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INTRODUCTION - References Map Christchurch Bay comprises a shallow embayment (average depth of -7 m CD) bounded by Hengistbury Head (Photo 1) to the west and Hurst Spit (Photo 2) to the east.
It therefore results from a complex interaction between sea level rise, sediment supply, storm overtopping events and the substratum (ie its capacity to support and contribute to the spit).
Longitudinal extension has probably been controlled by offshore water depths and rates of sediment loss.
Some sediment may be transported from Hurst Spit to the West Solent (Dyer 1971), but the major sediment flux appears to be south-west from Hurst Spit to feed the Shingles and Dolphin Banks and Dolphin Sand via the Needles Channel (Dyer 1970, Nicholls 1985, Velegrakis, 1994).
Recent studies of the rates of Chalk erosion; remnant buried and infilled palaeo- channels located in a west to east sequence offshore; the chronology of the lower terrace sequence of the Stour and Avon; early human occupation sites around Christchurch Harbour, and the depth, relief and inclination of the planation surface which truncates the Purbeck-Wight Chalk ridge indicate an early to mid-Devensian breach (West 1980, Wright 1982, Nicholls 1985, Nicholls 1987, Allen and Gibbard, 1993, Brampton et al 1998, Velegrakis et al 1999, Maddy, et al 2000).
An extension of the River Frome may have cut a gap through the western part of the ridge by the mid-Devensian, but later (Holocene) denudation of this feature appears to have utilised cols created by the headward erosion of rivers originally flowing southwards from its crest.
The thickness of some of these accumulations has recently been determined by Velegrakis (1994) enabling approximate volumetric calculations.
It is concluded that Christchurch Bay has been a virtually closed bedload transport system since the late Holocene except for probable (but diminishing) littoral drift input from Poole Bay and relatively small losses to both the outer bay and the West Solent (Nicholls 1985).
This might define the position of the ancestral coastline in mid-Devensian times, when sea-level was between -40 and -60 m OD, prior to the final breakthrough of the chalk ridge.