Technical assessment of culverts diameter on forest roads (Case study: Dehga District, No 35 of Tonekabon Basin)

Runoff generated by rain in forest areas and its flow on the roads can cause road damage and disrupt the operation and transportation. This research aims at minimizing the runoff damage to the road due to the flood by determining the optimal culverts diameter on the path of Tuska Kalam - Lyre Sar, Tonekabon, basin 35 in 23 forest sub-basins. Using the Curve Number method with different return periods, the maximum runoff flow of each sub-basin was calculated in cubic meters per second. Then, the optimal culvert diameter of the sub-basins was determined by using the Nomograph for determination of the diameter of concrete pipes. The results showed that on the B3 sub-basin, the maximum runoff for the return periods of 5,10,25,50 and 100 years is 3.37, 4.48, 6.04, 7.29 and 8.63 cubic meter per second, respectively, in which the culverts diameter is not suitable for the maximum passage runoff above the 5-year return period. On the B23 sub-basin, the maximum runoff for the return periods of 5,10,25,50 and 100 years is 2.24, 2.98, 4.02, 4.86 and 5.75 cubic meters per second, respectively, in which the culverts diameter, for the maximum passage runoff different return periods, is more than optimal. On the B9 sub-basin, the maximum runoff for the return periods of 5,10,25,50 and 100 years is 2.8, 3.73, 5.02, 6.06 and 7.17 cubic meters per second, respectively, in which the culverts diameter, for the maximum passage runoff different return periods, is optimal.