Targeted Retention of Contaminated

Sediment

in a Green Flood Retention Reservoir

Development of an Integrated Management Strategy

for Green Flood Retention Reservoirs and PoldersSven Wurms

4th international Symposium on Flood Defence

Managing Flood Risk, Reliability and VulnerabilityToronto, Ontario, Canada, May 6–8, 2008

Motivation

Emission

conflict situation! Land use (environmental damage/

loss of values…)

Immission Integrated management strategy for green flood retention reservoirs, polders and floodplains

• Quantity/ quality of deposited sediments?

• Factors influencing deposition in retention

reservoirs as well as on floodplains?

Flood event

Erosionmobilisationof pollutants

Deposition floodplains &

retention reservoirs

Overview

1. Aim

2. Procedure

3. Numerical model “Flood retention reservoir Horchheim”

4. Boundary conditions and scenarios

5. Reservoir sedimentation

6. Conclusions

Aim

Integrated management strategy

1. Quantity and spatial distribution of reservoir sedimentation2. Can this be influenced by

• modified operation rules (outflow discharge)• modified reservoir design?

maximum retention

(enhance situation downstream)

minimum retention

(enhance situation

within reservoir)

Options concerning deposition of potentially contaminated sedimentsin green flood retention reservoirs

spatially targeted retention

(enhance situation within reservoir)

retain present situation

andadapt land use

1. Flow2D-numerical modelling (TELEMAC-2D) of filling and emptying phase

• present conditions

2. Transport2D-numerical modelling (SUBIEF-2D) of suspended sediment transport

• sediment is considered to be medium of conservative contaminant transport (grain diameter: 20 µm)

• only sedimentation, no erosion

Gre

en

flood

rete

nti

on

reserv

oir

H

orc

hh

eim

20-,

50-

an

d 100-

years

flood

even

ts

3. Mass balance/ deposition patternsLong term accumulation of reservoir sedimentation (100 years)

Procedure

• modified operation rules

• modified reservoirdesign

Numerical model “Flood retention reservoir Horchheim”

• ordinary flood retention volume 1.16*106 m³

• area 590000 m2

• HQ100 (inflow gauge) 43.7 m³/s

21849 elements

lmin = 1.25m/lmax = 23 m

• kst river bed 20 m1/3/s• kst agricultural land 25 m1/3/s• kst grassland 20 m1/3/s• kst developed area 10 m1/3/s

0,00

5,00

10,00

15,00

20,00

25,00

30,00

35,00

40,00

45,00

50,00

0 10 20 30 40 50 60 70 80t [h]

Q [

m³/

s]

HQ20 (5h rising time)HQ20 (35h rising time)HQ50 (5h rising time)HQ50 (35h rising time)HQ100 (5h rising time)HQ100 (35h rising time)

Boundary conditions and scenarios

Flow BC: Two sets of hydrographs (5 h and 35 h time of rise)

Transport BC:

Assumption: Linear relation between inflow discharge and suspended sediment concentration with a maximum of 1 g/l.

scenario operation rule reservoir design

• present conditionsconstant outflow (30

m³/s)present condition

• modified operation rules

reduced outflow to use entire ordinary retention volume and maximise

deposition

present condition

• modified reservoir design

constant outflow (30 m³/s)

modified design for spatially targeted

deposition

HQ time of

rise

present conditions/ modified design modified operation rules

outflow [m³/s]

volume [m³]

duration

[h]outflow [m³/s]

volume [m³]

duration

[h]

20 5 h

30

7980 4 8.3

1.16 * 106

98

50 5 h 110940 14 12 76

100 5 h 260580 21 15.4 65

Reservoir sedimentation – present conditions

5 h rise 35 h riseDeposition accumulated over 100 years

5107 sediment inflow [t] 13629

145 deposition [t] 535

2.8 deposition [%] 3.9

Reservoir sedimentation – modified operation rules

5 h rise 35 h rise

10489 sediment inflow [t] 21218

5107 deposition [t] 5737

48.7 deposition [%] 27.0

Deposition accumulated over 100 years

0

2000

4000

6000

8000

10000

12000

5*HQ20 2*HQ50 1*HQ100 total

sedi

men

t m

ass

[t]

deposition

sediment inflow

0

2000

4000

6000

8000

10000

12000

5*HQ20 2*HQ50 1*HQ100 total

sedi

men

t m

ass

[t]

deposition

sediment inflow

present conditions modified operation rules

5 h rise

Reservoir sedimentation – modified reservoir design

presentconditions5 h rise

modified design 5 h rise

5107 sediment inflow [t] 5107

145 deposition [t] 163

2.8 deposition [%] 3.2

Deposition accumulated over 100 years

Vmod = 12500 m³

20 µm 40 µm

80 µm 150 µm

Sediment deposition

Grain size

[µm]

inflow

[t]

deposition

[t]

20 2560115

(4,5%)

40 2560233

(9,1%)

80 2560535

(20,9%)

150 2560761

(29,7%)

Deposition height [m]

HQ 5035 h time of rise

Reservoir sedimentation – varying grain size

Conclusions

• Knowledge of deposited masses of potentially contaminated sediments provides a basis for an integrated management strategy for green flood retention reservoirs

• Possibilities of taking influence on sedimentation in green flood retention reservoirs could be shown

• Maximizing/ minimizing of reservoir sedimentation can be done efficiently by modifying operation rules

• Deposition pattern can be influenced up to a certain extent by modifying reservoir design

• Numerical transport simulations should be performed for every grain size of interest due to varying sedimentation pattern

Related works presented on ISFD 2008

• Christoffels, E. et al.

An Integrated Management Strategy for Green Flood Retention Reservoirs, Polders and Floodplains - Taking account of contaminants

(Poster)

• Schönau, S. et al.

Erosion and Sediment Yield Estimation for Flood Protection

(Presentation)

Thank you very much for your attention!