Rainfall effects on marsh sediment redistribution, North Inlet, South Carolina, USA

Studies in terrestrial landscapes show that raindrop impacts and sheetflow entrain and transport sediment. There is a paucity of data, however, showing the effects of rainfall-induced sediment transport in marsh landscapes. We conducted sprinkler irrigation experiments during low tide to examine the sediment transport processes and sediment fluxes resulting from low tide rainstorms. We isolated 1×2-m plots in high marsh, low marsh, and tidal channel bank areas and irrigated them with freshwater to simulate low tide rainfall events, and with filtered saltwater to distinguish the effects of salinity on sediment yield. Also, for comparison, flood irrigation experiments with filtered saltwater were conducted on 1×3-m plots to simulate tidal flows. Sediment concentrations were 10–1000 times greater in runoff from sprinkler plots. Sprinkler plot concentrations peaked between 1000–36 000 mg/l within 5 min of irrigation, and most settled to near steady values between 100–10 000 mg/l within 15 min. These observations indicate that >67 t/km2 of marsh sediment can be mobilized within 5 min of a single low tide storm event. The chief mechanisms for sediment transport were detachment by raindrop impact and transfer by sheetflow. Although freshwater flux over salt marshes is negligible relative to the tidal prism, rainfall effects may facilitate the redistribution of disproportionately large volumes of marsh sediment.