Groins

Needs Address

• Coastal protection
• Measures against erosion, storm surges, flooding, inundation

Adaptation effects

• Enable coastal rehabilitation
• Stabilise shorelines to promote related livelihood activities

Overview and Features

Straight structures aligned perpendicular to the shoreline that are used to restore eroded foreshores. T-fences or groins can be emerged, sloping or submerged and can be arranged either by themselves or in groups. They do not inhibit sediment input and also dissipate wave action, create calm water conditions for sediment deposition and reduce erosion to provide an immediate solution to coastal threats in areas with extensive foreshore erosion. They can be constructed with rubble, concrete and timber.

Cost

• Costs for the construction of T-fences (e.g. in Vietnam – 50-60 USD/m); costs for the maintenance of T-fences (e.g. in Vietnam – about 7 USD/m per year) (Albers, Dinh and Schmitt, 2013)
• Human resources

Energy source

Construction

Ease of maintenance

Require upkeep following damage from storm surges and high waves

Technology performance

• Effectiveness depends on their extension i.e. how much of the littoral drift they block.
• The sand accumulation on the windward side and the lee side erosion depend on the type of groin employed.

Considerations

• Requires numeric modelling (based on actual data recorded on site) to determine boundary conditions and to ensure that down-drift erosion can be minimised as much as possible
• Lightweight materials required on soft mud topography
• Trapping of sediment can cause erosion on adjacent shores, particular when non-permeable structures are employed, therefore the use of this technology must be part of a comprehensive coastal management strategy
• Requires careful planning to avoid maladaptation and is often most useful for responding to temporary erosion only
• Due to variations in coastal characteristics, it is important to know the transport characteristics of the coastal section wherein the technology is being installed in order to enable prediction of the shoreline response.
• Construction requirements are site-specific, however, in general, the landward end of the groin must be constructed at the foot of cliffs or dunes so that it is not damaged by wave impacts where its height should be higher than the top of the backshore. The height of the groin further seawards can be lower, depending on the requirements for the sediment bypass.
• When employed in groups, the distance between each groin must be carefully defined to fit the transition area between the groins and the
• Unprotected sections of the beach

Co-benefit, suitability for developing countries

• Promotes fishery opportunities
• Can be constructed using local material such as bamboo poles for a cost-effective solution and aversion from the problems associated with building heavy concrete structures on soft mud.

Information Resources

Albers T, Dinh SC, Schmitt K (2013) Shoreline Management Guidelines – Coastal Protection in the Lower Mekong Delta. Published by Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ) GmbH. ISBN 978-604-59-0630-9. Vietnam.  Available at: http://czm-soctrang.org.vn/Publications/EN/Docs/Shoreline%20Management%20Guidelines%20EN.pdf [02 December 2014]

Jöhl Cadena, A. and Jacobs, K. 2013. The Second Annual Coastal Forum: Report on the Proceedings. Bangkok, Thailand: IUCN. 35pp.

IRIN, n.d. THAILAND: Sustainable approach to tackle coastal erosion. IRIN News. Available at: http://www.irinnews.org/report/79685/thailand-sustainable-approach-to-tackle-coastal-erosion [08 April 2015]