96 Sustainable drainage – the 6 REs

Lots of techniques have been developed over recent years for identifying suitable locations for sustainable drainage by using GIS mapping to bring together information including landuse, property density, terrain and soil type.? These generally focus on identifying locations for source control SuDS – removing surface water from piped drainage systems and infiltrating it into the ground.

I recently saw a Linked in post that took this a step further by including the location of empty or derelict property that could be de-developed to provide flood mitigation opportunities and also improve the local community by getting rid of urban blight.? The map (above) suggests that this was mainly considering watercourse flooding, but it could also apply to local surface water flooding.

This reminded me of an article that I wrote about 10 years ago pointing out that sustainable drainage wasn’t just about source control but applied to the whole source-pathway-receptor drainage systems.? I called it the “6 REs model” inspired by the reduce-reuse-recycle motto for waste minimisation.? I notice that the motto has now also expanded to be 6 REs with the addition of repair, remanufacture and recover.? I claim no credit for that.

So what is my 6 REs model?

Principles

• Surface water management should be compatible with the needs and expectations of the community it serves.
• The environmental, ecological and amenity benefits of surface water should be maximised including its use to reduce the urban heat-island effect.
• Surface water management should consider performance in extreme rainfall.
• When considering new development or urban regeneration, surface water management should be addressed at the start of the development process before the site layout is defined.

Source control – reuse and reduce

• Surface water should be managed at source to mimic the natural water cycle where most runoff infiltrates into the ground.
• Surface water should be recognised as a potential source of water and reused where practical.
• Relatively clean surface water should be kept separate from contaminated surface water and foul sewage. Wide-scale separation of existing combined systems is unlikely to be affordable but retrofitting measures to reduce surface water inputs to combined systems will be important. Wrong connections in existing separate systems should be eliminated as far as is practical, recognising that many of them are on the private elements of the system.
• Charging for surface water management should be used to incentivise separation and source control.

Pathway - retain and redirect

• Surface water should normally be conveyed on the surface unless this is unsafe or impractical. This increases the amenity potential and reduces the risk of wrong connections and pollution.? It can also reduce the need for pumping.
• Surface water conveyance and storage should utilise vegetation to reduce velocities, increase infiltration and trap pollutants.
• Surface water infrastructure should be designed for exceedance so that excess flows are safely conveyed or stored.? Surface conveyance of flows makes the transition to exceedance easier to manage.?
• Urban areas should be planned with space to accommodate flood pathways and flood storage areas and these should be designed and designated as such. These spaces can also fulfil other functions such as roads, paths or open space.
• Existing piped systems will be important in managing surface water for the foreseeable future.? Upgrading or improved operation of elements that restrict drainage capacity may therefore be appropriate.

Receptor - resist and re-site

Actions to minimise the impact of drainage systems and pathways on society and the environment.

• Impact on properties, infrastructure and the environment should be considered on a risk based framework that includes the true costs of impact to all stakeholders.
• Environmental impacts on watercourses include erosion as well as acute and chronic pollution.? Impacts on groundwater should also be considered. During extreme events, pollution control measures may be overwhelmed and bypass facilities should be provided. Eroding discharges into watercourses should be controlled even in extreme events, to avoid irrevocable damage.
• Properties and vulnerable infrastructure should be in locations that do not flood or the impact of any flooding should be managed by mitigation measures: Local flood defence works such as demountable barriers or flood gates. Improved building resistance such as anti-flood devices.? Improved building resilience by using water resistant construction materials.
• Where it is uneconomic to manage the risk of damage to property or infrastructure they may need to be relocated or abandoned.

What does it all mean

We need to consider the whole source-pathway-receptor system of urban drainage.? The management of exceedance flows has so far not been sufficiently considered.

We need to plan exceedance surface flow routes for existing and new developments.? For new developments; avoid putting properties or sensitive infrastructure on or near these routes.? For existing developments consider moving sensitive infrastructure and consider creating flow pathways.? Purchasing and de-developing redundant or derelict properties may be part of this.

The problem is that surface flow pathways are the responsibility of the local authorities and they have neither the resources nor necessarily the powers to take the necessary action.

The first draft of the 2010 Flood and Water Management Act included a clause that would allow the Environment Agency to dig a watercourse along any highway.? Unfortunately this didn't make it into law. Shame, as I would love to see it becoming commonplace.