Dimensioning of the hydraulic structures on the forest roads by using hec-ras software

The precipitation waters should be cleared from the forest roads quickly so as to make these roads durable and let them perform the expected functions in a sustainable way. Within this scope, maximum flow rate that may hit the intersections of the forest roads with the streams throughout the life span of the project should be known while hydraulic structures are constructed at this point and the span of the structure should be designed to pass this flow smoothly. In this study, 410 code numbered forest road which is located in Acısu Forest Management Directorate in Gerede Forest Management Directorate which is affiliated to Bolu Forest Regional Directorate has been specified as the working area and examined in respect to hydraulic structures that were built or should be built on this road. The road is not situated on the country maps as it was built in 2018. Because of this reason, the orthophoto map of the 410 numbered road was created by using an Unmanned Aerial Vehicle (UAV) platform and the points where there should be hydraulic structures on the forest road were identified on this map. Fifteen watershed areas of which water should flow through the 410-code numbered forest road was detected and it was found that six of them passed the road by placing corrugated pipes. It was also detected that construction was needed at nine more points. In consequence of the examinations on orthophoto, it was identified that a total of 648m landslide took place at 9 different points along the road which is 6+000 km long in total. It was also found that landslides that occurred at 8 points was because of the lack of transverse drainage structure or the narrowness of the sectional area in the existing structure. Besides, Grand Soil Group (GSP) map of Turkey was used in order to specify the soil types, stand data map was used to specify the land use, and country maps were used in order to specify the parameters for the calculation of the flow rate such as catchment areas, slope, stream axis length and stream slope. Using the data obtained, maps of Hydrological Soil Groups, Curve Numbers that are used in determining the rate of conversion to flow, and map of area-weighted average slope classes were created. Precipitation data, which has been recorded since 1963, were provided from by Meteorology Station. By using these parameters, design discharges for the watersheds were calculated with the precipitation to flow rate determination methods that calculate runoff from rainfall intensity. For HEC-RAS software to be able to be used in determining the span of the structures established at the stream-road intersections, hydraulic models of these points should be built. Digital Elevation Model (DEM) is required to generate geometry data which is one of vital input parameters of the model made up with HEC-RAS software. The high accuracy and resolution of the DEM data that will be used in the model affects the accuracy of the results as well. Because of this reason, DEM data which was obtained from the existing maps was not able to be used. DEM data with the targeted accuracy and resolution was generated from the images of the intersection points where existing structures were situated with the Unmanned Aerial Vehicle (UAV) systems. Thanks to simulations made with the hydraulic models, the structure span (sectional area) that will carry the flow smoothly was found out and we proposed the arrangements which ought to be made in the stream beds in order not to allow the overflow of the incoming water to the road. In the simulations which were made according to the calculated flow rates in the existing six structures, four of them were found incapable of carrying the flow that might come. Although two of them were capable of carrying the flow, they caused spreading of the flow since they did not have head and wing walls and the stream beds were not arranged. Moreover, their openness was calculated with the "Talbot Formula" which is the empirical formula that gives the direct sectional area of the structures. The sectional areas of the structures existing in the area were compared with the sectional areas that were calculated according to the "Talbot Formula" of the same structures and by using HEC-RAS in accordance with the flows. As a consequence of this, it was seen that the sections calculated according to the "Talbot Formula" were also narrow compared to the sections calculated by HEC-RAS. Keywords: Drainage, Hydraulic structure design, HEC-RAS, UAV-DEM, Forest roads, Rainfall-Runoff.