Credit 1.4: Groin Removal
This credit applies to removal of groins and any other structures that extend perpendicular to the shoreline (cross-shore) and impede sediment transport or tidal circulation (wherever removal is feasible, and would restore physical processes).
These types of structures include groins/groynes, boat ramps, jetties, large stormwater outfall pipes, and other structures which extend across the shore and trap littoral sediment on one side or significantly alter tidal circulation. The goal is for complete structure removal, although lower points are awarded for partial removal.
Where this credit applies
This credit applies to any site where a groin or similar perpendicular structure significantly impedes littoral transport or tidal circulation. Emphasis is on sites within littoral drift cells, although removal of a groin can benefit all types of shorelines.
Where this credit does not apply
This credit is applicable to groins, or portions thereof, that impede natural sediment transport, which occurs primarily in the intertidal zone. Groins that extend into the subtidal typically do not affect long-shore sediment transport, and in fact, may provide reef-type habitat. In most cases, subtidal groins or subtidal portions of groins should not be considered for removal.
Groin removal is also not feasible in locations where removal would cause significant up-drift or down-drift erosion in developed areas; for example, if a house is already located very close to a beach that is being held in place by a groin, it may not be feasible to remove the groin as part of renovations on the property. A qualified professional (coastal geologist or engineer) can advise on the stability of a shoreline and the effect of groin removal on that stability prior to considering this action.
Several beaches enhanced through beach nourishment have used “drift sills” which are low-elevation groins with the up-drift beach filled to the top of the groin with nourishment sediment. These drift sills were used in areas with impeded natural processes or where major buildings or infrastructure are located very close to moderately energetic shores. Removal of drift sills is typically not advised as the beach nourishment sediment may become unstable.
This credit offers up to 5 base points and 2 bonus points
|Removal of a large groin or other cross-shore structure that spans from the lower intertidal zone (below 1/2 of the local tidal range) to above OHWM||
|Removal of a groin or similar structure that spans the upper intertidal zone (above 1/2 of the local tidal range) to above OHWM||
|Bonus(available once the base conditions have been met)||
|Demonstrate that littoral sediment transport has been reduced or blocked by the structure to be removed, as evidenced by a higher beach level on one side of the structure.||
How to proceed
Since it involves working in the foreshore and fish habitat, groin removal may require permits or other approvals from regulatory agencies. Check your proposed project with local, state/provincial or federal authorities before getting started. Also be aware that removing a groin may affect your neighbor’s property—positively or negatively. If you suspect that groin removal might lead to a negative impact next door, seek the advice of a qualified professional before getting started.
Groin removal on Whidbey Island WA: before and after. Although removal was not complete, erosion on the left or down-drift side is somewhat reduced due to reduced littoral draft interruption; note that the shoreline to the left of the groin is less indented or offset after removal. J. Johannessen
Groin removal typically involves pulling out a structure, filling all depressions with appropriate beach sediment, and planting native vegetation if appropriate. It is important to remove all portions of the structure, including materials buried below grade, such that buried material does not become exposed on the beach surface over time. This avoids armor material or debris from surfacing in the future following adjustments in beach elevation.
Removal of groins of moderate or larger size can change accretion and erosion patterns. Assess shoreline processes at and around your site and how they might be changed as part of design plans (as described earlier in the sections “Existing Conditions Plan” and “Site Design Plan”). For small removal projects, do a general assessment of the effects on local erosion and deposition on your shoreline and your immediate neighbors’ shorelines; for larger removal projects, do a broader assessment of implications for the shoreline in your area—see Clancy at al. (2009) for details. Items to include in your assessment are:
- Identify the geologic setting of the site, sediment transport processes, general erosion/accretion trends for the project area and the role of the existing groin regarding sediment transport processes.
- Identify areas that are currently being influenced by the existing groin or beach structure.
- Consider the erosion potential associated with adjacent properties.
You may be able to find out if your structure is in a drift cell using maps published by the Dept. of Ecology in Washington State, the Island Trust in BC, or other sources as available (see “For more information” below).
You can determine if sediment transport is blocked or impeded by observing the beach level on each side of the structure to see if one side is higher by 1.5 ft. (0.5 m) or more.
Example 1.4 Marine: Photographic evidence of impediment of sediment due to a shoreline structure. Photo on the left was taken 35 years prior to photo on the right. It is clear the beach structure has impeded sediment movement (MSDG figure 4.5).
For more information
Clancy, M., I. Logan, J. Lowe, J. Johannessen, A. MacLennan, F. B. Van Cleve, J. Dillon, B. Lyons, R. Carman, P. Cereghino, B. Barnard, C. Tanner, D. Myers, R. Clark, J. White, C. Simenstad, M. Gilmer, and N. Chin, 2009. Management Measures for Protecting the Puget Sound Nearshore. Puget Sound Nearshore Ecosystem Restoration Project Report No. 2009-01. Published by Washington State Department of Fish and Wildlife, Olympia, Washington. see MM 7 Groin Removal and Modification chapter. Go to http://www.pugetsoundnearshore.org/technical_reports.html and scroll down to 2009-1.
In Washington State, to determine if a site is in a drift cell, check mapping at Washington Dept. of Ecology Coastal Atlas map server: https://fortress.wa.gov/ecy/coastalatlas/
In BC, the Islands Trust has mapped the shorelines of each of the member Gulf Islands. Go http://www.islandstrust.bc.ca/trust-council/projects/green-shores-for-homes.aspx and scroll down to the list of “Integrated Shoreline and Watershed Maps”.