Restoration of Subsided Sites and Calculation of Historic Marsh Elevations

Successful restoration of historic tidal marshes that have been diked and drained requires re-establishment of tidal flooding, a suspended sediment supply, and soils that will support vegetation in perpetuity. Drained and dried marsh soils shrink, lowering their surface elevation. Land subsidence and rising sea level during the time since such lands were diked further contributes to elevations, relative to tides, that are too low to support marsh vegetation. Some marsh plants, such as Spartina, require fresh mud substrate for colonization. Design of lasting conditions for plant habitat includes design of facilities for tidal flooding and raising the land surface elevation. Establishment of suitable soils may be accomplished by accumulation of sediment suspended in flooding waters or by partial filling with dredged material followed by deposition of a thin layer of suspended sediment. Issues to be resolved include inlet location and dimensions that provide a full range of tides, a desirable distribution of sediment deposits, and the expected time when conditions will permit plant colonization. Numerical models, such as the ones described below, are invaluable tools for resolving these issues.