Flood Consequences Assessment & Possible / Proposed Mitigation
Proposals.
–
Proposed Erection of Two New Detached Houses
on the site of part of the former car park.
Former Lifeboat Inn Public House.
Watkin Street. Sandycroft. Flintshire. CH5 2PE.
For
Kindertons Homes.
Introduction.
This document has been produced to accompany a planning application for
the above.
A Planning Approval Ref 049817 has recently been granted for the conversion
of the adjacent former Lifeboat Inn building to four, two bedroomed flats with
associated private parking.
The site of the application comes within the very extensive flood risk zone
relating to a potential breach of the River Dee flood defence embankments.
This flood risk zone is shown on the Local Authority UDP Plan map & also on
the Environment Agencies mapping. The identified flood risk zone includes
the whole of the existing Sandycroft residential area & also the existing
industrial / employment area to the north east. The River Dee at its closest
point, in a direct line, is some 700 metres from the applications site. In the
Sandycroft residential area alone, the potential flood event described will
significantly affect not only the application site but several hundred existing
dwellings also.
The planning application that this assessment accompanies follows a
Planning Refusal Ref 049773 for two houses on the same site. The sole
reason for refusal of the previous application related to potential future
flooding concerns ( allowing for Global warming & projected rising sea levels
). Specifically Ground floor habitable rooms were threatened as they had floor
levels below the future worst case projected possible flood datum level of
6.30m above Ordnance Datum.
The houses now proposed are a pair of identical, but handed, detached four
bedroom, two & a half storey ( rooms in roofspace ) high types. They are
positioned on part of the former pub car park. These houses have habitable
living spaces ( kitchen, lounge / dining area & bedrooms ) all well above the
worst case projected flood level, at first & second floor levels. At ground floor
level there is a tandem, 2 car length, integral garage, a Hallway with W.C. off
& a Utility room. None of these rooms / spaces are habitable & though we
consider we have designed them to be defendable with temporary proprietary
flood barriers should these be overcome there would be little danger to
householders life.
This document has been written with reference to the requirements & advice
given in Welsh Government Document TAN15 – Development & Flood Risk.
We have in particular noted that within the document on page 20 the
“assessment should be commensurate with the scale & nature of the
proposal”. We have endeavoured to do this for this relatively modestly sized
development.
Proposals below have been made with reference to DEFRA & Environment
Agency documents “Improving the Flood performance of new buildings” &
“Flood resistance & resilience solutions : an R + D scoping study”.
Further details & advice regarding the specifics of possible flooding of the
surrounding area have been provided by Ruairi Barry of the Environment
Agency & we would take this opportunity to thank him for his helpful
approach, assistance & advice.
Assessment of Flood Consequences.
This assessment is written in a predominantly non-technical form as we do
not dispute the identified flooding risk or figures provided by The Environment
Agency. We are concentrating efforts on investigating & providing mitigation
measures reasonable & commensurate, with scale & type of the proposed
development.
The existing River Dee flood banks have a typical crest level of approximately
7.20m AOD ( above ordnance datum ). Existing 0.5 % AEP ( 200 year ) tidal
levels give a figure of 6.62m AOD & when this is corrected by estimated
climate change sea level rises 100 years into the future the figure rises to
7.38m AOD. This level, at an extremely high & storm driven tide, would be a
clear overtopping & / or breach. This would be a truly catastrophic event
affecting a wide area & many hundreds if not thousands of existing properties
& dwellings. We feel it would not likely occur without prior warning as tides are
completely predictable & therefore future high risk periods can be identified.
Such an event would likely result in the need for widespread evacuation of
residents within the identified flood zone & result in significant levels of
damage to existing buildings & infrastructure. There is, we feel, one surprising
assumption in these figures in so much as it does not appear to allow any
factor for improvement to the flood banks or other flood defences in the next
100 years. We feel this is a highly unlikely scenario.
Examination of previous but relatively recently produced Flood Consequence
Assessments for other planning applications in the general Sandycroft vicinity
have given maximum floodwater levels due to a breach scenario of around
6.30m AOD. The application sites are further from the river & breach so
floodwater levels will likely be slightly less than the value indicated. A
computerised flooding model has not been specifically run for these proposals
as it is a very costly exercise ( quotes received are in excess of £3000 ) &
results gained will not likely be greatly different to the figures available &
indicated above. Certainly running a model specifically for this site will not
remove or significantly reduce the flooding risk. Velocity of flow of floodwaters
would not appear to be a significant factor in this instance as the distance
from the flooding breach point would be significant & flow velocities would be
broken down by intervening landscape features ( in this case principally the
railway line cutting / embankment which runs parallel to the river ) & built
structures.
Existing Ground levels on the essentially flat applications site are 5.25m AOD.
The ground floor level of the pub building is 5.35m AOD. The surrounding
residential area of Sandycroft has ground levels between approximately 5.0 &
5.4m AOD. Therefore floodwater on the site ( assuming no alteration of
ground levels ) could reach a depth of around 1.0m at worst. The whole of the
surrounding Sandycroft residential area would also be flooded up to 1.3m
deep at worst.
