Local Government Flood
Forum (LGFF)
Friday, 27 July 2012
Welcome & Introduction
Chair: Laurie Thraves, LGiU
Local Government Flood
Forum (LGFF)
Friday, 27 July 2012
Presentation by
Ben Mitchell & Daniel Hayes,
Peter Brett Associates LLP
Opportunities and Challenges to the
Delivery of SuDS
Through Effective Partnerships
Ben Mitchell and Dan Hayes
SuDS – Opportunities and Challenges
Presentation Content
 Part 1
What they are and why now.
PBA advice to government
 Part 2
The process, the pitfalls and the potential problems
 Part 3
Questions
SuDS – Opportunities and Challenges
What are SuDS?
 Nothing new except the name since the word
‘sustainable’ became popular
 Focus used to be on flooding, now joint focus
on water quality and flooding
 Not a panacea, just one tool in the box for the
flood risk practionioner
SuDS – Opportunities and Challenges
What are SuDS?
 Main principle is to replicate the natural
processes
 Blanket approach not always best for river
management
 Hard and soft solutions available
SuDS – Opportunities and Challenges
Why SuDS Now?




2007 Floods
Surface Water Flooding & Poor Management
Sir Michael Pitt Review
Flood and Water Management Act 2010 –
Royal Assent April 2010
 Defra and the PAB
 Consultation on draft Standards
 Implementation …?
SuDS – Opportunities and Challenges
Advice to Government (PBA/BPF)
The concerns:
 SuDS appropriate profile
 Balanced approach
 Parallel system
 SAB resources
 Counter intuitive
 Capacity Limited
 Blanket Approach
SuDS – Opportunities and Challenges
Advice to Government – Blanket Approach
SuDS – Opportunities and Challenges
Risk in the Development Process
 Risk and Cost
 SuDS approval process risk
 Administrative burden could lead to delay
 Early consultation to mitigate?
 Consultation process could be unduly biased
SuDS – Opportunities and Challenges
Additional Pre-Application Investment
 More design and site investigation
 Investment in time and consultant / contractor
fees
 Detailed design to demonstrate hierarchy
choices
 Hierarchy requirements could lead to ineffective
systems
 Counter productive in the context of affordability
SuDS – Opportunities and Challenges
Additional Pre-Application Investment
National Standards (NS) for SuDS
Part I – Principles
Functionality
• Surface Run-off is managed at source and on the surface where practicable
• Public space is used and integrated where practicable.
• Cost-effective to maintain over lifetime of development taking climate change
into account
Affordability
• SuDS systems should not cost more than an equivalent traditional design.
Part II – Design Hierarchies
•
•
•
Run-off Destination hierarchy - Building Regulations
Run-off Rate and Volume hierarchy – CfSH + first 5mm retained on-site
Water Quality hierarchy – treatment stages
SuDS – Opportunities and Challenges
Additional Pre-Application Investment
SuDS – Opportunities and Challenges
SAB Application Process
 What will the format of the application be?
 FRA in accordance with NPPF still required?
 Consistency and continuity of approach to
approval process.
 FWMA states consent cannot be withheld if
design in accordance with NS.
 Does the SAB need to consult?
SuDS – Opportunities and Challenges
SAB Operation and Approval
 Potential for non-determined applications
 SAB determination periods
• 12 week determination period for major applications
• 7 weeks for all others
 The Town & Country Planning appeals process
SuDS – Opportunities and Challenges
Adoption of SuDS
 No commuted sums payable for SuDS adoption
 Could SABs use S38 highway agreements?
 SAB have 8 weeks to ensure system is operating
 Automatic adoption point?
 Could SAB maintenance resources influence
design?
Local Government Flood
Forum (LGFF)
Friday, 27 July 2012
Presentation by
Julian Hatherall, Earth Systems
Andy Smith, WSP
A Methodology to Assist Local
Authorities and Developers in the
Evaluation of Potential Groundwater
Flooding Risk Resulting from
Development.
Julian Hatherall - Earth Systems Europe
July 27th 2012
www.earthsystemseurope.com
[email protected]
Structure
• Overview of groundwater (GW) flooding
• Groundwater flooding risk assessment process and supporting data
Acknowledgements:
• John Ashcroft at Sandwell Metropolitan BC for commissioning this study.
• Ola Holmstrom and Andy Smith of WSP who I undertook this work with.
www.earthsystemseurope.com
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Why is this of importance?
