Open Letter To WLBC Planning Committee From Burscough Flooding Group

Councillor Gareth Dowling (Chairman); Councillor Andrew Pritchard (Vice-Chair); Councillor Iain Ashcroft; Councillor Mrs Pam Baybutt; Councillor Alexander Blundell; Councillor Carl Barry Coughlan; Councillor Vickie Cummins; Councillor Noel Delaney; Councillor Cynthia Dereli; Councillor Terry Devine; Councillor David Evans; Councillor Susan Anne Evans; Councillor Julian Antoni Finch; Councillor David O’Toole; Councillor Edward Pope; Councillor Jane Thompson; Councillor Mrs Marilyn Westley; Councillor Mrs Joan Witter.

Dear Councillors

As you are all aware, both foul and surface water drainage networks in Burscough are known to flood in heavy rainfall due to a lack of capacity in both drainage systems. That problem is identified in the 2012 local plan and the current neighbourhood plan. No new capacity has been built, therefore, the frequency and severity of flooding which residents report to BFG has increased. It was once thought to be mainly due to development without SUDS, however, what is being hidden from residents is that developments like Yew Tree Farm (YTF) with SUDS have already significantly added to the flooding problems here and how that come about and why the application should be opposed by Councillors is explained following:

“Although the developer appears to intend to restrict runoff to pre-development (greenfield) rates, a review of the SUDS Manual CIRIA 2015 section 3 demonstrates that even if the peak flow from developed sites is attenuated so that it does not at any point exceed the greenfield peak flow; the overall volume and duration of flow is still increased.

“When a number of developments, such as phase 1 YTF, the nursing home on the A59, Booths, Victoria Park and Ainscough Mill all occur in the same catchment area, the combined effect is to increase the overall peak flow downstream because of the greater total volumes being discharged from each development. This means that the likelihood of flooding downstream still increases despite installation of SUDS.

“The NPPF requires that the risk of downstream flooding be considered and unless and until this is adequately addressed, it is contended that the proposals for discharge of surface water in this manner should be rejected.”

WLBC has recommended that YTF phase 2 (2019/1182/ARM and 2020/0293/CON) go ahead. Are any Councillors able to explain for the benefit of BFG and residents, how YTF phase 2 can possibly go ahead without increasing our downstream flooding problems?

Yours faithfully

Gavin Rattray – Secretary Burscough Flooding Group

Please see below the relevant extracts from the SUDS Manual.

Section 3.1 Designing for water quantity
Peak rates of surface water runoff discharged from a developed (i.e. relatively impermeable site, if left uncontrolled, are normally significantly greater than from the site in its greenfield state. This is because the runoff drains off the surfaces of the developed site much quicker than the greenfield site and there is much more runoff, as less water is able to penetrate the ground or be intercepted in other ways. On sites overlying sandy, well drained soils, peak rates could be at least an order of magnitude higher. This can have significant consequences for the receiving watercourse by increasing flow velocities and the likelihood of flooding and bank erosion. Where sites discharge to existing piped drainage systems, the risks tend to be even greater, as pipes have constrained capacities and are more sensitive to changes in flow rate.

3.1.2 Attenuation controls the peak runoff rate by extending the hydrograph. So, while the peak rate of runoff may not increase, the duration over which this peak rate occurs will be significantly longer than before development as a result of the additional runoff volume. This can also increase the likelihood of flooding in the receiving watercourse. Where sites discharge to sewer systems, changes in volumes are particularly important, due to the risk of sewer flooding and CSO spills.

Figure 3.3 shows the post-development discharge rate with attenuation in red. The volume of runoff is the area under the graph. This extended period of peak flows in the receiving watercourse can be damaging for both morphology and ecology, caused by greater erosion and movement of sediment. Therefore, controlling peak runoff rates from large storm events is extremely important, but it is not sufficient on its own to reduce the impact of the development on the downstream catchment.

Also, attenuation can only control relatively large rainfall events, and does nothing to address the problems associated with a development site generating runoff from all of the smaller rainfall events. With natural soil conditions, the runoff from the majority of such events (i.e. with a total depth of, say, 5 mm or less would have been lost through infiltration and/or evapotranspiration. Runoff from these frequent small rainfall events will usually just “pass through” attenuation systems with limited or no control.

At a catchment scale, the potential limitations of using attenuation alone are also evident (Figure 3.3). Although the runoff from each sub-catchment has been attenuated to limit flows to pre-development conditions, the peak flow downstream will continue to rise because of the greater total volumes being discharged from each sub-catchment. This means that the likelihood of flooding downstream still increases.

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