Tidal flows in salt marsh creeks

Tidal flows in salt marsh creeks must control both creek morphology and the overall sediment and nutrient budgets of the marsh, but little is known of the magnitude and frequency of such flows. Data are presented in this paper from north Norfolk, England, which indicate a threefold division of the tidal-flow regime. On Upper marshes the majority of tides are ‘below-marsh’ and, apart from a small initial pulse of velocity on the flood, they generate only modest flows in creeks. The level of the marsh surface provides a threshold at which, with higher ‘marshfull’ tides, velocities can increase to reach peaks shortly before and shortly after high tide. These velocity pulses are greater on the ebb than on the flood, but the flood pulse occurs at a higher stage. The even higher but infrequent ‘over-marsh’ tides, often associated with storm surges, have a substantial flood maximum in both velocity and discharge. It is hypothesized that only these extreme tides might be capable, on upper marshes, of achieving significant erosion and deposition, and that their much higher frequency on lower marshes may account for the observed rapidity of geomorphic change at this earlier stage in salt marsh development.