Improving culvert performance

Abstract

In Dutch polders, numerous structures like bridges, weirs, culverts, and pumping stations have been constructed over centuries to manage water levels. These structures play a crucial role in maintaining water levels within predefined targets. The flat topography of the Dutch landscape combined with the collective impact of head losses, induced by these structures may result in flooding of polders during high runoff scenarios. Over time, culverts and bridges may underperform due to alterations in the water system, increased pressure from climate change, evolved design rules, insufficient maintenance, and shifts in land use.
A challenge is the potential hydraulic underperformance of structures and the need for their premature replacement, which is costly. Waiting until the end of their technical lifespan may contribute to floods. Therefore this thesis focuses on improving existing structures to mitigate the need for replacement, specifically by streamlining inlet and outlet openings to reduce energy losses. This leads to the research question of this thesis: “How can the head loss over existing (too tight) culverts be minimised by adding an inlet or outlet profile and does this lead to a substantial enhancement in the performance of these culverts, providing a practical option to postpone the replacement of underperforming culverts?”
To answer this question, the problem is explored by looking into the fundamentals of energy losses, including entrance losses, friction losses, and exit losses. This gives an understanding of the conditions under which these losses manifest. However, these basic calculations have inherent limitations due to their reliance on predefined coefficients. This renders them inadequate for evaluating the effects of introducing new profiles onto an existing structure. To overcome this, a flume experiment has been performed to verify whether it is possible to measure water level differences for various profiles at the culvert entrance and exit. With a 3D Computational Fluid Dynamics (CFD) model (OpenFOAM), flows around different culverts are simulated. The results of the CFD model are compared to the flume experiment, after which the CFD model is used to simulate a variety of scenarios, with different profiles, culvert dimensions, velocities, and water depths.
As such, this thesis addresses challenges and uncertainties in quantifying head losses in culvert structures through experimental methods and CFD modelling. Experimental setups struggle with controlling all flow-influencing parameters, while CFD modelling offers flexibility but requires careful consideration of uncertainties and limitations. The discussion emphasizes the complexities of comparing experimental and model results, highlighting trade-offs and uncertainties in each approach.
The conclusion answers the central research question, confirming that specific profiles added to culverts can significantly reduce entrance losses up to 65%, thereby lowering headwaters for a constant discharge. The recommendations section outlines possibilities for further research, including optimizing profile dimensions and conducting sensitivity analyses of influential parameters. Practical recommendations involve aligning large-diameter concrete culverts with the socket end in the flow direction and integrating groove or rounded profiles during construction for cost-effective inlet loss reduction.