DIKE ENGINEERING - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 296

and aggressive. In the twelfth century, an enormous gap was made in the
Netherlands. The small lake Almere in the centre was enlarged into a substantial
inland sea (Fig 17.2b), the Zuyderzee. In that disaster many villages drowned.
Early dike building
The oldest dike made from sods of turf was found in Friesland in 1996 and it is
several hundreds metres long and dates from 200 AD. The reclamation and
exploitation of large peat-lands in the west and middle of the country created many
large lakes, which are clearly shown on maps of the 16th and 17th century. In the
17th century large-scale land reclamation started to dry these lakes. Dikes
surrounded low-lying areas and surplus water was drained out during low tides, or
eventually by windmill power. These areas are called polders.
In those times the discharges of the main rivers had increased significantly.
Annual inundations and floods had serious consequences. Since the 11th century
large efforts were undertaken to create defences against the rising water. In many
parts of the country dikes were build. The probability of inundation decreased and
man started to settle in previously uninhabitable places. Formerly, this was only
possible on hand-made mounds and sandbanks.
Yet the danger of inundation was not over. Particularly, poor or unsystematic
maintenance of the dikes frequently caused flooding. Pools along these dikes recall
the many dike bursts. Frequently, enormous amounts of ice accumulated against
the dikes often causing collapse.
Not always was the water regarded as dangerous. In war times, one breached the
dikes deliberately to prohibit passage for the enemy. The Spanish and French, in
particular, have experienced this strategic inundation during their invasions. The
region Holland remained safeguarded against occupation. Until after the World
War II this Holland Water Line was maintained. In 1939, Germany managed by
spying during inundation exercises to attack at weak spots along this water-defence
line (Grebbeberg). At present, parts of it are a tourist attraction.
Land subsidence and sea-level rise
It is by no means certain that the Netherlands will always be inhabitable, as it is
now, for a large working population. Some believe a time will come when the sea
will again inundate most of the Netherlands, as it already has on more than one
occasion in the distant past. The level of the sea is rising, slowly but surely. If
climatic change causes the polar ice caps to melt faster than at present, the rate at
which the sea is now rising could increase and rapidly create a critical situation. It
would only take one-tenth of today's polar ice to produce enough water to heighten
the level of the sea by three metres.
At present, measurements indicate a rise of about 0.50 m per century, but it is
not only a case of rise in sea level. Ground levels are getting lower at the same
time. Besides a small tectonic subsidence, the weight of the upper layers of soil is
compressing the softer peat ground underneath, and the intensified use of
groundwater means the ground is getting dryer, and dry soil compresses faster than
wet soil. Dry peat oxidises and vanishes. So the difference between sea level and
ground level is increasing. In total, the lowlands in the Netherlands may face an