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Drainage design tools
Richard Kellagher
© HR Wallingford 2012Page 2
This presentation
Designing drainage systems
> Conveyance
> Storage
> Infiltration
> Interception
> Exceedence
Assessing outline designs (planning)
Checking detailed designs
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Outline design illustration
Pipes
Swales
Infiltration basins
Attenuation basins
Permeable pavements
Attenuation basins
Flood zones
Overland flood flow
© HR Wallingford 2012Page 4
Conveyance (Pipes)
Pipe sizing
> Short duration storms
� Site specific hydrology
� FEH rainfall
> No flooding 1:30 years
Automated design
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Storage (basins & ponds)
Basin / Pond sizing
> Long duration storms
> 1:100 year containment� 1:1 yr &1:100 year for streams
� 1:1 yr & 1:30 yr for Sewers?
> Head discharge relationship dictates volume
> Initial design – www.uksuds.com
> Detailed design
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0
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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Vo
l.(m
3/h
a.m
m)
Proportion of impermeable surface
SOIL 1 SOIL 2 SOIL 3 SOIL 4 SOIL 5
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1
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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Vo
l.(m
3/h
a.m
m)
Proportion of impermeable surface
SOIL 1 SOIL 2 SOIL 3 SOIL 4 SOIL 5
Long Term Storage
Extra runoff due to development
Understand the basis of the criterion
> 1:100 yr 6 hour event
> Design as infiltration � If not…
> Design with 2l/s/ha maximum discharge rate
> Function of soil type
> Does pervious are contribute runoff?
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Long Term Storage
Requires detailed design modelling tools
> Prove volume is activated for events
� 1:100 year 6hr – 24 hr
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Infiltration
Maximise infiltration
> 1 – 100yr return period
> Long duration event
BRE365
Ciria RP156
Tricky integrated detailed design
> Requires explicit modelling of all components
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Permeable pavements
Voids ratio 0.3
Storage depth > 300mm
> 100+mm rainfall stored
> ~100 yr, 24hr event
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Interception
No official guidance (yet)
> Understand the basis for the criterion
> Deemed to comply
� Infiltration units
� Rainwater harvesting
� Swales
� Green roofs
� Permeable pavements
– Even lined
> Modelling? – probably not worth doing
� As long as all paved surfaces are specifically considered
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Rainwater harvesting
Water supply
> (Interception only)
Stormwater management?
> Y/D Contribute to LTS� Design to BS8515
– (not yet revised)
� SR736_Developing Stormwater Management using Rainwater Harvesting_R2-0
© HR Wallingford 2012Page 12
Overland flow / Exceedence
Site topography
> ‘Failure’
� Bigger events
� Failed systems
– Infiltration units
– Blockage
– Embankments
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Assessment Tools – www.uksuds.com
> Greenfield runoff rate
estimation
> Storage volume estimation
> RWH design for stormwater
control
> Other hydraulic tools
> Site evaluation
> Costing…. (in progress)
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Greenfield runoff estimation
Updated EA report: RF-RO management for
new developments W5-074
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Greenfield runoff estimate
Input data
> Location click
> Site area
> Over-ride options
� Rainfall parameters
� Soil type (HOST)
� Rainfall parameters (FSR)
� FEH / FSR uplift factor
� Climate change uplift
Output information
> Qbar
> 1:1 yr
> 1:30 yr
> 1:100 yr
© HR Wallingford 2012Page 16
Greenfield runoff estimate
Calculations
> IoH 124
� (FSR, 1994)
> FEH
� Statistical equation (2008)
> Uses SPRHOST in both
cases
> Hydrological region
growth factors
Assumptions
> Minimum flow rate 5 l/s
> Minimum Qbar 1 l/s/ha
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Storage estimate
Interception storage
Attenuation storage
Long Term Storage
Treatment storage
Also reporting
> Qbar and
> Discharge rates
> 1:1 yr
> 1:30 yr
> 1:100 yr
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Storage estimate
Input data
> Location click
> Site area
> Impermeable area
� >50%
> Long Term Storage
choice
Over-ride options
> Soil type (HOST)
> Rainfall parameters
(FSR)
> SAAR
> FEH / FSR uplift factor
> Climate change uplift
> Urban creep factor
> Interception depth
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Greenfield runoff estimate
Calculations
> IoH 124� (FSR, 1994)
> FEH � Statistical equation (2008)
> Uses SPRHOST in both cases
> Hydrological region growth factors
> Attenuation storage adjustment for not using Long Term Storage � Qbar = 2l/s/ha
Assumptions
> Minimum flow rate 5 l/s
> Minimum Qbar 1 l/s/ha
> Rainfall parameters� M560 - 14 / 17 / 20mm
� ‘r’ – 0.2 / 0.3 / 0.4
> Treatment storage� 15mm UK
� Vt (Scotland and NI)
> 80% paved, 0% pervious – infiltration for LTS
© HR Wallingford 2012Page 20
Rainwater harvesting tool - stormwater
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Tank Storage behaviour
Storage behaviour f(Y/D) must be Roof area
> Number of bedrooms
> SAAR (annual rainfall)
> Design rainfall depth
Over-ride information
> Statistical no. of
occupants
� Mean
� Standard deviation
> Number of events/yr
> Runoff loss coefficients
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Individual house
Output
> Tank size required
� Y/D ratio
� Compliance %
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Rainwater harvesting - Multiple properties
Multiple property types
Numbers of each type of house
> Number of bedrooms etc
> Output as for individual property
> % compliance enables calculation of
attenuation and Long Term storage
requirements
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Other Hydraulic tools
Infiltration design
> Design individual systems
� Soakaway / infiltration trench
Joint probability analysis
> Floodplain issues
� Critical duration of site
drainage & river
� Summer / Winter runoff
www.hrwallingford.com
HR Wallingford
Howbery Park, Wallingford, Oxfordshire OX10 8BA, United Kingdom
tel +44 (0)1491 835381 fax +44 (0)1491 832233 email [email protected]
