Rainfall effects on marsh sediment redistribution,

North Inlet, South Carolina, USA

M.J. Mwamba and R. Torres

Department of Geological Sciences, University of South Carolina, Columbia, SC 29208, USA

Received 12 July 2001; received in revised form 22 April 2002; accepted 23 July 2002

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 x 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 x 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.