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 582555james.berryman@microdrainage.co.uk

Micro Drainage LtdJacobs WellWest StreetNewburyBerkshireRG14 1BD