Hydrodynamic Controls on Hydroperiods in Salt Marshes

Design of successful marsh restoration projects requires careful consideration of the hydroperiods of the marsh system. If the hydroperiods are poorly defined, the duration of inundation of marsh grasses will be either too long or too short, the restoration effort may be less successful. Fortunately, proper design based on understanding of nonlinear tidal hydrodynamic processes can enhance the likelihood of success of marsh restoration. Tidal marsh systems are strongly nonlinear hydrodynamic filters. Astronomical tides entering salt marshes via tidal inlets may be inherently linear, but they are distorted strongly once they enter the marsh. This nonlinear distortion results in strong spatially-varying tidal heights and tidal phases. One representation of the strong nonlinearity is the generation of overtides and harmonics of the fundamental equilibrium tidal constituents found offshore. These nonlinear processes must be understood when designing a marsh, since the marsh channel design and the extent of tidal flats will control this nonlinear response. There are several fundamental controls on the hydrodynamics of marsh nonlinearities, which control the hydroperiods. The first nonlinear control focuses on depth of the channel; the ratio of tidal amplitude to water depth is the first order control on the degree of nonlinear energy transfer from parent tides to harmonics. The second control is the ratio of the area of the marsh system covered by vegetated tidal flat versus that dedicated to deeper tidal channels. Other tertiary controls exist as well. By careful design of the depth of the marsh channels, and the relative area covered by tidal flat vegetation, different hydroperiods can be encouraged, enhancing marsh growth.