Introduction — Deposition (geology)
Deposition is the process by which loose sediment, soil and rock fragments are laid down and accumulated to form layered deposits that alter surface topography and produce landforms such as beaches, barriers, deltas and floodplains. Material reaches depositional sites after being mobilized by agents including running water, waves, wind, ice and gravity; deposition occurs when those transport mechanisms lose the capacity to carry particles, commonly because the transporting medium loses kinetic energy and particles settle out.
The null-point hypothesis formalizes the physical threshold for this transition from transport to deposition. According to this concept, sediment will be deposited when the forces driving motion—such as fluid shear stress and lift—fall below the resisting forces of gravity and interparticle friction, so that net movement ceases and accumulation begins. This balance between driving and resisting forces also helps explain spatial contrasts in coastal behaviour: local variations in energy regime and sediment supply determine whether a shoreline erodes or accretes, as exemplified by Cape Cod where adjacent stretches show both cliff erosion and barrier-building deposition.
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Deposition encompasses more than mechanically transported clastic material. Biological and chemical processes commonly generate stratified accumulations: the settling of biogenic skeletal remains or the precipitation of minerals can produce substantial sedimentary deposits that later undergo diagenesis and lithification. Chalk illustrates biologically mediated chemical deposition—microscopic calcareous skeletons of plankton accumulate, alter the local chemistry during burial and precipitate additional calcium carbonate, ultimately forming rock. Organic-rich deposits such as coal begin with the accumulation and preservation of plant material in anoxic settings; limited decay under oxygen-poor conditions allows burial, compaction and progressive coalification over geologic time.
The null-point hypothesis describes how mixed sediments on a shore profile sort by grain size to reach positions where the net transport of a given size class is zero: coarser grains are retained toward the landward, high-energy parts of the profile while finer grains are exported seaward, producing systematic seaward fining. The concept—originally proposed by Cornaglia (1889) and elaborated in later quantitative work—identifies a distinct equilibrium depth or cross-shore position (the “null point”) for each grain size at which hydraulic forcing and downslope gravitational tendencies balance.
Sorting arises from the interaction of two principal processes. Downslope gravitational components and settling velocities drive particles toward lower elevations, but oscillatory wave asymmetries and tidal flows generate flow-dependent bed stresses and vortex dynamics that selectively suspend and advect particular size fractions. Fine particles, having low settling velocities, remain in suspension longer and are transported farther offshore or to deeper, lower-energy environments; coarse particles are constrained by bed friction, angle of repose and ripple-scale stability, which determine where they can resist downslope motion and remain on the shoreface.
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Ripple-scale hydrodynamics are central to the selective export of fines. Asymmetric wave-generated flows over ripples form lee-side eddies during onshore strokes that trap suspended material; when flow reverses these trapped vortices are released and the subsequent offshore stroke can carry a concentrated cloud of fines seaward. Where ripple morphology is symmetric or where wave orbital motion and tidal flow approach equilibrium, vortices form and dissipate more evenly and suspended material can be advected under tidal influence without a strong net onshore/offshore bias.
The resulting spatial pattern is evident in depth–grain-size relations: shallow, high-energy zones maintain and sort coarser sediments, whereas finer fractions predominate in calmer, deeper, or more seaward parts of the profile. Field studies have quantitatively confirmed null-point sorting in varied settings, including Akaroa Harbour (New Zealand), The Wash (U.K.), and Bohai Bay and West Huang Sera (China), demonstrating the theory’s predictive power across diverse coastal morphologies. Foundational theoretical and experimental treatments by Ippen and Eagleson (1955), Eagleson and Dean (1959, 1961), and Miller and Zeigler (1958, 1964) underpin the mechanistic formulations relating settling, bed friction and ripple-induced asymmetry to equilibrium positions.
Practically, the null-point framework links grain-size–dependent settling and bed mechanics with wave- and ripple-driven transport to predict deposition and erosion loci along shore profiles, thereby informing sediment budgets, habitat distributions and engineering responses on coasts exhibiting similar hydrodynamic and sedimentological conditions.
