Overview of the Bretforton application 013 / 01789.
This overview has been carried out following my concerns on the flood risk post
development and in order to make a recommendation to the Council on the
appropriateness of the proposals from a flood risk point of view.
Residents along Fallon Lane, which is adjacent to the proposed development, have
objected to the development on the grounds of flooding. They have produced some
clear photographs showing the flooding on a large part of the site and this is of
concern to me. The residents have also provided technical opinions from Richard
Kellagher in the form of two letters dated 17th Feb 2014 and a further letter dated 18th
April 2014 to address concerns in the original FRA August 2013 and the revised
version dated 7th March 2014 provided by the Planning Agent – Michael Robson of
Creda Planning.
Further more Michael Robson has commissioned an independent review of the
current documents dated 28th April 2014. This commentary on the FRA and
Drainage Strategy has been prepared for Michael Robson by Mr T.P Moore of Mayer
Brown Ltd and is dated 28th April 2014.
In addition to this the Drainage Consultant has been requested to comment on
Richard Kellaghers letter of 18th April 2014 and this has been received in the form of
an email forwarded by the Planning Case Officer dated 7 May 2014 from Nick Kholi
to Michael Robson.
Kellagher Letter Dated 17th Feb 2014
Mr Kellagher has stated that this was a cursory overview and that there was no
drainage system drawing for the site at the time of his review.
Some of the comments in this letter have now been revised in his second letter
following the revised FRA and drainage strategy. However he makes some very
good points on the groundwater levels and exceedence and planned flood routes
and questions the likelihood of exceedence routes being able to be constructed to
allow these flows to flow to the north of the site. He also comments that the FRA at
that time presupposes that the ground water is at 38.0m. He also mentions the key
problem of carrying out groundwater level surveys.
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His concluding sentence states,” you need to make sure the ground levels of the
development are no higher than Fallon Lane and that the flood routing is enabled so
that it passes to the stream to the north.”
Kellagher Letter Dated 18th April 2014
Mr Kellagher states that his letter has been submitted on behalf of a resident of
Fallon Lane as a result of the developer’s revised drainage proposals. He also gives
details of his experience and qualifications and states that it is provided as a
personal opinion and is not an output of his employer HR Wallingford Ltd.
Mr Kellagher gives an overview of the FRA calculations for attenuation storage and
questions some of the assumptions used.
Further comments are provided on the EA flood zone and the limitations on these for
this site given the extensive and frequent flooding of the site concluding, “it would
appear that it is very susceptible to groundwater flooding raising the ground level to
39.0m may prevent this taking place again, but it does mean that the ground is going
to be very saturated during wet winter periods, which again draws attention to the
non- conservative assumptions used in the run off modelling to determine the
attenuation storage”. In concluding this section of his letter he says, “It should be
noted that the runoff modelling is only related to rainfall and therefore there is no
provision for the drainage system for addressing groundwater flooding”.
Mr Kellagher restates his opinion on the exceedence flood flow path due to the
differences in existing ground levels along the track to the north of the site being
higher that the proposed raised level of 39.0. I believe he has used the term
blockage in this respect.
Mr Kellagher has made comments on SuDS v Traditional drainage and refers to the
forthcoming requirements of the Floods and Water Management Act 2010. By this
he means the introduction of SuDS under schedule 3 of the act which has still not
been implemented despite several dates being stated. His comments are quite valid
but there is still no sign of the SuDS implementation and we can only work with the
good practice of such legislation with the will of developers. But such systems have
to work and not put the development at risk.
His comments on the surface water sewers and the risks associated with this are
again valid. However it will be a matter for Severn Trent Water to satisfy them that
the system is designed not to create any flooding problems for them and once
adopted will be for STW to manage and maintain in relation to the adoptable
sections for the future. He has also made some comments based on how the system
might be constructed given that no construction details are provided at this stage and
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also the potential problems with ground water infiltration. The final detailed design of
the adoptable elements will be a matter for STW to sign off prior to the LPA
discharging any condition(s).
In summary, Mr Kellagher states, “ It would seem that there might be a viable
solution to actually building a surface water drainage system for runoff from rainfall,
although there is a lot of issues which seem to still need addressing --- . Also I feel
that, irrespective of the final arrangement of the surface water proposals, that the site
flooding is probably associated with groundwater which will make the effective
operation of the sewer system very doubtful during wet winter periods. Due to all
these uncertainties, I would suggest that if the proposed development does proceed,
that the 39.0m maximum ground level is applied rigorously as Fallon Lane residents
will have a considerably increased risk of flooding ( in my view) if ground levels are
raised to a level which would allow flooding to pass onto their properties from this
site”.
