Living Shorelines in Connecticut

Hybrid Approaches

On more exposed locations with high wave or wake energy environments, marsh plantings and beach nourishment may be unable to withstand wave conditions and reduce shoreline erosion. These environments will require temporary or manmade structures to attenuate wave energy to allow the establishment and maintenance of marshes and beaches. These structures include toe protection, sills or breakwaters constructed of natural materials such as rock, coir logs and matting, oyster reefs or other materialsAlternatively, manmade components such as synthetic matting, geotubes, and concrete wave attenuators can be combined with marsh plantings to reduce shoreline erosion while maintaining ecosystem services (Swann, 2008). This combination of vegetation and/or sediment with hard material is referred to as a “hybrid” living shoreline (Chesapeake Bay Foundation, 2007; VIMS-CCRM, 2015b). Unlike traditional coastal structures, hybrid living shorelines are designed to perform similarly to the natural ecosystem, rather than protect against it (Smith, 2008).

Fiber Logs

Coir logs are used to temporarily protect banks and marsh toe from erosion while planted vegetation develops strong root systems. The coir logs come in a range of sizes and grades, and may be placed in a single or multiple rows. Coir logs must be securely anchored to prevent wave and tidal current induced movement. Coir fiber is biodegradable and typically deteriorates in three to five years in low energy environments, sufficient time for the vegetation to become established (Chesapeake Bay Foundation, 2007; Hardaway et al., 2009; Hardaway, Milligan, and Duhring, 2010; VIMS-CCRM, 2006); they are not recommended for high energy saltwater conditions (Duhring, 2008b; Skrabel, 2013).

Marsh Toe Revetment Marsh toe revetment is a specialized riprap revetment designed to protect eroding marsh edges or banks from wave-induced erosion. Unlike traditional revetment protection, marsh toe revetment is low profile, only slightly higher than the existing marsh surface which is usually at or approximately one foot above MHW. The low profile protects the marsh edge from wave action but allows tidal inundation over and through the structure, thus maintaining the marsh ecosystem. Tidal gaps in long revetments provide the same function by allowing tidal exchange (Barnard, 1999; Duhring, 2008a; Hardaway, Milligan, and Duhring, 2010).

Marsh Sills

Marsh sills are very small, low profile stone breakwaters that are used to protect the seaward edge of a planted marsh (Broome, Rogers, and Seneca,1992). Constructed near mean low water (MLW), they are backfilled with sand to elevate and re-grade the slope, then planted with marsh vegetation to create a protective marsh fringe (Duhring, 2008b; Hardaway, Milligan, and Duhring, 2010). Marsh sills are appropriate for eroding shorelines where site conditions are suitable for marshes although no marsh currently is present (Duhring, 2008b).

Low marsh sills have been used extensively in the Chesapeake Bay and its tributaries; the design has remained fairly consistent (Hardaway et al., 2010). A wider and higher sill would provide more protection from coastal erosion; a too high sill will reduce or eliminate tidal exchange and the marsh behind it will become stagnant and die. If tidal flushing is not enhanced, the area landward of the sill may be unable to support aquatic species that need to migrate with the tidal cycle (Smith, 2006; Chesapeake Bay Foundation, 2007; Duhring, 2008?). Thus, poorly designed sills can do more harm than good to marine animals (Subramanian et al., 2008b). Slopes of 10 H:1 V and sill elevations near MHW have been recommended for the Chesapeake Bay (Duhring, 2008a; Hardaway et al., 2010). Hardaway and Byrne (1999) provide recommendations for marsh widths and sill construction; however, the Chesapeake Bay has a relatively small mean tidal range of 1-3 feet (Xiong and Berger, 2010). Therefore, these design parameters may need to be modified for locations with greater tidal ranges

