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.