SkyScape Vegetative Tray Hydraulics & Hydrologic Response
December 09, 2011
Green roof systems have been extensively researched and demonstrated to effectively manage stormwater runoff from roof tops. Research by the City of Portland demonstrated for a period of six years that eco-roofs/vegetative roof assemblies (green roofs) are effective at attenuating peak runoff rates from rooftops and reducing the total annual runoff volume by process of evapotranspiration. The SkyScape Vegetative Tray has been engineered to provide evapotranspiration.
Field Capacity
Runoff reduction is accomplished by temporarily storing higher intensity rainfall (which results in more extreme runoff) by two mechanisms. The first is storing the rainwater within the growing media (i.e., engineered roof soil) pores to the point where initial or apparent drainage occurs. This point is termed field capacity, or the amount of water a media can hold just before it begins to drain. The field capacity of a growing media relates to the textural class of the media and organic matter content.
Drainage
Once field capacity is reached, water is detained in the growing media pores and moves or percolates very slowly through the growing media until it reaches the drainage plane at the roof membrane surface. Once the drained water reaches this point it begins to drain laterally across the membrane until it reaches the internal roof drains and/or scuppers and drains away.
In the case where the media is saturated (i.e. all of the pores are filled with water) any additional rainfall will move through the growing media very quickly onto the drainage plane. However, full saturation is typically not achieved since the percolation rate through the growing media greatly exceeds the rate at which rainfall occurs. These characteristics are common to most green roof systems. Though the growing media depth and texture differ from one product to the next, the principles remain the same.
Water Storage
The second process is a reduction in the total annual runoff volume. This occurs by capturing the water and storing it at or below field capacity. Commonly occurring between storms, stored water is lost to the atmosphere through direct evaporation or plant transpiration. The combined effect is evapotranspiration (ET). The rate of ET is complex. It depends on air temperature, relative humidity, wind speed, solar radiation and plant characteristics. To date little data exists on ET rates of green roofs and typically are reported as a net loss over a monthly period.
SkyScape Tray
The SkyScape Tray is a 4.625” x 24" x 24" interlocking plastic tray that has multiple sets of metering holes which provide a restriction to the outflow of water. This mechanism increases the detention capacity and further reduces peak runoff.
Water Discharge Rate
The SkyScape Tray Stage Discharge curve below shows the hydraulic characteristics of the tray. The stage discharge curve shows the rate at which water will discharge from the tray given the depth of saturation. To this end, the SkyScape Tray that is completely saturated will release water at about 5 gallons per minute. The graph shown uses an empty tray. When the tray is filled with media, the discharge rate is reduced by the presence of the
growing media.
The drawdown curve is derived from the stage discharge curve and uses a 30% void ratio for the growing media, meaning 30% of the total volume of the tray is free water that will drain. In this case the volume retained is about 2.5 gallons of water and will drain out in about 13 minutes. This effectively raises the time of concentration (Tc) to about 18 minutes vs. 5 minutes; a rate which is commonly used on roof tops. This Tc reduction will substantially decrease the peak flow from a roof top and be an important part of meeting regulatory requirements which typically do not allow a peak flow rate exceeding predevelopment conditions.
Low Impact Development (LID)
Most tray and built up systems provide for some level of stormwater management by virtue of retaining some water for ET and attenuating peak flows by the presence of growing media and plant material. However, historically, green roofs have not been an integral part of the regulatory process for permitting, but rather a sideline player with the benefits focused more on energy savings, aesthetic values, LEED points, etc.
Recently, changes in permitting and regulatory processes have led to the implementation of Low Impact Development (LID) techniques to both attenuate runoff and to reduce it as well. The four principle cornerstones of these practices are bioretention, rainwater harvesting, green roofs, and permeable paving systems.
Summary
Green roofs are playing an increased role in the overall management of stormwater runoff for new urban development and retrofits. With this new focus, having defined and characterized hydraulic response to saturated and non-saturated conditions, the SkyScape Vegetative System is becoming critical to the up and coming hydrologic modeling techniques that are being adopted by regulatory agencies across the US.
References
City of Portland Green Roof Research:
http://www.portlandonline.com/bes/index.cfm?c=36055&a=148927
USEPA Green Initiative:
http://cfpub.epa.gov/npdes/home.cfm?program_id=298
State of Maryland Dept. of the Environment, 2007,
“Facts About Stormwater Management & Green Roof Technology":
http://www.mde.maryland.gov/assets/document/sedimentStormwater/SWM_greenroof.pdf
USEPA, December 2009,"Technical Guidance on Implementing the Stormwater Runoff
Requirements for Federal Projects under Section 438 of the Energy Independence and
Security Act", EPA 841-B-09-001, December, 2009