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Global check
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The global check as implemented in D-Flow Slide is based on the assessment schema described in VTV-2011.
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comprises the following steps, which can also be found in Toetsmethode zettingsvloeiing (version 2, 22 febr. 2016).
Step 1a : Would flow slide lead to damage on levee (i.e. is the "schadelijksheidscriterium" met)?
The damaging damage criterion liquefaction (geometrical check) is ("schadelijksheidscriterium" in Dutch) is met if at the so-called assessment level is situated within the slope (i.e. on landward side of) ("beoordelingsniveau") the actual slope lies landward of the so-called observation profile, in accordance with Annex 9.4 of the VTV-2006called assessment profile ("signaleringsprofiel"), see Figure below, which is adopted from Toetsmethode zettingsvloeiing (version 2, 22 febr. 2016).
1) Determination of
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the assessment profile
The required margin (the horizontal part from the outside toe the "invloedslijn" of the dike) depends on the presence of a revetment and is determined by the relation:M
- in case no revetment: M = 2 H
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- geul
- in case
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- a revetment: M = Max [ Mbestorting - Linfluence; 2 H
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- onbest) ]
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- (see also figure below)
where:
Hgeul is the channel depth in case no revetment is present and H = Honbest in case a revetment is present
Hvw is the thickness of the sensitive layer(s). The sensitivity is determined using the following criterion: sand (or gravel) layers with Ψ5m > -0.05 (when considering all the layers until 1.5*H below the toe of the submerged slope), are considered as sensitive.
Honbest is the depth of the channel below the top revetment.
Mbestorting is the horizontal projection of the length of the top revetment starting at the outer dike toe
Linfluence is the length of the influence zone from the outer dike toe
The influence zone is defined as follows: if this zone is damaged by an indirect failure mode, for example a flow slide, the safety of the dike drops below the required safety level, considering all direct failure modes. At surface level the influence zone is confined by the influence lines (in Dutch: "invloedslijnen").
Schadelijksheidscriterium - Determination of the observation profile ("signaleringsprofiel") when a top revetement is present
In case no slope protection is present, if over the entire channel depth softening sensitive sand is present, this this margin is thus equal twice the fictive channel depth (M = 2 H), assuming that entire submerged consists of sand that is sensitive to liquefaction and/or breach flow.
The inclined part of the observation profile in line with the horizontal portion. The inclinaison inclination of the slope depends on the channel depth:
- 1:15 if the fictive channel depth is less than 40 m (H < 40 m)
- 1:20 if the fictive channel depth is more than 40 m (H ≥ 40 m)
2) Determination of the assessment level
The assessment level is the lower boundary of the liquefiable sand layer, but has a minimum and maximum of H/3 and H/2 above the bottom of the channel respectively.
3) Comparison of the observation profile with the existing profile at the assessment level
The damaging criterium is not met if the liquefaction point SZV is situated landwards of the observation point Ssign.
where Ssign is the intersection point between the observation profile (from the foreshore) and the assessment level and
SZV is the intersection point between the existing profile and the assessment level.
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If the liquefaction point SZV is situated landwards of the observation point Ssign (so as in figure above), the criteria is met, so go to step 1c.
If the liquefaction point SZV is not situated landwards of the observation point Ssign, the criteria is not met, which means that the Global check PASSES.
Step 1b: Criterion on slope protection met (<1:2,5)?
Directly The slope directly in front of the toe of the dike or on the outside of the toe dumping or to the outward end of a dumping, the slope 1or outer boundary of the slope protection, may not cut a 1:2.5 may not be cut by the profileslope that starts at the toe of the dike or outer boundary of the slope protection respectively.
This check should be performed only if the damage criterion (step 1a) is not met (i.e. a flow slide does not lead to damage). The result of this check will not result in a "failure" in the safety assessment, but only indicates that the slope protection may become unstable.
As this check the answer to question 1 is "No". However, as the result has no influence on the result assessment, this check is not performed.
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final answer, it is therefore not performed and the answer is always "Not available".
Step 1c: Artificially underwater installed and non-compacted sandy foreland ?
The answer to this question nature of the submerged slope is given as input by the user , in the "General (Additional) Parameterparameters" .
Question 4: Flow slide possible based on geometry only?
The occurrence of a liquefaction is possible if one of the two following conditions is met:
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window.
In case of non-natural deposited slopes, both Global and Overall checks FAIL and it should immediately be switched to the Advanced methods.
In case of natural deposited slopes, go to step 1d.
Step 1d: Is flow slide possible based on the occurrence criterion of "the steepest slope over 5 m"?
If the previous step (step 1c) has not been passed, a check on the first of two occurrence criteria (“optredingscriteria”) must be performed.
A flow slide can occur if the following condition is met: the average slope is steeper than or equal to 1:4, over a height of at least 5 m.
If this occurrence criterion is met, both Global and Overall checks FAIL and it should be switched to Advanced methods.
If this occurrence criterion not met, go to step 1e.
Step 1e: Is flow slide possible based on the occurrence criterion based on average geometry?
If the previous step (step 1d) has not been passed, a check on the second occurrence criterion must be performed.
A flow slide can occur if one of the two following conditions is met:
- Liquefaction is possible: the ;The total slope (channel edge-channel bottom) is on average steeper than or equal to 1 : [7 .
Question 5: Is liquefaction possible?
The answer is "Yes" if Ψ5m > -0.05 between the water line and 0.5H below the toe of the submerged slope.
