Follow Up to Environment Agency re FRA

Your Reference: WA/2012/112201/01-L01


Thank you for your e-mail of 6th June 2012 with your responses to our document, Comments on the Flood Risk Analysis supporting the Planning Application 12/P/00645, (Guildford Borough Council). Upon reading your responses, we feel that further clarification is required on our part, and indeed to “complete the circle” further responses are requested from you.

In order to keep the understanding and follow the flow of each point in question, we have numbered below the points in our original document, followed by your response of 6th June in italics, followed by our comments of today.

Point 1 ASHRA wrote

The discharge of water into the ordinary watercourse at the north of the development is not recommended for a number of reasons.

This watercourse passes through a field (adjacent to the school playing fields) which is regularly flooded.

EA Response

In relation to the proposed development:

  • The proposal is reducing existing surface water runoff rates from the site and therefore will provide a flood risk benefit. The drainage scheme is restricting all surface water runoff rates up to the 1 in 100 plus climate change flood event to the greenfield QBAR rate of 11.2 l/s. The existing greenfield runoff rate from this site for a 1 in 100 storm event is 35.7 l/s.
  • The risk that proposed development will flood from this watercourse is reduced by the fact that the land along site the watercourse is designated a SANG. There are however two properties that are somewhat closer to the ordinary watercourse than the other properties on site. Surface water flooding is the responsibility of the Lead Local Flood Authority. If the council is concerned that these two properties may flood, then it may be advisable to raise the finish floor levels of these properties by 300mm to prevent the ingress of flood waters into the properties.

ASHRA Response

The flooding of this watercourse does not originate from the proposed site.  The flooding occurs during periods of moderate rainfall which due to the impermeable nature of the ground creates large lakes of water in the fields adjacent to the watercourse (not the school field) and for long stretches the watercourse is beneath the flood water.  When this happens the watercourse flows along the footpath as well as in the excavated ditch.  Therefore reducing the existing water runoff from the site will be of negligible benefit.

A point to note regarding the general flooding of this plot and its surrounds is that in August 2011 Ian Keen Ltd who were carrying out the tree survey for Taylor Wimpey on this site stated that “ the area was flooded at the time of the survey and so no further survey of the site was possible”.

Point 2 ASHRA wrote

The route described in the application is incorrect.  Having turned north the water course runs alongside the field, and then turns west under the school fence and joins the school drainage gulley before descending into a chamber.  It is then routed to the northern side of Bateman Grove where it then joins the drainage system (eventually the 1220mm drain described above).  The concern here is the water is being routed into school playing fields with possible water soluble contaminants from the development.  There is no connection between this watercourse and the one described in the application which would have taken the water in to the 1220 drain near Avon Close.

EA Response (Part 1)

As the alternative route has not been supplied in the submitted information the Environment Agency cannot comment on this.
ASHRA Response

We include diagrams to illustrate to you what was in the application and where this differs from the actual route of the watercourse.

Watercourse route as supplied by Taylor Wimpey

Watercourse route as supplied by Taylor Wimpey

Route as checked by ASHRA

Route as checked by ASHRA

EA Response (Part 2)

The drainage scheme proposed is using permeable paving with a gravel sub-base this will provide some water quality benefits as the gravel sub-base helps to remove and break down certain contaminants. All highways drainage will pass through a petrol interceptor to remove contaminants.

ASHRA Response

Our concern here was based on a statement by the developer in section 7.8 of the FRA that “The layout of the development is such that overland flows that exceed the design capacity of the formalised drainage system will be directed along overland flow routes such as roads and open space at the low lying catchment boundaries”.  It further goes on to say in 10.26 that “In total there are four points of discharge into the ordinary watercourse adjacent to the northern boundary of the site. The first caters for the adoptable highway, the second and third cater for the surface water runoff from a small number of dwellings to the north of the development. The fourth point of discharge in the north east corner of the site caters for surface water runoff from the remaining of the development”.
We can find no mention of a petrol interceptor catering for this situation in the application.
As this watercourse passes through the school field we were concerned about possible contamination from products typically used by car owners (on the public highway) such as alloy wheel cleaners etc.  Four products were examined and all have hazard warnings for extreme flammability, irritants to skin and eyes and danger from inhaling vapour.

Point 3 ASHRA wrote

The watercourse runs alongside South East Water main feed pipes of up to 14” in diameter adjacent to the site.  These pipes are in poor condition and have ruptured 3 times in the last 3 years.  Each time there has been extensive flooding of the field nearest the school, Ash Green Lane West, surrounding land and the associated watercourses.  The most recent rupture occurred at 2pm on 4th January 2012.

