The Netherlands has a unique system with a separate
layer of government dedicated solely to water management: the Waterschappen (literally Waterscapes,
but the actual translation is Regional Water Authorities). With about half the
country now below sea level this won't come as a surprise to most. These water
authorities have their own elections, their separate body of civil servants and
collect their own taxes and fines all relating to water management and waste
water treatment. The taxes are differentiated regionally. In the hilly
landscape of the south residents and companies pay for erosion prevention and
larger sewers, whilst people living in low-lying polders have to pay for the
upkeep of dykes and pumping stations. Only the major rivers (Rhine, Meuse,
Scheldt, Issel and Waal) together with the shipping canals and the coastal
defences are managed and maintained by a dedicated department of central
government: the Department for Water (Rijkswaterstaat) founded in 1789. Incidentally
the Waterschappen are much older; the
first one was set up as a cooperative body in the 12th century. These early
examples only concerned themselves with a single polder or stretch of dyke. The
first designated Water Authority was the Hoogheemraadschap van Rijnland of 1255
(the name translates as High Council of Residents of the Rhine Land).
As I've discussed before in the blog posts on watercities, Antwerp and Utrecht (on the floodplain) the first requirement for urban
development on a floodplain or between water courses is to create more
surface water to prevent flooding in the built-up area. This was mostly done by
digging canals and moats. A similar approach has been utilised by the Regional
Water Authorities in reviewing development proposals. It was enshrined in water
legislation (Keur) as hydrologisch neutraal
ontwikkelen (hydrologically neutral development).
Developing new land for housing often means ground
work. When developing in a hydrologically neutral manner this also means
creating more water. In some cases an attenuation pool can store water before
it is lead to a stream running besides it.
The development of housing and business premises or
offices often results in an increase in roof and paved surface. Rainwater
falling on these surfaces will have to be discharged via a sewer system or
surface water. Heavy rain can cause localised flooding. If the excess is
carried off too fast it can also lead to draught on higher ground as the water has
no time to naturally drain away replenishing ground water. This is why the
water authorities demand a water quantity analysis as part of the water test
within planning procedure. Clean rainwater should firstly be reused, secondly de
retained on site, thirdly be collected and stored, fourthly be discharged onto
surface water and only in fifth instance be discharged into a (rainwater)
sewer.
The hydrological measures taken in developing a site
for housing can be used to great effect. On the left houses on the waterside in
a polder in Almere. On the right apartment blocks built along a retention pool
on a sloping site in Arnhem.
Local authorities can impose rules regarding water
discharge. This is mostly relevant for water flowing over paved surfaces as the
rainwater can thus pick up contaminants. To prevent flooding it is vital that
water is kept in the location where it fell for as long as possible. An option
for realising this is to build water retention facilities where water can drain
into the soil. Other possibilities are: lowering ground level to allow for
controlled flooding (on roads, in parks, etcetera), using permeable paving, or
creating more surface water like a drainage ditch or a pond. Also green roofs
can dramatically lower the amount of rain water that needs to be lead away from
the roof. The idea is to compensate all added non permeable surface in any
given development by building dedicated water provisions. This prescribed
system of water compensation measures results in a hydrologically neutral
development.
Dutch paving is usually in sand on a bed of rubble.
This creates a semi permeable paved surface. The paving slabs can be placed
further apart with grit between to aid the infiltration of rain water (left).
This is especially important on higher ground. On the right a concrete gutter
that leads the water coming off the roof onto the pavement and beyond into an
infiltration ditch (again in Arnhem).
Hydrologically neutral development goes beyond
sustainable urban drainage systems (SUDS) as these are mainly focussed on
preventing flash flooding. In the Dutch approach the hydrological system is
changed but the characteristics are the same. One of the first housing
developments where this was implemented under this name was Westerpark in Breda.
But it is also part of the EVA-Lanxmeer estate and In Goede Aarde. These
projects are examples of an integrated approach to urban water management that
utilises the required provisions as features in the layout of the estate.
In low-lying areas more surface water should be
provided this can be permanent surface water (left) or take the shape of
incidental surface water (on the right in EVA-Lanxmeer) where a ditch fills up
with rain water after heavy rainfall that can then slowly drain away.
Another development is the separation of water streams
based on contamination category. well into the 1980s waste water was collected
in sewers together with rainwater and lead to water treatment facilities. In
instances of extreme rainfall the system would overflow on rivers, streams or
canals. This lead to a decreased ecological quality in these surface waters as
they became contaminated and nutrient rich. As sewer pipes are replaces every
15 to 30 years (the pipes below ground water level in polders are renewed more
regularly than those in sandy soils well above ground water levels) this system
is slowly being replaced by a system of separated water stream with the waste
water in one pipe and the rain water in a separate pipe. The rain water can be
lead towards an underground water retention bunker in densely built-up areas or
lead into a dedicated overflow pond or canal. On clay soils water doesn't
infiltrate much so more surface water is created, but on sandy and loamy soils
water can infiltrate by digging special ditches, by using perforates rain sewer
pipes or by lowering ground level in parks or road verges.
This housing development 't Ven in Veghel is located in a
formerly wet area on loamy soil (ven = fenn). The water retention pools are made a feature
of the design and also add habitat for animals and plants within this estate.
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