It should be noted that in a letter from Mr Don Bird to Mr I G Marshall Head of Legal
Services dated2nd May 2014, Mr Bird states that Mr Kellagher, “would not be able to
advise on ground water flow paths etc because that is not his area of expertise”. This
should not diminish in any way the comments he has made above. I would suspect
that this comment applies to everyone mentioned in this overview – including myself
- hence my request for seeking the views of a consultant with that capability.
Independent commentary by T P Moore of Mayer Brown Ltd.
Mr Moore introduces the independent review and lists the documents he has read
from the WDC’s planning portal.
He states that, “the overall conclusion of the studies and appraisals undertaken is
that the undeveloped site does present a risk of flooding from rising groundwater”.
On the drainage strategy he confirms that the proposal allow for the ground level to
be raised by up to 1.0m and the surface water runoff flows to be attenuated on the
site and then released to the STW sewers in the vicinity.
He adds,
“The high water table does mean that all potential drainage works for the
development will have to be constructed at relatedly shallow depth to be effective.
The groundwater monitoring showed groundwater pressure heads exceeding ground
level in places”. ---- “As a consequence, the detailed design of the site will require
that careful consideration of the site of the finished levels and overland flood routes
will be required”. --3
“The WDC Engineer has requested that a sequential approach be adopted, which
would be to avoid placing houses in the areas at highest risk; generally at the low
point of the site. Detailed overland flow paths would have to be prepared, so that it
can be demonstrated that if the groundwater rose above raised ground level, or
gravity drainage systems backed up, then the flood route would be away from the
houses themselves. It is anticipated that the site design would also require house
thresholds to be set above adjacent ground and with ground falling away from the
houses”.
“The permeable paving system will require careful design consideration to ensure
that it will remain effective when groundwater levels are high”. --Conclusions
“The consultations and design appraisal for the site indicate that although it is Flood
Zone 1 for most flood sources, the high water table does present a flood risk for the
developed site”.---“Residual flood risk will remain for the site, for occasions when storm events are in
excess of the “design event” or water table levels increase above those experienced
to date. These situations should be dealt with by careful planning of the finished site
level and ensuring overland flood routes which avoid residential properties.
“This approach will be lead to scheme approval by the local drainage authority”.
The reply from Cerda Planning’s Drainage Consultant Nick Kholi dated 7th May
2014 in response to the Kellagher letter dated 18th April 2014
FRA Calculations for attenuation storage
He appears to accept the comments from RK on the modelling and points out that
the calculations will be reconsidered at the detail design stage. He confirms that the
application is outline and thus the proposed layout is indicative with the storage
dependant on a number of factors and concludes if the storage needs to be
increased space is still available.
Classification of Flood Zone
Under this section the Flood Zone classification of flood zone is discussed but this
refers to fluvial flows and not pluvial groundwater flooding. It is argued that a greater
degree of surface water management will be brought about than currently exists.
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By raising the ground levels it is argued that it will provide a significant margin of
safety.
Provision of an Exceedence Flow Path.
Confirms that the flooding at the site will be reduced through the management of
surface water run off and the raising of ground levels above seasonally high ground
water levels. Further more in an exceedence event, runoff will eventually fall toward
the north where land levels are lower.
Confirms that some SuDS features may still be considered although not wishing to
introduce a risk of introducing ground water into the attenuation system.
Options for SuDS elements to the surface water system have been explored and will
continue to be considered at the detailed design stage.
Confirms the construction of the modular storage units and mentions that one option
as discussed in the FRA includes a pumped solution.
Conclusions
Following the deferral of the planning application from the April Planning Committee
agenda I have undertaken a fresh review of the case. As part of that review, I have
reconsidered the information submitted to the council by the applicant’s agent as well
as the information submitted by objectors.
This site has a history of flooding due to high ground water levels and evidence of
this has been provided in the form of photographs from the residents of Fallon Lane
and from the results of the ground water monitoring carried out on 17, 24 and 28
January 2014. I have also personally witnessed the flooding on the site since early in
the year.
Understanding ground water matters is difficult. Without a complete understanding
of these issues, uncertainty remains as to the implications of developing on this site
with a risk of consequential impacts on both the new homes to be built on the site as
well as on the existing neighbouring homes adjoining the site and on Fallon Lane.
My concern is that there is sufficient uncertainty given the information I have seen to
date for me to feel confident that flooding will not occur.
