BONFIRE HILL DRAINAGE ISSUES – PLANNING APPLICATION WA/2020/0363 (BONFIRE HILL)
1 I refer to your correspondence with Waverley Borough Council (WBC) in connection with the above planning application. I should explain that I am Chairman of Elstead Parish Council, which has serious concerns about the adverse impact of this proposed development on flood management in the village.
2 In your latest letter you say SCC as the Lead Local Flood Authority has no objection to the application, provided an acceptable SUDS scheme is put in place. I am not sure how familiar you are with the Bonfire Hill site, but I should explain that it is an area of elevated land which dominates the centre of the village of Elstead. It contains numerous springs and many of the names of the surrounding roads (Springfield, Springhill, Springhaven) reflect this characteristic of the land. In wet weather, the springs flow mainly to the north, north-west, and north-east towards the lower lying centre of the village, depositing large amounts of surface water into the quite inadequate ordinary water courses adjacent to the highways.
3 During recent flood events (notably this winter, in 2013/4 and in 2001) the amounts of water coming off Bonfire Hill have led to serious flooding of gardens, roads and on occasion dwellings in the village, in spite of action taken by residents to dispose of the excess. Your colleagues in SCC in highways and in flood control (Steve Lindsey Clark and Ian Fowler) will be very familiar with the issue. One neighbour has reported that in February this year he had to deploy pumps to remove floodwater at the rate of at least 5.6 litres/sec coming from just one of the many springs on Bonfire Hill, over a period of several days.
4 The proposed development would involve the construction of 30 dwellings around the lower slopes to the north, north-west and north-east of the site (ie cutting across the direction of flow of most of the springs). The drainage survey commissioned by the developer has determined that the surface water arising from the land cannot be disposed of using infiltration techniques, so the proposal is that this water (which will inevitably increase as a result of the construction of impermeable surfaces arising from the development) should be released into the foul drainage at a rate not exceeding 4 litres/sec. This is the maximum flow which Thames Water, the foul drainage authority, has agreed to accept.
5 It seems from the documents submitted by the developer and your own comments that no surveys have been carried out of the specific hydrology of the site. The calculations provided by the developer suggest that the amount of surface water to be disposed of is based simply on the area of land on the development site. In fact, the topography of the site and the adjacent land suggests that much of the water coming off the hill arises from the even more elevated land to the south, where the land continues to rise to a peak some 240 metres outside the development site. There is local evidence that the surface water generated during regular flood events is well in excess of the limit of 4 litres/sec. which Thames Water has agreed to accept. If this is indeed the case, it is difficult to see how any SUDS scheme restricted to an outflow into the sewer of 4 litre /sec could possibly cope with the amount of water involved.
6 SCC, as the LLFA, has a duty of care to offer advice to the LPA which will ensure that flood risks are appropriately addressed. SCC is moreover expected to apply due diligence in formulating this advice. Our experience in the village, sadly, is that development schemes have been allowed to proceed without due attention being given to flooding problems, both on the sites concerned and in relation to their effects on neighbouring properties (I refer you to the recent Croft and the Orchard Close developments). In the case of the Bonfire Hill application, there is much local concern that the proposals would lead to serious surface water flooding issue both for the development itself and for properties in the adjacent lower-lying areas. In view of this, I would be grateful if you could answer the following questions:
you or any other officer of SCC carried out a site inspection of the
relevant land or commissioned a hydrological survey of the site in
order to assess its specific characteristics?
you consulted your colleagues in Highways or Flood Management on the
application, and, if so, does your advice take account of their
you satisfied that a SUDS scheme based on a maximum flow of 4
litres/sec into the foul sewer would be adequate to cope with the
excess surface water during ‘normal’ flood events? If so, on
what is this judgment based, given the clear evidence that he flow
off the hill is likely to be much greater than this?
would legal liability rest if in the event of the application being
approved both the new dwellings and adjacent properties were to
suffer from surface water flooding owing to any inadequacy with the
approved SUDS scheme?
7 It is important to have answers to these questions so that the LPA (WBC) can reach an informed view of the adequacy of the measures proposed to deal with flooding issues. It would be helpful to have your response as soon as possible in view of the timetable for consideration of this application.
Elstead Parish Council