5.2 Delineation of the Development in Northern Germany
77
for implementation, for example, it is necessary to enhance the knowledge of failure
mechanisms and the failure probability of dikes and other structures.
Taking these remarks into account, the recent enhancement of the safety margin for
the secular sea level rise from 25 cm up to 50 cm announced in 2007 by the Minister of the
Environment, responsible for coastal protection, is a postponement of the issue at stake.
However, it serves as an approach to address the consequence of increased sea level rise
in Lower Saxony (and Bremen).
The development of different procedures on how to determine the crest height of a
dike is a tribute to the problem of calculating this height. An attempt to reduce or to
minimize the uncertainty of the extend of a worse event. Some of the parameters shown in
Fig. 5.8 contain uncertainties or at least the circumstances or conditions to measure them
are under discussion. For example, the value of the mean high water level is a calculated
value based on a certain time period of high water levels. The parameter highest high
water level differs from coastal stretch to coastal stretch. This parameter is influenced by
several circumstances which also contain uncertainties. The parameter e maximum wave
run-up also depends on global conditions (e.g., wave height in the North Sea, wind speed,
fetch) and local conditions (e.g., degree of shelter by islands or tidal flats, orientation of
tidal channels in front of the dike line). Consequently, the procedure to calculate the crest
height shows the best available approximation and is a reliable way to be dealt with by a
government. Nevertheless, engineers are continuously working on the improvement.
Another consequence is that if the crest height of a dike is calculated based on this procedure everywhere in the Federal State the safety level is the same. That means according
to the risk approach (see Sect. 6.1.2): is every area is protected by the same safety level the
(residual) risk differs. For example, some areas are densely populated and industrial areas
are protected by dikes. Other areas are dominated by farm land and are less populated.
The consequences of a dike failure would vary between both areas. The densely populated
area is more vulnerable than the farm land, but both are protected by the same safety level.
That means, even if the safety level stays the same risk differs along the coast depending
on the values and the equipment of the area of concern.
Going back to Fig. 5.4 in the 1970s another external trigger imposes the consideration
of ecological aspects within coastal protection. This is exemplified by Ley Bay case (East
Frisia, Lower Saxony, see in Fig. 8.3 within the description of problem 3 in the exercises
in Sect. 8.1):
During the 1960s three different reasons demanded for an improvement of the drainage system and coastal protection around the Ley Bay (Fig. 5.9). Caused by storm surges
the Ley Bay was enlarged approximately 40 km inland. Continuous land reclamation work
reduced the dimension and generated new fertile land for peasants until the beginning of
the twentieth century. The process of sedimentation continued and after World War II and
new land should be offered to the arriving refugees. Therefore, it was planned to reclaim
a large amount of new land at the Ley Bay. In the middle of the twentieth century a new
dike was built to protect the new land against flooding. Afterwards, the sedimentation process did not stop in the Ley Bay due to different reasons. The circumstances were getting
77
for implementation, for example, it is necessary to enhance the knowledge of failure
mechanisms and the failure probability of dikes and other structures.
Taking these remarks into account, the recent enhancement of the safety margin for
the secular sea level rise from 25 cm up to 50 cm announced in 2007 by the Minister of the
Environment, responsible for coastal protection, is a postponement of the issue at stake.
However, it serves as an approach to address the consequence of increased sea level rise
in Lower Saxony (and Bremen).
The development of different procedures on how to determine the crest height of a
dike is a tribute to the problem of calculating this height. An attempt to reduce or to
minimize the uncertainty of the extend of a worse event. Some of the parameters shown in
Fig. 5.8 contain uncertainties or at least the circumstances or conditions to measure them
are under discussion. For example, the value of the mean high water level is a calculated
value based on a certain time period of high water levels. The parameter highest high
water level differs from coastal stretch to coastal stretch. This parameter is influenced by
several circumstances which also contain uncertainties. The parameter e maximum wave
run-up also depends on global conditions (e.g., wave height in the North Sea, wind speed,
fetch) and local conditions (e.g., degree of shelter by islands or tidal flats, orientation of
tidal channels in front of the dike line). Consequently, the procedure to calculate the crest
height shows the best available approximation and is a reliable way to be dealt with by a
government. Nevertheless, engineers are continuously working on the improvement.
Another consequence is that if the crest height of a dike is calculated based on this procedure everywhere in the Federal State the safety level is the same. That means according
to the risk approach (see Sect. 6.1.2): is every area is protected by the same safety level the
(residual) risk differs. For example, some areas are densely populated and industrial areas
are protected by dikes. Other areas are dominated by farm land and are less populated.
The consequences of a dike failure would vary between both areas. The densely populated
area is more vulnerable than the farm land, but both are protected by the same safety level.
That means, even if the safety level stays the same risk differs along the coast depending
on the values and the equipment of the area of concern.
Going back to Fig. 5.4 in the 1970s another external trigger imposes the consideration
of ecological aspects within coastal protection. This is exemplified by Ley Bay case (East
Frisia, Lower Saxony, see in Fig. 8.3 within the description of problem 3 in the exercises
in Sect. 8.1):
During the 1960s three different reasons demanded for an improvement of the drainage system and coastal protection around the Ley Bay (Fig. 5.9). Caused by storm surges
the Ley Bay was enlarged approximately 40 km inland. Continuous land reclamation work
reduced the dimension and generated new fertile land for peasants until the beginning of
the twentieth century. The process of sedimentation continued and after World War II and
new land should be offered to the arriving refugees. Therefore, it was planned to reclaim
a large amount of new land at the Ley Bay. In the middle of the twentieth century a new
dike was built to protect the new land against flooding. Afterwards, the sedimentation process did not stop in the Ley Bay due to different reasons. The circumstances were getting