Naturally vegetated, rural ground is permeable and has the capacity to absorb high levels of rainfall through its surface. Development of this ground can greatly reduce permeability by the introduction of hard surfaces – asphalt, concrete paving and roofs. Less water infiltrates the ground and more water runs off the surface, more rapidly.
Flood potential
As development increases, so do run-off speeds and volumes. Their rapid delivery into the local water courses can cause flooding downstream, resulting in danger to life and damage to property, or erosion and destruction of natural habitats.
Pollution
Traffic, hard surfaces and increased run-off can increase the risk of pollutants, which are literally washed off the streets into the water catchment, contaminating the downstream water supply.
Climate changes
It is generally accepted that the UK’s climate is changing and that our weather is becoming more extreme. Paradoxically, in the short term, it is not the overall levels of rainfall that influence the need for sustainable drainage but the ferocity and frequency of the single rainfall events that may cause flooding.