Impact of shape and land use on sedimentation in green flood retention reservoirs

opossumoozeMechanics

Feb 21, 2014 (3 years and 3 months ago)

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Impact of shapeand landuseon sedimentationin
greenfloodretentionreservoirs
Developmentof an Integrated Management Strategy for Green Flood
Retention Reservoirs and Polders for Flood protection
Sven Wurms
5th International SedNet conference
Oslo, Norway, 27th –29th May2008
Motivation
Integrated
management
strategyforgreen
floodretention
reservoirs, polders
and floodplains

Quantity/ quality
of
depositedsediments?

Factorsinfluencing
deposition
in retention
reservoirsas well as
on floodplains?
Flood
event
Emission
Deposition
floodplains&
retentionreservoirs
Immission
Miningareas
Green flood
retention
reservoirs
Green floodretention
reservoirHorchheim/ Erft
Erosion
mobilisationof
pollutants
River
Erft
Overview
1.Aim
2.Quantifying transport processes in green flood retention reservoirs
3.Impact of reservoir shapeson 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.
sediment
concen-
tration
Operation
Instream/
bypass
Shape
Land use
Grain size
distribution
Flood
event
Sedimen-
tation
Background
„Integrated management strategy“
Enhancement of the environment
by targeted retention of flood induced,
released contaminated sediments
in 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 depositionin 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
database
quantification
time-and resource-intensivecomplexoften not available
large scope of investigations
(TELEMAC-2D, SUBIEF-2D) concerning factors influencing
sedimentation simplifications necessary
:
•sediment is considered to be medium of conservative contaminanttransport (no desorption/
degradation)
•basic reservoir shapes, simplified concentration graphs as wellas hydrographs
•no erosion (small flow velocities, vegetation)
Impact of reservoir shapes on sedimentation Objects under investigation:
Rectangular instreamgreen flood retention reservoirs, body of
water = 10
6
m³in eachcase, k
str,river
= 30 m1/3/s, kstr,floodplain
= 20 m
1/3/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 *10-2
2.7*10
-2
6.7*10
-3
criticalshearvelocity
fordeposition[m/s]
1.29*10
-2
4.21*10
-3
1.13*10
-3
2.75*10
-4
settlingvelocity[m/s]
150
80
40
20
Grainfraction[µm]
FlowBC
: idealisedinflowhydrographQ
in(t), Q
out
= const.in eachcase
Transport BC
forsuspendedsediments
0
10
20
30
40
04000080000120000160000
0
300000
600000
900000
1200000
[m³
]
[s]
[m³
/
s]
0
10
20
30
40
04000080000120000160000
0
0.1
0.2
0.3
0.4
0.5
Q_in
Q_out
c_in
[
m
³
/
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
grain
fraction
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
Depositedsediment(percentageof total incomingsediment);
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
04000080000120000160000
0
0.1
0.2
0.3
0.4
0.5
Q_in
Q_out
c_in
[
m
³
/
s
]
[
g/l
]
[s]
-5 h
+ 5 h
40.9%
41.4%
+5 h
36.4%
41.4%
-5 h
38.7%
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
04000080000120000160000
volume [

]
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
concentration [g/l]
c_in
portions
volume
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
04000080000120000160000
time of sediment input [s]
sedimentation [%]
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
04000080000120000160000
Qmax
Vmax
time of sediment input [s]
short circuit [%]
time of sediment input: 9h
0
0,1
0,2
0,3
0,4
0,5
3000080000130000180000
time [s]
concentration [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/s
random 1
kstr=10–25 m1/³/s
random 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 µm
Simulations with varying roughness distributions
kstr=10–25 m1/³/s
random 1
kstr=10–25 m1/³/s
random 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
Thankyouverymuchforyourattention!