Deposition (geology)

Large-grain sediments transported by either bedload or suspended load will come to rest when there is insufficient bed shear stress and fluid turbulence to keep the sediment moving;[4] with the suspended load this can be some distance as the particles need to fall through the water column. This is determined by the grain's downward acting weight force being matched by a combined buoyancy and fluid drag force[4] and can be expressed by:

${\displaystyle {\frac {4}{3}}\pi R^{3}\rho _{s}g={\frac {4}{3}}\pi R^{3}\rho g+{\frac {1}{2}}\mathbb {C} _{d}\rho \pi R^{2}w_{s}^{2}\,}$

Downward acting weight force = Upward-acting buoyancy force + Upward-acting fluid drag force[4]

where:

• π is the ratio of a circle's circumference to its diameter.
• R is the radius of the spherical object (in m),
• ρ is the mass density of the fluid (kg/m³),
• g is the gravitational acceleration (m/s²),
• C_d is the drag coefficient, and
• w_s is the particle's settling velocity (in m/s).

In order to calculate the drag coefficient, the grain's Reynolds number needs to be discovered, which is based on the type of fluid through which the sediment particle is flowing, laminar flow, turbulent flow or a hybrid of both. When the fluid becomes more viscous due to smaller grain sizes or larger settling velocities, the prediction is less straightforward and it is applicable to incorporate Stokes Law (also known as the frictional force, or drag force) of settling.[4]

The cohesion of sediment occurs with the small grain sizes associated with silts and clays, or particles smaller than 4ϕ on the phi scale.[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.[4] "The face of a clay platelet has a slight negative charge where the edge has a slight positive charge when two platelets come into close proximity with each other the face of one particle and the edge of the other are electrostatically attracted."[4] Flocs then have a higher combined mass which leads to quicker deposition through a higher fall velocity, and deposition in a more shoreward direction than they would have as the individual fine grains of clay or silt.

Akaroa Harbour is located on Banks Peninsula, Canterbury, New Zealand, 43°48′S 172°56′E. The formation of this harbour has occurred due to active erosional processes on an extinct shield volcano, whereby the sea has flooded the caldera, creating an inlet 16 km in length, with an average width of 2 km and a depth of −13 m relative to mean sea level at the 9 km point down the transect of the central axis.[5] The predominant storm wave energy has unlimited fetch for the outer harbour from a southerly direction, with a calmer environment within the inner harbour, though localised harbour breezes create surface currents and chop influencing the marine sedimentation processes.[6] Deposits of loess from subsequent glacial periods have in filled volcanic fissures over millennia,[7] resulting in volcanic basalt and loess as the main sediment types available for deposition in Akaroa Harbour

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]

Hart et al. (2009)[5] discovered through bathymetric survey, sieve and pipette analysis of subtidal sediments, that sediment textures were related to three main factors: depth, distance from shoreline, and distance along the central axis of the harbour. 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".[5] See figure 2 for detail.

Other studies have shown this process of the winnowing of sediment grain size from the effect of hydrodynamic forcing; Wang, Collins and Zhu (1988)[8] qualitatively correlated increasing intensity of fluid forcing with increasing grain size. "This correlation was demonstrated at the low energy clayey tidal flats of Bohai Bay (China), the moderate environment of the Jiangsu coast (China) where the bottom material is silty, and the sandy flats of the high energy coast of The Wash (U.K.)." This research shows conclusive evidence for the null point theory existing on tidal flats with differing hydrodynamic energy levels and also on flats that are both erosional and accretional.

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. Cheniers can be found at any level on the foreshore and predominantly characterise an erosion-dominated regime.[9]

The null point theory has been controversial in its acceptance into mainstream coastal science as the theory operates in dynamic equilibrium or unstable equilibrium, and many fields and laboratory observations have failed to replicate the state of a null point at each grain size throughout the profile.[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.[2]