• EU Floods Directive and Flood & Water Management Act 2010 places greater
responsibility on LLFA for GW flood risk management. The National Planning
Policy Framework reiterates that all flooding sources require consideration.;
• Groundwater Flooding not routinely assessed in SFRAs except for Clearwater
Flooding – sometimes out of sight out of mind BUT can be an issue in certain
circumstances and geographical areas.;
• Increase in SuDS means a greater proportion of rainfall and runoff discharged to
GW => higher GW flood risk.;
• SuDS Approving Bodies (SABs) will require reassurance as to sustainability of
SuDS on new developments.
• Mapping, SuDS suitability, limitations.
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Previous Work
A number of past studies and reports have referred to groundwater flooding
susceptibility and risk assessment methods – a few examples are:
•
•
•
•
•
Jacobs, 2004. Strategy for Flood and Coastal Erosion Risk Management: Groundwater Flooding Scoping
Study (LDS 23) Final Report
Defra 2005. Making Space for Water: Taking forward a new Government strategy for flood and coastal
erosion risk management in England.
JacobsGIBB, 2007. Making Space for Water: Groundwater Flooding Records Collation, Monitoring and Risk
Assessment (ref HA5) Consolidated Report.
Macdonald, D.M.J.; Bloomfield, J.P.; Hughes, A.G.; MacDonald, A.M.; Adams, B.; McKenzie, A.A.. 2008.
Improving the understanding of the risk from groundwater flooding in the UK. In: FLOODrisk 2008, European
Conference on Flood Risk Management, Oxford, UK, 30 Sept - 2 Oct 2008. The Netherlands, CRC Press.
BSI, 2011. BS 8533:2011. Assessing and managing flood risk in development – Code of practice.
As part of the current study, reference has been made to these and other reports
where possible. This work develops a framework utilising and building on some of
these ideas.
www.earthsystemseurope.com
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Groundwater Flooding Definitions
Defra’s “Making Space for Water” (Holistic Approach 5 or HA5 (Defra, 2005)).
“Caused by the emergence of water originating from sub-surface permeable
strata”.
British Geological Survey (Macdonald et al, 2008)
“The emergence of groundwater at the ground surface away from perennial
river channels, or the rising of groundwater into manmade ground under
conditions where the ‘normal’ ranges of groundwater level and
groundwater flow are exceeded”.
This study considers flooding from subsurface permeable strata close to and
away from surface water courses. It includes flooding of basements and
infrastructure.
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Groundwater Flooding Types
There are generally 3 main types of groundwater flooding:
1. Clearwater Flooding
2. “Superficial Deposits Flooding” or “Persistent Shallow Groundwater”
3. Groundwater rebound following cessation of abstraction or mine/quarry or
other dewatering
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Groundwater Flooding Situations
1. “Clearwater flooding”
Groundwater flooding in an unconfined Principal Aquifer setting
•
Large changes in seasonal GW
levels e.g. Chalk. Results in
emergence at ground surface.
•
Extended period of impact.
Courtesy: Macdonald, D.M.J.; Bloomfield, J.P.; Hughes, A.G.; MacDonald, A.M.; Adams, B.; McKenzie, A.A.. 2008. Improving the understanding of the risk from groundwater
flooding in the UK. In: FLOODrisk 2008, European Conference on Flood Risk Management, Oxford, UK, 30 Sept - 2 Oct 2008. The Netherlands, CRC Press.
www.earthsystemseurope.com
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Groundwater Flooding Situations
2. “Superficial Deposits Flooding" or “Persistent Shallow
Groundwater”
Flooding primarily in shallow sedimentary aquifers or Made Ground
•
May occur as a result of:
• Local connection to surface water;
• Changes in shallow or perched
groundwater levels from seasonal runoff
and recharge;
• Locally through infiltration from
soakways or changes to ground
conditions.
•
Generally shorter term and localised.
Courtesy: Macdonald, D.M.J.; Bloomfield, J.P.; Hughes, A.G.; MacDonald, A.M.; Adams, B.; McKenzie, A.A.. 2008. Improving the understanding of the risk from groundwater
flooding in the UK. In: FLOODrisk 2008, European Conference on Flood Risk Management, Oxford, UK, 30 Sept - 2 Oct 2008. The Netherlands, CRC Press.
www.earthsystemseurope.com
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Groundwater Flooding and Geology
Approx. location of
SMBC
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Assessment of risk – a framework
1. Use of BGS Susceptibility to Groundwater Flooding maps
A screening tool. Does not indicate risk but susceptibility only.
2. Development of Methodology for Site Specific Assessment of Risk
Uses a series of flow charts and a matrix with site and regional data for assessment.
3. Supporting Information
Summary tables and details of local geological and hydrogeological conditions.
