Regulating For Rainwater

Great Stour looking north towards Harbledown; the train is approaching Canterbury from Ashford; photo by Thomas Grant licensed under Creative Commons Attribution-Share Alike 2.0

In England, there are various regulations about the drainage of rainwater. It is good to know more about them if you are a property-owner, if you are a resident anxious about new housing development near you, or if you are a wild-water swimmer furious about sewage in the local rivers or seas.

From my earlier investigation of the causes of raw sewage in rivers, it is evident that excessive rain causes this pollution. The water companies are building larger holding tanks, but is there any way the building regulations could be adjusted to stop the surges?

Victorian sewage systems

The Victorians, faced with many rural migrants into the industrialising cities, built streets of terraced housing quickly. They also laid down pipes, in hand-dug trenches, which formed a network of sewers to drain away both the rainwater and the foul water (‘foul’ = effluent from toilet, kitchen and laundry). The vast majority of the UK is still served by these combined sewers.

Should it be the job of the property owner, local government or the privatised water company to replace this old sewerage? Clearly the water companies have a role to play, but we should all become more aware about what local government can do and is doing about flood risks.

Flood risk

Since the 1950s, it has been a requirement to separate foul and rainwater if possible. Over the years, various calculations have been made by civil engineers and planners to reduce flood risk. These calculations are anchored in a base figure of expected water flow in an exceptional storm (one in every 100 years). But climate change means such exceptional storms are coming more frequently.

So, some prudent modellers now add 45 percent to the calculations; but this is not yet a legal requirement. Note: this is for rainfall intensity, which varies across England. In Cornwall it is only 0.016 (gentle steady rain) whereas for most of Kent (and London, and Essex) it is 0.020. But in the coastal band of East Kent it is 0.022 which maybe explains the need for those extra tanks at Margate!

Kent County Council is the authority for flood management in Kent, and every borough has to produce a flood risk management plan. There is also a government site where a property owner can check for the flood risk from:

  • Rivers and the sea
  • Surface water
  • Reservoirs
  • Some groundwater

As this will affect the value of your house and insurance costs, it is prudent to check up on this.

Building on flood plains

Here in Ashford, there are flood maps that divide the borough into zones, from high risk to low risk. New housing developments come ‘on stream’ every year. It is noticeable that several of these have been placed on flood plains. The architects cunningly design the ground floor for parking, and the residential units are above the potential flood level.

Many are concerned about the sheer pressure of new housing on an already overwhelmed sewage system. I took a detailed look at a planning application which has been published this week: for 23 social housing units (one bedroom) to be placed on an under-used car-park on the Henwood Industrial Estate.

As it is fairly near the River Stour, most of it is within Floodzone risk 3a (flood every 20 years). The architect has duly placed storage units on the ground floor, with the units in the three floors above. There are also walkways to ensure residents could reach higher ground in the event of a flood.

But a red light flashed in my mind when I saw this statement in the application:

“The exception test report establishes the proposed development to provide wide sustainability to the community that outweighs the flood risk.”

These units are for local single residents in need of temporary accommodation. Many are likely to be youngsters just emerging from the care system. While some commentators have raised concerns that this might increase the risk of county lines running from such units, with ‘cuckoo’ dealers taking over, there has also been much praise of the location. It is within walking distance of the town centre, but it is set apart from other residential areas. However, the authorities state that police and local management will be able to keep an eye on it … including in flood times, one hopes.

Exception test

So what exactly is the “exception test” mentioned in the application? Ashford has policy ENV9 about drainage:

“All development should include appropriate sustainable drainage systems for the disposal of surface water in order to avoid any increase in flood risk or adverse impact on water quality, and to mimic drainage from the pre-developed site.”

On a greenfield site, the maximum discharge is 4l/ha (litres per hectare) but for a smaller site, defined as 0.25 of a hectare, a discharge of 2l/ha is allowed. The proposals for this site have fudged it by applying the more generous allocation for surface water of a small site to a site that is in fact 0.3 of a hectare. 

How bad is that? The building regulations state that rainwater shall discharge into an adequate soakaway, or into a water-course, or into a public sewer, in that order of preference. For this site, most of the drainage will be into the Stour. Using only ground drainage would be unsuitable because of impermeable soils beneath this site, i.e. clay. The designers are doing their best to put in systems that will slow rainwater rushing into the Stour.

Their plan shows “lined permeable paving providing treatment, attenuation and storage”. This means putting half a metre of gravel beneath the permeable paving to hold the water while it filters down toward the river.

In the compulsory report on trees, it is also stated that 23 trees will be planted. These are to replace some uprooted to make way for the buildings. Trees soak up rainwater beautifully. It is noticeable, however, that the criteria evident in this plan’s tree report are directed more at aesthetics than at biodiversity or flood risk. Is it therefore time to adjust the criteria?

Collective flood risk

There is a desperate need for suitable temporary accommodation, and many aspects of this planning application appear to be good. But, if similar applications all over Kent are fudging just slightly over the limits, what will this mean for the collective flood risk for our county?

National Government Building Regulations state clearly that there is “a preference” for separating foul water from rainwater, but then give a string of examples for when it is “unavoidable”. How lenient can we afford to be?

Regulating rainwater might seem a bit like asking Canute to rule the waves. But regulations have made our lives safer from floods in recent decades.


*Note: 1 hectare (ha) = 10 000m2 and is approximately equal to 2½ acres.