Longshore drift
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Longshore drift, sometimes known as drifting, longshore current, LSD (not common as it is more typically used in reference to a drug) or littoral drift is a process by which sediments such as sand or other materials, move along a beach shore. It uses the process of swash to push the material up the beach and backwash down the beach; until it reaches a groyne or another obstacle.
Where waves approach the coastline at an angle, when they break their swash pushes beach material up the beach at the same angle. The backwash then drags the material down the beach perpendicular (at a 90° angle) to the shore, following the line of the steepest gradient. This produces a zig-zag movement of sediment along the beach known as longshore drift. Largest beach sediment is found updrift, and the smallest material, which is more easily moved, downdrift. Groynes can be used for defending a beach against long shore drift.
Longshore drift causes environment problems as it can decimate beaches by literally washing them away
The effect of longshore drift is determined by many factors such as the direction and fetch of the prevailing wind. Erosion on the beach is constant and it works with longshore drift to straighten the overall shape of the beach for example : making it mould itself to the action of the waves so that any particles of sand that are not deposited parallel to the wave.[1]
There are many ways that longshore drift is prevented, such as groynes. Basically the idea of a groyne is to prevent longshore drift. Groynes are very useful and are the cheapest and most effective way of protecting a cliff and the beach behind it. The purpose of a groyne is to create and preserve a wide beach on its updrift side (the side in which the prevailing wind occurs). Groynes act as a barrier to physically stop sediment transport (sand + sediment) in the direction of longshore transport through the system. This causes a build-up of the beach on the groyne's updrift side.
Another way to prevent longshore drift is beachfeeding; this is when lorries/diggers transport the sand to the other side to even things out a bit. This is however an expensive method.
[edit] Notes and references
[edit] External links
- Photos, animation and explanation for schools
- Intranet.lissjunior.hants.sch.uk has a brief animation on longshore drift.
- USGS — Coastal Erosion on Cape Cod
- Shore drift
- Longshore drift in South Carolina
