A few years ago there were locations where rainwater harvesting was
frowned upon…if not illegal. With LEED (Leadership in Energy and
Environmental Design) taking a strong hold of the designed worlds of
construction and related disciplines, times have changed.
A proactive approach to the collection of water at a site, onsite, for use as irrigation water can help reduce the dependency on city water.
Skyrise rainwater harvesting project
A project that I have been working on, located in the middle of downtown Toronto, is designed to collect rain water at the roof level. The water will be stored on the 38th floor. Gravity will deliver the water to the multiple-terraced planted levels, of which there are in excess of 10 levels plus a green roof. This could become the greenest building in Toronto.
A pump is not required for this project for floors that are more than 20 feet lower than the source. (20 x .433 psi = 20 psi which 24 VAC irrigation valves and dripline can operate at.) With water delivered right to at grade landscape, pressures will be approximately 456 feet x .433 PSI = 198 PSI. Design and designer’s knowledge of irrigation component, operational parameters is critical for a system like this to operate correctly. Working with minimum valve operational pressures to high pressures, one must have a solid grasp of hydraulics to get it right.
Toro landscape drip line was selected as it is designed to be buried in the soil media as long as an air/vacuum relief valve is incorporated into the dripline system. Its operational pressure parameters allow for the low PSI at levels close to the rainwater collection cistern. Pressure-reducing valves will be required for lower floors with planted balconies so that the low volume irrigation components operate at optimum levels…and not excess pressure levels.
History of rainwater harvesting
“Water harvesting is the capture, diversion, and storage of rainwater for plant irrigation and other uses.”
Rainwater has sustained life on earth forever. The term and definition may be new, however, the practice is ancient. Unearthed, the House of Vetii, Pompeii, Italy revealed ancient irrigation and water features—more than 1,500 years old—powered by collected and stored rainwater. Collected as it fell into stone cisterns and released later by utilizing gravity, the water was used beneficially and artistically.
Rain or grey
It is only in recent times, since the development of centralized water supply systems, that rainwater has not been harvested for drinking, landscape and agricultural uses. In fact, there are many mixed feelings, definitions, and regulations governing the use and reuse of rainwater and, for that matter, grey water. Rainwater is self-explanatory—it is provided by none other than “Mother Nature.” Grey water is what we humans do to the water to clean things and wash ourselves. It is said to be the water from your sink, shower and laundry. The problem lies in what is in the soap we use to achieve the desired results.
Rainwater harvesting systems
There are many methods available for collecting, storing and using rainwater. They range from rain barrels, cisterns, mass-produced rainwater tanks, to galvanized culvert-style corrugated pipe above ground tanks. One way to harness rainwater which can be inexpensive for new construction is to reshape landscape areas to act as infiltration ponds to receive water rather than expelling water onto the hardscapes. Uncontrolled rainwater can wreak havoc—flooding streets and walkways eventually overflowing the storm drain systems, sending untreated waste into lakes, rivers and oceans. Inground custom designed and built multiple link tank systems are designed to meet site specific constraints, be they are self or regionally imposed.
Along the Amalfi Coast in Italy, fresh drinking water is so scarce that they have designed building roof areas as collection systems. The flat-shaped roof layout, sloped to one side, is where the water is collected in an inground cistern. The water is then pumped to the rooftop tanks (multiple small white tanks) held under pressure and used for drinking water, washing, toilet flushing, and other uses.
Typical system
A typical system for smaller projects collects the rainwater from the eavestrough. Troughs of five-inch width are recommended so that more rainfall can be captured. The downspout needs to be fitted with debris catcher and cleanout. As the water enters the cistern it needs to be calmed so that if any debris is lying on the bottom it does not get stirred up. Excess water is drained off to a soak-away pit. The water is then pumped into a distribution system where, here, it is pictured being filtered and then used for laundry, toilet and outside water use.
To be safe, it should be passed through a UV disinfectant process as well.
Tanks (cisterns)
Bushman rainwater harvesting systems is one of now many players in North America that supply rainwater harvesting tanks. Tanks can be above ground or in-ground. It all depends on the amount of catchment (water caught) that the system has been designed for. Tank sizing is an art. To get it right, the irrigation system should be designed and the water consumption calculated.
A 100-foot x 50-foot residential city property that has a landscape that requires supplemental watering could require up to 2,000 gallons of water per cycle. For simplistic sake a cycle is an entire watering of the landscape. Depending on location, environmental conditions, plant type and water requirements, soil water holding capacity, plant root depth (known as the soil reservoir or SR), water requirements vary considerably from site to site and locations.
Water application & scheduling
So, the skyrise that I have designed for requires plants to be provided with one inch of water per week at a minimum. The south-facing planters and those susceptible to the wind (on the west side) will require more water. With 10 floors with planting plus a green roof, all irrigated with dripline rated at one gallon per hour (gph), the irrigation stations will be required to provide 1 gph x emitters per planter (60) = 1 gpm.
The light FF soil with river rock over the top to hold the spoil in place will not hold water as well as, say, a loam soil. FF is a light soil with large particle sizes that do not hold water. Even through the action of adhesion to the soil particle and cohesion to the water particles, it is a tough soil to water adequately to provide sufficient moisture for plant survival. Scheduling with multiple short runtime cycles is the key. This is how desert areas that water into sand can have established plants, as long as moisture (irrigation water) is delivered at preset intervals.
GPH to GPM
OK, to convert gallons per hour to gallons per minute, multiply by 60. Now let’s say the plants require one gallon of water per day. This will take 1 gph x 60 = 1 gpm which equals one minute to apply one gpm. Caution here that we are calculating with 60 x 1 gph emitters that are inside the planter and are delivering water to the plants’ root zone. If not the case…do the math yourself. Always in this case apply more water than not enough. In other words, you need to know what you are doing to ensure a system such as this will do what you have designed it to do.
Tank sizing
We have 20 stations (zones). For simplistic sake, they require 1 gpm per day. 20 x 1gpm = 20 gallons per cycle per day x 7 days = 140 gallons of water for one week. I like to store for three weeks so 140 x 3 = 420 gallons. Size for a 500-gallon cistern or tank.
Simple, right? Not that easy. Each of these stations is not at one gpm. They will all vary and water calculations for each planter must be calculated based upon plant type and its water use requirements. Remember that the water use requirements are for ground level in ground landscapes. Skyrise planters do not react the same as the ground so you will need to double if not triple the amount of water storage capacity. The system discussed here has a 30,000-litre cistern on the 38th floor. I always design water storage for a three-week dry spell. Life in the new design world of skyrise plantings. Hope you will find this information useful. Need help? Let me know. Good luck.
Lorne Haveruk, principal, DH Water Management, educator and author is one of the country’s leading water resource consultants. DH Water is focused on all aspects of water resource management. For educational offerings and other services, please visit http://www.dhwatermgmt.com. To contact the author directly, email lorne@dhwatermgmt.com.
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