Hardwood forests of northern part of Iran are located at the latitude of 35° 46´ to 36° 58´ in mountainous areas. The annual rainfall of the areas is approximately 1000mm. Due to road constructions which are increasingly developing in these areas, it is important to recognize the hydrological considerations and to establish canals and cross-section drainage to prevent the destructive effects of runoffs and upper areas development, on roads. Therefore, 39 subareas were considered to determine the culvert diameters for Neka-Zalemrood catchment area. Flow gravity of each catchment was measured using a numerical curve method. Results of this study showed that culvert diameters changed from 11 inch to 62 inch during a 25-years returning period. Due to mountainous nature of the study area, sub-areas with culvert diameters below 24 inch were combined to reach the minimum diameter determined in the magazine number 131 of the Iranian Strategic Planning and Control Organization. Finally, the best culvert diameters were determined for each sub-area
