Rainwater needs to be drained away from urban areas to prevent streets and squares from flooding and to stop water pouring into tunnels and basements. Superfluous rainwater is removable in various ways. Traditionally, it has always been done by using a combined sewer system (to take away both domestic wastewater and rainwater), or through infiltration in the soil or flows into surface water.
Climate change is increasing the extremes in rainfall; there are likely to be more occurrences of extreme rainfall, when a lot of water will need to be removed in a short time. There will also be more extremely dry periods when water will actually be needed. One of the consequences of extreme rainfall is that it can cause untreated domestic wastewater to end up in surface water because of overspills (from the sewage cleanout drain to surface water).
It is the task of municipal authorities to check whether the draining of rainwater will still be adequate in their municipality in future. Handling rainwater smartly can also provide an answer to the need for water in dry periods.
If you are a municipal official responsible for rainwater drainage, you might wish to examine the use of natural processes.
The following natural processes are usable:
- infiltration of rainwater in soil
- use of green roofs
- creation of water bodies
- separate sewer system
Infiltration of rainwater in soil
Preconditions for good rainwater infiltration in soil include avoidance of closing off soil with things like buildings or a sealing land cover, like asphalt. Open or uncovered soil in the city comes in such forms as parks, green squares, cemeteries, open car parks, sports fields and urban farming. Increasing the number of distribution areas with open soil improves the natural drainage of water and also stops higher land from drying out. This increases the attractiveness of the surroundings and thus also the appeal of the area as a place for establishing business. Furthermore it has a positive effect on the tax-assessed value of homes in the municipality.
Use of green roofs
Green roofs retain rainwater and slow down its arrival in the soil. The rainwater also has more time to infiltrate the soil. Another advantage of green roofs is that they help to insulate buildings. And in the summer they contribute to a cooler climate in cities, because some of the water that falls on to the roofs will evaporate.
Creation of water bodies
Ponds and urban canals are usable to store rainwater. An infrastructure of ponds and urban canals enables fast removal of water to larger waters (canals, rivers and lakes). Ponds and urban canals contribute to a city's attractiveness and prevent heat stress during hot summers.
Transition to a separate sewer system
A separate sewer system will not have any difficulty coping with an abundance of rainfall. It will also eliminate the risk of wastewater spilling over into surface water. What's more, the wastewater will arrive in a more concentrated form at the sewage treatment plant and the plant will thus work more efficiently. The water drains of buildings can be disconnected from the sewer system, thus creating a simpler and smaller drainage system.
Use of a separate sewer system is a good option when building new neighbourhoods.
Municipal authorities are responsible for ensuring the efficient drainage of superfluous rainwater. This best-effort obligation is regulated under the Water Act. Municipalities are required to make the handling of rainwater (rain, snow and hail) part of their municipal sewage plan.
Platform 31 manages Agenda Stad (City Agenda) which brings together initiatives of various authorities and stakeholders. The Ministry of the Interior is formulating a new policy for the country's large cities, also under City Agenda.
The ‘Spatial Adaptation' knowledge portal provides guidance and tools with pointers for making a city more climate-proof. The ‘Climate-Proof City' charter sets out how stakeholders in the Netherlands intend to make cities climate-proof. The management of urban water figures prominently in the charter.
An attempt is under way in the Twekkelerveld district of the city of Enschede to disconnect the sewer system, to manage wastewater and rainwater separately and to store rainwater locally.