Longitudinal dams as an alternative to wing dikes in river …€¦ · insight into the range of...
Transcript of Longitudinal dams as an alternative to wing dikes in river …€¦ · insight into the range of...
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Longitudinal dams as an alternative to wing dikes in river engineering
Fredrik Huthoff
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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
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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
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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
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The Rhine
Wing dike lowering
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Wing dike lowering in the Rhine
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?
Lowering of groynes has already started!
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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
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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 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
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Effect on flood levels (WAQUA 2D)
• 2D hydrodynamic mode • Curvilinear grid (cell size ~ 80 by 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
-0.2
-0.18
-0.16
-0.14
-0.12
-0.1
-0.08
-0.06
-0.04
-0.02
0
865870875880885890895900905910915920925930935940945950955960
kribvlg tov refdamopbr tov refdamopsl tov refoptimale variant 1 tov refopeningen in dam
(2b) (1b)
(3)
(groynes)
Water level difference
River kilometer
Long. dams Lowered groynes Lowered groynes
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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
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Computational grid
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Case 2a: dam with side slope 1:2.5 (without openings)
Dam with side slope without openings
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Case 2b: dam with side slope 1:2.5 (with openings)
Dam with side slope with openings
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Input discharge hydrograph (8 discharge levels)
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Results (velocity field)
Q = 1409 m3/s
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Bed level changes After 1 year
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Bed level changes After 2 years
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Bed level changes After 5 years
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Bed level changes After 8 years
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Bed level changes After 10 years
Quick morphodynamic response (1-2 years)
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Bed level changes
Groyne lowering
With openings in dam
Bed level change (m
)
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Bed level changes
Groyne lowering
Without openings in dam
Potential steering parameter?
Bed level change (m
)
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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
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Questions?
Fredrik Huthoff
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Changes in discharge capacity (with respect to current situation)
Correlates well with bed level changes
Q = 3813 m3/s