The aim has been to design a methodology and process which is neither
onerous or overly costly.
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1. Susceptibility Mapping v. Risk
• BGS produced data set on the susceptibility of GW flooding (50m2). Areas where
geological conditions could enable GW flooding to occur.
– Essentially the map equates areas of shallow groundwater to higher risk.
– Map suitable for use for regional or national planning in conjunction with
other relevant information to inform land-use planning decisions.
– Local groundwater conditions likely to vary from the modelled data and
therefore there is the need for caution.
• To evaluate site specific risk of groundwater flooding requires
understanding of the development proposals together with local geological,
hydrogeological and hydrological conditions
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2. Assessment Methodology
• Based around Five Planning Stages with
appropriate technical assessment at
each. Sufficient details need to be made
available for Local Authority to make a
decision using the framework.
• Initial technical assessment “Screening”
at Level 1.
• Proposals discussed with LA and way
forward agreed.
• If all OK for LA then move to planning
application or further assessment at
Level 2 is required then submit.
ALL AT PRE APPLICATION STAGE
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Methodology – Level 1 technical
• Use of the flow chart, Susceptibility
Mapping and “Decision Matrix”
together with readily available
information to help determine any
issues and the degree of assessment
required.
• Collect available data from literature,
websites, previous and current SIs.
• Define conceptual model and assess
the risk qualitatively or semiquantitatively. May require specialist
input.
• Once assessed complete short and
concise report and discuss with LA.
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Methodology – Decision Matrix
DEVELOPMENT
PROCESS
CONSTRAINTS
Within V. High and High
Groundwater Flooding
Susceptibility Area
Within Moderate
Groundwater Flooding
Susceptibility Area
Within Very Low or Low
Groundwater Flooding
Susceptibility Area
Groundwater Flooding
Susceptibility not
Categorised (blank
mapping)
1
Area of known or logged
shallow groundwater
(<2mbgl) including
periodic seasonal
variations
2
Area
of historic groundwater
flooding outside of the
surface water flood plain
3
Development is on a site
containing steep slopes
4
Development within
proximity of a superficial
or solid geological
boundary
5
Development is within
proximity of a surface
water course
6
Development has existing
spring lines on site or
within close proximity of
the boundary
7
Development is within an
area of historically
depressed groundwater
levels
8
Significant seasonal
groundwater level changes
9
Is the site within an area of
Surface Water flooding
KEY
DEVELOPMENT
PROCESS
A
Development lies within a Installation of basements
mapped vulnerability zone or subsurface structures
B
Installation of features
likely to alter groundwater
flow
C
Lowering of existing
ground levels by more than
1m
D
Installation of SUDS
infiltration or unlined
balancing infrastructure
A
Development lies within a Installation of basements
mapped vulnerability zone or subsurface structures
Deta i l ed a s s es s ment requi red Deta i l ed a s s es s ment requi red Deta i l ed a s s es s ment requi red Deta i l ed a s s es s ment requi red Deta i l ed a s s es s ment requi red
CONSTRAINTS
Cons i der the need for further Cons i der the need for further Cons i der the need for further Cons i der the need for further Cons i der the need for further
a s s es s ment ba s ed on s i te
a s s es s ment ba s ed on s i te
a s s es s ment ba s ed on s i te
a s s es s ment ba s ed on s i te
a s s es s ment ba s ed on s i te
s peci fi c da ta i f thi s contra di cts s peci fi c da ta i f thi s contra di cts s peci fi c da ta i f thi s contra di cts s peci fi c da ta i f thi s contra di cts s peci fi c da ta i f thi s contra di cts
vul nera bi l i ty ma ppi ng
vul nera bi l i ty ma ppi ng
vul nera bi l i ty ma ppi ng
vul nera bi l i ty ma ppi ng
