DIKE ENGINEERING - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 300

The administrative department prepares policy and gives legal and financial
direction to all the work of water management to be carried out by the technical
department. The technical department cares for the design, execution and
management of dikes, pumping stations, purification plant, roads and buildings.
Dike types
There are, roughly speaking, four types of dikes in the Netherlands: river,
coastal, estuarine dikes and the embankments around bosom waters, i.e. the canals
and lakes used as storage basins to regulate polder water levels. These four types
differ in design and construction. When, in the past, the authorities had to decide
how high to build a dike, they took the highest known level as their standard. The
many erosion holes, scoured out by water swirling through breaches, now to be
seen as pools along the lines of the dikes, bear silent witness to our forefathers'
failure to get their calculations right. After the devastating 1953 flood, the
authorities decided that the height to which the dikes ought to be raised, should
conform, as far as possible, to scientifically calculated criteria.
Their basic principle was not that flood water levels should never be able to
exceed the height of the dike, since this would be impossible to finance, but rather
that the likelihood of floodwater overflowing the dike would be quite small. This
principle was laid down in the Delta Plan, drawn up in 1953. In some areas, for
example, an inundation risk of once in four thousand years is acceptable. The
heights derived from these base levels are known as draft levels. Besides the draft
level, the effect of surf rolling up the seaward slope must be taken into account
when calculating the height of the dike crest. This seaward slope will, if properly
designed, break the surf so it does not surge as high. Dikes in very exposed places
are designed with a 'sill' built into the slope at storm tide level.
The body of the dike must be protected against the force of the waves and the
effect of tidal currents. Dikes are mostly faced with turf as a basic covering.
Depending on the angle of the slope and the force of the waves' attack, basalt,
concrete blocks and/or asphalt are used as revetment. Fortunately, high tide levels
never last long. A normal tide cycle lasts twelve hours; consequently the water
hardly ever gets a chance to percolate deep into the dike so the angle of the inland
slope can be allowed to be fairly steep, provided the subsoil allows it.
Water levels on the non-estuarine upper reaches of the rivers are determined by
weather conditions upstream, such as rainfall and thaw. These days, the necessity
of dike improvement is based on exceeding a certain river discharge occurring
about once in two thousand years, a figure derived from statistics. High water on
the upper river reaches lasts considerably longer than in a storm tide area, and
waves do not generally play as great a role. These two factors are reflected in the
design of the most desirable river dike cross-section. The riverside slope may,
consequently, be angled more steeply and the revetment need not be of heavy
quality as on the coast. The angle of the slope may not, however, be too steep for
growth of proper grass cover or for a stone revetment to be satisfactorily placed.
The longer periods of high water give the groundwater table within the body of the
dike more opportunity to rise. In order to prevent seepage, the inland side of the
dike is often given a gentler slope.