1. Presented by Arne van der Hout Tom O’Mahoney Tommaso Boschetti
21 september 2018
Renovation of weirs in the Lower Rhine and Lek Computational models as support for weir operations in off-design conditions
Presented by Arne van der Hout
Tom O’Mahoney
Tommaso Boschetti
21 september 2018

2. Location of the weirs
The weirs Amerongen and Driel are on the lower Rhine. The river lower Rhine changes its name to the Lek just before weir Hagestein Function of the weirs is to maintain navigation depth along this course
Amsterdam
Hagestein
The Hague
Driel
Amerongen
Rotterdam
21 september 2018

3. Introduction to the weir
Views of the gates in operation And fully open
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4. Renovation of the weir gates
River canalized in 1960s with three weir complexes each with two large visor gates (and a small culvert regulated with a cylinder valve for small discharges < 100m3/s) The weirs were designed and tested using extensive physical scale model tests to determine discharge characteristics, necessary bed protection and vibration behaviour This year the gates are being renovated. So for some periods the discharge at each of the weirs will have to be regulated with only one gate instead of two (the other opening will be closed by a temporary dam)
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5. Project aims Two questions:
How should the weir be operated when only one gate is used?
At which position should the single gate be placed to control the waterlevels in the same way as the two gated weir
By how much should the bed protection be strengthened/lengthened to account for the higher velocities because only one opening is used?
What does the flow field look like downstream if only one gate is used instead of two
We aimed to answer these questions with modern numerical tools rather than scalemodels
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6. Technical drawings – side view
Source: Rijkswaterstaat
Note the presence of an energy dissipating sill downstream
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7. Question 1 - Weir programme
10 years of daily data available for waterlevels and valve positions
The weir at Hagestein has some influence from the tide at the downstream boundary which causes the water level fluctuation there
At large discharges (>~650m3/s) the weir gates are fully opened and the levels upstream and downstream are the same
21 september 2018

8. Discharge coefficient
Where: Q the discharge (m3/s) μ the discharge coefficient (-) Ak the throughflow area (m2) g the accleration due to gravity (m/s2) h1 the waterlevel upstream (m) h2 the waterlevel downstream (m) We calculate the discharge relation from the available data and hope to determine how the weir should be operated when only a single gate is available Note that actually we do not need to calculate the area under the viser or the discharge coefficient but only the product of the two (μAk)
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9. 21 september 2018
Calculated μAk
Linear trend as function of gate position – means that for situation of a single gate – just open the gate twice as far!
This answers question 1
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10. Question 2 - Velocity at bed
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Question 2 - Velocity at bed
The linear trend for the discharge means that the valve should be set twice as high for one gate as for two gates. What does this do to the velocity at the bed? It does not mean that this will be the same as in the two gate situation Need to look at how the sill works – for this we use Computational Fluid Dynamics
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11. 21 september 2018
CFD 2D
The sill works best for smaller openings where the jet is forced upwards. The bottom right picture shows the valve maximum valve position for two gates (for higher discharges the gates are opened and the flood plains are filled). When one gate is used the gate will be more often at this higher position and at positions that are even higher than this. Note: the bottom edge of the gate always stays underwater even if it is placed twice as high.
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12. 21 september 2018
Overview 3D CFD model
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13. 3D results - watersurface
The working of the sill can be seen from the disturbance of the water surface – in real life and in the CFD
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14. Comparison of 1 gate case with 2 gates
21 september 2018
Comparison of 1 gate case with 2 gates
Near-bed velocity
The flow does not spread quickly in the horizontal downstream of the single opening Extra bed protection is needed. Approx. 200m. Answer for question 2
The maximum velocity in each is the same because the effective area under the gates is the same (the smallest throughflow area). But away from the gates only half the canal is used so the velocities are higher in the single-gate operation. The sill also is used less effectively whereby the near bed velocities are still higher.
Note the worse situation is just before the gates are opened fully and the flood plains begin to be filled.
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15. Velocity distribution
Large concentration of flow in the middle, despite the expectation that the visor shape would provide radial outflow
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16. Conclusions
Q1 - As a first estimate, the single gate should be placed twice as far open as would be the case for double-gate operation
Q2 - More bed protection is needed, extra rocks should be added for up to 200m downstream of the sill or current bed protection
General conclusions
Sophisticated 2D and 3D computational tools were able to give a good understanding of the flow characteristics in a short time
Modern computational models even for complex geometries are not too large an investment for these studies
21 september 2018