vul nera bi l i ty ma ppi ng
C
Lowering of existing
ground levels by more than
1m
D
Installation of SUDS
infiltration or unlined
balancing infrastructure
Cons i der the need for further Cons i der the need for further Cons i der the need for further Cons i der the need for further Cons i der the need for further
a s s es s ment ba s ed on s i te
a s s es s ment ba s ed on s i te
a s s es s ment ba s ed on s i te
a s s es s ment ba s ed on s i te
a s s es s ment ba s ed on s i te
s peci fi c da ta i f thi s contra di cts s peci fi c da ta i f thi s contra di cts s peci fi c da ta i f thi s contra di cts s peci fi c da ta i f thi s contra di cts s peci fi c da ta i f thi s contra di cts
vul nera bi l i ty ma ppi ng
vul nera bi l i ty ma ppi ng
vul nera bi l i ty ma ppi ng
vul nera bi l i ty ma ppi ng
vul nera bi l i ty ma ppi ng
1
BGS da ta a s s umes l i ttl e to no
cha nce of groundwa te
fl oodi ng. As there wi l l be
a l oca l va ri a bi l i ty a check
s houl d be ma de to a s s es s i f
there i s s ha l l ow groundwa ter
a nd a ppropri a te a s s es s ment
ma de a s requi red
BGS da ta a s s umes l i ttl e to no
cha nce of groundwa te
fl oodi ng. As there wi l l be
a l oca l va ri a bi l i ty a check
s houl d be ma de to a s s es s i f
there i s s ha l l ow groundwa ter
a nd a ppropri a te a s s es s ment
ma de a s requi red
BGS da ta a s s umes l i ttl e to no
cha nce of groundwa te
fl oodi ng. As there wi l l be
a l oca l va ri a bi l i ty a check
s houl d be ma de to a s s es s i f
there i s s ha l l ow groundwa ter
a nd a ppropri a te a s s es s ment
ma de a s requi red
Area of known or logged
shallow groundwater
(<2mbgl) including
periodic seasonal
variations
BGS da ta a s s umes l i ttl e to no
cha nce of groundwa te
fl oodi ng. As there wi l l be
a l oca l va ri a bi l i ty a check
s houl d be ma de to a s s es s i f
there i s s ha l l ow groundwa ter
a nd a ppropri a te a s s es s ment
ma de a s requi red
BGS da ta a s s umes l i ttl e to no
cha nce of groundwa te
fl oodi ng. As there wi l l be
a l oca l va ri a bi l i ty a check
s houl d be ma de to a s s es s i f
there i s s ha l l ow groundwa ter
a nd a ppropri a te a s s es s ment
ma de a s requi red
N/A
N/A
Deta i l ed a s s es s ment requi red Deta i l ed a s s es s ment requi red Deta i l ed a s s es s ment requi red Deta i l ed a s s es s ment requi red
N/A
Check wi th the Envi ronment
Agency a nd Loca l Authori ty on
s peci fi c ri s ks a nd es ta bl i s h
ri s k l oca l l y
Check wi th the Envi ronment
Agency a nd Loca l Authori ty on
s peci fi c ri s ks a nd es ta bl i s h
ri s k l oca l l y
Check wi th the Envi ronment
Agency a nd Loca l Authori ty on
s peci fi c ri s ks a nd es ta bl i s h
ri s k l oca l l y
N/A
Cons i der the ri s k i n
conjuncti on wi th the geol ogy
a nd groundwa ter condi ti ons .
Ba cki ng up of groundwa ter
pos s i bl e.
Cons i der the ri s k i n
conjuncti on wi th the geol ogy
a nd groundwa ter condi ti ons .
Ba cki ng up of groundwa ter
pos s i bl e.
Steep pos s i bl e groundwa ter
Steep pos s i bl e groundwa ter
ta bl e a nd cros s i ng of
ta bl e a nd i ntercepti on of
i nterforma ti ona l geol ogi ca l
groundwa ter i s pos s i bl e
bounda ri es i s pos s i bl e where
where s ubs urfa ce fea tures a re
cutti ng a nd pl a tform
i ns ta l l ed
devel opments a re propos ed
2
Area
of historic groundwater
flooding outside of the
surface water flood plain
N/A
Cha nges to fl ow regi me a re
Cha nges to fl ow regi me a re
pos s i bl e cl os e to geol ogi ca l
pos s i bl e cl os e to geol ogi ca l
bounda ri es . Es ta bl i s h
bounda ri es . Es ta bl i s h
potenti a l of groundwa ter
potenti a l of groundwa ter
fl oodi ng from fl ow cha nges or fl oodi ng from fl ow cha nges or
ba cki ng up of groundwa ter.
ba cki ng up of groundwa ter.
N/A
Groundwa ter l evel cha nges
Groundwa ter l evel cha nges
Groundwa ter fl oodi ng to other
Groundwa ter fl oodi ng through
through ri ver i ntera cti on coul d through ri ver i ntera cti on coul d
properti es through ri ver
ri ver i ntera cti on coul d occur.
occur i nunda ti ng l owered
occur reduci ng SUDS
i ntera cti on coul d occur.
