The cyclic development of a macrotidal mudflat on varying timescales

Abstract

The development of a mudflat at Peterstone Wentlooge on the north shore of the Severn Estuary, UK, was observed over a two year period on a multi-timescale basis. The mudflat is located in an area with a macrotidal regime, strong tidal currents, exposure to short period wind waves and high turbidity. The mudflat consists of two Holocene sediment layers: relict clay extending over the entire width of the intertidal, overlaid by modern sediment located between the salt marsh edge and mean sea level. A combination of episodic and periodic mudflat development was observed on decadal, seasonal, spring-neap and tidal timescales. There was a clear seasonal development of the modern layer in terms of bed level, morphology and composition, with accretion during spring and summer and with erosion during autumn and winter. This periodic development was superimposed on a longer-term trend of continual erosion of the relict clay when its surface was exposed. The mean seasonal variation in bed level of the modern layer was in the order of 10 cm and was due to the presence or absence of short period waves. On the spring–neap and tidal timescales the development of the mudflat, due to tidally driven processes, was frequently modulated by episodic wave events. Variations of bed level during tidal inundation were observed autonomously and compared well with manual observations taken during exposure on a daily basis. Bed level variations of 1 cm were observed during single periods of tidal inundation and daily through measurements taken during mudflat exposure.