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Global check

The global check as implemented in D-Flow Slide is based on the assessment schema described in VTV-2014 and may comprise the following steps

NOTE: If the average state parameter Ψ_5m (as entered in the General Parameters window) is more  than -0,05, that means that liquefaction is possible. In such case the Global check fails, whatever the result of the steps described below. An information message will be given in the Overall Results window.

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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 criteria of harmfulness damage criterion ("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 [ M_bestorting - L_influence; 2 H

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• _onbest) ] 

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• (see also figure below)

where:

H_geul is the channel depth in case no revetment is present and H = H_onbest in case a revetment is present
H_vw is the thickness of the sensitive layer(s). The sensitivity is determined using the following criterion: sand (or gravel) layers with Ψ_5m > -0.05 (when considering all the layers until 1.5*H below the toe of the submerged slope), are considered as sensitive. (NOTE: For this check, the state parameter Ψ_5m entered per layer is used, not the value entered in the General Parameters window.)

H_onbest is the depth of the channel below the top revetment.
M_bestorting is the horizontal projection of the length of the top revetment starting at the outer dike toe

L_influence 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").

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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:

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If the liquefaction point S_ZV is situated landwards of the observation point S_sign, the  (so as in figure above), the criteria is not met, which means that the global check PASSES. so go to step 1c. 

If the liquefaction point S_ZV is not situated landwards of the observation point S_sign, the criteria is not met, so go to step 3.  which means that the Global check PASSES.

Step

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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 criteria on harmfulness the damage criterion (step 11a) is not met . However, as the result (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 has no influence on the result assessment, this check is not performedfinal answer, it is therefore not performed and the answer is always "Not available".

Step

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1c: Artificially underwater installed and non-compacted sandy foreland ?

The nature of the submerged slope is given as input by the user in the "General parameters" window.

In case of non-natural deposited slopes, both Global and Detailed Overall checks FAIL and it should immediately be switched to the advanced Advanced methods.

In case of natural deposited slopes, go to step 41d.

Step

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1d:

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 Is flow slide possible based

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on the occurrence criterion of "the steepest slope over 5 m"?

If the previous step (step 31c) has not been passed, a check on criteria the first of two occurrence criteria (“optredingscriterium”“optredingscriteria”) must be performed.

The occurrence of A flow slide is possible if one of the two following conditions can occur if the following condition is met:

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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 × (H_R/24)^(1/3) ], in which H_R 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, go to step 5.the Global and the Overall checks SUCCEED. 

Step 6: Are there layers present with a thickness of minimal 5 m, in which D₅₀ < 200 μm or D₁₅ < 100 μm averaged over a thickness of 5 m?

The answer is "Yes" if D₅₀ < 200 μm or D₁₅ < 100 μm where D₁₅ and D₅₀ 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.

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.

Step 7: Is breaching possible?

Liquefaction flow criteria:

H_R [m] is the fictitious height of the submerged slope in its most unfavourable situation during the assessment period and determined with:

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in which:

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Image Added (with h_dijk is dike height)

In which::

H_geul     depth of the channel [m]

Δh_onder  height of the slope above the water level during extreme low water: “niveau van geulrand” – “niveau LLWS/OLW/OLR” [m]

h_dijk       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 [-]

α_R         slope angle of the schematized fictitious under water profile [degrees]

α_boven    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.h_dijk In case of a "schaardijk", α’_boven = α_boven [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.

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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 neededshould be performed. In the case the slope height is over 40 is larger than 40 m a Detailed check is also needed.
If not then the check SUCCEEDED. 
 

Different cases are considered to determine the critical slope:

• a slope with or without  flat bankshorizontal steps ("berms")
• the sand type:for
  • very fine sand with  D_50,5m ≤ 200 μm and D_15,5m ≤ 100 μm
  • medium fine sand with
    D₅₀ > 200
  •  200 < D_50,5m ≤ 500 μm and 100 < D₁₅
  > 100
  • _,5m ≤ 250 μm
  for
  • coarse sand and gravel
  with   D
  • with D_50,5m > 500 μm and D_15,5m > 250 μm

NOTE: The case where the sand is finer than  200 μm is not checked because according to question 6 (see above) in this case a detailed check must be performed.

NOTE: D₅₀ and  D₁₅ are determined in the same way as in question 6.

where D_15,5m and D_50,5m 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 D_50;5m if a Detailed check is performed
• the calculated diameter D_15;5m and D_50;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 slopeTable A.4.2a from CUR113b, profile without flat berms:

 ^(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:

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40

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The above table is based on Table A.4.2b from CUR113b, profile with flat berms:

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from depth

[m +GL^(a)]

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to depth z
[m +GL^(a)]

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0

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Z1

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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

2a from CUR113b, In D-Flowslide only "without horizontal steps" are implemented, as defined in the CUR-report.