Other external factors that may impinge on the long-term ecological sustainability of a mitigation wetland include deleterious influences of natural pest species (e.g., intense grazing by herbivores such as migratory or resident geese) and large-scale disturbances such as hurricanes, fire, sea-level rise, and climate change (see Box 3–3). The committee recognizes that it is impractical to expect individual permittees to design for and be
Sea-Level Rise and Wetlands Placement
Sea-level rise, caused by both natural processes and increased concentrations of greenhouse gases, threatens the sustainability of coastal wetlands because (1) sea level will continue to increase for the foreseeable future, (2) a large rise in sea level will cause a net loss of wetlands, and (3) coastal development will block the natural inland migration of wetlands (Titus 1999). Global sea-level rise estimates vary dramatically, but 1.8 to 1.9 mm/yr may be a reasonable median rate, including correction for postglacial rebound (Douglas 1995, 1997). Based on current projections of greenhouse gas emission rates, with no future remedial reductions, sea level may rise from 31 to 110 cm by 2100 (IPCC 1990). A 50-cm rise in sea level would involve inundation of 24,000 square kilometers (km2) in the United States (Neumann et al. 2000). The areas most vulnerable to sea-level rise are in the mid- and south-Atlantic states and along the Gulf Coast, where land subsidence is also a concern, although parts of New England, San Francisco Bay, and Puget Sound also are vulnerable (Neumann et al. 2000). For example, it is estimated that 21% (22,000 acres) of Delaware's coastal emergent wetlands would be inundated (MARA Team 2000). Inundation would not be the only threat; storm frequency, intensity, and surge levels also would increase.
The contingency of climate change and sea-level rise argues for landscape-scale planning and implementation of wetland restoration, creation and enhancement, and preservation. The consequences of increased temperatures and reduced precipitation may need to be designed into mitigation projects particularly vulnerable to changes in flooding duration and frequency in wetlands such as prairie pothole and peatland wetlands. Drier wetlands, such as depressional, slope, flats, and river and lake fringe wetlands (Brinson 1995), may need additional design features to ensure protection of the proper hydrological regime. Inland migration of coastal emergent marshes, mangroves, forested wetlands, and seagrass and other submergent vegetation systems may need to be accommodated to some extent through, for example, “managed retreat” and reduction of armored shorelines. Strategic restoration of coastal marshes by breaching of dikes and levees may need to be advanced to accommodate an increased tidal prism and coastal erosion in estuaries. Marsh sediment accretion rates must be maintained by the preservation and enhancement of sediment sources and transport patterns and rates. Dams further impair sustainability of downstream wetlands by eliminating sediment transport that could counteract rising sea levels.