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  water storage &
irrigation

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Greenroofs must achieve a balance between adequate drainage and being able to store water and not dry out too quickly, and this is accomplished by choosing appropriate drainage products and growing media.  But the selection of a specific plant community, greater plant diversity, or simply the particular design parameters desired for certain greenroofs may require additional water.

According to Charlie Miller, "Ideally, even thin systems work optimally with two layers, separated by a geotextile.  The lower level is very light-weight granular mineral material (usually a fired clay).  The roots of the plants will penetrate through the geotextile and will concentrate along the bottom of this layer.  Here they find the best conditions for survival (cool temperatures and more consistent moisture).  If irrigation is included, we choose to introduce the water in the granular layer – further encouraging propagation of roots at the bottom.  If the substrate is chosen to have good water retention qualities, this system will support a variety of plants without irrigation.  When roots are encouraged to grow higher up in the profile, they are much more vulnerable to the effects of varying temperature and moisture," (Charlie Miller, personal communication, April 2000).

If the soil substrate/drainage system cannot hold a certain amount of free water, then additional forms of water storage may be necessary  and can be supplied by several methods.  Certainly, the most "ecologically correct" systems would be those that are considered sustainable, requiring no or little human intervention.

Completely self-sustainable passive water retention systems include using ponding elements welded directly to the protection membrane, and generally can be installed on roofs up to a 4% slope, as shown below, courtesy of Roofscapes, Inc., www.roofmeadow.com:

Passive Irrigation by Roofscapes, Inc.

 

Optigrun Active Irrigation

Or, a more active system incorporating the ponding ridges along with a drain with weir level regulator and an automatic irrigation control is seen above from Optigrün, courtesy of Roofscapes, Inc.

Solar Powered Irrigation by Optigrun

Photo Source: Optigrün Die Dachbegrüner Planungsunterlage 98/99 Brochure

Optima, Optigrün and ZinCo have developed active, sustainable, solar powered automatic water collection and irrigation systems.  These systems also have the added sophistication of a maximum and minimum on/off switch to make allowances for possible overnight rainfall filling the reservoir naturally.  For the ultimate environmental self-irrigation system, an additional cistern can be incorporated to store excess rainwater which can be recycled later (Optigrün, 1998/1999; and ZinCo, 1998).

The drawings below show a solar-powered rainwater collection system from Optigrün-Italy on the left and an irrigation detail by Roofscapes, Inc. on the right.

Solar Rainwater Collection System by Optigrun-Italy Recycle Irrigation System by Roofscapes, Inc.

 

ZinCo Greenroof Water Storage

Source: ZinCo International 3/98 Brochure

For additional water retention capabilities, a damming piece can be installed into the outlets, with inspection chambers or adjustable terrace grills for easy maintenance.

Shown at left is ZinCo's Floradrain FD 60, with its large drainage channels allowing up to 1.5" of water to be stored underneath.  The water reaches the plants by capillary action, and the inspection chamber allows access for maintenance.

Optima/Optigrun Greenroof Water Storage

Detail from Optima’s Planungs-Unterlage 9/97 manual

Optima has designed a self-regulating water retention, storage, and automatic irrigation system utilizing small tanks (1 ½’H) which retain rainwater that has percolated through the substrate. The system then hydraulically releases the stored water automatically when the water dips below the pre-determined minimum level.

As stated earlier, some drainage systems also incorporate water retention capabilities; the stored water reaches the plants by capillary action.  An optional reservoir board layer or retention fleece mat, available from some companies, can be installed to retain and store small amounts of water as well.  Additionally, either a simple automatic drip irrigation system with a manifold delivering water at the base of the profile can be installed, or a more complete (and heavy and costly) irrigation system can be incorporated into any intensive greenroof design which can withstand the added weight.

 

 

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