Es ta bl i s h groundwa ter/s urfa ce
ground. Es ta bl i s h
effecti venes s . Es ta bl i s h
Es ta bl i s h groundwa ter/s urfa ce
wa ter i ntera cti on a nd a s s es s
groundwa ter/s urfa ce wa ter
groundwa ter/s urfa ce wa ter
wa ter i ntera cti on a nd a s s es s
further a s neces s a ry.
i ntera cti on a nd a s s es s further i ntera cti on a nd a s s es s further
further a s neces s a ry.
a s neces s a ry.
a s neces s a ry.
N/A
Deroga ti on of protected ri ghts Deroga ti on of protected ri ghts
(e.g. a bs tra cti ons ) i s pos s i bl e, (e.g. a bs tra cti ons ) i s pos s i bl e,
a s i s cha nge i n fl ow pa tterns . a s i s cha nge i n fl ow pa tterns .
As s es s thes e i s s ues .
As s es s thes e i s s ues .
Ma y not ca us e ma jor i s s ues .
Cons i der the need for further
a s s es s ment ba s ed on
conceptua l model i f s peci fi c
ri s ks a re cons i dered to a ppl y.
Increa s ed i nfi l tra ti on ma y not
be a bl e to di s cha rge cl os e to
geol ogi ca l bounda ri es a nd
i ncrea s e the potenti a l of
groundwa ter fl oodi ng.
N/A
Check wi th the Envi ronment
Agency a nd Loca l Authori ty on
s peci fi c ri s ks a nd es tabl i s h
ri s k l oca l l y
Check wi th the Envi ronment
Agency a nd Loca l Authori ty on
s peci fi c ri s ks a nd es tabl i s h
ri s k l oca l l y
Check wi th the Envi ronment
Agency a nd Loca l Authori ty on
s peci fi c ri s ks a nd es tabl i s h
ri s k l oca l l y
Cons i der the ri s k i n
conjunction wi th the geol ogy
a nd groundwa ter condi tions .
Ba cki ng up of groundwa ter
pos s i bl e.
Cons i der the ri s k i n
conjunction wi th the geol ogy
a nd groundwa ter condi tions .
Ba cki ng up of groundwa ter
pos s i bl e.
Steep pos s i bl e groundwa ter
Steep pos s i bl e groundwa ter
tabl e a nd cros s i ng of
tabl e a nd i nterception of
i nterforma tiona l geol ogi ca l
groundwa ter i s pos s i bl e
bounda ri es i s pos s i bl e where
where s ubs urfa ce fea tures a re
cutti ng a nd pl a tform
i ns tal l ed
devel opments a re propos ed
Ma y not ca us e ma jor i s s ues . Increa s ed groundwa ter fl ow to
Cons i der the need for further
s pri ngs coul d occur through
a s s es s ment ba s ed on
SUDS us e or di vert thi s a wa y .
conceptua l model i f s peci fi c
Es ta bl i s h cri ti ca l na ture a nd
ri s ks a re cons i dered to a ppl y. a s s es s further a s neces s a ry.
N/A
Thi s i s unl i kel y to be pres ent Thi s i s unl i kel y to be pres ent Thi s i s unl i kel y to be pres ent Thi s i s unl i kel y to be pres ent
i n Sa ndwel l but i t i s
i n Sa ndwel l but i t i s
i n Sa ndwel l but i t i s
i n Sa ndwel l but i t i s
recommended tha t the current recommended tha t the current recommended tha t the current recommended tha t the current
s ta tus of rebound i s checked
s ta tus of rebound i s checked
s ta tus of rebound i s checked
s ta tus of rebound i s checked
wi th the Envi ronment Agency
wi th the Envi ronment Agency
wi th the Envi ronment Agency
wi th the Envi ronment Agency
a nd ri s k es ta bl i s hed.
a nd ri s k es ta bl i s hed.
a nd ri s k es ta bl i s hed.
a nd ri s k es ta bl i s hed.
N/A
Deta i l ed a s s es s ment requi red Deta i l ed a s s es s ment requi red Deta i l ed a s s es s ment requi red Deta i l ed a s s es s ment requi red
N/A
Deta i l ed a s s es s ment requi red Deta i l ed a s s es s ment requi red Deta i l ed a s s es s ment requi red Deta i l ed a s s es s ment requi red
3
Development is on a site
containing steep slopes
4
Development within
proximity of a superficial
or solid geological
boundary
N/A
Loweroverall risk is considered to apply but there consideration of the potential impact is required. A robust justification for not assessing
the risk would be required.
N/A
Generally low risk likely but a degree of consideration of the specific risks is required depending on the site conditions and development
proposals. The scale of this will be dependant on site conditions and the scale and type of development proposals.
