I have posted on this site some basic information about green roofs (Green Roofs and Water Management and Green Roofs and Urban Enhancement). Fellow Blogger Alan Harris has posted informative pieces about water capture for irrigation using various cisterns and water butts (Does this Make My Water Butt Look Big, When a Water Butt is Just Not Big Enough). With this post I hope to connect some currently disparate dots.
Aside from their aesthetic benefit, green roofs moderate and filter storm water run-off from roofs, reduce urban heat island effects, can provide a certain degree of thermal insulation for the structure, and even contribute to biodiversity in urban settings beset with pavement and hardscape. Water butts and cisterns provide the dual benefits of saving water and reducing the burden on storm drainage systems.
Until such time as they repeal the law of gravity, water flows down hill. As a result, cisterns and water butts are generally set at or below ground level such that they are out of reach for irrigating green roofs unless one introduces a pump into the system. Pumps use electricity which, for the die-hard tree hugger, is fraught with its own environmental baggage.
So, what’s a fella to do?
Recently I participated in a webinar on a popular European green wall system. I have long felt that green walls were a great concept provided that, like green roofs, their engineering and construction takes into consideration the limitations and site-specific conditions of the structure they are to be attached to. During this webinar, it struck me that green walls can benefit from and become part of an overall urban watershed program. What I envision goes something like this:
Green Roof: The green roof intercepts the first ½” to 1” of a storm event and holds it for its own use. Surplus rainfall beyond the roof’s initial capacity is filtered and flows off the roof via a passive gravity system. Think of this as run-off from high plateaus flowing into creeks and streams.
Upper Floor Cisterns: If we can impound a portion of the green roof’s surplus at or near the roof level in tanks or cisterns, that water could be discharged to irrigate green wall arrays. Think of these cisterns as upland ponds or tarns.
Green Walls: Water captured in the upper floor cisterns could be discharged in a controlled manner, and again by gravity, used to irrigate green walls on its way to ground level. Think of the green walls as lowland prairie areas.
Ground Level Cisterns: Any remaining surplus left after absorption and use in the green roof and green walls could still be captured at the ground level in more traditional cisterns and butts. Think of these as the lakes that hold our fresh water reserves. Note that this water could still be used for irrigation at ground level. Any surplus after irrigation use would at long last make its way into the storm drain system, greatly reduced in volume and relatively free of storm water of pollutants.
Introducing green walls into this cycle effectively mimics in miniature the functions of a natural regional watershed. In doing so it provides a program that can deliver additional benefits to man and nature:
- Increased bio-diversity in an urban setting
- Increased CO2 / Oxygen exchange
- Increased particulate pollution adsorption
- Decreased urban graffiti potential
- Enhanced acoustical dampening
- Enhanced thermal insulation to underlying structures
What’s not to like?
Yes there will be design and engineering considerations and challenges but the upside seems pretty positive to me. If this approach has not yet been implemented by anyone yet, then consider it my holiday gift to the design and development communities.