Deposition (geology)

Wind, ice, water, and gravity transport previously weathered surface material, which, at the loss of enough kinetic energy in the fluid, is deposited, building up layers of sediment.

[4] If these fine particles remain dispersed in the water column, Stokes law applies to the settling velocity of the individual grains,[4] although due to seawater being a strong electrolyte bonding agent, flocculation occurs where individual particles create an electrical bond adhering each other together to form flocs.

This resulted in the fining of sediment textures with increasing depth and towards the central axis of the harbour, or if classified into grain class sizes, "the plotted transect for the central axis goes from silty sands in the intertidal zone to sandy silts in the inner nearshore, to silts in the outer reaches of the bays to mud at depths of 6 m or more".

Kirby R. (2002)[9] takes this concept further explaining that the fines are suspended and reworked aerially offshore leaving behind lag deposits of the main bivalve and gastropod shells separated out from the finer substrate beneath, waves and currents then heap these deposits to form chenier ridges throughout the tidal zone, which tend to be forced up the foreshore profile but also along the foreshore.

[3] The interaction of variables and processes over time within the environmental context causes issues; "a large number of variables, the complexity of the processes, and the difficulty in observation, all place serious obstacles in the way of systematisation, therefore in certain narrow fields the basic physical theory may be sound and reliable but the gaps are large"[10] Geomorphologists, engineers, governments and planners should be aware of the processes and outcomes involved with the null point hypothesis when performing tasks such as beach nourishment, issuing building consents or building coastal defence structures.

This is because sediment grain size analysis throughout a profile allows inference into the erosion or accretion rates possible if shore dynamics are modified.

Planners and managers should also be aware that the coastal environment is dynamic and contextual science should be evaluated before the implementation of any shore profile modification.

Thus theoretical studies, laboratory experiments, numerical and hydraulic modelling seek to answer questions pertaining to littoral drift and sediment deposition, the results should not be viewed in isolation and a substantial body of purely qualitative observational data should supplement any planning or management decision.

Map of Cape Cod showing shores undergoing erosion (cliffed sections) and shores characterized by marine deposition (barriers).
Map of Cape Cod showing shores undergoing erosion (cliffed sections) in yellow, and shores characterized by marine deposition (barriers) in blue. [ 1 ]
Figure 1. Illustrating the sediment size distribution over a shoreline profile, where finer sediments are transported away from high energy environments and settle out of suspension, or deposit in calmer environments. Coarse sediments are maintained in the upper shoreline profile and are sorted by the wave-generated hydraulic regime
Figure 2. Map of Akaroa Harbour showing a fining of sediments with increased bathymetry toward the central axis of the harbour. Taken from Hart et al. (2009) and the University of Canterbury under the contract of Environment Canterbury. [ 5 ]