Systems Planning

Cisterns

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A cistern is a receptacle built to catch and store rainwater for irrigation during dryer periods. While they are similar to rain barrels in purpose, cisterns are usually much larger. Cisterns may be located underground, at ground level, or on elevated stands. It is recommended that they be watertight, have smooth interior surfaces, enclosed lids, and be large enough to provide adequate storage. They also should be fabricated from non-reactive materials such as reinforced concrete, galvanized steel, and plastic.

Requirements for credit

​​​​Installing a cistern can result in a savings of $6.35 per quarter.​​ Only one credit can be taken for utilizing​ a rain garden, dry well or cistern. 

Roof Drainage

At least 50% of your property’s roof area should drain to the cistern OR the cistern must capture runoff from impervious area on your property that is equal to 50% of your roof area. 

Size

​​66 cubic feet OR 500 gallons of capacity 

Infilltration

The cistern must be completely drained in no less than 24 hours and no longer than 48 hours after each rainfall event. Rainwater from the cistern must be applied to on-site vegetation and should not discharge from the property. Longer drainage periods may be acceptable if the cistern is larger than the minimum sized required for credit. 

Screen

The cistern must be equipped with screens, seals, or other suitable methods to prevent mosquitoes from entering them. 

Overflow

Cisterns must be equipped with an overflow or bypass device to divert runoff in excess of the cistern capacity to the storm drainage system without causing erosion and/or property damage. 

Above ground cisterns

Above ground cisterns have various capacities and can be made out of above ground small version of cistern drawing2.pngdifferent material.​ About 1/4 of the roof area drains rainwater into each of the cisterns. The image to the right is an example of a simple above ground cistern​ designed to take water runoff from a roof. ​


 

Below ground cisterns

Excavate the hole to the required dimensions. This is usually done with a back hoe. For example, 8 feet by 12 feet and 8 feet deep. Make sure the hole is dug to the dimensions you desire or your storage will be seriously compromised. 

Pour the cistern floor. First form up the floor of the cistern much as you would a sidewalk, driveway or other flat work. Construct a rectangular framework from 2 x 4's and secure it with 2 x 4 stakes driven into the ground at intervals of about 2 feet. 

Form the cistern walls. The cistern walls should be constructed by first building the outside forms and then installing #6 rebar wired together on an approximate 1 foot grid. Set the grid into the holes bored into the concrete floor of the cistern with a hammer drill. With the reinforcing grid in place build the inside forms. Make sure the walls are adequately braced and then pour the concrete. 

Let the concrete set for the required time period recommended by the manufacturer and then remove the forms. 

Seal the inside of the cistern, a Portland-based product with a latex additive is recommended, possibly Damt​ite, or another acceptable alternative available at your local building supply store. 

C​​reate the lid and hatches. The lid can be made of any acceptable material but should fit snuggly to keep potential pest from entering.

Resources

Planning and Development Services - A storm water management plan is required for residential construction projects with 200 plus square feet of impervious surfaces. Consult our residential stormwater code requirements site for more information ​about the code and to download an impervious​ area worksheet that will help you determine if your project will require storm water management as part of the grading permit application process.​