|Geomorphology and hydrology||
Maintained by UNEP
Breakwaters, piers and jetties are common engineering solutions for rectifying navigation problems in coastal areas also often providing easy access to the mainland for tourists and protected beaches for their enjoyment. A common characteristic of breakwaters, piers and jetties is their location in a dynamic, high- energy environment. Physical features of the environment where breakwaters and jetties are typically constructed reflect hydrodynamic and sedimentological conditions that have attained a dynamic equilibrium, a state of continuous change, which remains balanced around some average set of conditions. Construction of breakwaters and jetties may produce short- or long-term impacts on aquatic communities as a consequence of altered hydrodynamic conditions, sedimentation patterns, water quality parameters, and other physical or chemical factors. Eventually the system will return to a new set of dynamic equilibrium conditions. However, because the coastal zone is such a dynamic system that it is often impossible to accurately predict how the ecosystem will respond, over what length of time and what this new equilibrium actually means with respect to tourists' interests and needs.
Interrupting the movement of water, sand, seaweed and so on can cause either a build-up of sand, erosion or both. Structures like marinas or groins can cause significant accumulations of sand on their downdrift, sheltered side, and erosion on the exposed or updrift side. Changes in geomorphology and hydrology, in combination with habitat changes manifested by the presence of the structure(s) may affect the character of aquatic communities in the project area.
Picture courtesy of NOAA.
Where jetties, breakwaters and/or any type of coastal defence are deemed necessary it is important to attempt at building a structure most in harmony with the known geomorphological and hydrological conditions. In Morecambe, Northwest England, the use of simple offshore breakwaters was considered in order to provide suitable protection against storm waves, and reduce the impact of currents. However, with such an approach, the problem of strong shore-parallel currents due to ebb and flood flow and, according to hydrographic modelling, due to the confinement of flow between the shore and breakwater, current strength was actually predicted to increase. Long shore current effects and protection of the coastline against surge were removed by linking the offshore breakwaters to the shore through fishtail groyne shaped structures. By so doing it was hoped they would trap sediment and build-up the beaches leading to the provision of natural attenuation of storm waves along the Morecambe frontage. Furthermore, the expected crescent-shaped beach areas between the structures would provide an amenity beach to boost the area's tourism potential.
printed on 2013/05/24 09:34:31