EA Response

The water main keeps bursting and flooding the school site – this is outside of the Environment Agency’s remit to comment on.

ASHRA Response

EA seems to have misread our statement.  We actually said “The watercourse runs alongside South East Water main feed pipes of up to 14” in diameter adjacent to the site.  These pipes are in poor condition and have ruptured 3 times in the last 3 years.  Each time there has been extensive flooding of the field nearest the school, Ash Green Lane West, surrounding land and the associated watercourses.  The most recent rupture occurred at 2pm on 4th January 2012”.

There is no mention of flooding the school site – The areas that get flooded are Ash Green Lane West, the field immediately to the east of the school field, the proposed development site and the section of Spoil lane that abuts Ash Green Lane West.

Point 4 ASHRA wrote

The Environment Agency recommends (in the letter to the developers) quote “We understand that there is a ditch on site adjacent to the northern boundary of the site. This is not shown on our OS map but does appear to be visible on aerial photographs of the area. This would be deemed as an ordinary watercourse and as such should be crossed by bridges (not culverts) and ideally should have a 5m buffer left alongside it.”  We believe it will be impossible to achieve the 5m buffer anywhere along the route.

The watercourse route shown of photographic development plan is incorrect.

EA Response

Can’t achieve a 5m buffer zone away from the ordinary watercourse. This is an advisory figure, an aim the Environment Agency would encourage, we would not raise an objection to this proposal on these grounds.

ASHRA Response

No further comment necessary.

Point 5 ASHRA wrote

Odyssey Consulting Engineers FRA Statement states:
8.2 Extract:  Any overland flow during extreme flood events will flow unrestricted away from residential development via the road corridors and open space at the low lying catchment boundary to the ordinary watercourse.

The above events are termed Exceedance.

Definition from water authority SUDS manual:-
“Exceedance Routes When SUDS are overwhelmed by exceptional rainfall,exceedance routes protect people and property by providing unobstructed overland flow routes from the development.”

EA Response    
Concerns that exceedance events are unrestricted and will cause flooding. The National Planning Policy Framework (NPPF) only requires new development to provide mitigation for storm events up to the 1 in 100 plus climate change storm event. The proposed drainage scheme complies with this. Furthermore, should a storm event greater than 1 in 100 plus climate change event occur, any extra flood water will be routed to the proposed Sustainable Drainage System (SuDS) where this water is safely stored before being disposed via the SUDS device. The Flood Risk Assessment states that exceedance overland flow routes will be identified during the detailed design stage and retained as open. This is above and beyond normal drainage scheme requirements.

ASHRA Response

Our concern here relates to the areas along the River Blackwater that are at risk from flooding.  Your own telemetry shows that flooding between Tongham and Farnborough gauging stations can occur at flow rates a little over 1.25m3/sec.  We have carried out calculations on all the known entries to the large 1220 mm dia drain (into which the proposed watercourse for this application enters) and consider that this 1220 drain alone is more than capable of causing the Blackwater to flood.  Experience has shown that in the decade 2000 - 2009 these storm event conditions have occurred 4 times, ie on average once every 2.5 years (considerably more than the 1 in 100 storm event you state above).  In each case considerable flooding has occurred.
From experience these SUDS systems seldom live up to their claimed performance as highlighted by the events on the 62Ha Elvetham Heath site in July 2007.  It is our belief that with surface water flash flooding from surrounding areas, including water from Manor Road that the capacity of the SUDS pit or pond will rapidly be overwhelmed.

In early 2009 Michael Gove wrote to Thames Water expressing his concern that the improvements carried out to the downstream River Blackwater route had done nothing to alleviate the serious flooding problem at Lakeside.  Basically too much water enters the river upstream and the shallow route is overwhelmed by the volume and flooding occurs.

To conclude ASHRA have a deep concern for the residents of Ash and the adjoining areas of Hampshire whose lives are blighted by the flooding events.  Too much development has been allowed over recent years with one application after another getting the nod from the various official agencies.  As a result valuable green field sites which took the excess of surface water have disappeared under building work with little consideration to the effects on drainage and flooding.  The flooding of the area has definitely got worse over recent years with the frequency of flooding accelerating as the green fields disappear.
One such development in Guildford Road, Ash completed in 2000 has flooded 4 times in ten years.

Your comments on the above would be appreciated.

Ash Residents Association

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