Hydraulic capacity of culverts under sediment transport - Multibarrel Setup

As a part of the research program Natural hazards – Infrastructure for Floods and Landslides (NIFS), the hydraulic performance of the culverts are presently investigated in a scale model study carried out in the NTNU hydraulic laboratory. The background for this model study is the nonexistence of the culvert design guidelines with the sediment effect, and insufficient knowledge on culvert design in steep streams under consideration of sediment transport. The model consists of a collecting reservoir, an approach channel, an expansion section and a culvert system installed in a 45° embankment. The approach channel with a slope of 1:9 presents a steep stream in which the flow is always supercritical. The purpose of the model is to test the effect of different parameters on the culvert capacity under sediment transport conditions. The test parameters include length and width of the expansion section, inlet geometry, slope of the stream, size and amount of sediments, and the different ways by which sediments approach the culvert (i.e. continuous sediment transport and landslide transport in the stream) and number of the culvert barrels. Recent studies on the model investigated the effect of the length, the width of the expansion section and the slope of the approach channel. The present study focuses on the use of multibarrel culvert system. In each experiment, headwater was measured for different discharge increments, and the results were used to make performance curves or headwater-discharge curves for each parameter. Results showed that the inlet geometry is the most influential parameter on the culvert hydraulic and capacity, and the sediment transport. Both wingwalls inlet and cut inlet gave similar capacity results, but wingwalls inlet presented a more stable flow condition in the expansion section. The ability of these two inlet types to transport sediment was very poor. On the other hand projecting inlet gave a lower capacity but it was able to transport more sediments than the other two inlet types. However, by comparing the results with previous study, it was determined that the increased water level and a flow with reduced energy in the expansion section caused by the energy dissipater blocks was the actual reason for the low sediment transport by wingwalls and cut inlet. Both continuous sediment and landslide transport showed some elevation in headwater results but it was not significant enough to change the capacity of the culvert. However, increased amount and size of the sediment did not give any further significant elevation in the headwater. The main function of the reserve culvert in this multibarrel system was found to be the reduction of the headwater, as it did not have any effect on the pattern of sediment deposition. Sediments were deposited mostly on the centre line. However, in comparison with single barrel system it gave a flow with higher velocity towards the main culvert for the same headwater. The results of this study did not give a complete solution for culvert in steep rivers, but these results can be more helpful in combination with the other studies