Longitudinal dams as an alternative to wing dikes in river engineering Fredrik Huthoff
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
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
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
The Rhine Wing dike lowering
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?
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
Effect on flood levels: translate bed elevation to WAQUA 2D 2D hydrodynamic model Curvilinear grid (cell size ~ 80 by 20 m) Calculate water levels at design discharge (16000 m 3 /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
Effect on flood levels (WAQUA 2D) Water level difference 0-0.02-0.04 2D hydrodynamic mode Curvilinear grid (cell size ~ 80 by 20 m) openingen in dam Calculate water levels at design discharge (16000 m 3 /s) -0.06-0.08-0.1-0.12-0.14-0.16-0.18 Several cases considered: kribvlg tov ref (groynes) damopbr tov ref (2b) damopsl tov ref (1b) optimale variant 1 tov ref 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 Lowered groynes Long. dams Lowered groynes in the river bank section (3) -0.2 960 955 950 945 940 935 930 925 920 915 910 905 900 895 890 885 880 875 870 865 River kilometer
Morphodynamic effects: translate bed elevation to Delft3D Deltares open source package 2D hydro- & morphodynamic mode Curvilinear grid (cell size ~ 80 by 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
Computational grid
Case 2a: dam with side slope 1:2.5 (without openings) Dam with side slope without openings
Case 2b: dam with side slope 1:2.5 (with openings) Dam with side slope with openings
Input discharge hydrograph (8 discharge levels)
Q = 1409 m3/s Results (velocity field)
Bed level changes After 1 year
Bed level changes After 2 years
Bed level changes After 5 years
Bed level changes After 8 years
Bed level changes After 10 years Quick morphodynamic response (1-2 years)
Bed level changes With openings in dam Bed level change (m) Groyne lowering
Bed level changes Without openings in dam Bed level change (m) Potential steering parameter? Groyne lowering
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
Questions? Fredrik Huthoff
Changes in discharge capacity (with respect to current situation) Q = 3813 m3/s Correlates well with bed level changes