Not applicable
Detai l ed a s s es s ment requi red Detai l ed a s s es s ment requi red Detai l ed a s s es s ment requi red Detai l ed a s s es s ment requi red
Check wi th the Envi ronment
Agency a nd Loca l Authori ty on
s peci fi c ri s ks a nd es ta bl i s h
ri s k l oca l l y
Detailed assessment required as potential groundwater flooding risk is considered to be significant but is dependant on site conditions and
the scale and type of development proposals.
N/A
B
Installation of features
likely to alter groundwater
flow
Cha nges to fl ow regi me a re
Cha nges to fl ow regi me a re
pos s i bl e cl os e to geol ogi ca l
pos s i bl e cl os e to geol ogi ca l
bounda ri es . Es tabl i s h
bounda ri es . Es tabl i s h
potentia l of groundwa ter
potentia l of groundwa ter
fl oodi ng from fl ow cha nges or fl oodi ng from fl ow cha nges or
ba cki ng up of groundwa ter.
ba cki ng up of groundwa ter.
Ma y not ca us e ma jor i s s ues .
Cons i der the need for further
a s s es s ment ba s ed on
conceptua l model i f s peci fi c
ri s ks a re cons i dered to a ppl y.
Check wi th the Envi ronment
Agency a nd Loca l Authori ty on
s peci fi c ri s ks a nd es tabl i s h
ri s k l oca l l y
Increa s ed i nfi l tra tion ma y not
be a bl e to di s cha rge cl os e to
geol ogi ca l bounda ri es a nd
i ncrea s e the potentia l of
groundwa ter fl oodi ng.
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Methodology - Questionnaire
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Methodology - Level 2 technical
• If required, further detailed assessment
should be performed at Level 2.
• More data, calculations or modelling
may be required at this stage.and may
require specialist input.
• Model and modify proposals until flood
risk can be suitably reduced or
mitigated.
• Once completed the full assessment is
submitted at planning application stage
for assessment and a decision on the
application.
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Supporting Info – Geo & Hydrogeo
To aid the assessment details of the specific geological and hydrogeological
conditions have been outlined.
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Supporting Info – Constraints and
Processes
An importance ranking is given for constraints and development processes against
formation – it is a guide and not definitive.
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Supporting Info – Mitigation
Outline guidance on mitigation is provided. Similar framework is used for surface
water.
1.
2.
3.
4.
5.
Assessing and understanding the groundwater flood risk
Avoiding the risk
Substitution
Flood control and groundwater management
Resistant and resilient building techniques
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Summary
1. Groundwater flooding is the emergence of water originating from sub-surface
permeable strata and is not restricted to areas of “Clearwater Flooding”.
2. Groundwater can be a potentially significant source of flooding.
3. Groundwater flooding susceptibility is not groundwater flood risk.
4. BGS GW Flooding susceptibility mapping has been used to help in screening the
risk but the framework uses flow charts and matrix to determine level of
assessment required but professional judgment needed on a site specific basis.
5. Specific local data has been developed and the method can be adapted for other
geographical areas as well as for those with a specific portfolio of sites.
6. Tools such as the new BGS SuDS Suitability mapping is useful but likely to
require further site specific assessment.
7. Within SMBC the highest risk is posed by development in areas of superficial
deposits and across the significant areas of Made and Worked Ground and/or
“Persistent Shallow Groundwater”.
When in doubt – assess the risk.
www.earthsystemseurope.com
[email protected]
Thank You
Contact Details:
Julian Hatherall
• Principal Hydrogeologist, Earth Systems Europe
• Chartered Geologist
•
•
Tel: 0117 373 6155 or 07972 877519
[email protected]
•
www.earthsystemseurope.com
www.earthsystemseurope.com
[email protected]
Local Government Flood
Forum (LGFF)
Friday, 27 July 2012
LUNCH
Local Government Flood
Forum (LGFF)
Friday, 27 July 2012
Presentation by
Susana Ochoa-Rodriguez
Imperial College London
Urban Pluvial Flood Modelling,
Forecasting and Warning
Challenges, on-going work, discussion
Susana Ochoa Rodriguez, Imperial College London
WSP Group, Birmingham, 27th July 2012
CONTENTS
1. Urban pluvial flooding: what are the
challenges? why is it so difficult to model and
forecast?
