On the shape and widening of salt marsh creeks

We have developed a model that simulates aspects of initial channel formation in a youthful salt marsh environment. The model mimics the evolution of the cross section of a channel by coupling calculations of bottom shear stresses caused by tidal motions with erosion, taking into account the deposition of cohesive sediments. The simulations characterize flow in a reference cross section that includes an incipient channel zone and a marsh surface zone, with assigned water surface level and initial bottom elevation. This model mimics key characteristics of salt marshes where discharges due to tidal motion repeat in time with approximately the same magnitude and water surface level. Significant reductions in the tidal prism due to increasing bottom elevation above mean sea level, however, are not treated. Rather, the model is suitable for youthful salt marshes where relatively large water depths are maintained. Prolonged deposition reduces the area available for flow and thereby changes the shear stress distribution at the bottom, leading locally to erosion and alteration of the channel cross section. The simulations suggest that two mechanisms contribute to the longitudinal widening exhibited by salt marsh channels, which typically is disproportionately greater than that exhibited by river channels. The short duration of the maximum discharge (spring tide) and corresponding erosion rates, when compared with deposition rates, prevent the channel from reaching a deep, narrow equilibrium configuration. Furthermore, autoconsolidation of cohesive sediments, often occurring in salt marsh environments, leads to a downward increase in the resistance of the sediment to erosion. As scour occurs locally, the flow encounters more resistant sediment layers; so rather than deepening the channel over a narrow zone, flow and bottom stresses become more uniformly distributed leading to a wider channel than would otherwise occur in the absence of autoconsolidation. Based on flow and sediment properties estimated for the Venice Lagoon, Italy, simulations are consistent with observations of salt marsh creeks at this location.