Openings or gaps in marsh sills are recommended to allow tidal exchange and to provide marsh access for marine animals. However, the openings expose the marsh to waves which could result in increased erosion. Deposition of sediment in the gaps can also occur which could reduce or eliminate tidal exchange (Hardaway et al., 2007; Smith, 2008). Recommendations for mitigating these concerns include creating dog leg or offset openings, and varying the opening size and orientation of the sills to allow tidal flow exchange and access to the marsh habitat (Bosch et al.; 2006; Hardaway et al., 2007). In addition to sill gaps, access to the marsh takes place through interstitial spaces in the sill and by overtopping. The porosity of the sill may be as important if not more important to tidal exchange and species access than the size or number of gaps in the sill length (Hardaway et al., 2007). Although no scientific study of the effectiveness or design of sill gaps has been performed to date, empirical evidence suggests gaps approximately every 100 ft, although the final design will depend on local marine species, and wave and tidal conditions (Smith, 2008; Hardaway et al., 2010).

Oyster Reefs

Marsh sills are also formed with oyster reefs, constructed of bagged or loose oyster shell, to provide the same erosion control as rock sills, but with additional ecosystem benefits. Oyster reefs provide a substrate for oyster recruitment and thus are self-maintaining, building the reef dimensions and therefore, protection and restoration benefits with time so oyster reefs are sometimes referred to as “living breakwaters”. Like rock sills, oyster reefs provide habitat and foraging areas for aquatic species, however, as oysters are filter feeders they also improve water quality and clarity by removing sediment and algae, which improves light transmission and enhances the environment for submerged aquatic vegetation (SAV).

The effectiveness for shore protection of low profile marsh sills is limited due to the larger tidal ranges experienced in New England. Large scale oyster reefs are similar to traditional breakwaters but are seeded with oysters to reduce risk to coastal storms while providing ecosystem services enhancement (Rebuild by Design, 2015). The persistence and growth on oyster beds depend on wind, waves, tidal currents and ice. Currently, the Connecticut’s natural beds are only a few oysters deep and since most of the subtidal areas are designated harvest areas, the pyramid shape commonly found in the Chesapeake Bay does not exist in Long Island Sound (Getchis, 2015). In Long Island Sound, commercial oystering limits the feasibility of oyster reefs. Most of the nearshore sites suitable for oyster reef construction are designated town, state or privately held commercial harvesting beds. Additionally, the CT Bureau of Aquaculture has a policy of removing oysters when they reach 5-6 years old to reduce the potential occurrence of MSX (Carey, 2015). Thus, the feasibility of oyster reef sills and breakwaters for living shorelines in Long Island Sound is limited.

Breakwaters

Structural approaches to coastal erosion are not typically considered living shoreline approaches. However, offshore-gapped-headland breakwaters, as a component of a living shoreline, are used to create a pocket or crenulate

Wave Attenuation Devices

Reef balls, WADs, Coastal Havens, BeachSavers and Prefabricated Erosion Prevention (P.E.P.) reefs are marine suitable concrete structures designed to attenuate waves and provide benthic habitat. These wave attenuation devices may be used where appropriate instead of rock sills. Of these, Reef Balls are perhaps the best known with over 4000 projects worldwide; however, not all the installations were for erosion protection; many were to re-establish coral reefs. Wave attenuation devices are deployed as offshore breakwaters, to provide the hard coastal protection of a traditional breakwater with the ecological benefits of habitat creation and marsh restoration. As the wave attenuation devices become colonized with marine species, they provide recreational benefits such as fishing and snorkeling.

(photo credit: A. Dolan, Graduate Student (left) and J. Mattei (right), Department of Biology, Sacred Heart University)

Alternative Technologies

Although there are other examples of living shoreline approaches such as live fascines, branch packing, and brush mattresses, most are unsuited to the wave, surge and ice conditions experienced by New England coasts. Scientists, engineers and even private property owners are continually developing new technologies for responding to coastal erosion, storm surge and sea-level rise. Although property owners remain optimistic, no “silver bullet” has been produced that solves all these problems.