Tuesday, December 20, 2011

Construction of novel erosion control and habitat restoration project at Stratford Point is making excellent progress!

Hurricane/Tropical Storm Irene left Stratford Point's Long Island Sound-facing shore in shambles and during the month of August the site saw a lot of activity as this damage was repaired. Large trucks and several excavators moved mountains of large boulders into the storm-damaged revetment wall to stabilize and armor the 'high-energy' side of the site. The use of hard structures, such as rock or concrete walls, has traditionally been the method of choice to control erosion in tidal areas. However, in recent years this approach is being revisited and novel, 'soft' erosion control structures are being put to the test in many areas surrounding Connecticut. Connecticut Audubon Society, together with site-owner DuPont and specialized contractor 'All Habitat Services, LLC', is currently in the midst of constructing such a soft erosion control structure along the 'low-energy' north shore of Stratford Point - a novelty in Connecticut.

Contractors evaluate the projected contour of the new dune in an area affected by Hurrican Irene

As their name suggests, these 'soft' structures are relatively flexible and can absorb some of a wave's energy as it crashes onto the structure, rather than deflect it like a hard surface would. The latter scenario often results in undermining of the structure and/or excessive erosion at its base, causing a sea wall to become compromised over time. In areas where wave impact is relatively minor and in areas of sensitive coastal habitat, a soft erosion control structure may be a better fit. The northern cove of Stratford Point fits both descriptions since its orientation protects it from most direct wave impacts while the coastal habitat found on-site is among the most threatened in the state. Once completed, the newly constructed erosion control feature will be contoured, covered with a thick layer of sand and planted with 38,000 dune grass plugs to form an artificial coastal dune system extending the length of the north shore.

The first tube goes in to set the baseline

Historically, the intertidal zone of Stratford Point was densely vegetated with Smooth Cordgrass (Spartina alterniflora), rooted in banks of organic peat. A large-scale remediation in 2000-01 led to the removal of large amounts of lead shot from the site's upland and tidal areas - a remnant of the site's history as a trap and skeet shooting club. However, this environmental clean-up also caused the loss of all peat substrate and associated plant life from the intertidal zone. This loss of vegetation zone has destabilized beach sediment and led to erosion of the site's shoreline due to unabated exposure to wave action. Between 2001 and 2011, over 100 horizontal feet of upland was lost along sections of the north shore of Stratford Point, and erosion continued to be a problem.

Each additional layer of tubes is wrapped in sheets of geotextile and anchored in the
upland, while the entire stack is also tied together by a geotextile covering

In 2010, Connecticut Audubon Society's Conservation Biologist, Twan Leenders, carried out a feasibility study to evaluate whether the concept of creating erosion control structures that are integrated in coastal habitat elements was feasible at Stratford Point. Monitoring studies to assess the species of plants and animals currently found along the northern shore of Stratford Point, and additional surveys of nearby high-quality coastal dune habitats to investigate the potential target species composition for the newly created coastal dune were completed this past year. The latter study was carried out by CAS Science & Conservation staff and students from Sacred Heart University, with financial support from The Nature Conservancy. Once all the pieces of the plan were in place, by fall of this year, it was decided to move ahead as soon as possible to stop further erosion of the site's shoreline. The unseasonably warm weather in recent weeks has helped the project tremendously and extended our work window well into December. If this pattern holds, we should see a finished project before the end of the year!

The plantings arrive at Stratford Point: 38,000 beach grass plugs and their private chauffeur.
The plants are temporarily stockpiled here until the dune is ready to be planted. 

The process of constructing the basis for this artificial dune is worth a closer look. Rather than using traditional stone elements for a foundation, the entire structure is built around a series of sediment-filled geotextile tubes. Each tube is filled to capacity with a mixture of sand and organic compost to form a 12-inch diameter 'sausage' that runs the entire 750 foot length of the dune. Individual tubes are arranged in a step-ladder fashion, stacked up to nine tubes high in some places, and held together by sheets of additional geotextile fabric. The sediment-filled tubes are flexible and dynamic and settle snugly around the variable contours of the shoreline. They are placed against the face of the eroding shoreline and the entire structure is engineered to match the height of the existing bluff. Once completed, the tubes will be hidden from view by the sediment and plants that cover the dune, but they will provide stability and bulk to the entire system. The dune grass plantings will gradually be enhanced with a more diverse mixture of vegetation, but initially it is most important that these fast-growing grasses establish an extensive root system to stabilize all the sediment on top of the dune.

The tubes are in and a substantial section of the structure is covered with  sand already.
The new shoreline contour is taking shape...

Apart from the creation of the artificial dune system, the 2010 feasibility study also recommended restoring the historic tidal marsh vegetation lost during the 2000-01 large scale remediation at Stratford Point. Smooth Cordgrass plants growing in the intertidal zone move with the water's tidal motion and absorb energy from waves before they hit the shore. The combination of decreased erosion as a result of lower wave energy and the sand-trapping ability of salt marsh vegetation will promote the growth of a sandy beach habitat. Restoring the salt marsh fringe present at Stratford Point prior to 2001 will help to protect the newly created coastal dune system, while enhancing the tidal marsh habitat at the site. This salt marsh restoration project is next on the docket for 2012.

Beach Grass is being planted in the foreground, while the finishing touches are put on the dune contour
in the back. The end is in sight!   

Stratford Point's new dune system is designed to be dynamic and to allow for deposition and migration of wind-blown sediment. In the long term, the appearance of this coastal dune habitat will be shaped by the prevailing elements and will hopefully become an integral part of the site's coastal landscape. Continued monitoring of the area will tell us not only whether the soft erosion control structure does its job of stabilizing the shoreline, but we are also very interested to see whether plant and animal species that depend on coastal habitats will gradually occupy the newly created habitat. In the next couple of years, Connecticut Audubon Society staff will implement additional novel habitat management and conservation projects at Stratford Point and we will keep you informed on how they turn out. Through a series of creative habitat restoration and management projects we are hoping to see the recovery of local at-risk species while the site transitions from what was once an environmental hazard to a vibrant coastal nature preserve. Come see it for yourself some time!

Twan Leenders, Conservation Biologist
Photos by Twan Leenders

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