Impact of shape and land use on sedimentation in

green flood retention reservoirsDevelopment of an Integrated Management Strategy for Green Flood

Retention Reservoirs and Polders for Flood protection

Sven Wurms

5th International SedNet conference

Oslo, Norway, 27th – 29th May 2008

Motivation

Integratedmanagementstrategy for greenflood retentionreservoirs, poldersand floodplains

• Quantity/ quality of deposited sediments?

• Factors influencingdeposition in retentionreservoirs as well as on floodplains?

Floodevent

Emission

Depositionfloodplains &

retention reservoirs

Immission

Mining areas

Green floodretentionreservoirs

Green flood retentionreservoir Horchheim/ Erft

Erosionmobilisation of

pollutants

River Erft

Overview

1. Aim

2. Quantifying transport processes in green flood retention reservoirs

3. Impact of reservoir shapes on sedimentation

4. Detailed transport simulations

5. Impact of land use on sedimentation

6. Conclusions

Aim

Factors influencing sedimentation processes in green flood retention reservoirs

Susp. sedimentconcen-tration

Operation

Instream/bypass Shape

Land use

Grain size distribution

Flood event

Sedimen-tation

Background „Integrated management strategy“

Enhancement of the environmentby targeted retention of flood induced,

released contaminated sedimentsin green flood retention reservoirs?

Knowledge of effectiveness of factors influencing sedimentation and

their interaction is essential!

Investigation of the impact of shape and land use on sedimentation in

green flood retention reservoirs

Quantification of deposition in green flood retention reservoirs

• Q(t) (in, out)• c(t)• cross sections• geo data• land use data• sediment specific parameters

• (measured events)

• advection• dispersion• sedimentation• (erosion)• (sorption)• (degradation)

filling and emptying phases unsteady

modelling

• 2d-hn-modelling• physical modelling

processes databasequantification

time- and resource-intensive complex often not available

large scope of investigations (TELEMAC-2D, SUBIEF-2D) concerning factors influencing sedimentation simplifications necessary:• sediment is considered to be medium of conservative contaminant transport (no desorption/

degradation)• basic reservoir shapes, simplified concentration graphs as well as hydrographs • no erosion (small flow velocities, vegetation)

Impact of reservoir shapes on sedimentation

Objects under investigation: Rectangular instream green flood retention reservoirs, body of water = 106 m³ in each case, kstr,river = 30 m1/3/s, kstr,floodplain = 20 m1/3/s

Sediment properties (Sedimentation flux )

1000

m

500 m

245 m

1000

m

1500 m

745 m

1000

m

500 m

120 m

1000

m

500 m

490 m

1500

m

250 m

120 m

0.279.9 *10-22.7*10-26.7*10-3critical shear velocityfor deposition [m/s]

1.29*10-24.21*10-31.13*10-32.75*10-4settling velocity [m/s]

150804020Grain fraction [µm]

Flow BC: idealised inflow hydrograph Qin(t), Qout = const. in each case

Transport BC for suspended sediments

0

10

20

30

40

0 40000 80000 120000 1600000

300000

600000

900000

1200000

[m³]

[s]

[m³/ s

]

0

10

20

30

40

0 40000 80000 120000 1600000

0.1

0.2

0.3

0.4

0.5Q_in

Q_out

c_in

[m³/

s ]

[g/l]

[s]

Q_in

Q_out_14

volume 14

48.9 %56.3 %60.4 %58.2 %70.6 %80 µm

44.3 %43.1 %42.5 %42.1 %38.7 %20 µm

46.6 % 49.0 % 50.4 %49.0 %56.4 %40 µm

51.8 %64.9 % 69.3 % 66.5 %78.3 %150 µm

grainfraction 10

00 m

500 m

245 m

1000

m

1500 m

745 m

1000

m

500 m

120 m

1000

m

500 m

490 m

1500

m

250 m

120 m

Deposited sediment (percentage of total incoming sediment); Qout = 14 m³/s

Deposition pattern; Qout = 14 m³/s

Deposition height [m] Deposition height [m]

d = 20 µm d = 150 µm

0

10

20

30

40

0 40000 80000 120000 1600000

0.1

0.2

0.3

0.4

0.5Q_in

Q_out

c_in

[ m³ /

s]

[g/l]

[s]

- 5 h + 5 h

40.9 %41.4 %+5 h

36.4 %41.4 %-5 h38.7 % 42.1 %0 h

1000

m

500 m

1500

m

250 m

Time dependency of sedimentation processes

Transport BC

temporal shift of concen-tration

0 h

time [s]

Detailed transport simulations

Input of several portions of suspended sediment at different levels of filling into the reservoir instead of continous sediment input 1 simulation per input portion (d=20 µm, Qout=14 m³/s)

0

200000

400000

600000

800000

1000000

0 40000 80000 120000 160000

volu

me

[ m³]

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

con

cen

trat

ion

[g

/l]

c_inportionsvolume

0 h

3 h

6 h

9 h

12 h

18 h

24 h 30 h 36 h 42 h 48 h

0

10

20

30

40

50

60

70

0 40000 80000 120000 160000

time of sediment input [s]

sedi

men

tati

on [

%]

Qmax Vmax

Result: Deposition (percentage of total incoming sediment); f (time of sediment input)

d = 20 µm, Qout = 14m³/s

0

10

20

30

40

50

60

70

80

90

100

0 40000 80000 120000 160000

Qmax Vmax

time of sediment input [s]

shor

t ci

rcu

it [

%]

time of sediment input: 9h

0

0,1

0,2

0,3

0,4

0,5

30000 80000 130000 180000

time [s]

conc

entr

atio

n [g

/l]

20 µm input

20µm_output

short circuit

input

Result: Short circuit (percentage of total incoming sediment); f (time of sediment input)

Example of deposition pattern f (time of sediment input)

Impact of land use on sedimentation

• Impact of seasonal changes/ different types of land use on sedimentation?

• Degree of uncertainties concerning calculated deposition masses due to estimation of roughness parameters?

Simulations with varying roughness distributions within green flood retention reservoir

Range of amount and pattern of deposited sediments

kstr=10–25 m1/³/srandom 1

kstr=10–25 m1/³/srandom 2

kstr=10 m1/³/s kstr=20 m1/³/s kstr=35 m1/³/s

Deposition:46.0 %

Deposition:45.6 %

Deposition:43.5 %

Deposition:45.3 %

Deposition:46.0 %

grain fraction d = 20 µmSimulations with varying roughness distributions

kstr=10–25 m1/³/srandom 1

kstr=10–25 m1/³/srandom 2

kstr=10 m1/³/s kstr=20 m1/³/s kstr=35 m1/³/s

Deposition:63.0 %

Deposition:63.4 %

Deposition:57.2 %

Deposition:58.1 %

Deposition:63.4 %

grain fraction d = 150 µm

Conclusions

• Effectiveness of sediment retention in green flood retention reservoirs strongly depends on the combination of grain size and reservoir shape

• The bigger the grain size, the smaller the area of deposition and the smaller the impact of shape and land use on sedimentation pattern and masses

• One mean value is not suitable for the description for the effectiveness of sediment retention for a given reservoir due to the strong dependence on time of sediment input

• Different shaped reservoirs have their maximum effectiveness of sediment retention at different stages of filling (knowledge is important for an adaptive reservoir operation)

• The short circuit of suspended sediment flow reduces the effectiveness of sediment retention enormously, this can be influenced by modified operation or reservoir design

• Land use has only a small influence on sedimentation, modelling errors due to rough estimation of parameters are small

Thank you very much for your attention!