determination of culverts diameter in forest roads

construction of forest roads disorders the natural trend of water movement. it is vital to make ditch and cross line drainage culvert to prevent the area from being destroyed and conduct the water of streems and vallies. determination of culvert diameters depends on the information of area discharge, physiographic characteristic, vegetation cover and site land use. in order to determine the amount of sub basin water discharge and the diameters of cross culvert in forest roads, for drainage operation and preventing from land destruction, a study has been carried out in zalemrood watershed area in 27 sub areas which were created using arc view software. the flow intensity of each sub area has been calculated as foot cube per second, using curve number method (cn) with time intervals of 25 year. then the diameter of each sub area culvert was achieved, using cement culvert diameter determination graph. the results show the dimensions of the culverts were different from 38 to 156 centimeters. to achieve the best culvert dimension in mountainous area, the culverts sub area with diameter less than 60 centimeter were combined together so that the best diameter can be achieved for each sub area. thus, according to the results, in some sub basins the diameter of the available culverts were not equal to the calculated diameters of culverts and need to be increased or decreased, which leads to decrease in the costs of culverting and fixing the roads having improper culverts.