Agenda item

(This report was presented to Cabinet on 22 October 2020 and is for Information only)

(1)       Mr Bourner introduced the report which had previously been considered by Cabinet on 12 October 2020. He said that Kent was experiencing intense rainfall events on an increasingly frequent basis, with recent rainstorms generating a volume and intensity of rain well beyond that of the design capability of highway drainage systems. Summer ‘flash flooding was also becoming an increasingly significant risk to the highway authority.  For example, over 40 mm of rain had fallen in the Sittingbourne area on 15 August 2020 in the space of just 45 minutes.  A perspective on the extraordinary nature of the event could be gained when noting that the average amount of rainfall in this region was just 56.3 mm for the entire month of August.

(2)       Mr Bourner then said that the burden on Kent’s highway drainage systems could be exacerbated by many other factors. These included the age and condition of highway drainage systems (some systems were more than 100 years old and be operating beyond their original design life);  operational issues arising from budget limitations for ongoing routine maintenance; capacity issues of drainage systems which were not under the control of the Highway Authority, including public sewers or private ditches and watercourses which they connected into; structural damage to drainage systems by third parties or site environs that could remain  unnoticed until significant rainfall occurred;  poor maintenance of drainage features in land adjacent to the highway; “Urban Creep” effects such as additional run-off onto highways from the paving of front gardens; and increases in the peak intensity of rainfall brought about by climate change.

(3)       Mr Bourner said that KCC’s highway drainage systems were designed to drain water from the highway surface only and were not generally intended to be flood defences. They, nevertheless, still played a key role in managing local flood risk. They were usually designed to cope with a 1 in 5 - year event, such as a storm producing approximately 20 mm of rainfall in a one-hour period. In recent years, however, many occurrences had exceeded that design standard.

(4)       When such events occurred, run-off often used the highway as a conduit to escape to lower ground, either as “overland flows”  which following the topography or as “exceedance flows” where a drainage system was unable to cope.  This could lead to highway flooding or property damage in a location that was remote from the original source of the flood water.

(5)       Mr Bourner added that this often gave the impression that the run-off had originated solely from the highway and should therefore have been dealt with by the drainage system in that location.  As a result, the Highway Authority was often blamed for flooding that may have been outside of its reasonable control.

(6)       Mr Bourner then addressed KCC’s role as the Lead Local Flood Authority for Kent. A range of Surface Water Management Plans (SWMPs) had been produced with the aim of increasing the understanding of local flood risks and providing a high-level action plan to identify measures to mitigate local flooding risks. The majority of these plans had been produced in 2012 and 2013 which meant that they predated some notable surface water flooding events that had occurred in recent years.

(7)       Mr Bourner said that the current one and two-year programme of works for capital drainage improvements known as the “Well Managed Highways” approach was based upon a Geographic Information System (GIS) analysis of customer enquiries involving highway flooding and properties damaged by flood.  This had allowed an initial focus on areas with existing reported issues rather than reliance on the SWMP action plans, which were considered to be out of date and did not cover the entire county.

(8)       In the last two years, schemes had also been jointly funded or delivered by the Highway Drainage Asset Management Team and the Flood and Water Management Team which piloted the use of Blue-Green Infrastructure as described in the Appendix to the report.

(9)       Mr Bourner then said that it was essential to manage KCC’s existing assets appropriately in order to reduce the risk of flooding occurring. It was also important for KCC to protect its investment in areas where capital funded repairs and drainage improvements were carried out. This was likely to require additional future revenue funding and smarter use of existing funding.

(10)     The Highway Drainage Asset Management Team had been exploring better drainage management through the “Live Labs” project in order to seek a more encompassing software platform, dedicated to the complexities of drainage, which had the function of supporting KCC’s maintenance activities while communicating as much data as required to the Pitney Bowes Confirm system (WAMS) already in operation.

(11)     Mr Bourner said that research had highlighted several areas where there was a significant financial opportunity for better management of the drainage network. In comparison to similar county councils, KCC’s average cyclical/scheduled crew productivity was 65 gullies per day, as opposed to their 99, which represented a 52% opportunity for improvement.

(11)     Mr Bourner added that Kaarbontech had been identified as the appropriate platform for KCC and their trial included several stages and options as part of an approach to manage drainage differently in Kent. The Borough of Maidstone had been chosen as the trial area. The broad goals of the project included collecting an inventory of drainage assets; attributing historic information from other council systems to these assets; defining and prioritising zones of interest; Risk profiling maintenance based on prioritised assets; assessing if and how handheld devices could play a part in future maintenance; allowing ongoing data collection to automatically feed into risk profiling; investment in the asset management software platform to map all KCC’s drainage assets, including the final outfalls (this would reduce cost as future investigations would not be required as the asset would have already been plotted, including all CCTV surveys.

(12)       Mr Bourner said that several smart gully sensors from different manufacturers had been installed across the County as part of the ongoing Live Lab works, to record data which would also be factored into future proactive cleansing.  The sensors judged to be the most effective in performance and costs would be installed across the County as future funding became available.

(13)       Mr Bourner then turned to the question of future capital investments. He said that KCC intended to develop a map of the locations where the risk of surface water flooding was high and/or where climate change impacts might affect the risk of flooding in future. This would allow KCC to take a more proactive asset management approach instead of focusing solely on customer enquiries.

