During the period of industrialisation and uninhibited growth of towns and cities, people started leading sewer water away from residential areas for reasons of hygiene. In most cases, this also included draining away rainwater via underground sewers. Today, the increasing levels of precipitation over short periods of time, caused by climate change, mean that many existing sewer systems are not capable of handling the volumes involved. In recent times this development, combined with the realisation that clean rainwater need not necessarily be led off in underground sewers and that storing it above-ground can also benefit urban areas, has caused policies and practices to be reconsidered.
The policy for the urban water system has always been to slow the runoff of rainwater by capturing it for as long as possible or allowing it to infiltrate immediately. The reason is that the pumps and ditches do not have sufficient capacity to process large volumes of rainwater if it is led off at an accelerated pace. The purpose is to approximate the natural water balance as closely as possible. Even better than buffering it or delaying the runoff of rainwater is capturing and using it, which also saves on drinking water. In Belgium this policy has already been officially adopted. [Vlario, 2010]
The heavier rainfall caused by climate change will lead to more overflows in districts without sufficient buffering facilities. Rainwater buffering facilities can be realised by reducing the proportion of hard surface areas and introducing green roofs, rainwater ponds, underground reservoirs and decentralised infiltration systems.
The form of infiltration or buffering that is selected depends on the permeability of the ground, the groundwater level, the quality of the runoff and the space available for infiltration or buffering facilities.
Two methods exist for reducing the area of impervious surfaces. Firstly, roads and other hard surfaces should be no larger than strictly necessary. Secondly, where possible, for example on footpaths, semi-permeable materials should be used. Reducing the area covered by impervious surfaces offers several other advantages besides having a buffering effect and supplementing groundwater. Not only do areas with fewer paved surfaces have more lively designs, the microclimate is also improved by evaporation and because plants filter dust particles out of the air. The microclimate is also better for the life forms living in the soil. Habitats are created for plants and animals, both on the surface and in the ground.
Water balance with unpaved surface:
- more evaporation by plants
- more slow drainage through shallow groundwater
- less runoff through surface
Influence of pavement on the water balance:
- less evaporation
- less seepage to groundwater
- strongly enlarged runoff through surface