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Can the roads cope with more water?

Can the roads cope with more water?

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.