Flooding in urban areas is a growing worry, especially for policy- makers. The problems caused by increasingly erratic rainfall have been made worse by a sharp rise, particularly in London, in the numbers of front gardens that have been paved over to provide off-road parking.
By one estimate, two-thirds of the capital's gardens - which amounts to an area the size of 22 Hyde Parks - are already at least partially paved over, greatly increasing the load on London's often creaky drainage infrastructure.
The widespread floods of summer 2007, which are estimated to have caused damage of around £3.3bn nationally, spurred the Government into taking action, and last October a law made any further paving over contingent on planning permission.
The problem is not restricted to flood risk. Driveway run-off also carries pollutants from cars into drains and from there it flows on into streams and rivers. And as RHS principal horticultural adviser Leigh Hunt points out: "Without gardens, urban temperatures rise in summer as paving acts like a storage heater, soaking up the sun's energy and giving it off at night - which is worrying, given the climate change predications."
However, under the new regulations, greater soil-moisture retention will also benefit street trees, which by some estimates are kept watered only through leaking underground drainage in many cases.
Communities & Local Government, however, does not record numbers of planning applications made under the new law, thus making it difficult to assess the impact it will have.
But homeowners needn't feel that adopting a more considerate approach to their front gardens will leave them out of pocket.
Hunt explains: "To concrete over a terrace front garden measuring an average of 24sq m would cost the homeowner in excess of £2,500. However, if they decided to gravel or pave over half of the area with permeable material to create a parking space and plant up the rest of it with wildlife-friendly plants, they could save hundreds of pounds."
Furthermore, a survey last year by the HTA showed that an attractive front garden can add up to £5,000 to the value of a property - yet nearly half of all estate agents thought most homeowners didn't make the most of their frontages.
Association of Professional Landscapers vice-chairman Mark Gregory welcomes the new law. "It's about time," he says. "I think it will have a positive impact - in fact, I can't see any negatives.
"It makes customers think about the issue and also gives landscapers extra arguments for good-quality design. For them it's another string to their bow, an opportunity to think creatively."
Gregory points out that, while it was already illegal to discharge water into the street, this was rarely if ever enforced. "The new law makes (paving) subject to planning, so applications for dropped kerbs will flag it up," he says.
"But even now it will be incredibly difficult to police. Like a lot of planning, it will rely on neighbours shopping neighbours. And there's a grey area when it comes to renewing existing surfaces."
Part of the problem currently is a lack of awareness of the law, even among professionals, he explains. "There's a lot of educating to be done - of the consumers, the planners and the landscapers. It would be nice to see a technical helpline to encourage people to talk about it.
"Councils are becoming aware of it, but they have a lot of other planning changes to contend with as well."
Indeed, implementation is likely to vary nationally, Gregory adds, with urban local authorities making it a higher priority than rural ones.
The type of solutions adopted will also vary, he thinks. While there are a variety of surfacing options available, the choice will partly depend on local conditions. For example, a pronounced slope will make gravel impractical as it will tend to slip downhill.
And Gregory points out that solid clay found in areas like Highgate or Hampstead does not allow much water to soak in rapidly. In such areas, one solution is to connect outflow to the house-roof water drain, where it is permitted.
Gregory welcomes the opportunity for the landscape industry to make a positive impact. "Our front gardens are the worst in Europe," he says. "Everyone is too in love with their cars. But there is a market for good design and planting."
The garden design shows have already served to raise awareness of the issue among the public. Last year's RHS Hampton Court Palace Flower Show featured a design, installed by Gregory's Landform Consultants for sports car manufacturer Porsche, incorporating a "lift" which allowed cars to be parked underneath the garden.
"If you have a £150,000 car, you don't want to leave it on the street," he says. "And you'd rather have the garage as another room, particularly where you have very expensive ground rent."
For the company that supplies the technology, interest has been "astronomical", Gregory adds. "The limiting factor is that they can only make so many."
For those not in possession of high-end sports cars, there are affordable solutions, which this year's RHS Chelsea Flower Show will highlight.
The Yorkshire and Humberside region has the second-largest concentration of housing in flood plains after Greater London and two bodies from the area are each using this year's flower show to promote legislation-friendly front-garden designs.
Leeds City Council, exhibiting for the seventh time, is working with Yorkshire engineering firm Hesco Bastion to highlight a range of flood-alleviation techniques. The garden, recessed in the earth, includes storm water planters which channel water into a central stream, as well as two "bio-retention ponds" or soakaways and a permeable scree surface. And Yorkshire Water is sponsoring Future Nature, which shows a variety of ways garden design can adapt to meet the challenge of a changing climate.
Meanwhile, show sponsor and hard-landscaping supplier Marshalls' Living Street will exemplify how sustainable surfacing can be incorporated into a range of small urban front gardens, meeting the needs of different households.
The company sees itself at the forefront of the move towards greater sustainability.
"A new driveway is an infrequent purchase and, fortunately, most consumers do their research first," says Marshalls group marketing director Chris Harrop.
"The big issue is enforcement," he adds. "So far, the implementation is very patchy, which undermines the intent of the law."
However, the company is doing its bit. "It's vital that everyone in the supply chain, from builders' merchants to installers to consumers, understands what's necessary and what's possible. That's why we aim to train everyone on the Marshalls Register by the end of the year. We have already trained around half of the members, who number nearly 1,000."
Other hard-landscaping suppliers are also developing products. Bradstone provides a rainwater-harvesting kit as well as a new range of permeable aggregates. And Stonemarket has launched Permeapave, a four-colour range of permeable block paving, along with a porous sub-base.
Meanwhile, Netlon Turfguard, distributed by Derbyshire-based TDP, adapts a thermoplastic mesh used in the construction industry to enable turf to cope with car parking.
Gregory adds: "There are some interesting solutions being developed and we will see more 'domesticated' products."
WHAT THE LAW SAYS
Since October last year, any householder wishing to pave over an area of front garden greater than 5sq m, or to replace existing hard standing, must seek planning permission.
This involves submitting an application form, scale plans and a fee of £150 to the local planning authority. Decisions are usually made within eight weeks. So far this law only applies in England.
However, any form of surfacing that retains rainwater within the curtilage of the property is exempt from this requirement.
The RHS and Environment Agency have together drafted suggestions for alternative surfacing. While these take many forms, they basically boil down to either letting rainwater flow through the surface or allowing it to run off, but still dealing with it within the garden.
- Permeable or porous surfacing: At its simplest, this can consist of grass reinforced with plastic mesh or a concrete lattice to prevent rutting - although even hard-wearing turf mixes will struggle if cars are parked for long periods.
Gravel underlaid with a porous sub-base and membrane provides another low-cost option.
As with conventional impermeable surfaces, porous blocks sit on a sub-base, but this will be open grade, known as 4/20 or Type 3, designed to absorb and retain higher volumes of water.
- Impermeable surfaces: Run-off can be minimised by paving only narrow strips of 30-60cm for cars to park on. Again, any planting such as turf between the strips may suffer if cars are to be parked for long periods.
For larger paved areas, thought needs to be given to dealing with the run-off on-site. This can take the form of soakaways adjacent to the paved area. Alternatively, water can be harvested in under-soil tanks for later use.