Performance of grass strips for sediment control in Okinawa

Reddish fine sediment runoff from upland fields has been one of the causes of coastal environmental pollution in Okinawa, Japan. Planting grass strips is a well-known measure to reduce runoff of non-point-source pollutants from agricultural fields. In this study, we investigated the performance of a centipede grass (Eremochloa ophiuroides (Munro) Hack.) strip for reducing sediment runoff from an upland field in Okinawa, focusing on strip length and sediment aggregate size as factors affecting performance. Field experiments were conducted in field plots with a 4.0-m by 31.5-m bare source area on a Kunigami-Maji soil using three strip lengths (0.5, 1.5 and 3.0 m) under natural conditions. The sediment removal efficiencies were 24% for the 0.5-m strip, 36 to 54% for the 1.5-m strip and 73% for the 3.0-m strip. The strips trapped well the sediment aggregates larger than 0.02 mm in diameter, regardless of strip length. The longer strip trapped more aggregates of the 0.002-0.02-mm size class, which were dominant in the eroded sediment runoff from the plots. The strips poorly trapped aggregates smaller than 0.002 mm. The sediment trapping resulted primarily from deposition of sediment due to slowdown of surface flow.