Comparison of the effect of riprap, herbaceous textile and grass cover on reduction of sediment yield from the ditch of forest roads

Lateral ditches which are constructed to drain surface flow and protect the road structure under the sever rainfall are eroded by runoff velocity and then sediment as suspended load are transmitted to the river and aquatic ecosystem. Therefore, use of the cost effective, efficient and environmental techniques is essential for protecting ditches. The objective of this research was to compare the effect of different protective treatments on reduction of sediment yield from the ditch of forest roads as well as the assessing the effect of flow intensity on sediment concentration. At first a segment of lateral ditch with a length of 50 meters was selected along the roads of district one in Bahramnia forestry plan. The longitudinal slope and soil type (texture, porosity and moisture) of selected segments were same. Five plots each of with the length of 10 meters were considered for establishing the protective treatments including riprap, grass cover by Festuca, cotton geotextile, picket and wattle by local slash and control. Runoff simulation was done for ten minutes at two intensities of 5 and 10 L S-1 using a tank with capacity of 6000 liters equipped with motor-pump. Sediment sampling from the runoff was carried out at the end of each plot and at each minute. Samples were transported to the laboratory and there sediment concentration was calculated in gram per liter. Runoff flow velocity in different treatments was measured using electromagnetic micro molinet. Sediment concentration and runoff velocity in treatments of riprap, grass cover by Festuca, cotton geotextile, picket and wattle by local slash was significantly lower than that of control. Conversion of flow intensity from 5 L S-1 to 10 L S-1 didn’t cause the significant change in sediment concentration (except for control and riprap treatments) and runoff velocity (except for control and cotton textile treatments). Sediment yield from riprap and control treatments under the flow intensity of 10 L S-1 was significantly more than that of 5 L S-1. Moreover, runoff velocity from cotton textile and control treatments under the flow intensity of 10 L S-1 was significantly more than that of 5 L S-1. Riprap, grass cover by Festuca, cotton geotextile, picket and wattle treatments can be used in ditch of fores roads with flow intensity less than 5 L S-1 and picket and wattle treatment can be used in flow intensity more than 5 L S-1 because of their high efficiency in reducing sediment yield and runoff velocity.