December 2006
guest feature

The Green Roof Stormwater Management Tool ~ a Free Community Resource

By Kelly Luckett, LEED AP

As I was studying to take the U.S. Green Building Council's LEED Accreditation Exam in November of 2003, I noticed that green roofs and their runoff coefficient were absent from the list of surfaces listed in the sustainable sites section of the reference guide, although there were designations for surfaces ranging from undisturbed forest floor to concrete.  While green roofs were given attention under the heat island section of the LEED rating system, there seemed to be a lack of data allowing credit towards stormwater management.  The most recent revisions of the LEED rating system have addressed green roof usage to capture credits outside the heat island section, but without the runoff coefficient, the green roof is difficult for design teams to factor into a project’s stormwater modeling tools they use to prepare the stormwater management plan.  For several years we simply concentrated on stormwater retention values on a project-by-project basis, primarily to calculate the structural load of the green roof assembly.

Stormwater runoff coefficients speak to the rate and volume at which water percolates into the ground rather than running over the ground’s surface.  Where the surface of an undisturbed forest floor allows water to quickly be absorbed into the ground, a concrete or asphalt parking lot would instead repel water across the surface to lower neighboring grades.  The difference in the permeability of the surface is sometimes referred to as imperviousness of the surface.

Hydrologic Cycle Diagram; Source:  JetStream - An Online School for Weather

Discussions of stormwater runoff are typically concerned with the impervious surfaces within a certain development or a certain watershed.  Many municipal planning and zoning authorities have implemented policies restricting the amount of stormwater runoff a new development is allowed to generate.  Some mandate no increase in the pre-development runoff volumes for new projects under consideration for building permits.

Redrawn from Bruce Ferguson's "Introduction to Stormwater: Concept, Purpose, Design," 1998.

One such municipality requested my company, Green Roof Blocks, to make a presentation to the Planning Board to help evaluate a proposed condominium project.  During the discussion the focus of the proposed green roof changed from that of merely an amenity to a component of the project’s stormwater management plan.  The board instructed the city’s engineer to compile the data quantifying the stormwater runoff reduction attributed to the green roof. Subsequent meetings with the city engineer began the process of developing a method of calculating the stormwater runoff coefficient that would allow the engineer to produce the data for the Planning Board.

An internet search turned up a paper published on October 19, 2005 by Charlie Miller, a well respected authority in green roof design and construction.  The paper discusses the possibility of calculating the runoff coefficient for green roofs but suggests that particular model was based on a specific green roof design and results may be somewhat flawed should this same model be used to calculate the runoff coefficient for different green roof designs.  The paper includes some complex formulas used to calculate the runoff coefficient based on percolation rates of the growth media.  I contacted Mr. Miller to discuss his conclusions and to gain some insight to his calculation process, and took meticulous notes during the conversation to relay to the city engineer.  After several clumsy attempts to articulate what Mr. Miller had explained to me, the city engineer suggested that we were making this more difficult than it needed to be.

Knowing my background in roofing and sheet metal, the engineer asked me, “What do you do for a living?”  I hesitated, not knowing where he was going with this, so he answered for me. “You manage water flowing across the roof top,” he said.  Then he asked, “How do you size the downspouts?”  I started to understand where he was leading me.  We size downspouts for particular regions based on the area of the roof and the regional rainfall intensity values given for 10 year and 100 year storm events.  There is a chart listing these values for regions throughout the United States in the Sheet Metal and Air Conditioning Contractors National Association manual.  Values listed in the chart allow us to determine a known volume of water falling on a given roof area.  This is significant because from here we can now determine the amount of water to run off of the roof once saturation points of the green roof growth media, drainage layers, and moisture retention materials are reached.  By subtracting the volume of water required to saturate these green roof components from the known volume of rainfall, we can determine the stormwater runoff volume. We now have everything we need to calculate the runoff coefficient.

The runoff coefficient for a 100% impervious surface is 1.0, as all rain falling on this surface runs off.  The runoff coefficient speaks to the fraction of the rainfall that runs off if some volume is absorbed into the surface.  By dividing the retained volume by the total rainfall amount we come to the fraction of stormwater that is absorbed into the green roof.  By subtracting that value from 1 we come to our runoff coefficient.  That is, the decimal equivalent to the percentage of stormwater that runs off of the green roof assembly.

I wanted to be certain that the assumptions we had made were correct and I was uncertain if we had properly accounted for the effect of stormwater percolation into the green roof system. I decided to seek a second opinion from Dan Wind, a civil engineer in St. Louis who works closely with the St. Louis Metropolitan Sewer District to develop stormwater management strategies for regional construction projects.  I put all of the calculations into an Excel based spreadsheet and asked Dan to take it for a test drive.  He verified that the formulas were correctly calculating each of the values and agreed with our assumptions.  We then discussed whether we should make any changes to account for the effect of percolation.  Since the green roof assembly requires water to drain through the growth media before exiting the rooftop and since we calculated for total saturation of the green roof components, we assume 100% percolation through the green roof assembly, so we agreed that no changes were necessary.

The final version of the Green Roof Storm Water Management Tool has cells designated for user input data with instructions that pop up in information balloons as the user rolls the cursor across each cell.  The user friendly format allows a green roof designer to instantly see the effect each of the green roof components has on stormwater retention.

The Green Roof Storm Water Management Tool

While this model may not accurately account for all stormwater characteristics of green roofs with aggregate drainage layers or deeper intensive green roofs, it is reasonably accurate for most extensive green roof designs.  To help further promote green roof projects in the United States, we decided to make this tool available free of charge for anyone wishing to use it.  The Excel based spreadsheet is available for download from the “Downloads and Calculators” page of the Green Roof Blocks website www.greenroofblocks.com.

Kelly Luckett is president of GREEN ROOF BLOCKS and Green Paks, subsidiaries of Saint Louis Metalworks, and a member of the USGBC.  Kelly has been involved in the industry since 1980 and is a LEED Accredited Professional.  He is also Greenroofs.com's "Roving Exhibitor" Contributing Editor.

Green Roof Blocks are self contained portable units that are compatible with all roof membranes and hold 4 square feet of drought resistant sedum foliage growing in lightweight engineered soil. Green Paks are knitted polyethylene green roof modules, pre-filled with lightweight engineered soil and plants, and each Green Pak covers 5.2 square feet of roof area.  Visit the websites at www.greenroofblocks.com and www.greenpaks.com.  Contact Kelly by phone at 314.972.8010, or email him at either RovingExhibitor@greenroofs.com or kelly@saintlouismetalworks.com.

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