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Used singularly, or in groups known as groin fields, their primary purpose is to trap and
retain sand, filling the areas of shoreline between them. Groins interrupt the movement
of sand along the shoreline. Ideally, the area between the groins is filled, then sand can
continue moving along the shoreline. Sand usually accumulates between the groins with
more accumulation on one side than the other dependent on the direction of the
movement of sand along the shoreline. The accumulation of sand between the groins
acts as a barrier that waves can attack and erode without damaging upland areas. An
adequate quantity and movement of sand along the shoreline is necessary to produce
accumulations of sand between the groins. If sand transport is equal in both directions,
groins may not be effective.
Site Characteristics
The shoreline should be a gently sloping beach. It is important to consider the direction
and amount of sand moving along the shoreline before choosing groins as a method of
shore erosion control
Construction Materials
Stone, concrete, gabions, timber and steel are the primary materials used in the
construction of groins. Many other materials are also used but are not as dependable
as these. Quarry stone should be considered where locally available. The structural
form of a stone used in construction of a groin is about the same as for a stone
revetment. Filter cloth should be installed under any stone or rubble groins.
Timber and steel groins do not require the use of filter cloth. Recommendations for
construction materials contained in the section for sheet pile bulkheads applies to groin
construction. If the movement of sand along the beach is mixed with too much clay or
silt, filling the area between the groins with sand from an outside source is necessary.
Design Considerations
Important design considerations for groins include:
• Height
• How far they should extend into the water or onto the land
• Spacing between structures, if you are constructing a groin field
• How much sand and water should be allowed to pass through the groins
Groins may be built either high or low in relation to the existing beach profile. High
groins effectively block the movement of sand along the shoreline, provided sand
cannot pass through them. Low groins, built so waves can wash over them, permit sand
to pass over the structure and nourish adjacent beaches. The extension of a groin must
be sufficient enough to create a desired beach while allowing adequate passage of sand
around the end. A groin extending beyond the area of the shoreline where the waves
break, forces sediments too far offshore to be returned to the adjacent shorelines. The
groin should extend inland far enough so that storm waves cannot erode around the
upland side, making the structure ineffective.
The correct spacing of groins depends on their length, wave strengths in the area, the
amount of sand moving along the shoreline and the desired final shoreline shape.
Properly designed groins are spaced so that sand accumulates along the entire length
of the area between groins. The shoreline erodes in some areas between groins that
are positioned too far apart. Groins placed too close to one another may not allow for
sand accumulation. Generally, groins are spaced two or three groin lengths apart.