Depositional Coastal landforms

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Depositional Coastal landforms

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Spits

Spurn Head

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Spits – Spurn Head & Orford Ness

Spurn Head Drift aligned Aided by rapid input from glacial material Little or no refraction from F.H. Proximal retreat in balance with the

Holderness coast Humber channel “forced” south

Orford Ness• Single spit• 15km long - vegetated• Diversion of R. Alde• Dynamic growth:

• 64m p.a. 1962 to 1967• 183m p.a. 1804 to 1812 • 2.9km 1812 to 1821

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Hooked/recurved/single spits

Distal needs ↑ sediment as it reaches across deeper water

Multiple hooks

Reworking

Sheltered sedimentation zone

Hurst Castle

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Double spits

Poole HarbourDefinitions Front

Double spits

Christchurch Harbour

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Poole Harbour

1785 channel outflow through “Little Sea”

By 1875 entrance migrated Managed to maintain channel

Migration due to off shore bar inshore migrating

Christchurch Harbour

1880 spit extenison from south to north almost to Highcliffe Castle

1886 & 1935 spit breach by easterly storm

Theory?

Northern spit is remnant of bar?

PH = swash aligned, CH = drift aligned

Double spits

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Barrier beaches

Slapton sands

Created during the Flandrian Transgression (10k-5k BP)

Relic feature – positive eustatic change

Ley fed by three streams, shallow lake –usually higher than sea level

Bar beach easily breached – 2001 – normal conditions; 4m tidal range, 0.5m wave height (storm up to to 2m)

Future impact of eustatic change?

Slapton Ley

Slapton Ley

Composition – flint pebbles – provenance 30/40km offshore

@10k BP offshore barrier moved onshore by “roll over” process

@5 k BP Intermediate step of forming barrier islands

@3k BP emergent coastline created headland divisions of Start Bay

Dynamic equilibrium –

local sea level rise = 2-4mm p.a.

evidence from current storm roll over = 0.5m p.a. inward migration

Slapton Ley

LSD north and south in equilibrium over centuries if not smaller scalesShort term storm:Roll over crest & remove forebeachForebeach rebuilt over time.↑ storms → reduced barrier

Swash aligned beaches;·Smoothly curving concave beaches.· Beach face is orientated parallel to the fronts of the dominant waves.· Beaches which face the waves

are termed swash aligned.

Definitions

Drift aligned beaches;Some beaches show obliquealignments to the dominant wavefronts.Usually occurs where the beachgradient is steep and the wavelengthis short.This is because the crest breaks atdifferent times along the beach.

Spits;e.g. Spurn Head, Holderness, Yorkshire.Dominant waves push material along the coast in longshore drift. Coast turns inwards, and the material continues to be deposited in line with the coastal trend. Finer materials are deposited in the sheltered side of the spit. Marshes can build up, and this forces the river to the other side of the estuary.

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Spits II;As spits build further in to deeper water, they require more sediment to build above the HWM. The waves have greater energy to attack the distal end, turning it inwards. Sometimes, the curves are due to the changing of the predominant wind direction to another, causing waves, and thus deposition to occur an a different angle. These recurvesare sheltered by the spit and become prominent features, e.g. Hurst Castle Spit, The Solent.

Barrier beachesBeach which stretches across from one embayment to the other, and encloses a lagoon behind it, e.g. Slapton sands, Cornwall. Feature is swash aligned, and has probably been driven landwards from the offshore zone.