Rainwater cisterns


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Bronze Construction Services Inc. www.bronzeconstruction.com blog article on construction of rainwater cisterns.

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Rainwater cisterns

  1. 1. An overview
  2. 2.  A cistern is a container with a waterproof lining that is designed to store rainwater.  Can be underground or above ground.  Can range in complexity from a barrel at the bottom of a downspout to more extensive systems with covered storage tanks, pumps, and water treatment.
  3. 3.  Collected rainwater can technically be used for indoor, including drinking water (if properly filtered or purified) and outdoor use.  According to the City of Portland rainwater harvesting code guide, collected rainwater cannot be used for drinking unless it is treated and meets both state and federal standards for potable water.  http://www.portlandoregon.gov/shared/cfm/ image.cfm?id=68627
  4. 4.  Saves money on water bills  Clean water without chlorine and other additives  Less dependence on public water supply  May help reduce storm water fees  Water conservation – helps the environment  About 1 inch of rainfall on a 2,000 sq ft roof can create 1,250 gallons of reusable rainwater
  5. 5.  The total cost varies depending on the size, need, and material used.  A complete system with filtration and water capturing for 800 gallons of water at a single residential home may cost $1000 for parts (not including installation/labor).  Savings over time can be great in an area with regular rainfall, such as Portland. For example, if an average water bill is $100 per month for a family of 4, the savings could be $5000 over a period of 5 years ($6,000 minus $1,000 for the initial cost of the cistern).
  6. 6.  The water collection system (roof, gutter, and downspout)  The filter to keep out debris, such as bits of leaves  The water storage vessel (cistern). The size can vary in range depending on user needs. The cistern should be made out of stone, concrete, or another non-corroding, non- contaminating material. Wood and metal are not recommended.
  7. 7. The steps involved in designing a complex water harvesting system include:  Site analysis  Calculation (not absolutely required)  Design (above or below ground, for example)  Construction
  8. 8.  Draw the site and all site elements to scale.  During rainfall, observe the area to find out the existing drainage flow patterns. Show the water flow direction with arrows. Indicate high and low areas on your plan.  Look for catchment areas for water harvesting, such as a roof or paved area.  For outdoor use, find planted areas or potential planting areas that require irrigation.  Locate above or below ground storage near planted areas.  Decide how water will be moved from the catchment area to the holding area or storage container. Rely on gravity to move water whenever possible.
  9. 9.  This step is not required, but it is recommended for more elaborate systems.  Calculate the monthly supply (rainfall harvest potential) and the monthly demand (plant water requirement) for a year.  Next, calculate your monthly storage requirement if you are designing a more complex system.  Here is a website that is helpful with the calcuation step: http://www.green- trust.org/rainwater/
  10. 10.  Use the site analysis info and potential supply and demand calculations to size and locate catchment areas.  Roofs or shade structures can be designed or retrofitted to maximize the size of the catchment area.  For efficient use of harvested water, group plants with similar water requirements together.  Use gutters and downspouts to transport the water from the roof to the storage area.
  11. 11.  Select gutters that are 5 inches wide.  Select galvanized steel (26 gauge minimum) or aluminum (.025 inch minimum) gutters.  Slope gutters 1/16" per 1' of gutter, to enhance flow.  Use an expansion joint at the connection, if a straight run of gutter exceeds 40 feet.  Keep the front of the gutter one-half inch lower than the back.  Provide gutter hangers every 3 feet.  Do not exceed 45 degree angle bends in horizontal pipe runs.  Select elbows in 45, 60, 75, or 90 degree sizes.
  12. 12.  Space downspouts a minimum of 20 feet apart, a maximum of 50 feet apart.  Provide 1 square inch of downspout area, for every 100 square feet of roof area.  Select downspouts in different configurations -- square, round, and corrugated round, depending on your needs.  Use 4-inch diameter Schedule 40 PVC to convey water to the storage container or filter.
  13. 13. For more in-depth information on installing a rainwater cistern, the following resources may be helpful:  Planning, design, installation, and more: http://www.treepeople.org/install-cistern-or-rain- barrel  City of Portland Code Guide – Office of Planning and Development Review: http://www.portlandoregon.gov/shared/cfm/image.c fm?id=68627  Calculations: http://www.green-trust.org/rainwater/  More on calculations: http://www.lid- stormwater.net/raincist_sizing.htm