Some stretches of the Danish roads are not ready for the increased quantities of water expected in the future, while others have been designed to cope.
In the future more frequent and heavier downpours will lead to
more flooding of roads and viaducts. However, it is not just the
increasing amounts of water from the heavens that will cause
problems for the road infrastructure. If the climate-scenario
prognoses for increasing seawater levels hold true, the groundwater
level will also increase in many parts of the country.
Together these two effects may mean that vulnerable stretches of
road will erode and stretches of road along the coast and inland
will be at risk of collapsing. This in turn will mean a greater
risk of accidents, long-term inconvenience for road traffic as well
as considerable road-repair costs for local road authorities.
Rain ripped roads in two
On 20 August 2007 southern Jutland was hit by a heavy
cloudburst. In the course of just a few hours the area
received 150 mm rain. The most obvious effect of this extreme
weather incident was the collapse of the road connecting the two
towns of Graasten and Sønderborg and the breach in a railway
embankment. A train had actually only just crossed the embankment's
weak spot before it collapsed.
The Danish Meteorological Institute has referred to the
incident in southern Jutland as a "convective bomb" - a
short, extremely heavy and completely local shower. Photo:
Municipality of Sønderborg
The incident at Graasten was a combination of several special
circumstances, and both the stretch of railway and the road have
been reconstructed to prevent this type of accident in the
future. Therefore the Municipality of Sønderborg
has not found it relevant to inspect the entire road infrastructure
on the basis of this incident. However, the Technical and
Environmental Administration is working on several initiatives that
will provide a better overview.
"Together with Sønderborg Forsyning A/S (the local utilities
company, ed.), the Municipality will map the vulnerable areas in
the Municipality (blue spot, ed.), so that we will know in advance
which areas, and therefore also road stretches, are particularly
vulnerable to increases in water levels and extreme downpours.
Based on these maps, we hope to develop a tool that can show in
more detail which areas need to be dealt with first and which areas
can wait," says Inge Olsen, director for the Technical and
Environmental Administration in the Municipality of
Sønderborg.
"At the same time the entire Municipality is very focussed on
energy and climate issues when replacing and maintaining its
facilities, as well as when e.g. putting up new buildings.
This is why we are raising the terrain height by one metre in the
area in Sønderborg Harbour which is to accommodate a new
sustainable urban development," says Inge Olsen.
Flooded six weeks a year
A case from the Municipality of Viborg illustrates that it can be
quite costly to repair even just short stretches of road. Here a
stretch of road 100-200m long across a river valley has been
flooded repeatedly over the past years. In 2008 alone the stretch
was flooded for a total period of about six weeks.
According to the Municipality, the frequent flooding is due to the
fact that approximately 600m of the road rests on peat and silt
deposits at a depth of 10m or more. Furthermore, in connection with
a new water regulation for the watercourse in 2004, cutbacks on
maintenance were introduced leading to rising water levels.
Moreover the Municipality expects that increasing downpours due to
climate change will lead to higher water levels in the river valley
in the long term.
The Municipality of Viborg has prepared several proposals to deal
with the situation by raising the road by 1.3m. This is
estimated to cost somewhere between DKK 6.7 and 9.3 million.
However, to ensure that the best solution is chosen, geo-technical
surveys of the soil conditions of the stretch of road in question,
and the consequences of the expected increases in water levels,
will have to be examined more closely first.
In the mean time, the Municipality will put up signs warning that
there is water on the road on stretches where the water level is
0-15cm above the road, and will close the road when water levels
exceed this. Finally, children from the area will be given the
option of going to school by school bus in periods with
flooding.
Roads with built-in drainage
All in all it makes good sense for municipalities to include
climate change considerations in their future traffic and
infrastructure planning. Both in order to avoid flooding, but also
to be better at controlling water flows. As an example, the
Municipality of Roskilde has done this when planning the new
housing development at 'Trekroner Øst', which is situated just next
to Roskilde University.
The framework local plan for Trekroner Øst dictates that rainwater
is to be used to create a "blue structure" in the area by leading
the surface water away along the surface. So, instead of laying
down the traditional subsurface pipelines, rainwater will flow
through ditches and gutters and remain in the local area.
Even Trekroner Parkvej, a 1,300m long road that leads into the
area, has been built according to this principle.
Trekroner Parkvej with drainage trough in the central reserve.
Drawing courtesy of Rambøll. Click on the drawing to enlarge.
"The road has been designed with two separate lanes divided by
a central reserve," says Jan Villumsen, chief consultant and
landscape architect with the Danish engineering company
Rambøll.
"The new road, Trekroner Parkvej, slants towards the centre and a
drainage trough has been built into the central reserve to collect
water from the road. The trough has been built incorporating a sand
filter at the bottom and a drain with slits so that water from the
road is filtered in the filter box and then seeps down to the
drain. Rain water is collected via wells at every 50-100m, and is
then led to a flood retention basin," says Jan Villumsen.
This new road design may be the solution for the future, because,
according to a new analysis by the engineering company Grontmij |
Carl Bro, approximately 10,000km of roads in Denmark, including
motorways, dual carriageways and municipal roads, risk flooding and
or having foundations undermined. This corresponds to more than 10
per cent of the total Danish road infrastructure.