DIKE ENGINEERING - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 306

inaccuracy, scale). This is practically possible by the definition of dike-rings. For
each ring a safety norm is determined and the safety norm of all other elements in
the dike-ring system are related to this norm. The scheme shown in Fig 17.7
clarifies the relation between the conventional inundation safety approach and the
new way as foreseen in the New Water Retainment Act. Four different levels are
distinguished: water level exceedence, inundation likelihood, inundation risk, and
inundation safety. They are interconnected in a comprehensive manner, and they
will give place to evaluating required and actual safety. Hence, the inundation
approach comprises the following aspects
- Loading (tides, waves, earthquakes);
- Mechanisms (failure mechanisms per type of structure and element);
- Calculation methods (probabilistic models, partial safety factors);
- Inundation scenarios, damage evaluation;
- Decision models accounting for “values” and “loss” (damage).
The dike-ring approach is essentially different from the traditional dike building
method. The original dike improvement concept, applied until the first Delta law,
after the flood of 1953, was raising the height of the dike by one metre above the
latest highest water level (Fig 17.7a). Here the only strength aspect considered was
the height. Flood safety was based on intuition and calamity.

STRENGTH STRENGTH

STRENGTH STRENGTH

LOADING LOADING

Likelihood
of flooding

Likelihood
of flooding

HIGHEST
LEVEL

HIGHEST
LEVEL

Figure 17.7a Dike building approach before 1953

During the flood of 1953, it was noticed that in many cases the dike inner slope
had failed, and the first following Delta Law distinguished the likelihood of
different failure mechanisms. The probability of flooding was therefore redefined
into: inundation likelihood = water level exceedance (loading) failure likelihood

(strength). The symbol K refers to convolution of stochastic loading (water level /
waves) and each dike ring, each case and each failure mode, such as overtopping,
wave overtopping, erosion outer slope, instability outer slope, leakage, settlement,
erosion inner slope, instability inner slope, uplift, and/or piping (see Fig 17.5 and
17.6). The approach is shown in Fig 17.7b in a schematic way.

K

LOADING LOADING

OVERRUN OVERRUN OVERRUN

STRENGTH STRENGTH

STRENGTH STRENGTH STRENGTH

LOADING LOADING LOADING SLIP FAILURE SLIP FAILURE SLIP FAILURE

likelihood = convolution of loading
and strength

likelihood = convolution of loading
and strength

PIPING PIPING PIPING

Figure 17.7b Dike building approach after the first Delta law.