Transcript Urban Flooding, Trees & Sustainable Drainage Systems

Urban Flooding, Trees &
Sustainable Drainage Systems
John Young BEng MSc (Eng) CEng MICE MCIWEM
Introduction
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Why and what are Sustainable Drainage Systems (Suds)
Hydrological properties of trees
How can we introduce trees into formal drainage systems
How and where we can use trees at a source, site and regional level
Potential barriers and problems
Why Suds?
In the mid 1990s there was
a growing recognition that
continuing urban
development would place
significant pressure on
existing drainage
infrastructure
WHAT WOULD HAPPEN?
pressure on our drainage
infrastructure: much of it
aging & Victorian
Why Suds?
FLOODING
Increases in frequency and
magnitude of flooding from
urban storm water systems
THEN CLIMATE CHANGE
•More intense & frequent rainfall
•Economic loss
•Social & health impacts
Why Suds?
WATER FRAMEWORK DIRECTIVE
Degradation of existing rivers and
diffuse pollution first flush from
roads and other impermeable
surfaces:
•BOD
•Heavy metals
•Chemicals, pesticides etc
•Suspended sediments
Suds used to mitigate problems
with diffuse pollution.
Why Suds?
FOUR CROSSES
A483 Improvement
Why Suds?
IMPERMEABLE
SURFACES
•Reduce natural
infiltration to ground
•Increase volume of
surface water run-off
•Increase speed of
surface water run -off
How we do Suds?
OBJECTIVE OF SUDS
•Replicate closely the
response of a natural
catchment.
•Limit discharge rates
to the equivalent
“green field” rates of
run-off
•Improve water quality
Suds Components
•Attenuation Ponds
•Strom Storage Cells
•Hydrobrake systems
•Filter Drains
•Swales
Suds Components
WHAT DO THEY DO?
•Intercept rainfall at source
•Temporally store water
•Attenuate discharge
•Allow infiltration to ground
•Improve Water Quality
•Green Roofs
•Filter strips
•Filter drains
•Swales
•Soakaways
•Permeable paving
•Bio-retention systems
Hydrology & Trees
WHAT DO TREES DO?
•Intercept rainfall at source
•Temporally store water
•Attenuate discharge
•Allow infiltration to ground
•Improve Water Quality
IS A TREE CANOPY A GREEN ROOF?
Probably
Hydrology & Trees
WHAT DO DESIGN ENGINEERS
WANT?
Precipitation
Evapotranspiration
•Canopy interception rates
•Storage volumes
•Retention times
for individual and groups of trees and
for different species throughout the
growth life of the tree.
Stem Flow
Leaf Drip
Through Fall Sap Flow
Source & Site
VISITOR CENTRE DEVELOPMENT
Run-off from
Visitor centre roof
Public squares
Car Parks
POSSIBLE USE OF TREES
Interception & Storage
Delay of discharge to stormwater system
Water quality improvements
Regional
STREET TREES
Run-off from roads
Car Parks
POSSIBLE USE OF TREES
Shading of impermeable surfaces
Interception & storage
Delay of discharge to stormwater system
Water quality improvements
Water Quality
STREET TREES
Bio-retention within a treatment train
Potential Difficulties
•Flooding is more prevalent in winter than summer. Loss of leaf
during winter effectively removes the benefit associated with the
canopy.
•Space within the urban environment to incorporate trees.
•Rate of tree growth and canopy development
•Potential clashes with utilities including foul sewers, gas,
telephone and electricity.
•Structural damage associated with tree roots and perception
associated with the risk of collapse
•Adoptability by the Local Authority or Water Company
Conclusions
•Trees can be used effectively within a Suds strategy at
source, site and regional level
•Research is required to establish the engineering design
parameters
•Codes of Practice and design guides must be developed to
give Suds designers the tools to incorporate trees into a
formal drainage systems at site and regional level
•Cross discipline design is essential including architects,
landscape architects, aboriculturalists and engineers are
involved in urban design.