Global check

The global check as implemented in D-Flow Slide 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 damage criterion ("schadelijksheidscriterium" in Dutch) is met if at the so-called assessment level ("beoordelingsniveau") the actual slope lies landward of the so-called assessment profile ("signaleringsprofiel"), see Figure below, which is adopted from Toetsmethode zettingsvloeiing (version 2, 22 febr. 2016).

1) Determination of the assessment profile

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

• in case no revetment: M = 2 H_geul 
• in case a revetment:   M = Max [ M_bestorting - L_influence; 2 H_onbest) ]  (see also figure below)

where:

H_geul is the channel depth

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").

Schadelijksheidscriterium - Determination of the observation profile ("signaleringsprofiel") when a top revetement is present

In case no slope protection is present, 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 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

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 (so as in figure above), the criteria is met, 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, which means that the Global check PASSES.

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

The slope directly in front of the toe of the dike or outer boundary of the slope protection, may not cut a 1:2.5 slope 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 has no influence on the 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 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 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 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, the Global and the Overall checks SUCCEED. 

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:

in which:

 (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.

Breach flow criteria:

This criterion is adopted from 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 sensitive to breaching. This means that the check fails and a Detailed check 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 horizontal steps ("berms")
• the sand type:
  • very fine sand with  D_50,5m ≤ 200 μm and D_15,5m ≤ 100 μm
  • medium fine sand with  200 < D_50,5m ≤ 500 μm and 100 < D_15,5m ≤ 250 μm
  • coarse sand and gravel with D_50,5m > 500 μm and D_15,5m > 250 μm

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 slope

 ^(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.