Drainage is a natural process.  Rain falls onto the ground and either soaks into soil, or runs off the surface where there is no soil or it is already water logged.  Whichever route the rainfall takes, it ultimately enters a water course and flows away.

Human activity inevitably interrupts this natural drainage.  In our towns and cities development is so extensive that we have covered most of the soil with hard surface (roofs, pavements and roads) which prevent the rain from soaking away.  To deal with the water that can no longer soak away we have built drains and pipes to quickly carry it downstream.  This sounds like a sensible solution but it comes at a price.  This was most clearly seen in the floods of the summer of 2007 across the UK, where more than 25% of the impact was as a result of surface water flooding.  This not only leads to inundation of properties but also major pollution and environmental harm.  A similar problem occurs in our rural areas where soils have been compacted as a result of changes in agricultural practice to the point where their capacity to absorb rain has been dramatically reduced.

Sustainable Drainage Systems (SuDS) are surface water drainage schemes designed to mimic natural drainage.  Natural hydrology is disrupted in developed areas by paved surfaces.  Rainwater can no longer soak into the ground and replenish groundwater which in turn means less supply to springs and river network.  In the longer term, this means lower flows and increased likelihood of drought during dry periods.

In addition, by intercepting run-off and routing it through piped networks, we accelerate the passage of flow to the receiving watercourses, resulting in higher peak flows and flooding.  Another effect of this is increased erosion and damage to river banks and beds.