerial photographs.

The data resulting from the analysis conducted in the GIS has been compared against an engineering and meteorological timeline comprising all the major documented events occurring within the vicinity of Christchurch Bay, and more specifically the study area of Naish Farm.