Build barriers to protect littoral brackish/salt marshes from rising water levels and severe weather
Overall effectiveness category Awaiting assessment
Number of studies: 5
Background information and definitions
Littoral wetlands (on sea or lake shores) are vulnerable to physical damage from strong waves, and from excessive flooding linked to storm surges and rising water levels. These threats may be reduced by building barriers such as dykes, walls, breakwaters, reefs, groynes, coconut-fibre rolls or even additional marshes or swamps. Barriers can provide some immediate shelter from wave energy. They can also encourage sediment deposition, potentially helping existing marshes or swamps to keep up with sea level rise or building a surface at suitable elevation for emergent vegetation (Wetlands International 2016). Barriers could be installed temporarily to protect colonizing vegetation, or could be used as permanent protection.
Caution: Littoral areas are naturally exposed to wind, waves and flooding. Some disturbance from these elements may be necessary to maintain a diversity of coastal habitats and normal wetland functions. For example, restricting tidal influx to coastal marshes may limit their natural tendency to accumulate sediment and organic matter to keep up with sea level rise (Redfield & Rubin 1962; Nyman et al. 2006). If barriers are left in place for many years, it may not be possible to ever remove them without flooding the marshes or swamps they were intended to protect.
Related actions: Build barriers to protect littoral areas from boat wakes; Divert/replace/block saltwater inputs; Use artificial barriers to block pollution; Use fences or barriers to protect planted areas.
Nyman J.A., Walters R.J., Delaune R.D. & Patrick W.H. Jr. (2006) Marsh vertical accretion via vegetative growth. Estuarine, Coastal and Shelf Science, 69, 370–380.
Redfield A.C. & Rubin M. (1962) The age of salt marsh peat and its relation to recent changes in sea level at Barnstable, Massachusetts. Proceedings of the National Academy of Sciences USA, 48, 1728–1735.
Wetlands International (2016) Mangrove restoration: To Plant or Not To Plant? Available at https://www.wetlands.org/publications/mangrove-restoration-to-plant-or-not-to-plant/. Accessed 1 February 2020.
Supporting evidence from individual studies
A controlled study in 1994–1995 of two salt marshes in northern Italy (Scarton et al. 2000) reported that a salt marsh behind a sediment fence contained more plant species than an exposed salt marsh, but found that there was no significant difference in vegetation cover or distribution. After 17 months, three plant species were recorded in the fenced marsh, compared to only one species in the exposed marsh. The marshes did not significantly differ in terms of vegetation cover (fenced: 29%; exposed: 19%) or distance between the physical edge of the marsh sediment and the vegetation (fenced: 7.5 m; exposed: 9.1 m). Methods: The study involved two intertidal salt marshes behind embayments. In May 1994, a fence (vegetation bundles behind wooden posts) was built across the mouth of one embayment to trap sediment and protect the marsh behind from waves. The other embayment was left open. In October 1995, vegetation was surveyed along the edge of each marsh (14–16 points/marsh; species and cover within a 1 m radius around each point).Study and other actions tested
A replicated, randomized, paired, controlled study in 2002–2003 of brackish marsh around a lake in Louisiana, USA (Piazza et al. 2005) found that installing offshore oyster shell reefs reduced the rate of shoreline retreat, but had no significant effect on vegetation cover or biomass within the marshes. Over one year, the rate at which the vegetated shoreline receded was lower for marshes behind oyster shell reefs (8 cm/month) than for unprotected marshes (12 cm/month). Over the year, vegetation cover and above-ground vegetation biomass were statistically similar in marshes behind oyster shell reefs and in unprotected marshes (data not reported). Methods: The study used twelve sections of shoreline (six in a high-energy area, six in a low-energy area) around one coastal lake. All had brackish marsh landward. Oyster shell reefs (25 m long and exposed at low tide) were deposited <5 m offshore of six random sections (three high-energy, three low-energy). The other six sections were left unprotected. Vegetation was surveyed for one year after reefs were installed: three measurements/section/month for shoreline position (i.e. edge of marsh vegetation); nine 1-m2 quadrats/section/month for cover of each plant species; three 0.25-m2 quadrats/section/quarter for biomass (vegetation cut, dried and weighed).Study and other actions tested
