1. 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

Step 1 : Would flow slide lead to damage on levee (i.e. is the "schadelijksheidscriterium" met)?

The criteria of harmfulness ("schadelijksheidscriterium" in Dutch) is met if at the so-called assessment level is situated within the slope (i.e. on landward side of) the so-called observation profile, in accordance with Annex 9.4 of the VTV-2006.

1) Determination of the observation profile

The required margin (the horizontal part from the outside toe of the dike) depends on the presence of a revetment and is determined by the relation:

M = 2 H_vw + 1.5 (H - H_vw)                                 in case no revetment is present

M = Max [ M_bestorting ; 2 H_vw + 1.5 (H - H_vw) ]     in case a revetment is present

where:

H 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 revetment.
M_bestorting is the horizontal projection of the length of the revetment.

In case no slope protection is present, if over the entire channel depth softening sensitive sand is present, this margin is thus equal twice the fictive channel depth (M = 2 H).
The inclined part of the observation profile in line with the horizontal portion. The inclinaison 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

S_sign is the intersection point between the observation profile (from the foreshore) and the assessment level and S_ZV is the intersection point between the existing profile and the assessment level.

If the liquefaction point S_ZV is situated landwards of the observation point S_sign, the criteria is not met, which means that the global check PASSES.

If the liquefaction point S_ZV is not situated landwards of the observation point S_sign, the criteria is met, so go to step 3.

Step 2: Criterion on slope protection met (<1:2,5)?

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 1:2.5 may not be cut by the profile.

This check should be performed only if the criteria on harmfulness (step 1) is not met. However, as the result has no influence on the result assessment, this check is not performed.

Step 3: 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 checks FAIL and it should immediately be switched to the advanced methods.

In case of natural deposited slopes, go to step 4.

Step 4: Flow slide possible based on geometry only (i.e. is the "optredingscriterium" met)?

If the previous step (step 3) has not been passed, a check on criteria of occurrence (“optredingscriterium”) must be performed.

The occurrence of flow slide is possible if one of the two following conditions is met:

• The average slope is steeper than or equal to 1:4, over a height of at least 5 m;
• The total slope (channel edge-channel bottom) is on average steeper than or equal to 1:7.

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.

Step 5: Is liquefaction possible?

Liquefaction is possible if the state parameter Ψ_5m > -0.05 .

If Ψ_5m < -0,05 that means that liquefaction is unlikely.

Ψ_5m is defined as the lowest average value of state parameter over total of 5 m between the water line and 0.5H below the toe of the submerged slope.

NOTE: for this check, the state parameter Ψ_5m as entered in the General Parameters window is used, not the Ψ_5m values per layer.

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?

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 a Detailed check is needed. In the case the slope height is over 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 banks
• for medium fine sand with   D₅₀ > 200 μm and D₁₅ > 100 μm
• for coarse sand and gravel with   D₅₀ > 500 μm and D₁₅ > 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.

Table 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").

Table A.4.2b from CUR113b, profile with flat berms: