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Flood Risk Assessment and Sustainable Drainage Systems
4 Phillimore Terrace, London W8
Introduction
This site-specific flood risk assessment has been carried out in accordance with National
Planning Policy Framework (NPPF), RBKC Core Strategy Policy CE2 and other relevant
guidance, in support of a planning application for 4 Phillimore Terrace, London W8.
Policy CE 2, Flooding
The Council will require development to adapt to fluvial flooding and mitigate the
effects of, and adapt to, surface water and sewer flooding.
To deliver this the Council will:
d. require development at risk from flooding in Flood Risk Zones 2 and 3, in areas with
critical drainage problems(…) to incorporate suitable flood defence or flood
mitigation measures in accordance with the recommendations of the site-specific
Flood Risk Assessment.
A review of the Environment Agency's Flood Map in February 2015, suggests that the
proposed development site lies within Zone 1, which is a low risk area.
A flood risk assessment (FRA) has been produced to generate an understanding of the potential
flood risk at the proposed development.
Policy CE 2, Flooding requires
e. (…) sustainable urban drainage (SuDS), or other measures, to reduce both the
volume and the speed of water run-off to the drainage system ensuring that surface
water run-off is managed as close to its source as possible in line with the hierarchy in
the London Plan.
The buildings currently occupy the complete site and there are no impermeable areas. The
proposed is for a basement wholly under the existing footprint of the main house (not rear
extensions). The rear extensions will be demolished and patio will be provided with permeable
areas.
In this circumstance the RBKC SuDS calculation form is not capable of providing information,
as confirmed by RBKC who have stated that any increase in permeable are will be acceptable.
To comply with the requirement for SuDS it is proposed to provide approximately 6m2 of
permeable paviours/planters to the rear garden, where currently this is constructed over. This
will reduce the rate of surface water discharge from the site and decrease the flood risk to the site
itself and elsewhere.
The FRA follows the NPPF Planning Practice Guidance, Flood Risk and Coastal Change, SiteSpecific Flood Risk Assessment: CHECKLIST.
1 Development description and location
a. What type of development is proposed (e.g., new development, an extension to
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existing development, a change of use etc.) and where will it be located?
This is a residential development, which involves the creation of a basement level within the
footprint of the main building and front and part rear gardens.
b. What is its flood risk vulnerability classification?
The basement extension forms part of the existing singly occupied dwelling and is not a selfcontained basement dwelling. Despite this, the relevant Flood Risk Vulnerability Classification
would be ‘Basement dwelling’, which are defined as ‘highly vulnerable’.
Source: NPPF Planning Practice Guidance, Flood Zone and Flood Risk Tables, Table 2:
Flood Risk Vulnerability Classification
As described in Table 3, of the NPPF Guidance showing the Flood risk vulnerability and flood
zone ‘compatibility’, residential development in Flood Zone 1 is considered acceptable in
flood risk terms.
c. Is the proposed development consistent with the Local Plan for the area? (Seek advice
from the local planning authority if you are unsure about this).
Flood Zone 1 – Low Probability
Flood Zone 1 is the area outside Flood Zone 2 and 3 – for RBKC this is the majority of the
borough. It includes the area north of Kings Road and a large portion of the area to the south.
Flood Zone 1 equates to a flood event within less than a 0.1% chance of occurring each year
(1 in 1000 year event).
In accordance with NPPF Tab. all development (…highly vulnerable…) is permitted in Flood
Zone 1. All development proposals should consider the following about the sites:
- Their vulnerability to flooding from other sources as well as from river and sea
flooding
- Their potential to increase flood risk elsewhere through the addition of hard surfaces
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and the effect of the new development on surface water runoff.
Source: RBKC Strategic Flood Risk Assessment2 p.27
The site lies in Flood Zone 1. There is no fluvial flood risk within RBKC and the tidal flood
risk in Zone1 is low probability.
d. What evidence can be provided that the Sequential Test and where necessary the
Exception Test has/have been applied in the selection of this site for this development
type?
As this site lies within Flood Zone 1, it is deemed to have passed the Sequential Test, in
accordance with NPPF.
Source: RBKC Strategic Flood Risk Assessment2 p.6
e. Will your proposal increase overall the number of occupants and/or users of the
building/land, or the nature or times of occupation or use, such that it may affect the
degree of flood risk to these people?
(Particularly relevant to minor developments (alterations & extensions) & changes of
use).
No
2. Definition of the flood hazard
a. What sources of flooding could affect the site?
Surface and Sewer Water Flooding. However the recorded data for surface water and sewer
floods at the site being low.
b. For each identified source in 2a above, can you describe how flooding would occur,
with reference to any historic records where these are available?
Blockage/excessive inlet of surface/foul water
No recent records indicating such flooding.
c. What are the existing surface water drainage arrangements for the site?
100% of the site is impermeable and all surface water is collected and drained into public
sewer.
3. Probability
a. Which flood zone is the site within?
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The delineation of NPPF Flood Zones classifies the site as Food Zone 1 with a low probability
to flooding.
