Facilitate tidal exchange to restore/create brackish/saline swamps from other land uses
Overall effectiveness category Awaiting assessment
Number of studies: 2
Background information and definitions
This action includes actions that facilitate tidal exchange, in order to restore or create swamps from other land uses. The action may be a single permanent one (e.g. breaching sea walls or embankments, installing or widening culverts, excavating tidal creeks) or a reversible one (e.g. opening sluice gates once per day). However, the intervention must affect an area that does not retain substantial characteristics of the target habitat. This could be an upland area (e.g. the thousands of square kilometres of farmland that has been reclaimed from salt marsh in the Netherlands since the Middle Ages; Wolff 1992), an unvegetated wetland (e.g. mudflats), or a wetland other than the target type (e.g. marsh, where the habitat used to be a swamp). “Planned retreat” and “managed realignment” fall within the scope of this action.
Tidal wetlands may be brackish/saline (e.g. mangroves, coastal marshes) or freshwater (e.g. at the upstream end of estuaries, as in the Mississippi, Yangtze, and Elbe rivers; Baldwin et al. 2009). Studies of accidental realignment, such as when coastal defences are breached by a storm, have not been summarized as evidence (e.g. Onaindia et al. 2001; some sites in Williams & Orr 2002).
Related actions: Facilitate tidal exchange to restore degraded swamps; Reprofile/relandscape or Remove surface soil/sediment, both of which can alter patterns of tidal exchange; Facilitate tidal exchange to complement planting.
Baldwin A.H., Barendregt A. & Whigham D. (2009) Tidal Freshwater Wetlands. Backhuys Publishers, Lieden.
Onaindia M., Albizu I. & Amezaga I. (2001) Effect of time on the natural regeneration of salt marsh. Applied Vegetation Science, 4, 247–256.
Williams P.B. & Orr M.K. (2002) Physical evolution of restored breached levee salt marshes in the San Francisco Bay estuary. Restoration Ecology, 10, 527–542.
Wolff W.J. (1992) The end of a tradition: 1000 years of embankment and reclamation of wetlands in the Netherlands. Ambio, 21, 287–291.
Supporting evidence from individual studies
A before-and-after study in 1993–2004 in an estuary in New South Wales, Australia (Howe et al. 2010) reported that after removing culverts to improve tidal exchange to an island, the area of mangrove vegetation increased. Mangrove forests covered 1 ha of the study area two years before culvert removal, 5 ha three years after culvert removal, and 12 ha nine years after culvert removal. Mangroves benefitted from the expansion of intertidal habitat, which provided a suitable physical environment. Other habitats present in the study site included salt marsh vegetation (before: 44 ha; after nine years: 53 ha), tidal pools/mudflats (before: 33 ha; after nine years: 32 ha) and upland pasture (before: 42 ha; after nine years: 22 ha). Methods: The study focused on an island in the Hunter River Estuary, which had been partially drained for agriculture. In 1995, two 0.5-m-diameter culverts in a tidal inlet were removed, restoring full tidal exchange to approximately one fifth of the island. Tidal exchange was slightly improved across the rest of the marsh, where culverts remained in place. Habitats were mapped from aerial photographs taken in 1993, 1998 and 2004.Study and other actions tested
A study in 1999–2005 in a former shrimp pond in Thailand (Matsui et al. 2010) reported that six years after restoring tidal exchange (along with reprofiling and planting mangrove seedlings), 1,797 unplanted trees of 15 different species were present. The most abundant species were grey mangrove Avicennia marina (842 trees), Bruguiera cylindrica (486 trees) and Ceriops decandra (267 trees). Four species were represented by a single tree. Methods: In June 1999, full tidal exchange was restored to an abandoned 6,525-m2 shrimp pond by levelling the banks surrounding the pond. Previously, water could only flow in and out through a 10-m-wide channel. The pond was also filled in. In September 1999, seedlings of four mangrove species were planted in the pond (500–800 seedlings/species, 1.5 m apart). The study does not distinguish between the effects of these interventions on naturally colonizing vegetation, of restoring tidal exchange, reprofiling and planting. In October 2005, mangrove trees that had spontaneously colonized were recorded in a 300-m2 section of the site.Study and other actions tested