Rainfall events intensify

City design in the Netherlands is based on current climate conditions and does not account for climate change. Due to climate change, rainfall intensities are expected to increase. Therefore, the urban water system should be designed in such a way that the extremer events of the near future do not cause significant damage.

Exploring ways to buffer and reuse rainwater

Possible interventions for a rainproof city are: green roofs; blue roofs for water storage; more green private gardens; more surface storage. Another aspect of a rainproof city is the use and reuse of storm water. Examples for storm water use are irrigation, flushing of toilets and washing machines. Water that is infiltrated into the subsurface can be used by vegetation for transpiration in dry periods. This can contribute to reduction of the air temperature in summer and thereby reducing the heat island effect. All these measures reduce the demand for water supply of the city district.

Assessing the performance of water-storing roofs and increase of permeable surface area
To explore the positive effect of interventions, using an urban water balance model, four restructuring scenarios for the area ‘Kop Grasweg’ in the Buiksloterham district of Amsterdam were analysed:

Business as usual (no measures)

Water storing green roofs (equipping all roofs with a storage of 20 mm in growth medium and 50 mm in drainage layer)
More green permeable gardens (paved area on private property is reduced from 13% to2%)
Combination of scenario 2 en 3

Green roofs proved to have a larger effect on preventing pluvial flooding than permeable gardens. As can be expected, combining the two has the best outcome.

Scenario Recurrence time of pluvial flooding(year)
Traditional garden and roofs 2
Green roofs 7.5
Green gardens 2.3
Both 10

Recurrence time of pluvial flooding as a result of intervention

Similar conclusions were drawn based on calculations with the 3Di model, which adds a highly detailed spatial and temporal component. The model shows the maximum water depth two hours after a heavy rainfall of 50 mm in two hours.

Maximum water depth after heavy rainfall in 3 scenarios: without measures, with water storing roofs and with permeable road surfaces

Maximum water depth after heavy rainfall in 3 scenarios: without measures, with water storing roofs and with permeable road surfaces

Rainproof measures interact with many aspects

Several measures are effective in reducing pluvial flooding. But their implementation depends not only on ‘water’ arguments. It involves many other aspects, like mobility, attractiveness, investment and maintenance. Therefore different rainproof scenarios should be constructed and discussed with the stakeholders.

Online models support awareness and investments

Engaging citizens in making the city rainproof can be stimulated by good communication about the problem and solutions, e.g. a water label for houses or financial stimulants, but also with online tools which show and quantify the positive effect of measures that citizens implement for making their property Rainproof.

Download the full ACC report on “Circular Solutions Part 3 – The Rainproof City” here