Flashcards in Coastal Protection Deck (27):
Describe the usage of sea walls as a method of sea defence.
Seawalls protect the base of the cliff from the action of the sea and are intended to reflect and/or refract wave energy. Ideally seawalls should be placed behind a large beach, with the sea only reaching the wall during storm conditions. Therefore seawalls are the final defence from flooding and erosion. However, in some conditions seawalls can be subjected to wave action more regularly and when in these conditions, they require more frequent repairs.
Describe the usage of Groynes as a method of sea defence.
Groynes are designed to create and maintain a healthy beach which protects the land behind. They act as a barrier to physically stop longshore sediment transport, which causes a build-up of beach on one side of the groyne. A series of groynes (groyne field) interrupts the tidal flow forcing the tidal current further offshore beyond the groyne end. This causes a slower inshore tidal current which encourages the deposition of sediments on the beach.
Describe the usage of Revetments as a method of sea defence.
Revetments are sloping timber structures
which run along the top of the beach,
parallel to the cliff. They are designed to
absorb the wave’s energy and reduce
erosional impact on the cliff. Revetments
are also less detrimental to beach levels
than seawalls, for they reduce beach scour
by dissipating wave energy rather than
reflecting it. Often sediments – including
sand, shingle and pebbles – are carried
through or over the structure and, due to the
less turbulent water behind, are deposited. This creates a buildup of beach at the base of the cliff helping to protect further against erosion.
Describe the usage of Gabions as a method of sea defence.
Gabions are large metal wire cages filled with rock and are usually used above seawalls. They act as an additional barrier to the sea when on occasion it overtops seawalls. Gabions lessen the damaging impact of a wave by intercepting it and reducing the energy of the wave before it reaches the cliff toe. In addition gabions strengthen and add weight to the toe of the cliff in order to prevent landslides. In normal conditions gabions perform well, however in storm conditions when they are subject to direct wave action they may easily be damaged.
Describe the usage of reefs as a method of sea defence.
Sea Palling is the only location on the North
Norfolk coastline that has offshore reefs. The
purpose of a reef is to prevent storm waves
from reaching the beach and to alter natural
coastal processes so that a wider beach
is able to develop. Reefs at Sea Palling are
supplemented with beach renourishment in
order to manage wider coastal impacts.
Describe the usage of rock armour as a method of sea defence.
Rock armour dissipates wave energy and can be used on beaches and in front of seawalls. The rocks are specifically placed with spaces amongst them. The spaces allow the water to flow around the rocks causing the wave to release energy. Subsequently, by the time the wave reaches the cliff toe, much of the wave energy has already been released and the wave has a greatly reduced impact on the cliff face.
Describe the usage of beach nourishment as a method of sea defence.
If beach levels become low, it may be beneficial to undertake beach renourishment. This involves adding large quantities of sand/shingle to a location in order to create wider, more gradually sloped beaches. This reduces the destructive impact of waves as long sloping beaches cause the waves to break earlier and lose much of their energy before they reach sea defences and cliffs.
Describe why beach nourishment needs maintaining.
Once the material has been added to the beach, it will be subject to natural sediment transport and therefore it may be necessary to carry out further renourishment over time in order to maintain the beach at the desired level.
Name a place sea walls are used and state the cost of sea walls.
Concrete seawall at Walcott.
Cost: minimum £4000 per metre.
Name a place groynes are used and state the cost of groynes.
Groynes at Cromer.
£100,000 - 150,000 .
Name a place revetments are used and state the cost of revetments.
Revetments at Overstrand,
protecting the cliff behind.
Cost: minimum £2500 per metre.
Name a place gabions are used and the state cost of gabions.
Gabions at Overstrand improving cliff stability.
Cost: minimum £700 per metre.
Name a place reefs are used.
Offshore reef at Sea Palling.
Name a place rock armour is used and state the cost of rock armour.
Rock armour at Happisburgh absorbs
wave energy and protects the cliff toe.
Cost: minimum £750 - £1600 per metre.
Name a place Beach Renourishment are used and the cost of Beach Renourishment.
Beach Renourishment in action at Clacton.
Cost: minimum £6500 per metre of beach.
What are the benefits of Sea walls?
• Effective long term protection
• Can incorporate amenities and promenades
• Many different types suitable for different locations
• Withstands extreme weather more effectively than other
sea defence types
What are the considerations involved with sea walls?
• Prevents erosion and release of
sediment from cliffs and thus reduces
• Repairs and maintenance required
• Supplem entary defences often
• Not always aesthetically pleasing
What are the benefits of groynes?
• Can be effective in building natural beaches
• Long term protection
• Can be constructed in a range of materials and designs
• Relatively quick to build
• Can be altered or contain features (i.e. groyne gates) to
control sediment movement along foreshore
What are the considerations involved with groynes?
• Need to be deployed in groups
otherwise only a limited beach is
likely to only form
• Some designs can trap too much
sediment and can increase down-drift
• Disturbs natural movement of
• Can require frequent maintenance
What are the benefits of revetments?
• Cheaper alternative to sea wall
• Minimal wave energy reflected so results in very little
beach scour, if any.
• Trapped sediment provides further protection to the cliff.
What are the considerations involved with revetments?
• Dependent on beach levels to keep
structure from being exposed.
• Sediment can be trapped behind
the revetment removing it from the
• Potentially high maintenance costs
What are the benefits of rock armour?
• Effective at dissipating wave energy
• Easy to re-place if moved in storms
• More resistant than local sediment/materials
• Long lasting and requires little maintenance
• Can be re-deployed if priorities change
• Quick to install compared to other sea defences
What are the considerations involved with rock armour?
• Expensive to install
• Aesthetics – does not fit with North
• Rocks can be moved in heavy storms
• Can create hazard to beach users
What are the benefits of gabions?
• Cheaper than many sea defence types
What are the considerations involved with gabions?
• Difficult to remove
• Do not effectively withstand strong
weather conditions and storm events
• Expensive to maintain
• More effective when used for cliff
stability than for sea defense
• Most effective when used in
conjunction with other defences
What are the benefits of beach nourishment?
• A more natural approach to coastal protection
• Aesthetically pleasing
• Uses natural processes
• Reduces erosion and pressure on defences
• Can improve sediment transport and benefit down-drift
• Possible benefits to tourism and provision of amenities