Of the two, ie floods and droughts, the influential International Panel on Climate Change reports that future droughts pose the greater economic and
environmental threat to the UK, suggesting that the impact of future water shortages must not be overlooked given the current priority which is to avoid floods.

Exacerbating the problem is the increased weather tendency for long dry spells to be broken by short periods of intense rain.  The intensity of these semitropical downpours is hard to manage from the perspectives of both flood-avoidance, and of water-capture for subsequent use.

Impact of Development

These twin rainfall-related threats are increased by manmade factors, such as population-growth and the development of new homes and jobs. More people and jobs increases water demand, whilst development potentially increases flood-risks, compared with greenfields which absorb more water.

The flood avoidance side of the equation is already well-reflected in national policies, with Building Regulations requiring all new developments to meet sustainable drainage (SuDS) criteria, designed to ensure that down-stream flood risks are not increased.

Enlightened Welsh Government environmental polices have expanded on that requirement to embrace the notion that the ideal way to avoid flooding is to collect and store rainwater for re-use instead.  The water collected locally can then be used to feed services that do not require drinking water, such as toilet flushing, clothes washing, vehicle washing and irrigation.

At a stroke this Welsh policy helps to tackle both sides of the surface water management equation, by avoiding floods and at the same time reducing the demand for mains-water.

Practical Implications

Implementation of the Welsh Government’s policy, which must surely be a pointer for other drier and more densely populated regions of the UK, hinges around use of very simple rainwater harvesting (RWH) technology to collect, store and re-use the water.

Although this obviously helps to reduce stresses on mainswater supplies, in isolation a RWH system does not mitigate flood risks as, at the time of peak weather event, its storage tank may already be full. This limitation can be overcome, however, by arranging on a housing development, for example, for the overflow to feed into communal systems serving smaller/higher density properties. These might otherwise collect insufficient water from their own roofs to satisfy occupants’ potential usage of non-potable water.

In extreme weather conditions, however, even these communal (or commercial system) tanks might also need to overflow into eco-friendly balancing swales and/or ponds. These add value to a development by providing environmental and attractive amenity benefits.

SuDS and Schedule-3

An important aim of the 2010 Flood and Water Management Act is to ensure the ongoing effectiveness of SuDS on new developments, by putting in place arrangements for their ongoing maintenance. This aim has yet to be achieved, due to practical implementation issues, but becomes politically increasingly important with each new flood event.

Schedule-3 of the Act addresses this aim which, when implemented, will require the SuDS system on a new development to be adopted, akin to what already happens with roads and sewers. In this process, the Developer will pay a fee to the adopting agency, which thereafter assumes responsibility for the ongoing maintenance and performance of the system.

From all points of view, the easier the system is to maintain the better, particularly for the Developer who will be bearing the life-cycle cost through the adoption fee. Taking this into account, a SuDS design approach that integrates with water re-use, in line with Welsh policy, will come become much
more cost-effective when the adoption fee is taken into account alongside capital and installation costs.

The schematic above illustrates this combined approach, which applies equally to both commercial and housing developments. As the rainwater harvesting
components of the overall SuDS system will require their own servicing regime, which will be the responsibility of the residents’ management company, the “adopted” side of the overall scheme can be made easy and relatively inexpensive to maintain, and thus straightforward to adopt.