3D hydraulic modelling of the flow control structure and drop shaft upstream of a retention basin
As part of a new retention basin project, a French metropolitan authority needed to validate the design of the basin’s inlet, made up of a side-weir overflow and a stepped drop shaft. 3D EAU built a detailed 3D model of the structure to assess its discharge capacity and provide the input data for the structural design.
Optimising a basin inlet with 3D modelling
The client wanted to validate the sizing of a retention basin's inlet at the pre-study stage, before detailed design. The inlet combines a flow control structure with a drop shaft of 3 steps, each 7.5 m high.
The 3D model had to answer three key questions:
- Does the proposed design perform as intended for 10-year and 20-year rainfall events?
- What force is applied to the steps?
- Can the design be optimised, and how do the alternatives perform?
Why model a drop shaft in 3D ?
Drop shafts, whether vortex or stepped, are typically used to guide the flow when effluent has to drop several metres.
They are usually sized using empirical methods based on experimental data. Yet every structure is unique: depth, flow range and site constraints all differ, so it is hard to be sure that published results can be extrapolated with confidence.
When a drop shaft falls outside the range of commonly tested configurations, 3D modelling can confirm its flow capacity and optimise the design where needed.
From hydraulic analysis to 3D simulation
Hydraulic analysis
Preliminary review of the hydraulic behaviour to identify the weak points of the design.
Structure modelling
Digital reproduction of the geometry and mesh generation.
Flow simulation
3D numerical solution of the Reynolds-averaged Navier-Stokes (RANS) equations.
Results analysis
Interpretation of the simulations, extraction of key parameters and optimisation proposals.
Design issues revealed by simulation
The simulations made it possible to assess all the main hydraulic phenomena at work in the structure.
Watch our summary video
Key findings
The transit pipe and drop shaft discharge the flow effectively.
The overflow is governed by the intake capacity of the transit pipe, so the weir crest is partially or fully submerged for both design flows.
The inlet and retained pipes are at risk of partial surcharge (Q10) and full surcharge (Q20).
Install a conical (tapered) inlet at the entrance of the Ø1200 transit pipe to increase its intake diameter.
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