Green Roofs - Living Architecture Monitor - Fall 2013 - (Page 13)

CREATIVE STORMWATER LANDSCAPING MANAGING STORMWATER WHERE CONVENTIONAL GREEN INFRASTRUCTURE OPPORTUNITIES ARE LIMITED BY: CHARLIE MILLER T he hallmark of green roof technology, and the feature that sets it apart from all other green infrastructure strategies, is the way it harnesses very large surface areas of plant foliage. Other best management practices rely primarily on infiltration as the mechanism for runoff volume reduction. On green roofs, simply put, runoff volume reduction is equivalent to evapotranspiration (ET). If ET effectiveness can be optimized, then so will stormwater management performance. Green roofs are best understood as shallow interflow groundwater aquifers. Based on our observations, we find that ET on green roofs is strongly influenced by the con- tact time of percolated water with the media and plant root systems. The time it takes for rainfall to percolate vertically through an extensive green roof may be a few minutes, while the time that it takes to flow horizontally through the system toward discharge at the roof drain may be many hours. Consequently, we at Roofmeadow are arguing for a reconsideration of the metrics that we use to describe green roof assemblies. Recognizing that the horizontal pathways through the green roof assemblies may be the most important feature of the design, we are advocating for the consideration of hydraulic residence time (HRT) as a critical design variable. HRT is a measure of the average time that a drop of water is in contact with the green roof materials. HRT can be manipulated by controlling: 1) transmissivity of the basal layers of the green roof assembly, 2) length of the flow path, 3) tortuosity (passage) of the flow path, and 4) opportunities for detention storage within the green roof. It is an interesting observation that the effectiveness of green roofs in controlling both runoff volume and runoff rate is only weakly proportional to media thickness. The reason is that most runoff percolates to the base of the assembly comparatively quickly; conditions at the bottom of the profile, however, most directly influence performance. In short, extensive green roofs are much more efficient, pound for pound, than intensive green roofs in managing stormwater runoff. With this realization as a starting point, it is easy to make the leap to applying this same strategy on the ground. Opportunities to infiltrate rainfall in cities are limited. Roofmeadow is leveraging its experience with green roof hydrology to engineer similar systems at grade. The results are Veneer Stormwater Management Practices™ (Veneer SMPs™), shallow noninfiltrating measures that can overlay existing impermeable or contaminated substrates where effective infiltration is not possible or advisable. Examples include vacant lots,

Table of Contents for the Digital Edition of Green Roofs - Living Architecture Monitor - Fall 2013

Green Roofs - Living Architecture Monitor - Fall 2013
From the Founder
Bookshelf – Eat Up: The Inside Scoop on Rooftop Agriculture
Green Roofs Boost Efficiency of Solar Panels
Grey to Green a Great Success
New & Upcoming GRHC Courses
On the Roof With… Industry Leaders
West Coast Green Innovation
Industry Research Collection
The Green Façade Inquiry
Creative Stormwater Landscaping
Leadership – Green Roof Leadership
Plant – Selecting Plants for Living Walls
Conference Agenda — CitiesAlive in San Francisco
Project – Green Walls for Greener Cities
Project – Living Architecture That Feeds
Project – Reduce, Reuse, Recycle
New Corporate Members
GRHC Buyers Guide

Green Roofs - Living Architecture Monitor - Fall 2013