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

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!

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 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

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

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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

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)

Results (velocity field)

Q = 1409 m3/s

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

Groyne lowering

With openings in dam

Bed level change (m

)

Bed level changes

Groyne lowering

Without openings in dam

Potential steering parameter?

Bed level change (m

)

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)

Correlates well with bed level changes

Q = 3813 m3/s