Question 6: Are there any sensitive to liquefaction layers present?
The answer is "Yes" if D50 < 200 μm or D15 < 100 μm averaged over a thickness of 5 m between the water line and the toe of the channel slope.
Question 7: Is unstable breaching possible?
- × (HR/24)(1/3) ], in which HR is defined below.
- Breaching is possible: see paragraph below called "Breach flow criteria"
If the criteria of occurrence is met, the Global check FAILS and it should be switched to the Detailed method.
If the criteria of occurrence is not met, the Global and the Overall checks SUCCEED.
Liquefaction flow criteria:
HR [m] is the fictitious height of the submerged slope in its most unfavourable situation during the assessment period and determined with:
in which:
(with hdijk is dike height)
In which::
Hgeul depth of the channel [m]
Δhonder height of the slope above the water level during extreme low water: “niveau van geulrand” – “niveau LLWS/OLW/OLR” [m]
hdijk height of the levee with respect to the outer toe of the levee [m]
B width of the foreland. In case of a so-called "schaardijk", B = 0 [m]
cot(α) cotangent of the slope α [-]
αR slope angle of the schematized fictitious under water profile [degrees]
αboven slope of the outer slope of the levee [degrees]
α’boven slope between incision of channel and the fictitious outer crest of the levee with a height of 2.hdijk In case of a "schaardijk", α’boven = αboven [degrees]
Other symbols in the figure below are (in Dutch):
LLWS meerjarig gemiddelde van het laagste springlaagwater ten opzichte van NAP, geldig in het kustgebied en de estuaria.
OLW Overeengekomen Laag Water ten opzichte van NAP, geldig in het benedenrivierengebied (in Waal stroomafwaarts van Tiel).
OLR Overeengekomen Lage Rivierstand ten opzichte van NAP, geldig in het bovenrivierengebied (in Waal stroomopwaarts van Tiel), hetgeen overeenkomt met de Overeengekomen Lage Afvoer bij Lobith.
Breach flow criteria:
This criterion is adopted from step This criterion corresponds to step 5 of CUR Aanbeveling 113, 2008.
When considering the geometry of the slope, are there parts of the slope with a height as given in the first column of the tables below which is equal to or steeper than the slope given in the second column?
If so then the slope is breaching sensitive, so the check FAILED, so is sensitive to breaching. This means that the check fails and a Detailed check is needed.if not then the check SUCCEDDED.should be performed. In the case the slope height is larger than 40 m a Detailed check is also needed.
Different cases are considered to determine the critical slope:
- a slope with or without flat bankshorizontal steps ("berms")
- the sand type:
- very fine sand with D50,5m ≤ 200
- μm and D15
- ,5m ≤ 100 μm
- medium fine sand with 200 < D50,5m ≤ 500 μm and 100 < D15,5m ≤ 250 μm
- coarse sand and gravel with D50,5m > 500 μm and D15,5m > 250 μm
- for sand finer than 250 μm , there is no rule
NOTE: D50 and D15 are determined in the same way as in question 6.
Table A.4.2a from CUR113b, profile without flat berms:
For ):
where D15,5m and D50,5m are the minimum averaged values (only for sand and gravel soils) over a thickness of 5 m between the water line and the toe of the channel slope.
The type of sand (particle size) is a user-defined parameter: very fine sand, medium fine sand or coarse sand/gravel.
The program checks that the user-defined parameter is coherent with:
- the calculated diameter D50;5m if a Detailed check is performed
- the calculated diameter D15;5m and D50;5m if an Advanced Breach flow check is performed
In case of a incoherence, the Overall result gives a Warning message.
NOTE: In the current version of the program, it is not checked if the thickness is minimal 5 m. The average is performed over a thickness of 5 m, by not taking into account the none-sandy layers.
NOTE: In case the user-defined sand type is "very fine sand", the breach flow criteria cannot be checked. The program indicates breach flow is possible and a warning message will be given in the Overall Results window.
Table with th average and local slope
from depth | to depth z [m +GL(a)] | Average slope | ||||
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For not too Medium fine sand D50 > 200 μm and D15 > 100 μm | Fairly coarse sandCoarse sand/Gravel D50 > 500 μm and D15 > 250 μm | |||||
Local slope | Average slope 0 - z | Local slope | Average slope 0 - z | |||
0 | -5 | 1:2 | 1:2 | 1:2 | 1:2 | |
-5 | -10 | 1:3 | 1:2.5 | |||
-10 | -15 | 1:4 | 1:3 | 1:3 | 1:2.5 | |
-15 | -20 | 1:5 | 1:3.5 | |||
-20 | -25 | 1:6 | 1:4 | 1:4 | 1:3 | |
-25 | -30 | 1:8 | 1:4.67 | |||
-30 | -35 | 1:10 | 1:5.43 | 1:6 | 1:3.75 | |
-35 | -40 | 1:10 | 1:6 |
without flat berms (Table A.4.2a from CUR113):
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Slope height
(m)
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Average slope
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5
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1:2
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10
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1:2.5
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15
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1:3
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20
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1:3.5
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25
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1:4
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30
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1:4.67
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35
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1:5.43
(a) In D-Flow Slide, the ground surface level (GL) (maaiveld in Dutch) is the lowest level between the water level and the top of the channel slope (called "Insert river channel").
NOTE:
The above table is based on Table A.4.2a from CUR113b, In D-Flowslide only "without horizontal steps" are implemented, as defined in the CUR-report.
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40
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