Longitudinal dams as an alternative to wing dikes in river engineering

1. Longitudinal dams as an alternative to wing dikes in river engineering Fredrik Huthoff

2. Contents • Introduction • Why consider longitudinal dams? • “Room for the river” in the Netherlands • The pilot study • The Dutch Rhine • Wing dike lowering • Pilot study for longitudinal dams • Numerical model • Results • Impacts on flow • Morphodynamic effects • Discussion & Conclusions

3. Introduction • Are longitudinal dams a suitable alternative for wing dikes? • Functions of wing dikes • Protection of banks • Maintain required flow depths for navigation during low-discharge situations • Wing dikes force the flow field towards the main channel in the river during low flows • Larger flow depths • Less sedimentation in the main channel • Disadvantages of wing dikes • during high-discharge conditions wing dikes obstruct the flow and may lead to higher flood levels • Design is not flexible • Wing dikes cause local disturbances to flow and river beds

4. Longitudinal dams: a pilot study • A study for longitudinal dams in the Rhine • Use dams instead of (lowered) wing dikes • Computational study to investigate hydrodynamic and morphodynamic effects • Consider different types of dams in order to get insight into the range of possible effects

5. Wing dike lowering The Rhine

6. Wing dike lowering in the Rhine Lowering of groynes has already started! Room for the river • Main objective: increase safety by reducing water levels on the Rhine • The target is approximately 10 cm water level lowering at the design discharge of 16000 m3/s • This can be (partly) achieved by lowering the wing dikes by approx. 1,5 m over a length of 76 km (DHV 2011) • Is a longitudinal dam a suitable alternative for wing dikes?

7. Pilot longitudinal dams • Remove 37 (lowered) wing dikes on inner bends and add longitudinal dams in a river section of 11 km length • Wing dikes on outer bends are not lowered • Height of dam is comparable to height of the (unlowered) wing dikes • Consider variations in dams: • vertical wall and dam with side slope • with and without openings

8. Effect on flood levels:translate bed elevation to WAQUA 2D • 2D hydrodynamic model • Curvilinear grid (cell size~80by 20 m) • Calculate water levels at design discharge (16000 m3/s) • Several cases considered: 1a: dam as vertical wall (without openings) 1b: dam as vertical wall (with openings) 2a: dam with side slope 1:3 (without openings) 2b: dam with side slope 1:3 (with openings) 3: dam with side slope 1:2.5 (with openings) and removal of obstacles in the river bank section

9. Effect on flood levels (WAQUA 2D) (groynes) (2b) • 2D hydrodynamic mode • Curvilinear grid (cell size~80by 20 m) • Calculate water levels at design discharge (16000 m3/s) • Several cases considered: 1a: dam as vertical wall (without openings) 1b: dam as vertical wall (with openings) 2a: dam with side slope 1:3 (without openings) 2b: dam with side slope 1:3 (with openings) 3: dam with side slope 1:2.5 (with openings) and removal of obstacles in the river bank section (1b) (3) Water level difference Long. dams Lowered groynes Lowered groynes River kilometer

10. Morphodynamic effects: translate bed elevation to Delft3D • Deltares’ open source package • 2D hydro- & morphodynamic mode • Curvilinear grid (cell size~80by 20 m) • Refined grid in pilot study area (factor 3) • Sediment transport • Uniform sediment • Van Rijn • Morphological calibration • Adopt grain size to get agreement with measured sediment transport loads • Study the RELATIVE EFFECT of longitudinal dam • Compare to projected development of current situation

11. Computational grid

12. Case 2a: dam with side slope 1:2.5 (without openings) Dam with side slope without openings

13. Case 2b: dam with side slope 1:2.5 (with openings) Dam with side slope with openings

14. Input discharge hydrograph(8 discharge levels)

15. Results(velocity field) Q = 1409 m3/s

16. Bed level changes After 1 year

17. Bed level changes After 2 years

18. Bed level changes After 5 years

19. Bed level changes After 8 years

20. Bed level changes After 10 years Quick morphodynamic response (1-2 years)

21. Bed level changes With openings in dam Bed level change (m) Groyne lowering

22. Bed level changes Without openings in dam Bed level change (m) Potential steering parameter? Groyne lowering

23. Conclusions • LDs may be a suitable alternative for wing dikes • Maintain flow depths & sediment transport rates during low flows • Quick morphodynamic response (1-2 years) • Advantages over wing dikes are: • LDs give less resistance to flow during high discharge events (lower flood levels) • Additional flow area can be created behind the dams to lower flood levels • LDs allow easy readjustment of inlets/outlets to correct for unwanted morphodynamic effects (minimize dredging efforts) • Future studies of LDs should also focus on • Combination of LD-designs and monitoring strategies to optimize LD-design (openings in dam) • morphodynamics behind the LD in order to get insight into stability (or required maintenance) of LD-designs

24. Questions? Fredrik Huthoff

25. Correlates well with bed level changes Changes in discharge capacity (with respect to current situation) Q = 3813 m3/s