But very popular in London – technical guidance available.
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
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
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 (Pallaet al. 2008, Stovin et al. 2007, Denardoet 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