2. The RainGain Project: tackling the challenges
3. Discussion: needs, expectations, potential
solutions for enhanced urban pluvial flood
forecasting, warning and event management
1. URBAN PLUVIAL FLOODING
Extreme rainfall events
exceed the capacity of
the drainage system
1. URBAN PLUVIAL FLOODING
• Insufficient capacity of sewer
system
• Surface flow (overland system)
• Dynamic interactions between the
two systems
• It’s localised and happens quickly –
“flash floods”
Model Assembly for Pluvial Flood Modelling, Forecasting
and Management
Rainfall Estimation /
Forecasting
Flood Modelling /
Forecasting
Management (urban
planning, emergency)
Same “framework” as other types of flooding, but for urban pluvial flooding
each step is a bit more complex
Rainfall
Estimation /
Forecasting
Flood Modelling /
Forecasting
Management
(urban planning,
emergency)
•
The rainfall events which generate pluvial
flooding are often associated with
thunderstorms of small spatial scale
(~ 10 km), whose magnitude and spatial
distribution are difficult to monitor and
predict (also: lead time vs. accuracy)
•
Rainfall estimates/forecasts with fine spatial and temporal resolution
required
Rainfall Estimation
/ Forecasting
•
•
•
•
Flood Modelling /
Forecasting
Management
(urban planning,
emergency)
Urban “jungle” is complex
Interaction of sewer and overland systems
Since flooding is localised, models must have fine spatio-temporal
resolution
Model detail vs. Runtime
Effective rainfall
Bi-directional
interaction
Surface
component
Sub-surface
component
Sewer flow
Rainfall Estimation
/ Forecasting
Flood Modelling /
Forecasting
Management
(urban planning,
emergency)
•
Urban catchments change constantly
•
Complete flood records for calibration and verification are
seldom available
•
High uncertainty in boundary conditions
•
High operational uncertainty (blockages, pipe burst, pump
failure, change in geometry of roads and other channels, etc.)
•
Individual sources of uncertainty are magnified by small scale
Rainfall Estimation
/ Forecasting
•
•
•
•
•
•
Flood Modelling /
Forecasting
Management
(urban planning,
emergency)
Uncertainty in modelling and forecasting hinders decision making
Low awareness
Given rapid onset and short forecasting lead-times, the public
become the principal responders, but they are not so willing to
respond
Lack of coordination between stakeholders involved
Budgetary cuts
…
2. Tackling challenges:
RainGain Project
Advanced observation and rainfall prediction
for urban pluvial flood management
(Sep 2011 – Jul 2015)
RAINGAIN
Project Objective
To improve fine-scale measurement and prediction of
rainfall and to enhance urban pluvial flood prediction
in order to enable urban water managers to adequately
cope with intense storms, so that the vulnerability of
populations and critical infrastructure can be reduced.
Project Partners
1)
2)
3)
4)
5)
6)
7)
8)
9)
10)
11)
12)
13)
TU Delft (NL)
Zuid-Holland Province (NL)
Gemeentewerken Rotterdam (NL)
KU Leuven (B)
Aquafin NV (B)
Ecole des Ponts ParisTech (F)
Marne-la-Vallée (F)
Seine-St.-Denis (F)
Météo France (F)
Imperial College London (UK)
Met Office (UK)
Local Government Flood Forum (UK)
Véolia (F)
Work Packages
• WP1: Acquisition, installation and testing of X-band radars and highquality radar protocols in pilot locations.
Lead: ParisTech, Daniel Schertzer
• WP2: Acquisition of rainfall data at the detailed time and spatial scales
that are essential for urban rainfall and flooding prediction
Lead: KU Leuven, Patrick Willems
• WP3: Implementation of rainfall data in existing urban water models to
enhance short term pluvial flood modelling and prediction
Lead: Imperial College of London, Cedo Maksimovic
• WP4: Implementation of detailed rainfall data and flood modelling results
into enhanced urban water management strategies at the short and long
term
Lead: TU Delft, Marie-claire ten Veldhuis
Pilot Sites
Leuven (BE)
Rotterdam (NL)
Marne-la-Vallée (FR)
Seine-St.-Denis (FR)
Croydon (UK)
Redbridge (UK)
Torbay (UK)
Cooperative Work
•
Knowledge exchange between partners
•
Field visits pilot locations
•
Workshops on development of common methods and
training for practical application
•
Demonstration tools (radar, flood model), applications
(radar results, model results), solutions (early warning
systems, operational control, storage basins) to other
partners
•
Sharing of experiences in FRM
Rainfall
Estimation /
Forecasting
•
•
•
•
•
Management
Flood Modelling /
Forecasting
Acquisition and testing of X-band
radars
Development and testing of rainfall
downscaling techniques
Improved radar signal processing
Meteorological Radar
Numerical Weather Prediction: UM/MM5
10 - 30 km
Merging of rainfall data from
different sources (e.g. X-band and
C-band radars and raingauges)
Testing of different nowcasting
techniques
(urban planning,
emergency)
C-Band
1 - 2 km
1 km
STATISTICALLY
DOWNSCALING
i
T = Current
T = Future
t
100-500 m
1 km
Spatial
i
Temporal
X-Band
CALIBRATION
t
Ground Raingauge Network
Benefits of gauge-based rainfall
adjustment for flow simulation
Benefits of gauge-based rainfall
adjustment for flow simulation
Rainfall Estimation
/ Forecasting
• Monitoring of case studies
• Setup of 1D/2D and 1D/1D
models
• Development of hybrid models
Flood Modelling /
Forecasting
Management
(urban planning,
emergency)
Rainfall Estimation
/ Forecasting
Flood Modelling /
Forecasting
• Overall uncertainty analysis and
risk-based model calibration
• Analysis and definition of local
pluvial flood triggers (for
translating rainfall alerts into
flooding for a given area)
• Setup and testing of pilot
forecasting systems
Management
(urban planning,
emergency)
Forecasting - Workflow
Rainfall Estimation
/ Forecasting
Flood Modelling /
Forecasting
Management
(urban planning,
emergency)
•
The modelling and forecasting tools will support decision making
•
A number of FR reduction measures will be analysed/tested for the case
studies (including RT control, SUDS/source control, pathway control, etc.)