Source: RBKC Strategic Flood Risk Assessment2 p.26, App. B, fig.1
b. If there is a Strategic Flood Risk Assessment covering this site (check with the local
planning authority). Does this show the same or a different flood zone compared with the
Environment Agency’s flood map? (If different you should seek advice from the local
planning authority and, if necessary, the Environment Agency).
RBKC Strategic Flood Risk Assessment - same zones
c. What is the probability of the site flooding, taking account of the maps of flood risk
from rivers and the sea and from surface water, on the Environment Agency’s web site,
and the Strategic Flood Risk Assessment, and of any further flood risk information for
the site?
The property is within zone 1 which indicates a 1:1000 year event probability.
d. If known, what (approximately) are the existing rates and volumes of surface water
run-off generated by the site?
Not known.
4. Climate change
How is flood risk at the site likely to be affected by climate change? (The local planning
authority’s Strategic Flood Risk Assessment should have taken this into account.
Further information on climate change and development and flood risk is available on
the Environment Agency’s web site.)
The Surface Water Management Plan has taken climate change into consideration. The surface
water model was run for a 1 in 1000 year probability event (0.1% AEP) to include the effect of
climate change. Based on current guidance an increase in peak rainfall intensity of 30% was
assumed for this model scenario.
5. Detailed development proposals
Where appropriate, are you able to demonstrate how land uses most sensitive to flood
damage have been placed in areas within the site that are at least risk of flooding
(including providing details of the development layout)?
N/A – only one land use.
6. Flood risk management measures
How will the site/building be protected from flooding, including the potential impacts of
climate change, over the development’s lifetime?
Detail assessment of the site has shown that the water flood hazard and water flood depth is
low
Despite the low probability we assessed the flowing flooding scenarios:Surface water flooding from the Street
The entrance step threshold level will be 150 mm above hardstanding and provide a
natural flood defence for the main entrance and front light well.
Surface water flooding from the garden
The basement is sealed to the rear with no possibility of water ingress.
Sewer flooding at basement level
The manholes will be equipped with a sealed covers. The basement drainage is
protected from sewer flooding through the installation of a suitable pumped device
with backup and one way flap.
Groundwater penetration
A cavity drain membrane will manage any ground water permeating through the
concrete shell. This will be pumped using a sump-pump system designed to cope with
the ground conditions.
Any service penetrations will be sealed.
7. Off-site impacts
a. How will you ensure that your proposed development and the measures to protect
your site from flooding will not increase flood risk elsewhere?
We are using existing site features and passive measures to protect the site from flooding and
therefore do not expect to increase flood risk elsewhere. In addition additional areas of garden,
permeable paving/planters will be provided.
b. How will you prevent run-off from the completed development causing an impact
elsewhere?
The proposed basement is not extending under the complete garden. Whilst the development
does increase the impermeable area on site, this is limited and still allows for adequate
drainage as indicated by the SuDS calculation as offered by RBKC.
c. Are there any opportunities offered by the development to reduce flood risk
elsewhere?
Planning Policy CL7, Basements
Basement development should:
i. include a sustainable drainage system (SuDS), to be retained thereafter;
j. include a minimum of one metre of soil above any part of the basement beneath a
garden;
Development should utilise sustainable urban drainage systems (SuDS) unless there are
practical reasons for not doing so and should aim to achieve greenfield run-off rates and
ensure that surface water runoff is managed as close to its source as possible in line with the
following drainage hierarchy:
store rainwater for later use
use infiltration techniques, such as porous surfaces in non-clay areas
attenuate rainwater in ponds or open water features for gradual release
attenuate rainwater by storing in tanks or sealed water features for gradual release
discharge rainwater direct to a watercourse
discharge rainwater to a surface water sewer/drain
discharge rainwater to the combined
sewer. Source: London Plan, Policy 5.13 |
Sustainable drainage
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Reducing the rate of surface water discharge from urban sites is one of the most effective ways
of reducing and managing flood risk. SuDS are defined as a sequence of management
principles and control structures designed to drain surface water in a more sustainable fashion
than conventional piped drainage techniques.
The Royal Borough of Kensington and Chelsea have developed their own guidance and
selection tool for managing runoff from small developments
http://www.rbkc.gov.uk/planningandbuildingcontrol/planningpolicy/flooding/SuDSforsmalldev
elop ments.aspx
8. Residual risks
a. What flood-related risks will remain after you have implemented the measures to
protect the site from flooding?
Failure of pumping equipment used to evacuate any surface water.
b. How, and by whom, will these risks be managed over the lifetime of the
development? (E.g., flood warning and evacuation procedures).
Residents will be presented with this flood risk assessment to ensure that they are aware of the
risk of flooding at this location and it is their responsibility to maintain the pump in working
order.
Registration online with the Environment Agency’s Floodline Warnings Direct
service https://fwd.environment-agency.gov.uk/app/olr/register
If you would prefer to register by telephone, or if you need help during the registration
process, please call Floodline on 0845 988 1188.
Electricity will be supplied via RCD protection and will switch off in the event of flooding.
The drainage one way valves will require maintenance annually.
There are no habitable areas proposed at basement level.