Road run-off – what does happen to it?
[Continued from our summer 2024 newsletter]
Of course, some road run-off will run directly into an adjacent river, and some will end up in waste water treatment works due to misconnected pipes, or because the drainage system predates swales, drainage ponds etc.
The New Civil Engineer article queries the cleaning records of drainage ponds around the M25. They submitted a freedom of information request from the Highways Agency regarding the frequency of the drainage ponds being cleaned. National Highways appear to have insufficient data to identify when and where such cleaning activities are undertaken.
National Highways could only provide the most recent maintenance dates for seven of its 94 runoff ponds on the M25, with the remaining 87 having their maintenance described as ‘not yet required’. National Highways stated the cleaning was undertaken by contractors.
Jo Bradley , Director of Operation at Stormwater Shepherds is quoted in the New Civil Engineer article arguing that National Highways ‘is failing in its responsibilities as a public body. This is increasing both flooding and pollution risk’.
Storm Water Shepherds also say that ‘the accumulated sediment from ponds like these should be removed every 10-15 years to retain the functional capacity of the device and to prevent pollutants from being flushed out’. They continue, ‘This list shows no evidence that these ponds (apart from four of them) have ever been emptied since they were built which, for some of them, was over 30 years ago. This failure to remove sediment means that the devices no longer provide the flood risk management for which they were designed, nor prevent pollution effectively.’
If you are interested, please read the section of the article where Jo Bradley and John Bryden, Head of Improving Rivers at Thames21, refer to the claim used by National Highways that sediment removal is ‘not yet required’; they question the criteria used for and how National Highways Agency are making their decisions.
These articles got me thinking about what happens to the potentially toxic sediment once it is removed from the separators and drainage ponds, so I researched what was in the Highways Agency Network Management Manual. On page 15 it states that:
‘3.3.9 Vegetative drainage systems for highway runoff. Vegetative drainage systems are examples of system described elsewhere as sustainable drainage systems that are suitable for highway use for the conveying, storing and treating highway runoff. They are designed to enable the Highways Agency to comply with pollution protection legislation so as not to pollute receiving water courses. As a consequence of this, maintenance of these systems is essential for the continuing protection and must take priority.
‘The effectiveness of vegetative treatment systems can be easily and seriously impaired. Silting in ponds causing a loss of storage capacity and an accumulation of heavy metals that may increase the risk of pollution.
‘3.3.11 Linear Drainage Systems (page 16). Linear drainage systems are shallow in depth and are generally at the edge of pavements, in nosings to slip roads and in central reserves. These systems are prone to accumulation of silt where the flow speed is insufficient to self-clean the system. Therefore, these items may need to be emptied of silt and other detritus to avoid solids entering the drainage system. Cleaning is normally carried out by large volume, low pressure, water flushing. Silt and other solids arising, from emptying and cleaning operations may cause pollution. Material must be disposed of in accordance with the relevant waste management regulations and legislation.’
So, as yet I have not found an answer to my question of what happens to the silt other than it ‘must be disposed of in accordance with the relevant waste management regulations and legislation’.
In April 2004, the Highways Agency issued Area Management Memo 45/04 seeking information from service providers to fully understand and quantify the implications of the Landfill Directive on their operations. The responses revealed a variation in disposal procedures.
I can only conclude from what I have read in the New Civil Engineer article and the 82 pages of the Highways Agency Network Manual that the Highways Agency have set the standards for managing surface road run off, clearly stating that potentially toxic substances in the water must be removed and treated. They do not, however, discuss the range of potential toxic pollutants, nor specify how they must be treated, neither does it seem that anyone closely monitors cleaning activities.
Going forward, I will try and source the missing pieces of the Road Run Off jigsaw puzzle. If any of you reading this article can complete the jigsaw puzzle I would be most interested in hearing from you.
See also 3.17 Pages 58 and 50 The Landfill Directive and Liquid Waste, and Page 60 Environment Agency Directive Interpretation Note 1 (version 1.0) July 2002.
See also Highways Agency Network Management Manual. 3.17 Pages 58 and 50 The Landfill Directive and Liquid Waste and Page 60 Environment Agency Directive Interpretation Note 1 (version 1.0) July 2002.