Sedimentary processes over an intertidal flat: A field investigation at Hythe flats, Southampton Water (UK)

Abstract

The sedimentary processes acting over an intertidal flat (Hythe, UK) located in Southampton Water, Hampshire, UK are investigated on the basis of hydrodynamic, morphological and sediment transport field measurements. Bed level changes, patterns of suspended sediment transport and shell movement over the tidal flats were examined in order to identify sediment transport pathways and salt marsh–mudflat morphological changes. Overall, sediment transport patterns at Hythe can be described in terms of suspended sediment and bedload-shell transport. The hydrodynamic complexity of the tidal flow over the flats determines the patterns of suspended sediments transport. During the young flood phase, tidal currents present a SW orientation (shoreward). About 1 h into the flood, a change occurs and the currents start flowing in a SE direction (estuary mouthward). This veering of the tidal current controls the hydrodynamic pattern over the mudflat inducing sediment transport towards the estuary mouth. Conversely, shell tracer experiment showed a net landward movement. Shells are transported over the mudflat, deposited at the base of the cliff, and eventually transported over the salt marsh where they form extensive chernier deposits. Shell transport rates can reach up to 0.7 m/tide over the upper mudflat. Shell movement over the salt marsh (on cheniers) occurred only during storms (up to 3 m in a storm event). Shell-induced erosion can become an important parameter on morphological changes of a coastal area. Shell transport and deposition over the salt marsh surface can contribute to vegetation deterioration, as shell deposits migrate over the marsh surface. As a result, the seaward limit of the salt marsh showed deteriorated vegetation, which leads to scouring and consequent surface lowering. In terms of bed level changes, no seasonal pattern was observed. A relationship between mudflat bed erosion and cliff retreat is suggested. During periods of high cliff erosion there was a tendency for the mudflats to be stable and, conversely, mudflat bed erosion was greatest when cliff retreat rates were lowest. Hence, morphological characteristics of Hythe intertidal area appear to be controlled by a combination of prevailing hydrodynamic conditions and shell transport over the salt marsh.