How to reduce highway water runoff and related pollutants.

The popularity of the Sir David Attenborough-fronted television series Blue Planet did a great job of raising public concern about pollutants in water.

While much of the show’s focus was on microplastics, it’s fair to say greater impetus was given to policy on water pollution of all kinds - and highways have a major role in this, even though they can very often be overlooked.

When a storm sends water across a well-designed highway, it quickly disperses, thereby preventing the development of large puddles that would present hazards to drivers. But a well-designed road needs to do more than simply remove water from surfaces quickly; it has to do so in a way that minimises as much as possible any environmental damage.

And this is more difficult than it may initially sound.

A serious concern

Highway authorities, construction companies, term maintenance contractors and designers all have an obligation to think about what is happening to water, even if their understandable priority is to get it off the road.

Runoff—the water the flows across and off roads—is a danger to the environment because it carries pollutants onto adjacent land, as well as into watercourses and drains. In this new climate of increased public concern, it will surely not be long before an organisation suffers reputational damage, or even litigation, because of pollution from a highway affecting its land or water supply.

Hydrocarbons, as well as an array of toxic chemicals, may be carried from highways during heavy rains into watercourses where—depending on the local circumstances—they can accumulate, causing chronic pollution incidents, which could also prevent fish from reaching the gravel in which they spawn.

There are multiple potential sources of such contamination, including rubber from tyres, dust from brake and clutch pads, metal from engine wear, exhaust emissions, and also fuel leakages, and all of these must be fully taken into account if the correct solutions are to be found, and the most appropriate environmental protection decisions are to be made.

Tackling the problem

One particularly strong example of an organisation responding to public concern is the Thames 21 charity’s project on road runoff. It has established that one cause of pollution of London’s rivers is ‘sediment, heavy metals, petrol and other pollutants from cars that are washed into the waterways from roads when it rains’.

This sort of pollution can be particularly pronounced during rainfall events after prolonged dry periods, with hydrocarbons and other chemicals liable to build up on roads over the course of many weeks or months, before being swept into watercourses when it rains.

Thames 21 is seeking to quantify the pollution from roads entering rivers so that it can develop a ‘heat map’ to determine where green infrastructure such as sustainable urban drainage systems (SuDS) should be located.

SuDS lore

When it published its SuDS for Roads guide, the Scottish Environmental Protection Agency (SEPA) explained that historically, surface water drainage systems have not been designed with sustainability in mind, and most paid ‘insufficient regard to water quality, catchment flooding, water resources, site amenity, and potential for habitat enhancement’.

In recent times, SuDS have been seen as a means of redressing this. They can manage surface water runoff in a way that minimises the impacts of road runoff, while maximising amenity and biodiversity.

In England, the Environment Agency (EA) has stated that ‘sustainable drainage systems can significantly reduce the harm to our water resources, and improve the quality of our built environments, by moderating flows and filtering runoff’. 

It also notes that SuDS can be cost-effectively designed to work with retained natural features such as ditches or ponds and, where circumstances permit, can play an active role in enhancing nature conservation, and helping to make best use of water resource.

The options

SuDS may include permeable pavements, described by the agency as encouraging runoff through porous materials before slowly soaking into the ground. The EA believes that swales—shallow channels with gently sloping sides—can do more than simply provide stormwater channels when needed, but can actually play a vital role in maintaining landscaped areas. They can also be installed alongside roads to replace conventional kerbs, thereby minimising construction and maintenance costs. 

Infiltration trenches—sometimes referred to as percolation trenches—are also worthy of consideration. These are stone-filled reservoirs where stormwater runoff can be directed, and from which the water gradually enters the ground. These are generally used to support footpaths and car parks, where there is limited space available to manage excess water.

Key considerations

The EA suggests that the choice of a SuDS approach will depend on the pollutants present in runoff, the size of the catchment area, local hydrology, as well as soil conditions and any contamination.

Highways England has used, amongst other approaches, road-edge surface water channels, while South West Water has detention basins; open areas of grass that are normally dry, but which after heavy rainfall fill with water for a short time before slowly soaking away. This approach ensures that sewers are not overwhelmed, and also means the ground will not become saturated.

Water UK, the water industry body, has stated that retention ponds can be used in residential development to attenuate flows, reduce flood risk, improve water quality, enhance biodiversity, and create habitats from water received from roofs and roads.

Thus, while there are choices to be made about which SuDS technique is best for a given situation, where practical they will be expected by planning and environmental authorities to reduce pollution from runoff, and indeed to prevent runoff from overwhelming watercourses and drains during particularly wet periods.

The conclusion

Public concern about pollution, and regulatory requirements, will make it difficult to avoid using SuDS where needed. Not only that, but making better use of water—which, though plentiful, is still considered a valuable resource—is becoming increasingly essential.

Hydro International has the expertise to advise on the approaches, equipment, design, and installation of the most appropriate SuDS solution for a given situation.