Hydrological Modelling of the Ilabekken Catchment, and Flood Simulations of Ilabekken using HEC-HMS and HEC-RAS

As the global climate is changing, the local climate in Trøndelag is expected to be wetter and more extreme in the future. Flooding and erosion are huge challenges in Norway, leading to severe damages each year which lead to large costs for the society. As a result, the society demands a greater focus on the climate change impacts. The aim of this thesis is to investigate how the climate change will impact the risk of erosion and flooding in Ilabekken, an urban stream in Trondheim. This will be done by creating a hydrological model of the catchment upstream of Ilabekken, and a river model of the upper part of Ilabekken, using the software HEC-HMS and HEC-RAS. The thesis includes a calibration of the HEC-HMS model by using observation data from Kobberdammen, located in the upper part of the catchment. To be able to establish the HEC-RAS model, surveying the river geometry has been conducted. The stability in the stream can be evaluated by investigating the hydraulics in the stream. Computation of the stable stone sizes in parts of the stream exposed to erosion has been done using the Shields equation and the HEC-11 method. The 200 year flood event is computed to be 17.14 $m^3/s$. This magnitude is based on a 6 hours synthetic storm event computed by statistical precipitation data from Voll observation site. The simulation shows that Theisendammen will have a damped effect on the runoff. Since there are three larges lakes in the catchment, these will likely have a reducing effect and contribute to a delay of the runoff. During a 200 year flood event it will likely be flooding in the lowest part of the modelled reach. The simulations shows that increasing flood magnitudes could lead to a flow pattern producing hydraulic jumps at several places along the stream which could possibly cause erosion. The main trend in the computations of $D_{50}$ indicate that increasing runoff will induce larger stable stone sizes, however this is not the case in all part of the stream. In shallow areas the size of $D_{50}$ will decrease with increasing runoff due to higher water depths and decreasing velocity. The computations of stable stone size show a great difference between the methods. The results shows that the HEC-11 method is very sensitive when the angle of the side slope approach the angle of repose. This method should be used carefully when estimating stable stone sizes in steep side slopes. Transport of sediments can lead to sedimentation in the lower part of the stream where the velocity decreases due to more shallow areas. It has already been observed in the lower part of the stream. A consequence is poorer biological diversity in the area, due to accumulation of sediments