hard engineering Flashcards
how a groyne works
-wooden walls on beach at right angles to coast to slow down LSD
-make beach wider and higher so waves can expend energy on it rather than backshore
advantages of groyne
-maintains beach size
-enhance beach for recreational purposes
-less expensive than sea walls
disadvantage of groynes
-expensive (5-10,000 per m)
-obstacle to moving freely down a beach
-not natural so seem unsightly
-greatly reduce LSD and cause sediment starvation
how a sea wall works
-recurved sea walls use shape of wave to direct water into following wave to reduce wave energy
-stepped seawalls use point edges to break up a wave as it hits the wall
-often combined w rip-rap or steps at base to prevent undercutting
advantage of sea wall
-long lasting concrete
-give ppl confidence and security
-tops can be used as promenades
-prevent high water levels from moving inland
disadvantage of sea wall
-most expensive defence (5000 per m)
-impact beach access
-seem unsightly
-scouring by waves makes it necessary to have deep pilings at base
-reduce supply of sediments by redirecting wave energy out to sea
how riprap works
-usually granite or specially designed concrete shapes
-large surface area break up waves so dissipate wave energy
-hold back mass movement on unstable cliff
advantage of riprap
-long lasting and flexible in use
-can be used as breakwater for groynes or protect base of sea wall/cliff face
disadvantage of riprap
-expensive (100,000 per 100m)
-unsightly due to contrast w local geology
-create access and be dangerous to climb on
-seawater can move through so some erosion may take place on back shore
how do gabions work
-pebbles in wire baskets
-when tied together can make wall where great surface area absorbs wave energy
advantage of gabion
-flexible in terms of placement
-cheap and easy to install
disadvantage of gabion
-not the strongest, limited use on high energy coastlines
-need frequent repairs
-may be unsightly
what are revetements
-sloped walls often made of wood
-placed parallel to the back shore but a short distance in front of it
-take the force of a breaking wave
advantage of revetments
-absorb wave energy
-trap sediment behind reducing removal due to erosion or LSD
-LSD can still continue
-cheaper than sea walls
disadvantage of revetments
-can cost up to 4500 a metre
-reduce access to sea and limited beach width
-unsightly
-constant maintenance
what are offshore breakwaters
-rock boulders dropped and aligned in short lengths in shallow water parallel to the shore
-dissipate wave energy before they have chance to damage backshore
advantage of offshore breakwaters
-effective at vulnerable sections of coastline
-can protect entrance to harbour (like skerries bank)
disadvantage of offshore breakwaters
-costly, between 1 and 2 million
-may need other engineering to complement them e.g. seawalls in between gaps
-create deposition on landward side, reducing LSD
-impact other section of coast if material dredged from there (e.g. start bay)