Deposition of non-cohesive sediments occurs when hydrodynamic forces fall below the threshold needed to keep grains entrained: coarse particles transported as bedload (rolling or sliding along the bed) settle when bed shear stress and near-bed turbulence diminish, while grains in the suspended load may travel substantial horizontal distances because each particle must first fall through the water column under gravity.
The terminal settling of an idealized spherical grain is set by a balance between its weight and the opposing buoyancy and hydrodynamic drag. In mathematical form,
(4/3)πR^3ρ_s g = (4/3)πR^3ρ g + (1/2)C_d ρ πR^2 w_s^2,
where R is the sphere radius (m), ρ_s and ρ are the solid and fluid densities (kg m^-3), g is gravitational acceleration (m s^-2), C_d is the dimensionless drag coefficient, and w_s is the terminal settling velocity (m s^-1). The drag term uses the projected area A = πR^2 and the dynamic-pressure factor (1/2)C_dρw_s^2.
Rearranging gives an explicit form for the inertial-dominated settling speed,
w_s = sqrt[(8/3) R (ρ_s − ρ) g / (C_d ρ)],
which highlights dependence on particle size, density contrast, gravity, fluid density and the drag coefficient. Because C_d itself depends on the particle Reynolds number Re (for a sphere, Re ≈ 2Rρw_s/μ, with μ the fluid dynamic viscosity in Pa·s), the drag coefficient and settling velocity are coupled; C_d must be evaluated as a function of Re and therefore cannot be prescribed independently.
In the viscous (low-Reynolds-number) limit the flow-induced drag is linear in velocity and the Stokes solution applies. Using F_d = 6πμR w_s and balancing viscous drag against the effective weight yields the Stokes terminal velocity
w_s = (2/9) (ρ_s − ρ) g R^2 / μ,
valid only when inertial effects are negligible (very small particles or high viscosity). In higher-Re regimes the drag scales approximately with w_s^2 and empirical C_d–Re relations (or iterative numerical solution) are required to obtain realistic w_s; misidentifying the flow regime leads to substantial errors in predicted settling speeds.
Practically, computing settling velocities therefore entails either iteration (solving the coupled Re–C_d–w_s problem) or use of empirical C_d–Re curves. Accurate regime identification is essential because the choice between viscous and inertial formulations controls predicted settling distances and hence the spatial patterns of sediment deposition.
Deposition of cohesive sediments
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Cohesive behavior becomes dominant for very fine sediments—chiefly silts and clays smaller than 4ϕ—because interparticle forces rather than inertial or gravitational effects control their transport and settling. When individual clay and silt grains remain dispersed in the water column they settle very slowly, with terminal velocities that approximate those predicted by Stokes’ law for viscous-dominated settling of small particles. However, the high ionic strength of seawater screens electrostatic repulsion between particles and promotes flocculation: dissolved ions facilitate electrical bonding so that particles adhere and form aggregates. At the mineral scale, clay platelets display complementary charges (a slightly negative face and a slightly positive edge), so face–edge electrostatic attraction drives the formation of flocs. These aggregates have a larger effective size and greater combined mass than their constituent grains, which increases their settling velocity and causes them to deposit more rapidly and in more shoreward (nearshore) positions than would occur if the fine particles remained dispersed.
The occurrence of null‑point theory: Akaroa Harbour case study
Akaroa Harbour (43.800°S, 172.933°E) occupies the flooded caldera of an extinct shield volcano on Banks Peninsula, New Zealand. Marine transgression into the volcanic depression has produced a sinuous inlet approximately 16 km long and 2 km wide on average, with bathymetry that deepens toward the central axis. Bathymetric surveys along the central transect record depths to about −13 m (relative to mean sea level) near the 9 km mark, and sediments characteristically transition to mud at depths of 6 m and greater.
