Tag Archives: GSI Mandate

Cement Cisterns Collect Urban Rainwater

With limited space in urban areas, cement cisterns are being built beneath homes as a rainwater harvesting solution.

tank linerWith the new “Green Storm Water Infrastructure” or GSI mandate, Seattle based RainBank Rainwater Systems is seeing increased interest in rainwater collection.  But, with limited space on an urban building lot, the storage of rainwater in above ground tanks is not an option when required to mitigate thousands of gallons of roof runoff.

With many new residential construction projects building on small city lots, there is just not enough room for a rain garden. One way to solve this issue is with a cement vault, constructed beneath the house, which stores the captured rain water for potable use.

The average 2,000 square foot roof area will yield over 44,000 gallons of water.  However, a cement vault cistern needs special engineering and considerations. Water weighs approximately 8 lbs per gallon and 1 cubic foot equals approximately 8 gallons. Calculations of weight and sheer need to be engineered for Seattle’s seismic zone. Additional ventilation should be considered along with aeration.

waterproofing tankCement water tanks are not naturally waterproof; cracks in concrete can result years after construction. The use of a liner, whether spray on epoxy, or a poly bag will prevent groundwater from entering the water tank as well as improve water quality.

Potential Unexpected Costs From GSI Mandate

Potential Unexpected Costs From GSI MandateThe GSI mandate or “Green Stormwater Infrastructure” can create unexpected costs for new commercial and residential construction. Controlling storm water through GSI compliance requires on-site infiltration and/or use.

Roof runoff, along with impervious surfaces, contributes to overwhelmed stormwater systems, which may result in pollutants entering our waterways and wetlands. A design using both methods of rainwater collection and infiltration can be the most cost-effective, while being beneficial to the environment.

Potential Unexpected Costs From GSI Mandate2Directing roof runoff for irrigation, toilets, laundry, or even pre-infiltration potable water, will reduce costs from municipal water supplies. During the rainy season, collected water can supply these uses while overflow from the cistern can be directed to infiltration. Irrigation in the summer months being supplied by the desired amount allocated from storage will enable the rain garden or infiltration system to be sized for a smaller amount of runoff while supplementing city water usage.