Highway Drainage Amethyst Surveys

By Nick Orman – expert in Wastewater Infrastructure

A recent report in New Civil Engineer (NCE) highlighted a skills gap in highway drainage

It is 44 years now since I first started work as a graduate civil engineer and for one of my first projects, I was given the job of sorting out the drainage for a major highway junction improvement project in Bury, Greater Manchester. As I walk and drive around, I can see evidence of this skills gap around me. The effects of this are felt by road users (pedestrians, cyclists, motorists etc.) but also property owners and water companies, who often get the short straw from poorly designed and poorly maintained highway drainage.

Here are some myths about highway drainage:

1. Highway drainage is just about getting the water off the carriageway as quickly as possible. It is all very well getting it off the surface but if there is not enough capacity downstream in the sewer – or river – it will either come back up or cause a problem somewhere else downstream.

2. It is all about the pipes and the gullies. On the Bury project, most of my time was spent on designing the alignment of the road to get the flow paths to the gullies without causing ponding or ice hazards. Then once the water is in the pipes it still needs to go somewhere and if the downstream system does not have capacity, then the highway drainage will be ineffective.

3. A good gully system will keep the road free from surface water. Gullies are small holes on a large surface. They are designed for a 1 in 1-year or 1 in 2-year rainfall event and they won’t take much more than that, regardless of the capacity in the downstream pipes. Even well-maintained gullies will block with debris sometimes. It is better still to use swales and filter strips to take the water off the carriageway if you can. They are much less likely to block and have much more capacity than gullies. Even then, the design must also include an alternative overland flow route for extreme rainfall events that does not cause property flooding.

4. Road drainage should normally be connected to the public sewer system. If the public sewer is a combined sewer, then adding further surface water to the system will increase discharges to rivers from Combined Sewer Overflows (CSOs). It will also increase the risk of sewer flooding. Highway authorities do not have a right to connect road drainage into sewers. They need to get permission from the water company, but how many even ask when they are doing a road improvement that could significantly change the flow into the sewer or just the point of entry?

5. Connecting to the public sewer is free so it is best value to our council taxpayers. Although water companies cannot charge the highway authority, every water bill contains an itemised charge for highway drainage, so it is not free to council taxpayers. Using Sustainable Drainage Systems (SuDS) for new highway drainage will ultimately benefit council taxpayers through their water bills. Even if the runoff is connected to the sewer system, SuDS will minimise the impact on the sewers by attenuating the peak flows. If it is feasible to divert existing highway runoff from the public sewer during major highway works, this should be considered as well.

6. Rainwater runoff is clean water. Surface water is often discharged directly to rivers; however, it is not always clean. Runoff from roads and parking areas in particular can be polluted with oil and heavy metals as well as organic materials. Road runoff can therefore cause pollution in our rivers. Gully pots offer only minimal treatment even if they are maintained. SuDS offer better treatment performance. The Environmental Permitting Regulations allow highway drainage to discharge to rivers without permit, but where it causes pollution, the Environment Agency can serve a notice requiring a permit. As there is more pressure to improve river water quality, discharge of untreated highway drainage directly to rivers or indirectly via public surface water sewers may not be acceptable in the future.

7. Design the road first, then design the drainage. That might work on a major rural road but not in an urban area. In an urban area the design of the vertical alignment is crucial to limit the risks to pedestrians and neighbouring properties. If the road is higher than the floor level of adjacent buildings, then there will be a flood risk to those buildings. Pedestrian crossings, bus stops and other places where pedestrians need to wait should be away from low points and other places where ponding might occur.

8. Gullies can be used to convey the runoff from 1 in 100-year rainfall into an underground attenuation tank. No – if you need to store flow from extreme events you need something other than gullies – unless they are the size of cattle grids! I have seen too many planning applications where this was the drainage solution proposed and they were still passed!

9. The system will be maintained. There was a time when road gullies were cleaned out twice a year. This may still happen on some major highways but even then, debris from a storm can soon block a gully, particularly at a low point. I know of roads where more than half the gullies are blocked. The result is that the flow of water along the road is wider, increasing risk to pedestrians from passing traffic. Ponding can cause rapid deterioration of the structure of the road I know of roads that continually develop potholes due to regular flooding.

So, if I can think back, my 21-year-old self knew little if any of this. How much do the highway drainage designers of today know? The NCE article suggests probably not enough. Since design is integrated, we cannot just pass it on to drainage engineers, we need to develop the skills of our highway engineers so they can design effective highway drainage for all road users and the environment with the help of specialists where necessary.

For more information on Drainage Surveys contact Amethyst Surveys

The original version of this article was published in www.wrcgroup.com