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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 Roofs  Amenity space  Urban heat sink  Aesthetic  Enhanced biodiversity  Improved insulation  Pollutant removal

• Final validation – Storm Water Management

Overcoming the Sceptics • 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

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

Attenuation (Lag) Storage Cumulative Area Discharging from Green Roof 1.2Three 1were


Time Area Diagram (TAD) options considered:

Time Area Time Area Diagram 0-4 minute entry



Area (ha)


Diagram 0-4 min32 entry “Unit Hydrograph” (time to peak of Unit mins and a time base of 90 mins) Hydrograph



0 8 16 24 32 40 48 56 64 72 80 88 96 104 112 120


Time (min)

Calibration and Validation

Exponential Method Incorporated into WinDes • 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

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

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 582555

Micro Drainage Ltd Jacobs Well West Street Newbury Berkshire RG14 1BD

Green Roof Congress  

Presentation by James Berryman to the World Green Roof Congress

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