(14)        Mr Bourner explained that a GIS analysis had been undertaken to identify and score “flood cells” across Kent. This ensured that multiple factors were taken into consideration when assessing a site.   He referred to an example in the report of a flood cell at Swanscombe which illustrated the area which might contribute to a flooding issue.

(15)       Mr Bourner said that the GIS analysis provided a high-level overview of the risk as well as the area where surface water run-off might contribute to it. Each flood cell location would require a more detailed review in the future. This would inform KCC’s 3 to 5-year capital works programme as not every site identified would require drainage improvement works to reduce the risk of flooding. There might be instances where minor repairs or an enhanced maintenance regime sufficed.  In other circumstances, there might not be a viable solution or one that was not within KCC’s control to deliver.

(16)     Mr Bourner then gave examples of opportunities for mitigation such as:-

(a)         enhanced maintenance regimes where the existing drainage system was in sound operational order but liable to blockage from leaves or silt.  These areas could potentially be linked into future trials following the “Live Labs” project;

(b)       a like-for-like replacement of existing assets where operational or structural issues were found but where existing reports of flooding were minimal;

(c)      use of modern techniques to extend the life of existing drainage assets, such as trenchless and no-dig cast in place pipe and culvert lining and stabilisation;

(d)       the retrofit of Sustainable Drainage (SuDS) features and Blue-Green Infrastructure such as permeable paving, rain gardens, open attenuation for exceedance flows;

(e)       the replacement or supplementing of existing assets with new or upsized assets (for example larger or additional soakaways) where greater resilience was required;

(f)        the attenuation of surface water to accommodate additional run-off volume with a controlled discharge back into the network in order to avoid increasing flood risk elsewhere; and

(g)      the separation of surface water from existing sewers and its redirection to an alternative outfall to ease sewer capacity issues.

(17)     Mr Bourner added that future improvements would need to be cost-beneficial and that any improvements made were unlikely to completely eliminate the risk of surface water flooding, as all measures could be overwhelmed by a rainfall event of sufficient extremity.  It was also necessary for KCC to work closely with the various water and utility organisations in order to develop co-operative programmes which aligned KCC’s operational needs to their ongoing asset modernisation and water management obligations.  For this reason, areas of interest would be included in the next update of the Local Flood Risk Management Strategy.

(18)     Mr Bourner informed the Committee that the Highway Drainage Asset Management Team now benefitted from an increased capital budget of £15m over a 3-year period between 2019/20 and 2021/22. He said that this was likely to be sufficient to deliver the current highway works and two capital programmes. He warned, however, that this was subject to the current significant uncertainties on future funding allocations.

(19)      Mr Bourner said that the government had announced in April 2020 that it would double its investment in flood and coastal defences in England to £5.2 billion over the next six years. This gave KCC an opportunity to seek external funding for some drainage schemes which offered a good cost benefit ratio and/or be match funded by KCC. The previous scheme had been changed by the introduction pf a new risk category which enabled schemes that prevented surface water flooding to qualify for more funding.

(20)     Mr Bourner concluded his presentation by saying that KCC needed to continue to seek investment in its highway drainage infrastructure to support the delivery of improvements.  Match funding was usually required to enable delivery. Potentially significant investment was required to support the investigation and design of drainage schemes before any bids for external funding could be made.

(21)     In response to a question from Mr Chittenden, Mr Bourner said that there were 36,000 gullies in Maidstone Borough which had been plotted on the GIS system by Carbondata on KCC’s behalf.  Information such as siltage levels and flood zones had also been entered.  As a result, 17,000 gullies in the Borough had been cleansed to date at a cost of £8 per gully.  The normal result for Maidstone had previously been 6,000 gullies at £35 per gully. 

(22)      Mr Bourner replied to a question from Mr David Brown by saying that he would be interested in receiving data from Kent Fire and Rescue on areas of flood. 

(23)     In response to a question from Mr Bowles on flooding in Linsted, Mr Bourner said that the £15.2m allocated to the Highways Drainage Team would enable the smarter working project to be extended over most of the rest of Kent in the next three years.  Additionally, multi-agency work with the water companies would be developed to overcome the problem that KCC could install modern drainage systems only to find that the water company had not done the same in respect of its sewage drainage, resulting in continued flooding of the highway. 

(24)     The Chairman commented that recent meetings of the Southern Regional Flood and Coastal Committee had seen greater willingness from the water companies to undertake improvements to its assets than had previously been the case. 

(25)     Mr Tant said that the issue in Linsted was not one where highway run-off ended up in the sewer.  Both the highway and the sewer were affected by overland flow and the pumping station, which had originally catered for less than half as many properties as now existed, was often unable to cope with it. 

(26)     Mrs Hurst referred to a pilot scheme at George Park in Margate, where a green grass area had been naturally developed, including a sequence of ponds to facilitate good drainage. She asked whether it was intended to roll out more projects of this nature across the County. 

(27)     Mr Tant replied that the George Park scheme had been the third such project. One of the areas shown to be prone to flooding in the report was likely to see a similar scheme.  It was not clear whether it would be possible to roll out such schemes across the entire County for a number of reasons, including cost.  It was often difficult to identify an area of sufficient size to produce significant benefits.  Even when the potential site was large enough, other factors could prevent its development.  An example of this was one large site which could not be naturally developed because of a high voltage cable running below it. 

(28)     RESOLVED that the report be noted.