Hydrological circle in nature works efficiently and effectively. Precipitation slowly infiltrates into the ground and replenishes the groundwater levels. Another part of the water runs on the ground surface, part evaporates and part is released back to the atmosphere through evapotranspiration.
When we started to develop our cities, we replaced huge parts of this cycle to fit our needs. All the rainwater and additional water imported into our households is supposed to be discharged through sewer and rainwater pipes. If the system is not designed properly, large volume of runoff can fill the streets and cause floods. At the same time, groundwater levels are affected and keeping the system running costs a significant amount of money.
The goal of water-sensitive urban design is to incorporate elements from the natural hydrological cycle back into our built environment. These elements and design features reduce the need for water and restore the balance in our environment. Ideally, the runoff is minimized and water discharged through sewer and rainwater pipes is reduced, lowering the maintenance cost.
There are many ways how to improve the hydrological cycle in the cities. So let’s take a closer look at some of the possibilities, so we can kickstart our projects with a great ideas planted in our minds.
Wet and dry basins
Basins act as a reduction of runoff water. When there is an excessive discharge, the basins store the additional water and dispose it over a period of time.
Wet basins have permanent water level which varies depending on the amount of water entering and leaving the basin.
Dry basins contain water only when there is a need, otherwise it is dry.
The water from basins can be partly absorbed by the ground, partly evaporated and partly discharged to the sewer system at slower rate, depending on the design and local code and design priorities.
Permeable surfaces
In order to reduce runoff from horizontal landscaping surfaces, we can use permeabloe surfaces. Those surfaces work well for walking or driving on as well as allow rainwater to seep through.
Traditionally, when we talk about permeable surfaces, we often imagine grass paving, however, now there are new permeable materials such as porous asphalt.
Grass pavings combine the infiltration and growth properties of grass with durability and load-bearing capacity grater than ordinary grass. They come as concrete flagstones with holes for grass to grow or grass paving grid with a honeycomb structure. The holes are filled to just below the edge with a suitable growth medium such as a sand-humus mix, and the grass is sown.
Soakaway tank
Soakaway tanks are the invisible solution. Pits in the ground, stabilised with a porous material and covered with topsoil and vegetation. Water from roofs and impermeable surfaces can be led underground, into the tank. From there, the water is slowly allowed back into the ground.
If such solution is chosen, connecting the property to the rainwater system can often be ommited, offsetting the cost of the soakaway tank.
Green roof
Green roofs allow the rainwater to be partially retained, reducing peak loads on the rainwater system. At the same time, part of the water from green roof evaporates and part is released back into the atmosphere through evapotranspiration.
On top of that, green roofs have very long lifespan, offsetting the initial cost by limiting the maintenance and replacement cost.
Conclusion
It is essential to consider how our projects influence the hydrological cycle and choose the solutions that help with rebalancing the natural cycle of water. Not just because it helps the nature, but also because many of these solutions help to reduce the cost for sustaining our built environment in the long run.
At the same time, many of these solutions reintroduce greenery into our cities, helping to mitigate other problems, such as heat island effect.
We need to re-imagine the cities. Not just greener, but bluer as well. I yes, I am aware of the fact that it is terribly hard to pronounce bluer, which might be one of the reason we do not talk about re-introducing water into our cities and making it part of the environment.
Take care and start adding water features.