As more land gets covered with concrete and buildings, which are impervious to water, we are facing a double problem. Firstly when it rains, we have to quickly get rid of the large and quickly appearing streams of rainwater through centralized rainwater sewer, secondly, we deplete ground water level in the urban areas.
In the summer, when the chance of thunderstorms is high, some urban areas face flash-floods as the infrastructure cannot handle the rapid influx of water. In recently constructed buildings, the runoff from roofs has to be collected and infiltrated locally, in a decentralized manner.
especially in the autumn, winter and the spring, pavements are plagued by puddles,
While cycling through the streets of Olomouc, I noticed an important fact. While the roof runoff is already being tackled, the larger problem- runoff from the streets, is still being managed in the old-centralized way. While roofs take up a large proportion of an urban area, roads and pavements in my opinion even larger.
On a more positive note, I was pleased to see that areas surrounding the shopping centre Globus built about 15 years ago were designed in a very forward thinking way, which raises a question, why can’t this be done everywhere? (WHAT FORWARD THINKING WAY?
What’s wrong ?
Even though large areas of cities are completely paved, some green areas remain separating the road and pavements. Usually, they are covered with grass, sometimes planted with hedges. What seems lacking logic, is that they are placed above the road and above the pavement. This means, that the only permeable section of the ground in the whole area is the least exposed to water. All water that runs off the road and the pavement has to be directed into the sewer. The rainwater takes rubbish and grit with it, which clogs the sewer. The sewer inlets hold large baskets which collect some solids from the runoff, but still, they need regular de-gritting and de-trashing, just like wastewater sewers, which is costly. The construction of the sewers has certainly been an expensive venture too.
How to fix it?
Let me describe the current situation and the two proposals on a Sketch-up visualisation. The status quo is such that the road and the pavement are, in this example, separated by a thin green strip of ground. The pavement is above the road level and the grass is above the pavement level. Sometimes, there is more grass behind the pavement line followed by an open space, fence or a wall of a house. In case of rain, water from the roads run off into the rain sewer. Water from the pavement penetrates in the gaps between the tiles. However, especially in the autumn, winter and the spring, pavements are plagued by puddles, since the surrounding wet ground is situated above the pavement, water accumulates at the lower spot, on the pavement. All runoff from the roads- dust, grit, trash, leaked oil, precious metals from catalytic converters, go straight into the sewer and either into a river or into a sewage treatment plant. In the first case, they pollute the streams and in the second case put a huge strain onto the plants, lowering the cleaning efficiency and increasing the pumping costs.
Thankfully, the current situation may be relatively easily alleviated. Firstly, the grass-covered ground has to be lowered below the paved areas. Holes and troughs can be dug and the ground either taken away, or placed behind the dug-out trough. Then, slots need to be cut into the curb, to allow runoff. Since the capacity of these troughs is limited, an overflow leading to a sewer can be placed. The advantage of these ditches is that they will lower or delay the influx of water in case of a rapid rain, they collect and concentrate trash, which can be cleaned from the surface and they purify ran-off water by soil infiltration. The rate of infiltration into the soil is between 5-30 liters per square meter per hour, with sandy soils being more permeable than clay.
The second solution, is based on a similar principle, lower the surrounding ground below the pavements or raise the pavements above the surrounding ground. The effect is the same. The second option is hardly applicable to fixing the current problems, but can be applied in newly designed and built up areas. In this case, you can avoid the use of a sewer altogether, for example by fitting the pavement with a through channel, which will let water drain into a large grassland area, alternatively this area, can be continued by a wetland.
What are the benefits of decentralizing?
Lower rain sewer loading, and less maintenance costs- de gritting, de-trashing. Increasing the ground water lever, which is beneficial for trees and plants. Keeping roads and pavements puddle free, which is good for pedestrians and cars. Easier collection and re-using of grit after the winter and certainly many others.
Are there downsides ?
I can think of several factors, which might discourage the use of decentralized rainwater runoff management. The first one is that soil, when it gets saturated by water, can significantly change its properties, and this could lead to destabilization of buildings or roads. This is especially the case, where top soil is lined by impervious clay. In this case, experimenting, soil surveying and calculations may help determine the correct progress. The second one is the creation of ephemeral ponds, which will promote breeding of e.g. mosquitos. This is not necessarily desirable, but it’s an effect that can be controlled.
Water running off paved areas should be left where it falls, or at least as close as possible. This will save us work fixing the sewers, and will promote a more diverse natural world. Let’s get calculating, how much the proposed changes would cost and if they are therefore worth undertaking. As far as urban design goes, I would recommend that every new project is fitted entirely with decentralized rainwater management system.
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