Helen Mitchener

Erosion of mud/sand mixtures

Abstract The prediction of sediment erosion is an important issue in coastal engineering projects. There are methods for predicting the erosion of cohesive sediment (mud) and non-cohesive sediment (sand), but there are presently no relationships for mixed sediments. However, natural sediments rarely consist of only mud or sand and the erosional properties of combined mud and sand sediments are required so that the whole spectrum of natural sediment size combinations can be modelled. This paper attempts to characterise the erosion behaviour of mixed sediments in a way that can be used for predictive models. In this paper mud (or fines) is defined as clays and silts, which pass through a sieve of size 62.5 μm, and sand is defined as the fraction retained. The collaboration between European researchers in the framework of the MAST G8M project has resulted in the accumulation of an extensive amount of data on the erosion of mud/sand mixtures. The data, which originate from both laboratory and field experiments, has been used to examine the physical processes behind the erosion behaviour of mud/sand mixtures. It was found that adding sand to mud, or vice versa, increases the erosion resistance and reduces the erosion rates when the critical shear stress for erosion is exceeded. The highest values for the erosion shear stress of homogeneously mixed beds occurs at a maximum in the region 30 to 50% sand by weight. The most significant effect on erosion resistance occurs on the addition small percentages of mud by weight to sand. The mode of erosion also changes from cohesionless to cohesive behaviour at low mud contents added to sand, with a transition occurring in the region 3% to 15% mud by weight. The erosional properties are also strongly dependent on the history of the bed and it is common that mud and sand segregate under typical deposition conditions owing to their different settling velocities in water which creates discrete layers. The erosion of these segregated beds should thus be modelled as a sequence of mud and sand erosion “events”.

The influence of bedforms on flow and sediment transport over intertidal mudflats

Abstract A range of bedforms of different types are found at a range of scales on intertidal mudflats. The different types include (a) channels, creeks and gullies, (b) ridge–runnel systems, (c) ripples and other micro-topography, and (d) cliffs. The main features of these bedform types are illustrated with examples found at a number of NW European mudflat sites studied during the EC INTRMUD project. Some comments are made on their environment of formation and observations of bed sediment properties and flow and suspended sediment processes are presented. Finally, the influence of bedforms on the hydrodynamics, shallow water tidal flow and wave propagation, and sediment processes are discussed.

Settling and consolidation of mud/sand mixtures

Abstract The formation of a cohesive sediment bed is a combination of settling and consolidation processes. These processes strongly influence the structure, properties and erodibility of the sediment bed. At low values of the bed shear stress, suspended sediments, individual particles or flocs, deposit onto the bed. Subsequently, during the consolidation process, the flocs and aggregates rearrange themselves to form a denser structure. The pore water, initially supporting the particles, is being expelled. In an extensive set of laboratory experiments the influence of sand on the settling and the consolidation of mud has been studied. The deposition of mud/sand mixtures has been closely followed in settling column experiments for different types of mud at different sand contents. During the experiments settling rates, density profiles and pore water pressures have been measured. The presence of a sand fraction in the initial suspension has a large impact on the bed formation processes. The results show that the heavier sand particles settle faster and form a separate layer at the bottom of the column as long as the mud does not form a continuous network structure that prevents this segregation. A continuous mud matrix is formed at low sediment supply rates or at high initial suspension concentrations, when the concentration of the mud fraction in the mixture exceeds the gel point density. The settling rates of the mud/sand suspension increase with increasing sand content. The local bed densities within the consolidating bed increase with increasing sand content of the initial suspension, even for the sand-free top layer. Sand addition also speeds up the consolidation process and permeability increases. However, these effects seem to be limited to a maximum sand content above which no additional effect is found. Using the experimental results, guidelines for modelling the settling and consolidation of mud/sand mixtures have been formulated.

Observations of the morphodynamic behaviour of an intertidal mudflat at different timescales

Abstract This paper concerns the morphological (bed level) behaviour of the intertidal mudflat at Portishead on the Severn Estuary and discusses how the prevailing hydraulic and cohesive sediment processes contribute to observed changes in the bed level. The characteristics of the mudflat behaviour during a fortnightly cycle is presented, based upon a continuous high resolution time series of the bed level measured at one location during 31 tidal immersions. During spring tides the sediment supply to the mudflat is increased and the tidal-mean bed elevation is at least 10 mm higher following the peak of spring tides than on the previous or following neaps. On a tide by tide basis the bed level varies by 10–20 mm and the preservation of fresh mud deposits on the intertidal area is controlled by the hydraulic conditions prevailing during the shallow water phase of the tide (less than 0.5 m depth) and the processes operating during the dry part of the tide. Hence, the bed level is controlled by the phasing of the tidal range (ambient sediment supply) with local water depth and wave activity. In comparison, on the annual timescale the bed level variation on the middle and upper mudflat is of the order of 100 mm and seasonal changes in storminess, subaerial environment and biostabilization appear to play a significant role.