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Sediment inputs are dominated by local volcanic basalt and windblown loess derived from glacial episodes. Over millennial timescales loess has progressively infilled volcanic fissures and augmented the harbour’s fine‑sediment budget. Hydrodynamically, the outer harbour is exposed to southerly storm wave energy with effectively unrestricted fetch, whereas the inner harbour is relatively sheltered; local sea breezes, however, generate surface currents and chop that influence nearshore transport and deposition.
Hart et al. (2009), under contract to Environment Canterbury, integrated detailed bathymetry with sieve and pipette analyses of subtidal samples to quantify links between physical setting and sediment texture. Their results identify three principal controls on sediment grain size: water depth, lateral distance from the shoreline, and longitudinal position along the central axis. A consistent fining trend is observed with increasing depth, greater distance from shore, and proximity to the axis, reflecting the combined effects of reduced near‑bed stress and channelized circulation.
Transects along the central axis reveal a systematic grain‑size succession driven by bathymetry and hydrodynamics: silty sands in the intertidal, sandy silts in the inner nearshore, silts in outer bay areas, and mud dominating where depths reach or exceed ~6 m. This vertical and longitudinal sorting is consistent with differential entrainment, transport and settling under spatially varying fluid forcing.
Comparative field evidence supports the applicability of null‑point theory to tidal and subtidal settings subject to differing energy regimes. Wang, Collins and Zhu (1988) showed that increasing fluid forcing corresponds with coarser bed material across low‑energy clayey tidal flats (Bohai Bay), intermediate‑energy silty flats (Jiangsu coast) and high‑energy sandy flats (The Wash), indicating that hydrodynamic intensity can impose predictable equilibrium grain‑size distributions on both erosional and accretional flats. Complementary process descriptions (Kirby 2002) emphasize that fine particles are preferentially suspended and exported offshore while coarser biogenic fragments form lag deposits; waves and currents then concentrate those lags into chenier ridges along and up the foreshore—a diagnostic signature of erosion‑dominated regimes.
Taken together, the morphological setting, sediment sources, measured texture gradients, and comparative studies indicate that Akaroa Harbour exhibits the spatial sorting and equilibrium partitioning of sediments anticipated by null‑point theory, with bathymetry and spatially varying hydrodynamics acting as the primary agents controlling subtidal grain‑size distributions.
Null-point theory proposes that hydrodynamic sorting concentrates particular grain sizes at discrete positions on shore profiles where transport forces and settling equilibrate. Although the mechanics are clear in narrowly controlled settings, the theory’s reliance on stable equilibrium states is contested because natural shorelines frequently exhibit dynamic or metastable behaviour that undermines the assumption of permanent balance.
Empirical testing has produced inconsistent results: many field studies and laboratory experiments do not find the idealised pattern of distinct null points for every grain size throughout a profile. These inconsistencies arise primarily from the interaction of multiple, time-varying controls—wave climate and storm variability, tidal regime, sediment supply and source characteristics, bedform evolution, and anthropogenic interventions—that modulate transport pathways and residence times and complicate pattern formation and observation.
For coastal planners, engineers and regulators these limitations carry practical consequences. Decisions about beach nourishment, development approvals and coastal defences depend on accurate forecasts of sediment redistribution; overreliance on simplified null-point expectations can produce erroneous predictions of shoreline response and maladaptive interventions. Sediment grain-size profiling, particularly continuous vertical sampling through a profile, remains a valuable diagnostic: distributions record past and recent hydrodynamic sorting and can constrain likely erosion or accretion under altered forcing. However, such data must be interpreted as a conditional indicator rather than a deterministic predictor.
Effective management therefore requires treating the coast as a dynamic, context-dependent system. Prior to modifying shore profiles, practitioners should assess local environmental variability and the limits of predictive models. Quantitative approaches—analytical theory, laboratory experiments, numerical and hydraulic modelling—are essential tools but should not be used in isolation; robust planning and management demand their corroboration and supplementation with qualitative observational evidence, including targeted field surveys, long-term monitoring and incorporation of informed local knowledge.