2B.5 - Sediment transport and depositional landforms

Moving sediment along and across the shore

Sediment travels only when water or wind supplies enough energy to overcome the resistance of its particles.

When waves break obliquely to a beach, swash carries sediment up the shore at an angle. Gravity returns backwash approximately down the steepest beach slope. Repeated movement creates a net alongshore path called longshore drift. Breaking oblique waves also generate an alongshore current which transports suspended sediment; the visible zigzag model does not explain all movement below the water surface. If incoming wave direction changes, drift can weaken or reverse. Its net annual direction reflects the combined wave climate, rather than the direction seen on one visit.

Coarser particles can roll or slide along the bed by traction, smaller particles can hop by saltation, and fine material is carried in suspension. Dissolved material is transported in solution. Wave-driven currents move sediment across the shore between beach and bars. Flood and ebb tides exchange material through estuary mouths; strong channel currents can prevent a spit from closing an inlet. Currents driven by wind, tides and differences in water level also redistribute sediment offshore.

Deposition occurs when transport capacity falls below what is needed to carry the supplied sediment. Coarse particles generally settle before fine particles, though turbulence and grain shape affect sorting. An estuary may receive mud from rivers and marine sources, depositing it at slack water while stronger currents redistribute it later. Identify the transport route, energy change and available supply before explaining a landform.

Different arrangements of deposited sediment

Depositional landforms differ because shoreline geometry, wave direction, islands, currents and sediment supply create different places where transport slows.

Plant succession can stabilise landforms where sediment is exposed long enough for colonisation. Pioneer dune plants trap dry windblown sand on upper ridges; salt-marsh plants trap fine sediment in sheltered water behind spits and barriers. Roots and stems reduce mobility, but vegetation does not create the initial island shelter or reverse an unlimited sediment deficit.

A hooked spit is evidence of deposition and changing transport near its end. It is not proof that drift always has one direction, nor that it will inevitably seal an estuary. A tombolo must actually connect island and mainland; a nearby spit pointing towards an island is not enough.

Sediment cells, budgets and feedback

A sediment cell groups coastal sources, transfers and stores to explain changes that cannot be understood by looking at one beach alone.

Sources include eroding cliffs, rivers, offshore deposits and beach nourishment. Transfers include longshore drift, tidal transport and onshore/offshore exchange. Sinks or stores include beaches, dunes, bars, spits, marshes and deeper-water deposits. A store can become a source later, and grain sizes may follow different routes. Prominent headlands and changes in wave/current patterns help define cells and smaller sub-cells.

The Portland Bill to Selsey Bill sector of southern England is the specification's regional example. Portland and Selsey mark major changes in coastal orientation and sediment pathways. SCOPAC's published sediment-transport research describes several interacting local subsystems, with offshore banks and tidal currents complicating the Selsey boundary. Do not imagine one uniform conveyor carrying all sediment eastward across the whole sector: identify the relevant local pathway and grain size. This evidence explains system complexity; its historical measurements are not presented here as current annual rates.

An original illustrative beach receives 8,000 m³ from cliff erosion and 3,000 m³ from alongshore transport in one year. It loses 7,000 m³ alongshore and 6,000 m³ offshore. Its change is $11{,}000-13{,}000=-2{,}000\,\mathrm{m^3}$: a storage deficit. This does not prove every point on the beach lowers equally or that the exported material has left the larger regional cell.

Dynamic equilibrium means continuing movement and adjustment around a changing balance, rather than no movement. A storm can move sand into a nearshore bar, where breaking waves dissipate energy farther offshore and reduce additional upper-beach erosion. This negative feedback counteracts the disturbance, provided the bar remains in a protective position. Positive feedback amplifies change: dune vegetation loss exposes sand; increased wind erosion makes plant establishment harder, exposing more sand. Feedback describes the direction of the response, not whether people like it. A large disturbance or persistent sediment deficit may push the system beyond its previous equilibrium.

Testing differences between pebble samples

A statistical difference between samples helps evaluate a transport explanation, but the test alone does not identify the process that caused it.

Compare pebble long-axis diameter at independent sites along a drift-aligned beach, or in separate zones from foreshore to above the storm beach. Use the same measurement axis, selection rule and sample size, select without choosing conspicuous large pebbles, and record position and tidal state. Pebbles are a sample of sediment, not automatically a representative sample of every storm or every beach. Do not enter wave danger to obtain measurements.

A two-sample Student t-test compares means. The null hypothesis here is that the two sampled populations have equal mean diameter. The equal-variance form assumes independent observations, approximately normal diameter distributions and comparable population variances. Small skewed samples or measurements of the same pebbles twice need different treatment. Numeric diameter is suitable; named roundness categories are ordinal and should not be treated as equal-interval measurements without justification. A genuinely continuous shape measure, such as a consistently measured axis ratio, can be tested if its assumptions are met.

In an original illustrative dataset, six diameters at Site A are 30, 32, 34, 36, 38 and 40 mm; six at Site B are 18, 20, 22, 24, 26 and 28 mm. Means are 35 and 23 mm. Each sample variance is 14 mm², so pooled variance is also 14 mm². The denominator is $\sqrt{14(1/6+1/6)}\approx2.160\,\mathrm{mm}$; therefore $t=12/2.160\approx5.55$. Degrees of freedom are 10. Against a supplied two-tailed 5% critical value of 2.228, 5.55 is larger, so reject the equal-means null hypothesis under the assumptions.

The evidence supports a difference in mean diameter between these populations. Down-drift attrition is one possible explanation for smaller material, but hydraulic sorting, different sediment sources and storm deposition can also change size. Repeat sampling, roundness observations and source mapping are needed to distinguish explanations. Statistical significance is neither the probability that attrition is true nor proof of a causal mechanism.

The same method can compare a continuous shape measure across the littoral zone. Define an axis ratio as short-axis length divided by long-axis length, measured consistently: a lower value indicates a flatter or more elongated particle. An original illustrative independent sample of ten foreshore pebbles has mean ratio 0.65; ten pebbles above the storm beach have mean 0.50. If both approximately normal populations have comparable variance and the pooled sample variance is 0.0125, the denominator is $\sqrt{0.0125(1/10+1/10)}=0.05$, so $t=0.15/0.05=3.00$. With 18 degrees of freedom and a supplied two-tailed 5% critical value of 2.101, reject equal means. This supports a difference in this defined shape metric between zones; selective storm transport, source differences or abrasion may contribute. An axis ratio does not itself measure edge roundness, and a statistical test of these illustrative samples does not certify a learner's actual fieldwork.