ABSTRACT
Interest in sedimentary processes within coastal marshes has been stimulated by an increasing awareness of their delicate adjustment to tidal elevations, and of their obvious engineering significance in the context of coastal protection schemes. In tide-dominated marshes, typical of the UK, a large proportion of the tidal exchange of water and materials occurs via well-defined channel systems which frequently drain completely at low water. Flows within these are correspondingly complex. In particular, estimation of turbulence parameters known to be important in the entrainment and suspension of fine sediment (channel sand and externally derived silt) is rendered problematic by virtue of the need to eliminate severe nonstationarity due to flow unsteadiness over a range of non-turbulent scales. In addition to tidal variation, large velocity transients occur on both flood and ebb phases as the marsh surface undergoes inundation and drainage. Aftercareful attention to series de-trending, however, a large number of near-bed turbulence measurements have been obtained. These indicate significant variation in the intensity and ‘event structure’ of turbulence within and between individual tidal periods, although extreme short-term flow unsteadiness partly obscures longer-term pressure gradient effects. High rms turbulence intensities and streamwise shear stresses persist after marsh surface inundation; these may be significant in entraining sediment and in maintaining vertical concentration profiles, thus facilitating the introduction of silt-sized material to the adjacent marsh surfaces via lateral momentum exchange across channel margins. Neap tides, which do not inundate the marsh, exhibit lower turbulence intensities. These findings offer additional insight into channel - marsh interactions, and suggest new avenues of research that might lead to more sophisticated mathematical models of these environments, and to more accurate prediction of their response to natural and human-induced change.
