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M = Max [ M_bestorting ; 2 H_geulonbest) ]     in case a revetment is present

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H_onbest is the depth of the channel below the revetment.
M_bestorting is the horizontal projection of the length of the revetment

NOTE: in WBI2017 in case of a revetment the margin is defined in a slightly different way. Instead of H_onbest, H_geul is used: M = Max [ M_bestorting ; 2 H_geul) ] 

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 inclination of the slope depends on the channel depth:

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