… 21st century water management …

About the author

Glyn Hyett is the Managing Director of water re-use specialists 3p Technik (UK) Ltd, and a founder-member and Director of the UK Rainwater Management Association.

water-supplies under stress …

DEFRA’s recently published progress report on the Government’s 25-Year Plan for the Environment, makes interesting reading for any enviro-conscious developer, not least where it relates to water.  Acknowledging existing stresses on water supplies, and the additional increased pressures likely to arise in coming years from population growth and climate-change, one of the key aims of the Government’s Plan is to ensure adequate future supplies of good-quality water for both people and the environment.

The growing challenge in this respect is illustrated by the “growing conditions” graphic which shows the water-related difficulties already being faced by agriculture and, by implication, the natural environment.

win-win benefits …

The farming sector’s response to this is already well underway in the drier regions of the country, where we are witnessing farm-level reservoirs being constructed to harvest winter rainfall for subsequent summer re-use.  Quite apart from keeping agriculture productive/viable in the worst-affected regions, these reservoirs also offer the potential to help reduce flood-risks by holding back water during peak weather event that might otherwise cause floods.

To realise this potential, however, Government needs to join-up its surface-water management thinking.  As a straightforward water store, these reservoirs are likely to be completely full by late Autumn, after which they will simply overflow leaving down-stream flood risks unchanged.  With Government intervention, however, farmers could be encouraged to include “attenuation” capacity in their reservoirs, capacity which is kept empty to accommodate peak weather events.  Subsequently, this water is then allowed to drain away at a rate at with which downstream infrastructure can cope, leaving spare capacity for the next peak event.

win-win benefits …

The farming sector’s response to this is already well underway in the drier regions of the country, where we are witnessing farm-level reservoirs being constructed to harvest winter rainfall for subsequent summer re-use.  Quite apart from keeping agriculture productive/viable in the worst-affected regions, these reservoirs also offer the potential to help reduce flood-risks by holding back water during peak weather event that might otherwise cause floods.

To realise this potential, however, Government needs to join-up its surface-water management thinking.  As a straightforward water store, these reservoirs are likely to be completely full by late Autumn, after which they will simply overflow leaving down-stream flood risks unchanged.  With Government intervention, however, farmers could be encouraged to include “attenuation” capacity in their reservoirs, capacity which is kept empty to accommodate peak weather events.  Subsequently, this water is then allowed to drain away at a rate at with which downstream infrastructure can cope, leaving spare capacity for the next peak event.

making better use of available water …

Given the stresses on water supplies already being experienced in the agricultural sector, it is hardly surprising that the Government’s plan for the environment also intends to tackle the challenge, in part, by aiming further to reduce domestic mains-water consumption below current Building Regulations requirements.

By far the most straightforward way of doing that would be to make rainwater harvesting (RWH) systems as normal a feature of new UK developments, as is already the case on mainland Europe in countries experiencing similar water-supply stresses.

Rainwater harvesting systems are very straightforward to install in new buildings; they operate simply by gathering roof water via standard guttering and downpipes, then filtering-out solid matter before storing it underground for later re-use.  The storage tanks are sized according to the roof area and occupancy of the property concerned, to provide around 20-days consumption in dry weather from full.  The stored water is then delivered through dedicated pipework to services which do not require potable water; typically these are the toilets, clothes-washing machines and garden taps.

saving precious mains-water …

Current Building Regulations require new homes to be designed and built so that they use no more mains-water than 125-litres per person per day, on average; generally, this can be achieved by a combination of water-efficient appliances, smaller baths and wash-hand basins/sinks, and water-efficient taps and shower-heads.

To reduce average daily consumption significantly below the current requirements will mean having, either new homes without baths, or the substitution of harvested rainwater for some of the mains water that would otherwise be used.  Typically, this latter approach would reduce the mains-water consumption down to around 80-litres per person per day.  And, better still for keen gardeners, if designed-in from the outset, the system can also be a source of garden irrigation water – even when hose-pipe bans are enforced.

integration with SuDS …

Unlike the farming case, where responding to water-shortages can have a beneficial effect on flood risks, in the urban environment Building Regulations water related requirements are first and foremost focussed on avoiding floods, with most new developments needing to incorporate a sustainable drainage system (SuDS); these must demonstrate to the local Planning Authority that the development will not increase downstream flood risks.

As stand-alone systems, SuDS potentially involve design complexity and unwelcome additional cost; so this is therefore not a good starting point for advocating the further complexity and cost of adding rainwater harvesting systems to the project.

A more enlightened approach, by Government, Designers and Developers, would be to take into account both sides of the surface water management equation at the same time, and start the design process through the prism of water-shortages, rather than flood-risks.

Investments in new housing or commercial developments should always be tackled from the perspective of development costs needing to translate into development value.   From a surface-water management perspective, this equation is optimised when design consideration is given first to the use of rainwater harvesting (to address stresses on water supplies), then amenity value is added to the development by using “green” attenuation measure such as water gardens and/or balancing ponds.

optimal design solutions …  

What this results in, taking a housing development as an example, is shown in the schematic illustration with the larger properties justifying, in cost-effectiveness terms, their own independent RWH systems.  As with the farmers’ reservoirs, however, these will potentially overflow during peak weather events, ideally into larger communal systems serving a number of smaller or higher-density properties.  This provides the added benefit of reducing the mains-water consumption of properties which might otherwise be incapable of harvesting sufficient rainwater to satisfy their non-potable demand.

Just like the upstream independent systems, the storage tanks of the communal systems will also need to overflow during peak weather events, either to an attenuation tank or, much better, amenity/development value-boosting surface features such as swales or a balancing pond.

The end result of tackling surface-water management in this way is to optimise the cost vs value equation for the development, particularly when long-term ease/cost of system maintenance is taken into account.  And, of course, in the process you will be helping to meet the Government’s 25-year Environment Plan.