A recent study published in CIWEM’s Water and Environment Journal has shown that permeable hardstanding solutions benefit the urban drainage cycle and should be promoted through legislation, education and incentives.
Urban development generally leads to an increase in roads, roofs and other impermeable surfaces, causing a corresponding reduction in permeable surface area. Following construction, the impermeable area continues to increase as residents install or enlarge patios, extensions and driveways, a phenomenon commonly known as ‘urban creep’. In London, it is estimated that two-thirds of front gardens are at least partially paved over to provide off-road parking spaces. This gradual increase in impermeable surface area in urban conurbations has clear consequences for flood risk.
The development of impermeable surfaces reduces the opportunity for rainfall to infiltrate into the soil, and consequently lowers the time taken for runoff to enter sewer systems or watercourses. At the catchment scale, the consequences are that peak flows are more pronounced and runoff volumes are greater. This can lead to increased flood risk, increased combined sewer overflow discharges, modification of river morphology and habitat degradation. At the site scale, the increase in impermeable area can lead to premature surcharge of the drainage network and subsequent flooding.
In contrast, the use of permeable options can reduce the amount of water discharged into the environment after rain. Depending on the type of hardstanding and sub-base, relatively moderate rainfall events can be totally infiltrated into the soil, while significant reductions in total runoff volume can be achieved for more intense events.
In addition to the impact on water quantity, urban creep can also have a significant impact on water quality, as deposited pollutants are washed off during rainfall events. These can include hydrocarbons, herbicides, fertilisers and heavy metals. Conversely, permeable surfaces retain pollutants, primarily by filtration of the suspended particles and to a lesser extent by absorption to substrate.
The primary aim of the authors’ research was to investigate the impact of hardstanding on flood risk and water quality in Scotland, therefore contributing to the evidence base necessary to support any proposed changes to relevant Scottish building regulations.
A review of current practice and a stakeholder consultation highlighted that the installation of impermeable hardstanding in Scotland is sufficiently widespread to justify measures to discourage such development. Although the general public are aware of the impact that impermeable hardstanding has on flood risk, and would generally support a change in legislation to mitigate against detrimental environmental impacts, very few households are planning to turn their driveway back into a garden.
The authors recommend that improved guidance on the design, specification and installation of permeable hardstanding is provided, as well as further research and/or long-term monitoring programmes to gather more data to ensure that the evidence base surrounding the long-term performance of permeable hardstanding is comprehensive.
Source: The Chartered Institution of Water and Environmental Management
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