1. Global assessment method (VTV-2014)

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

Question 1 : Would flow slide lead to damage on levee?

The criteria of harmfulness ("schadelijksheidscriterium" in Dutch) checks if the “signaleringsprofiel” does not have overlap with the so-called zone of influence of the levee (“invloedszone”).

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.
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

The criteria of harmfulness is not met if the liquefaction point S_ZV is situated landwards of the observation point S_sign.
where:

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.

Question 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 answer to question 1 is "No". However, as the result has no influence on the result assessment, this check is not performed.

Question 3: Artificially underwater installed and non-compacted sandy foreland?

The answer to this question is given as input by the user, in the "General (Additional) Parameter".

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:

• 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.

Question 5: Is liquefaction possible?

The answer is "Yes" if Ψ_5m > -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 (i.e. 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 averaged over a thickness of 5 m between the water line and the toe of the channel slope.

Question 7: Is unstable 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.

if not then the check SUCCEDDED.

Different cases are considered to determine the critical slope:

• a slope with or without  flat banks
• for sand with   D₅₀ > 200 μm and D₁₅ > 100 μm
• for sand 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:

  For ):

without flat berms (Table A.4.2a from CUR113):