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Transcript of May 6-8, 2008 1 PROBABILISTIC SENSITIVITY ANALYSIS OF DUNE EROSION CALCULATIONS 4 th International...
![Page 1: May 6-8, 2008 1 PROBABILISTIC SENSITIVITY ANALYSIS OF DUNE EROSION CALCULATIONS 4 th International Symposium on Flood Defence, Toronto, Canada Kees den.](https://reader035.fdocuments.net/reader035/viewer/2022062515/56649cfa5503460f949cbfd3/html5/thumbnails/1.jpg)
May 6-8, 2008
1
PROBABILISTIC SENSITIVITY ANALYSIS OF DUNE EROSION CALCULATIONS
4th International Symposium on Flood Defence, Toronto, Canada
Kees den Heijer, Jan van de Graaff and Pieter van Gelder
Section of Hydraulic Engineering
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May 7, 2008 2
Contents
• Introduction• Model description• Probabilistic model• Dune erosion model
• Approach• Results• Conclusions
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May 7, 2008 3
Introduction
• approximately 350 km Dutch coastline• 254 km dunes• 34 km sea dikes• 38 km beach flats• 27 km of boulevards, beach walls etc.
• Design conditions very extreme (10-5 year-1)• No prototype data for these extreme situations• Policy to maintain coastline of 1990
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May 7, 2008 4
Introduction
Current method for Dutch dune safety assessment:
• Empirical model• Applied in semi-deterministic way• Conditions based on probabilistic investigations
(WL | Delft Hydraulics, 2007)
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May 7, 2008 5
Objective
• This study aims at more insight in the sensitivity of the rate of dune erosion for the stochastic characteristics of various variables, currently taken into account.
Remarks:• 10-5 year-1 probability of exceedance• DUROS-plus dune erosion model• Simple cross-shore profile
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May 7, 2008 6
Model descriptionProbabilistic model
Prob2B (former 'Probox'; Courage & Steenbergen, 2007)•FORM method•Easy to couple with other software
Input:Stochastic characteristics
Limit state function
Probabilistic model Dune erosion model
Generate set of input values based on stochasts and
results so farCalculation
Result:Retreat distance
Process results of calculation
If result ≠ design point
Result:Probability of failure
Parameter values in design point
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May 7, 2008 7
Model descriptionDune erosion model (DUROS-plus)
-200 0 200 400 600 800 1000 1200
-3
0
3
5
15
Cross-shore distance [m]
Le
vel w
.r.t. MS
L [m
]
1 : 2
.51 : 16.7
1 : 58.3
1 : 150
-200 0 200 400 600 800 1000 1200
-3
0
3
5
15
Cross-shore distance [m]
Le
vel w
.r.t. MS
L [m
]
Computational water level
-200 0 200 400 600 800 1000 1200
-3
0
3
5
15
Cross-shore distance [m]
Le
vel w
.r.t. MS
L [m
]
Computational water level
1 : 1
1 : 12.5
Parabolic shape depends on: WL, Hs, T
p, D
50 and profile fluctuation
-200 0 200 400 600 800 1000 1200
-3
0
3
5
15
Cross-shore distance [m]
Le
vel w
.r.t. MS
L [m
]
Computational water level
A
-200 0 200 400 600 800 1000 1200
-3
0
3
5
15
Cross-shore distance [m]
Le
vel w
.r.t. MS
L [m
]
Computational water level
AA A includes a contribution for storm duration and model accuracy
-200 0 200 400 600 800 1000 1200
-3
0
3
5
15
Cross-shore distance [m]
Le
vel w
.r.t. MS
L [m
]
Computational water level
AA A includes a contribution for storm duration and model accuracy
Retreat distance
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May 7, 2008 9
Approach
• Stochastic characteristics of reference situation are mainly based on the probabilistic investigation of WL | Delft Hydraulics (2007)
• The stochastic characteristics have been changed one by one, keeping the others the same
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May 7, 2008 10
OverviewReference situation
Parameter Mean value Uncertainty/variance Distribution type
Water level Related to Pexceedance - Conditional Weibull
Wave height Related to water level 0.6 m Normal
Wave period Related to wave height 1 s Normal
Grain size 225 m 10% of mean (22.5 m) Normal
Profile fluctuation 0 60 m3/m1 Normal
Surge duration 0 10 % * A Normal
Model accuracy 0 15 % * A Normal
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May 7, 2008 11
Reference situation
10-6
10-5
10-4
10-3
10-2
10-1
20
30
40
50
60
70
80
90
100
Probability of exceedance [year-1]
Re
trea
t dista
nce
[m]
33.6
45.6
60.3
78.8
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May 7, 2008 13
Overview of investigations
Variable Mean value Uncertainty/variance
Water level Reference situation + [-0.5, -0.25, 0, 0.25, 0.5] m -
Wave height Reference situation + [-0.5, -0.25, 0, 0.25, 0.5] m [0, 0.3, 0.6, 0.9 and 1.2] m
Wave period Reference situation + [0, 1 and 2] s [0, 0.5, 1.0, 1.5 and 2.0] s
Grain size [200, 225, 250, 275 and 300] m [0, 5, 10 and 15] % of 225 m
Profile fluctuation [-20, 0 and 20] m3/m1 [0, 30, 60, 90 and 120] m3/m1
Surge duration [0, 5 and 10] % * A [0, 5, 10, 15 and 20] % * A
Model accuracy [0, 5 and 10] % * A [0, 5, 10, 15 and 20] % * A
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May 7, 2008 14
Results: Sensitivity water level
-0.5 -0.25 0 0.25 0.560
65
70
75
80
85
90
Re
trea
t dista
nce
[m]
Water level change [m] w.r.t. reference situation
Reference case
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May 7, 2008 15
Sensitivity mean grain size
200 225 250 275 30060
65
70
75
80
85
90
Re
trea
t dista
nce
[m]
D
50
[m]
Reference case
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May 7, 2008 16
Standard deviation grain size
0 11.25 22.5 33.7560
65
70
75
80
85
90
Re
trea
t dista
nce
[m]
D
50
[m]
Reference case
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May 7, 2008 17
Standard deviation peak wave period
0 0.5 1 1.5 260
65
70
75
80
85
90
Re
trea
t dista
nce
[m]
Tp
[s]
Reference case
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May 7, 2008 18
Conclusions
• Water level and grain size distribution are the most important variables in current safety assessment method for the Dutch dune coast
• Proper field data of these variables are crucial• The other variables do have their contribution,
but changing their stochastic char. does not have so much influence