There is a lack of information in relation to groundwater matters. Essentially this
relates to the uncertainty of how ground water moves through the site. It is doubtful
that it will ever be known unless the applicant commissions a groundwater survey
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over a longer time span than has been carried out to date. This is my key point of
concern.
Uncertainty about the groundwater levels means that I have concerns that the
proposed new build levels on the site would be sufficient. I note that in Appendix I
(of the applicant’s FRA) groundwater levels have been recorded across the site at
38.6m. I understand that the proposed development is to be raised to 39m. Even
with the information provided by the developer, the concern is that the development
would only be 0.4m above potential water table levels. As above, I am not
convinced that there has been sufficient analysis or understanding of the water table
to feel confident in relation to the 38.6m measure recorded by the applicant; in any
even this is relatively small gap which leave little tolerance; finally there is no
accounting for possible change and increased risk resulting from climate change.
The provision of an exceedence flood flow route has not been provided as part of the
development and I am unable to establish if such a route can be delivered by the
developer as part of this planning application. Notwithstanding this matter, the lack
of information in relation to the flow of water through the site means that there is
insufficient information to determine if the flow of water from the site to the north can
be effective it what it seeks to achieve. Uncertainty remains as to whether a
functional and effective flood flow route design can be delivered to meet water
management needs. I have concerns that the boundary to the north of the site
appears to be at a higher level than the site, and as above, appears to be outside of
the applicant’s red line site and therefore outside of the applicant’s control. Any
planning conditions to secure this route could not be reasonably imposed given the
uncertainty as to the delivery of the route. If the ground levels are raised without
establishing a properly designed and effective flood flow route this would lead to an
increased risk of flooding to the new homes proposed for the site and some of the
properties in Fallon Lane. I am aware that there is a history of ground water flooding
to some properties in Fallon Lane and this has occurred long before any
development.
I have reservation in relation to the proposed storage system. I accept the
applicant’s view that there will be some betterment to the existing flood mechanism
by rain falling on the site not going to ground. However, my concern relates again to
the fundamental matter that there is uncertainty as to the groundwater levels on the
site. Given the lack of information in relation to this aspect I can not confidently say
that such a system will be effective especially following prolonged periods of rainfall.
If the water table level increases on the site, the proposed system may be inundated
– with water from ground water sources combining with surface level run off into the
storage system. The potential is for the system to be overloaded creating the risk of
flooding to the proposed dwellings (and possibly beyond onto the properties on
Fallon Lane).
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However the developer plans to use a storage system to hold the surface water
runoff back and release it at a rate accepted by Seven Trent Water. The problem of
this is that the system has a finite capacity and situations often arise when the
system fills up during prolonged wet periods which could lead to surface water
flooding which would then go to ground.
Overloading the system may result in a technical failure to the system which relies
on water within the storage area being slowly put back into the ground in a managed
flow. Notwithstanding these concerns, I will need to have reassurance that the
system will be maintained competently to reduce risks. In my view, the need for this
design solution, given the difficult water management issues on the site, is another
matter which should be weighed in the decision making process. Surely there is
other sites available in the district to meet housing needs which do not rely on such a
system which has the potential for a technical failure and its consequences on future
and current residents.
I would suggest the following reason for refusal.
The Strategic Flood Risk Assessment carried out as part of the evidence base for
the emerging South Worcestershire Development Plan identifies the application site
to be at risk of surface water flooding. The proposed development is therefore
considered unsustainable by virtue of evidence of pluvial flooding of the site and a
high water table. It is not considered that the submission, including the Flood Risk
Assessment undertaken has satisfactorily demonstrated that the risk of flooding has
been addressed, with no adverse impact on the proposed homes or the existing
homes close to the application site. As such, it is considered that the proposed
development is contrary to saved Policy ENV18 of the Wychavon District Local Plan
(June 2006).
It has been disappointing that despite being informed by the Planning Agent Michael
Robson that another indicative plan was in the process of being prepared and would
be submitted – meeting with Local member on 25th March 2014 -to my knowledge it
has not been submitted. This was requested when the developer was asked about
moving houses away from the highest risk area.
Given that other available land is within the District for housing development, my
view is that the uncertainty and risks in relation to water management with this case
are so sufficient as to warrant the refusal of the planning application. I accept there
is a need for new homes in the district but there needs to be a judgement as to the
merits of development against any harm. This is of course a matter for the planning
authority. In my opinion this application should be refused.
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A.H.Jones
Engineering Services Manager
14th May 2014
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