Post on 18-Jan-2017
Why we all need a structured approach to surface water
management
There is no doubt that there is a need to think differently about how surface
water is managed if flooding is not to be
a persistent problem.
Flood Risk AreasSouth EssexWest MidlandsLeicesterHullGreater ManchesterLiverpoolBristolBrighton & HoveLondonMedway
SURFACE WATER MANAGEMENT PLANS
SWMP Guidance 2010“A SWMP should establish a long-term action plan to manage surface water in an area and should influence future capital investment, drainage maintenance, public engagement and understanding, land use planning, emergency planning and future developments”
Many LLFA’s have completed SWMP’s up to “Risk Assessment” stage or beyond.
A straight forward approach (already being adopted by a number of Water & Sewerage Companies) has the potential for widespread adoption as it is very user friendly.
The approach is : Sequential Transparent Easily Understood Identifies the best cost benefit solutions
STRUCTURED APPROACH: THE SWMM PYRAMID
FLOODING OPTIONS: THE SWMM PYRAMID
Flow ReductionDiversion
Storage
Exceedance
ProtectionConveyance
Flow
Red
uctio
n
Dive
rsio
n
Stora
ge
Exceedance
Prote
ction
Conv
eyan
ce
Increasing Cost
Incr
easi
ng B
enefi
tsCOST VS BENEFIT: THE SWMM PYRAMID
Flow Reduction
Diversion
Storage
Exceedance
Protection
Conveyance
SWMP OPTIONS: FLOW REDUCTION First preference to establish
whether the flows can be reduced– Suds– Infiltration– Green Roofs– Separation– Source Control– Rainwater Gardens
Areas which could be Permeable Pavements
Existing flat roofs whichcould be ‘Green Roofs’
Excessively wide footpathswhich could be reduced
Chevron areas
could be gravel
Working together at a variety of scales to deliver multiple
benefits
We can also retrofit in urban areas
THE BENEFITS OF RETROFITTING (USING SUDS)
Limit flows entering drainage systems
Maximise capacity of
the drainage system
Manage flows above ground
Improve resilienc
e
Surface water
used as a resource
Surface water used
as an amenity
Improve water quality
Increase extent and viability of
green infrastructure
RAIN GARDENS
Especially where pluvial runoff from rural outskirts flows in to urban areas
Flows have to be diverted to a safe area
Often used in conjunction with storage solutions
SWMP OPTIONS: DIVERSION
Flow Reduction
Diversion
Storage
Exceedance
Protection
Conveyance
Diversionchannel
Particularly cost effective option Usually above ground
storage, in fields or parkland either within or upstream of urban areas.
Important to consider size, height and visual impact
SWMP OPTIONS: STORAGE
Flow Reduction
Diversion
Storage
Exceedance
Protection
Conveyance
STORAGE Rainwater Butts
Rainwater Harvesting
Enhancing existing flood pathways to become more formal exceedance routes
May need measures to ensure that the water stays within the pathway; Continuous kerbs Profiling roads
SWMP OPTIONS: EXCEEDANCE
Flow Reduction
Diversion
Storage
Exceedance
Protection
Conveyance
Exceedance is flow that is conveyed or stored on the surface because the capacity of a drainage system carrying storm water (including as a result of a blockage to an inlet) has been exceeded
DESIGNING FOR EXCEEDANCE
DESIGNING FOR EXCEEDANCE
DESIGNING FOR EXCEEDANCE
DESIGNING FOR EXCEEDANCE
DROP KERBS
Note how footpaths and drives fall away from kerb towards houses
SWMP OPTIONS: PROTECTION
Conducted at property level Generally only individual or small
number of properties benefited Relatively expensive
Flow Reduction
Diversion
Storage
Exceedance
Protection
Conveyance
SWMP OPTIONS: PROTECTION
Traditional external flood barriers
Modern range ofWatertight doors
Upsizing sewers / widening or deepening river channels
Reduces the time for water to pass through parts of the catchment
Can cause issues downstream Existing channels can be altered to
increase capacity
SWMP OPTIONS: CONVEYANCE
Flow Reduction
Diversion
Storage
Exceedance
Protection
Conveyance
SUMMARY Structured approach allows for comparison
of measures on a variety of criteria Avoids an instinctive response towards
selecting certain types of measure for cost Conveyance measures such as upsizing
sewers & widening/deepening river channels have their place but are not the only option
There is a hierarchy of other possible solutions