c/o Belinda Iliff-Wells
Spring Gardens
Fallon Lane
Bretforton
Worcestershire
WR11 7GZ
63 St Helens Ave
Benson
OX10 6RU
18th April 2014
Dear sir
Ref Planning ref 013/01789 Land off Station Road
This is my second letter commenting on the proposed drainage system at this proposed
development. This has been asked for by Belinda Iliff-Wells as a result of a re-submission by
the developers of their drainage proposals.
I will reintroduce myself again; I am a Chartered Civil Engineer with over 30 years’
experience in stormwater drainage. My principle area of expertise is in computer modelling
of drainage systems and Sustainable Drainage Systems (SuDS).
I am the author or co-author of a number of the national guidance documents in the UK in
relation to the design of surface water systems, including:

Advising Defra on the technical aspects of the SuDS Standards and supporting
Guidance for the Floods and Water Management Act

BS 8515 Code of practice on rainwater harvesting (co-opted expert specifically for
the storm water management aspect).

BS 8582 Code of practice for surface water management for development sites

Drainage guidance manuals including the CIRIA SuDS Manual C697

Environment Agency Guidance policy document “Rainfall Runoff Management for
Developments” (RRMD)

Carrying out the research which led to the principles on stormwater control which
was then produced as the RRMD as Environment Agency policy.
I am currently employed as a Technical Director at HR Wallingford Ltd. However this letter is
provided as a personal opinion and is not an output of HR Wallingford Ltd.
I have carried out a review of the revised FRA document which now includes a brief
overview of the proposal which now plans to use a standard piped drainage system and tank
storage to attenuate extreme rainfall events. The following issues are points that I feel are
worth making.
The FRA calculations for attenuation storage
The information on calculations is fairly limited, but the calculation for Qbar and the
associated factors for climate change, runoff coefficient etc. all appear to be in line with
normal practice with those who use MicroDrainage. It should be noted that the winter
runoff coefficient of 84% for paved surfaces is fairly a universal practice, but it is generally
considered by drainage engineers to be too low as there is no runoff allowed for from
pervious areas. The Environment Agency policy guide (SC030219) on development drainage
analysis assumes 100% for paved runoff which effectively provides some allowance for
pervious runoff. (The derivation of 84% is based on an incorrect application of an equation
which is no longer in use in drainage modelling due to the fact that it under predicts runoff).
It is also common that these simulation models, at this stage in planning applications, do not
provide for the head-discharge relationship which will require an additional 10% – 30%
storage. However I note that the gravity version of the proposal is around 20% larger so it is
possible that some provision has been made in this case. This is particular relevant in this
situation due to the depth of the tank (which is discussed shortly). Finally there is normally
an allowance for urban creep which usually requires an increase of 10% in the paved surface
areas. This may have been applied, but as the paved fraction is 59% this seems unlikely.
Therefore, overall, I would expect that detailed design of this system should probably be in
the region of 1100 – 1500m3 based on using the UKSuDS tool (which applies the EA
methodology).
Classification of flood zone
The FRA correctly considers the EA flood maps, but it seems extraordinary that the
extensive and frequent flooding of the site leads to the conclusion it is in flood zone 1 just
because it is on a map which a national scale study produced. Clearly it may not be being
flooded by the river, but 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 runoff modelling to determine
the attenuation storage.
I am really very surprised that this proposal is continuing to progress due to all the historical
evidence, and I feel that a professional assessment of the reasons for the flooding should be
made. 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.
Provision of an exceedence flow path
As I made a point of in my previous letter, as there seems to be a high risk of flooding on the
site (even if the ground is made up) there needs to be an exceedence flow path to the north
towards the river. It is evident that current flooding is blocked from passing towards the
river, and this should be positively provided for in any solution.
SuDS V. Traditional drainage
Unsurprisingly, with high groundwater levels in winter, reliance on infiltration has been
abandoned. However there is no provision for any form of treatment of surface water
runoff which will soon be a requirement of the Floods and Water Management Act. Shallow
swales and pervious pavements could still have been used. During summer periods and
when large summer storms take place volumes of runoff into the STW sewerage system
would be very much reduced. However if groundwater levels get above 38.5m (currently the
assumed maximum) this would then add groundwater to the STW system – an anathema for
water companies. Also the EA would not be happy as the unsaturated depth below these
units will be minimal in the winter. Lining permeable pavements is therefore an option,
though infiltration for reducing summer flows to the sewer is then not possible.
I was surprised to see that the surface water drainage system draining to a foul sewer. This
might be a combined system, but this is not stated on the drawing. It is very uncommon for
surface water to be drained to a foul sewer especially when there is a watercourse which is
so close; particularly in this case, where it is so difficult to drain back against the gradient to
get to the sewer.
Effectively the principles of the proposed SuDS Standards and other existing best practice
guidance are not being followed.
Drainage and attenuation storage arrangement
I note that the ground level is 39.0m and the invert level of the connecting sewer is 37.81m.
Assuming the longest drain run is at least 200m and there is 900mm cover on any part of
the drainage system, this allows a fall of 300mm. This is a gradient of 1:700. Even taking into
account the proposed attenuation tank system, the drainage gradient is not feasible for
meeting the requirements of Sewers for Adoption. Is STW going to adopt this system?
Regarding the tanks; assuming the bottom of the tank is only 100mm above the invert of
the connecting manhole, and also that at least 600mm soil cover is needed over the tank
(and more if it needs to be under a road), This leaves only 500mm depth for the tank. This
means that it would need to cover an area of between 2000m 2 and 3000m2. This seems to
have been provided by visual inspection in option 2. However it does mean that when the
tanks are part full, the pipework draining to it will all be surcharged. What this implies is that
the surface water system is most likely to need pumping – which is shown as option 1.
However the use of pumps for surface water is regarded as being a provision of last resort.
Given this situation there are two questions; firstly why does the drainage system not
consider draining to the river, and are STW going to adopt a surface water system which has
a pumping station? If not, this has serious implications with regards the ownership and costs
of management of the drainage system for the new residents.
The final point regarding the surface water storage system is that this is usually provided
using geocellular crate systems wrapped in a water-proof membrane. Where ground levels
can be up to 38.5m (as measured this year), it will be very difficult to make the whole
system (membranes and pipework) fully water tight. In time the system will act like a
groundwater collecting system passing very large volumes of groundwater into the sewer in
winter (unless the pumping station is switched off!). When they are first built, the tanks may
be quite water tight. Design engineers will be fully aware of the floatation risk of the tanks
and this will require sufficient surcharge of soil to keep these from lifting.
In summary, it would seem that there might be a viable solution to actually building a
surface water drainage system for runoff from rainfall, although there are a lot of issues
which seem to still need addressing. However I find it very surprising that STW seem happy
to take surface water runoff when the industry as a whole is trying very hard to reduce
surface water flows into their foul systems, especially in this case when they are seriously at
risk of getting large volumes of groundwater infiltration. Also I feel that, irrespective of the
final arrangement of the surface water drainage 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.
Regards,
Richard Kellagher BSc, MSc, MICE, CIWEM, CEng