Justification & Possible Mitigation Measures for the Development.
Policy Document TAN 15 identifies in Section 6.1 “flexibility is necessary to
enable the risks of flooding addressed whilst recognising the negative
economic & social consequences if policy were to preclude investment in
existing urban areas, & benefits of reusing previously developed land. Further
development in such areas, whilst possibly benefitting from some protection,
will not be free from risk”. We feel the proposed conversion & reuse of the
Lifeboat Inn building clearly fits into this above definition. The building is a
local landmark of some character & presence. Further its upper storey has
always been in residential usage.
The applications site is clearly within an established residential area & other
potential uses ( business etc… ) would be at no less risk of flooding but would
likely be less acceptable in all other planning respects ( noise, potential
nuisance, traffic generation etc… ). Not permitting the reuse / change of use
of the existing building solely on flooding grounds ( & this would currently
appear to be the case ) would essentially totally “blight” the site.
Redevelopment of the site will not exacerbate the flooding issue elsewhere.
The following mitigation proposal options have been considered / are
proposed where indicated :1. Raising site ground levels around the proposed new houses to above
the flooding level. This is not practical given the proximity of
surrounding existing dwellings etc.. The resultant new houses would be
visually obtrusive with ground floors around 1.2 metres above all
existing surrounding dwellings. This will create needless relationship &
overlooking issues to existing surrounding dwellings. External access
would also be problematic, requiring stepped approaches. These would
not be Building Regulations or Disabled Access compliant, external
steps not generally being permitted where sites slope at less than 1 in
12. We are of the opinion that a scheme using this design principal
would be totally unacceptable to the Planning authority. We also
consider that if this approach were used in a catastrophic flooding
event as identified though the residents of the new houses would be
safely above flood water level they would be surrounded in all
directions of the compass for significant distances by deep flooding
from which escape by walking would not be possible ( this is generally
considered as 600mm maximum ). We would suggest that in the
catastrophic flooding event as all surrounding dwelling residents would
be evacuated so would the residents of the new dwellings. They
wouldn’t simply be left alone on their own “island” home. For all these
reasons we consider the idea of artificially raising ground levels of the
proposed development above the potential flooding level to be
somewhat unrealistic. We therefore reject this option.
2. Raising ground floor levels of the new houses built using either
traditional / normal or heavy duty cavity walling to a level at which flood
waters may be successfully defended against ( at doorway openings
etc ) with proprietary flood barriers. This approach was what we
suggested in our previous Planning Application for this site which was
refused. The sole reason for refusal related to the possible flooding
issue & this approach was considered not acceptable by The
Environment Agency. We therefore reject this option.
3. Retaining existing site ground levels but amending the design of the
new houses to avoid all habitable rooms at ground floor level. Here
flooding would essentially be assumed to in a worst case scenario
encroach on the ground floor of the houses. Design measures will be
incorporated though to defend against future possible flooding to less
than worst case scenario levels. This option would result in houses with
integral garages & servicing rooms / spaces only at ground floor level.
All living / dining & bedroom accommodation is at first & second floor
levels. This style of house needs to be an absolute minimum of 3
bedrooms layout to be a viable sales prospect. This will almost
certainly result in a 2.5 / 3 storey house layout. This is the preferred &
pursued design option.
4. Proprietary temporary slot into openings flood defence barriers / doors
will be provided for all of the house external doorway openings. These
will give a minimum of 1.0m high flood protection. These will be
approved to BS PAS 1188 - 2009 Demountable & Building Aperture
Flood Defence Systems.
Further detailed design details will be incorporated as follows:-
1. No low level ground floor / below ground floor voids (suspended
floors, air bricks etc..).
2. No low level ( below projected flood level ) electrics installations.
3. The provision of non-return valves on the foul drainage
connections….
All these measures will incorporate products that will be to BS PAS
1188 - 2009 Demountable & Building Aperture Flood Defence
Systems.
So we can see proposed levels will be as follows:5.25m Existing Site Ground Level.
5.30m Proposed Ground Floor Level. +0.15m
5.90m or 6.30m Protected / Defendable Level. +0.6 or +1.0m
7.90m Minimum Proposed Habitable Room Floor Level ( 1st floor, being
2.60m above ground floor level ).
6.30m Projected Maximum Flood Level in the area of the site.
All levels are given above national ordnance datum.
5. All residents /owners of the proposed properties will be signed up for /
advised of The Environment Agency Floodline Warnings Direct Service
( Tel 0845 988 1188 ) so they can be advised of forthcoming high tidal
events.
Summary.
Mitigation measures as detailed above in 3, 4 & 5 will be undertaken /
incorporated into the construction of the proposed houses.
We consider that the above is a realistic & reasonable set of mitigation
proposals that would provide protection against the worst case scenario
possible flooding event projected 100 years into the future (allowing for
projected climate change sea level rises). We would suggest that it is likely
within that time frame some work will be done to improve the Dee flood
defences & therefore the breach & subsequent flooding risk will reduce.
C. Harwood. For Kindertons ( Homes ) Property Development Ltd.
Revised February 2013.