•
Development of generic workshop pack for ranking of alternatives in flood
risk management (for awareness raising and collaborative FRM)
•
Regular meetings with a variety of stakeholders to discuss developments,
communicate results and enhance uptake of results
•
Recommendations for pluvial flood modelling, forecasting and management
based on results and sharing of experiences amongst project partners
Mitigation solutions?
Improved Forecasting and Event
Management
Advanced (Water Sensitive) Urban
Planning + Improved Management
3. DISCUSSION
• I’d like to focus on urban pluvial flood
forecasting
• I’d like to know your views, needs and
expectations on this matter, staring from your
perception of the currently available Extreme
Rainfall Alerts (ERA)
ERAs: Extreme Rainfall Alerts
• State of the art tool for predicting extreme rainfall events at county level
likely to lead to SWFs; however, it does not provide specific SWF warnings.
• Alerts will be issued when there is a 20% or greater probability of the
following thresholds being exceeded:
– ƒ30mm per hour
– ƒ40mm in three hours or
– ƒ50mm in six hours
• Based on averages of FEH storms with 1 in 30 yr probability for rainfall
values for 1, 3 and 6 hours in eight UK cities (it is assumed that 1:30 yr
design rainfall intensities is likely to overload urban drainage systems).
Topics for discussion
• In general, do you find the ERAs useful? If not, why?
– Is it because of the large uncertainty associated to them?
– Because they are not “local” enough?
– Or because the notice is too short to react?
– Anything else?
• Do you currently take any actions upon receipt of an ERA? If
so, what actions? Do you have a clear action plan?
Topics for discussion
• Currently, do you communicate ERAs to any community
members, such as flood wardens?
• Do you think the current ERAs should/could be communicated
to the general public?
• Does your response to ERAs vary according to the probability
associated to it?
• Is it clear to you and other responders in your area how the
ERA translates into a potential flood?
Topics for discussion
• If more localised warnings were available, what levels
of uncertainty would you tolerate:
– What’s the minimum level of confidence/reliability of the
forecast required for you to take action upon receipt of a
rainfall alert?
– What’s the minimum level of reliability/confidence of the
forecast before warnings can be communicated to the
public? How should it be communicated?
Topics for discussion
• What’s the minimum lead time that would enable you to take actions?
• In your local area, does the location of flooding change significantly from
event to event?
• Would you like to see rainfall thresholds linked to local runoff models?
Would knowing the relationship between rainfall and runoff thresholds
change the way in which you react to warnings?
• Inform about previous flood events, insurers.
• Who would provide this forecast? LAs? EA? How to warn?
• Public engagement is critical
Topics for discussion
In general, there are 3 options through which rainfall thresholds are
related to flooding thresholds and impact:
1. Rainfall-based alerts based on either nationally set or locally set
rainfall thresholds
2. Rainfall-based alerts using locally specific runoff thresholds
3. Flood warnings linked to runoff and local drainage
Considering criteria such as feasibility, uncertainty, effect of spatial and
temporal distribution of rainfall on flooding, etc. which option do you
consider most suitable for implementation of surface water flood
warnings?
Thank you
Susana Ochoa Rodriguez
[email protected]
Local Government Flood
Forum (LGFF)
Friday, 27 July 2012
Presentation by
Tim Farr
Midlands regional Flood & Coastal Committee
Thank you