A replicated, paired, site comparison study in 2001–2004 of six salt marshes in North Carolina, USA (Currin et al. 2008) found that restored marshes – protected with breakwaters and planted with cordgrasses Spartina spp. – typically contained less, and shorter, smooth cordgrass than natural marshes. Averaged over the 22 or 31 months after intervention, smooth cordgrass cover was lower in restored than natural marshes in three of three comparisons (restored: 10–26%; natural: 33–46%). Smooth cordgrass density was lower in restored than natural marshes in two of three comparisons (for which restored: 70–162 stems/m2; natural: 150–222 stems/m2; other comparison no significant difference). Smooth cordgrass plants were shorter in restored than natural marshes in three of three comparisons (restored: 50–62 cm; natural: 64–82 cm). Methods: Between autumn 2001 and summer 2002, three degraded salt marshes were restored. Rocky breakwaters were built offshore, then cordgrasses Spartina spp. (mainly smooth cordgrass Spartina alterniflora) were planted. The study does not distinguish between the effects of the breakwaters and planting on non-planted vegetation. For each protected/planted marsh an adjacent, physically similar, natural marsh was selected for comparison. Smooth cordgrass was monitored along transects each spring and autumn for up to 31 months after intervention. Cover was estimated in 1-m2 plots, stems were counted in 0.25-m2 subplots, and the three tallest stems/plot were measured.Study and other actions tested
A replicated, randomized, paired, controlled study in 2007–2009 of two salt marshes in Alabama, USA (Scyphers et al. 2011) found that installing offshore oyster shell reefs had no significant effect on the rate of shoreline retreat. Over approximately two years, the vegetated shoreline receded by a statistically similar amount whether it was behind an oyster shell reef (3.1–5.1 m retreat) or left unprotected (4.5–5.5 m retreat). Methods: The study used eight sites across two rapidly eroding shorelines. At four sites (two random sites/shoreline), oyster shell was deposited just offshore to form a breakwater (three 5 x 25 m sections; top exposed during low tides). The shell was placed on geotextile fabric and anchored in place with plastic mesh. It was colonized by oysters Crassostrea virginica. The other four sites were left unprotected. Stakes were inserted at the seaward limit of emergent vegetation when the reefs were constructed. Retreat relative to these stakes was measured over 24–27 months.Study and other actions tested
A replicated, site comparison study in 2011–2013 of salt marshes in the Netherlands (van Loon-Steensma et al. 2015) reported that degraded marshes behind low sea walls developed similar plant/algal communities to natural salt marshes within 15–22 years, but contained fewer plant/algal species. The overall plant/algal community composition in protected salt marshes fell within the range of the community composition of natural marshes (data reported as graphical analyses; statistical significance of similarity not assessed). In both protected and natural marshes, the most common species were glasswort Salicornia europaea (present in 59–66% of quadrats), saltmarsh grass Puccinellia maritima (59–63%) and seablite Suaeda maritima (58–62%). However, only 85 species of plants and algae were recorded in the protected salt marshes, compared to 155 species recorded in natural salt marshes in the region. Protected marshes were missing some of the rarer species present in natural marshes. Methods: In 2011 and 2013, cover of every plant and algal species was recorded (in 148 circular 4-m2 quadrats) across two coastal salt marshes in the Dutch Wadden Sea. The marshes had developed behind low sea walls (10–60 m from the salt marsh edge, extending 1 m above mean sea level) built in 1991 and 1998 to protect remnant, eroding marsh vegetation. Previously published data, from 6,198 quadrats in natural marshes across the Dutch Wadden Sea, were used for comparison.Study and other actions tested