Hurst Spit Flashcards
What are spits?
Spits are narrow ridges of sand and/or shingle attached to the shore at one end and terminating in open water at the other (“beach extension”).
They are associated with coastal orientation changes and occur in areas with a low tidal range (less than 3 metres).
Broadly, what two types of spit can be recognised?
Parallel – those aligned to the shore.
Divergent – those that change angle to the shore.
How long does it take for spits to develop?
It takes hundreds of years for spits to develop
Different spit material is produced by different waves
Sandy Spits are formed by constructive waves.
Shingle Spits are formed by destructive waves.
Composite Spits are shingle deposited before the finer sand (made of both constructive and destructive waves).
Hurst Castle spit stretches for how long?
he spit extends some 4 kilometres across the Solent and provides a sheltered environment in which the mud flats and marshes have formed.
What material is hurst spit made out of?
It is a shingle spit comprising of mainly large pebbles.
The main process, which causes spits, is what?
Long-shore drift. This is the movement of material along the coast parallel to the shoreline.
Its occurrence depends upon the oblique approach of a wave to the shoreline, for the wave may carry material up a beach at an angle approximately perpendicular to the wave crest, but gravity will cause the material and the backwash to take the steepest gradient seawards which in an oblique wave will be a different course from that taken by the swash.
The drift departs from the beach and drops its sediment material onto the shallow waterbed; this gradually builds up the sediment gradually forming the eventual spit. This develops further at the high tide where the inner stages of the spit can be developed by further sediment deposits.
What demonstrates longshore drift the best?
Best demonstrated where accumulation has taken place against groynes, or any other man-made obstacle that juts out into the sea.
Hooked or Re-curved Spits
As spits build out into deep water they require increasing volumes of sediment to build above the high mark. The tip or distal turns towards the land where it’s shallower. If it built out in deep water it would be eroded. Once formed hooks are sheltered from the dominant waves by the spit and become permanent curves also formed by the second dominant wind.
Where did Hurst’s spit originally come from?
Hurst Castle used to be replenished by beach material from Christchurch Bay to the west, together with some eroded sediment from the local cliffs at Milford-on-Sea. This was reduced as a result of extensive defence works involving rock armouring and land drainage at Barton-on-Sea, the construction of a sea wall at Milford-on-Sea and groynes which prevented long shore drift.
What did Hurst’s spit reduced sediment supply mean?
With its supply of sediment reduced, Hurst Castle spit began to suffer erosion and it became much more vulnerable to high seas during storms.
It is estimated that some 30,000 tonnes of sand and shingle were being lost from the spit each year. In the 1960’s the spit was breached several times, causing flooding in the village of Keyhaven.
After research had been completed and a cost-benefit analysis conducted, a £5 million stabilisation project was undertaken in 1996. The scheme involved a number of features including…
Rock armouring, revetments and the construction of a rock breakwater.
Since this stabilisation project has been completed, he spit has not suffered any breaches.
Despite the engineering works, the feature maintains many of its ‘natural’ characteristics.
Reasons why the spit is important:
Flood protection - the spit helps to protect Keyhaven and Lymington from the full force of the sea.
Conservation - the spit provides a protection against a much-valued salt marsh and wetland habitat. Breaching of the spit would destroy this habitat.
Recreation - the spit is used for walking and fishing, and the sheltered creeks are used for sailing and windsurfing.
Commercial fishing - a small fishing fleet is harboured at Keyhaven. If the spit were breached, the harbour would no longer be such a safe mooring.
How do salt marshes grow?
Above ground, mineral sediment settles out of the water column and onto coastal wetland soils during periods of tidal flooding
Plant shoots influence mineral sediment deposition by slowing water velocities, and add organic matter to the soil surface deposition rates highest in low-elevation marshes (inundated for long periods of time) lowest in high-elevation marshes (rarely flooded). More plants -> slower -> more deposition.
FEEDBACK: Supply of nutrients = plant growth…
Below ground, the balance of plant root growth and decay directly adds organic matter to the soil profile, raising elevation by sub-surface expansion
Houser (2009) Summary of beach-dune interactions
Most barrier islands and spits are in a state of transgression (land- ward retreat) in response to a combination of eustatic sea-level rise and local land subsidence.
