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Modelling a Green Roof and the Benefits to
Storm Water Management
James Berryman
16th September 2010
Introduction
• Micro Drainage develop the industry-standard drainage design software in the UK and Ireland.
• Its WinDes® drainage design software is relied upon by engineers around the world, with over 14,000 software modules currently in use globally.
• WinDes allows engineers to design drainage solutions that meet the evolving challenges of flood management, environmental legislation, climate change and sustainability.
Storm Water Management?
• Established benefits of Green RoofsAmenity spaceUrban heat sinkAestheticEnhanced biodiversity Improved insulation Pollutant removal
• Final validation – Storm Water Management
• The What Ifs? – Fair tests
• Drainage Standards– Single storms (no tests for
continuous analysis/Antecedent Conditions)
– New systems, no allowance for :• FOG• Sediment• Root ingress• Blockages (both internally and
at inlets)• Deformation
Overcoming the Sceptics
Specification
• Static Volumes v Real Time Analysis
• Pragmatic approach
Two key variables to generate a runoff model
• Roof Area
• Substrate Depth
University of Sheffield : Field Test
• Virginia Stovin selected the most suitable modelling
approach based on reviews of existing research and test
rig results
• Rainfall/runoff data obtained from the University of
Sheffield’s Green Roof test rig was compared with model
outputs tested and calibrated using in WinDes
(1 x 3 m, standard Alumasc extensive sedum configuration on ~105 mm substrate + drainage layer).
Two Forms of Storage• Interception (Depression) Storage
– Depth of water that is retained in the roof (only lost through evapo-transpiration)
• Attenuation (Lag) Storage – Conceptually equivalent to a unit hydrograph for
the vertical flow through the substrate, the drainage layers and into the storm water network
It was recognised that the total storage will vary with the Antecedent Dry Weather Period (ADWP)
Interception (Depression) Storage
Field tests and other academic research (Palla
et al. 2008, Stovin et al. 2007, Denardo et al.
2004) indicated:
– That the retention to be expected after 2 days ADWP
would provide a reasonably conservative
perspective.
– An Interception value of 5% of substrate depth would
be a reasonable average for an ADWP of 2 days.
Interception (Depression) Storage Applied
• 1ha Green Roof with 100mm of soil substrate applying a depression storage of 5%
• This will result in the first 5mm of rainfall being held in the roof
• Equating to 50m³ of storage
• Continuous Analysis and evapo-transpiration
To scale
Attenuation (Lag) Storage
Three Time Area Diagram (TAD) options
were considered:
• Time Area Diagram 0-4 minute entry
• “Unit Hydrograph” (time to peak of 32
mins and a time base of 90 mins)
• Exponential
Calibration and Validation: i
Calibration and Validation: ii
Calibration and Validation: iii
• A = factor required to scale the curve to provide the correct total catchment area
• e = exponential• k = the decay
coefficiency• t = the time in
minutes
Exponential Method Incorporated into
WinDes
Summary: Benefits for Storm Water Management?
– Consider a 2 ha warehouse roof discharging to storage tank.
– If a Green Roof is used the volume of storage reduces from 450m³ to 300m³
To scale
Further Research• Future scope to calibrate the method:– Varying roof construction materials
/techniques– More locations, slopes etc– Increased/reduced lag affect?
El Struthio
Contact Information
James Berryman
Tel: 01635 [email protected]
Micro Drainage LtdJacobs WellWest StreetNewburyBerkshireRG14 1BD