STORMWATER DETENTION
INSIDE THE FLOODPLAIN | OUTSIDE THE FLOODPLAIN

What Is It?

Cost: Dependent on size and material

  • Galvanized steel: $950 for 2,000 gal.
  • Polyethylene: $1,100 for 1,800 gal.
  • Fiberglass: $10,000 for 10,000 gal.
  • Fiberglass/steel composite: $10,000 for 5,000 gal.

(Costs for system only, does not include installation/other associated costs)

Applications: Can be installed in retrofits or in new construction
Service Life: Extensive (20-50 years)

A 1,000 square foot roof receives approximately 600 gallons of water from 1 inch of rainfall. Rainwater harvesting allows for some of this rainwater to be captured and reused in non-potable applications. Rain is captured in barrels (approx. 55 gallon capacity) and cisterns (10,000 gallons or more), which reduces the quantity of stormwater runoff and non-point source pollution. Rainwater storage tanks can be located above ground, underground, or inside building basements. Rainwater that exceeds a harvesting system’s capacity can be diverted to a storm drain as needed. Annual cleaning is necessary to remove biological contaminants that collect in the storage tank. Downspout seals, installed to prevent mosquito breeding in the standing water, should also be inspected periodically.

Rainwater captured in cisterns has limited use. Graywater can be used for irrigation, though rainwater is slightly acidic (pH 5.6-5.7) and should be used on appropriate vegetation. Rainwater can be used in buildings for toilet flushing or for HVAC processes (e.g. boilers, air conditioning). However, care must be taken to ensure that potable and non-potable water sources are separated and that all plumbing within the building is properly labeled. Successfully reused water can significantly reduce building water use: Atlantic Wharf installed a rainwater harvesting system utilizing a 40,000-gallon storage tank that has, in conjunction with other water conservation efforts, yielded a 63.1% reduction in irrigation-related water use and 15% reduction in process water use compared to similar buildings.

Stormwater reinfiltration systems have a permeable base, which allows water to pass through into the ground. When the soil is saturated, the excess water is retained within the system and allowed to infiltrate slowly back into the earth as the water table drops back to normal levels, reducing the risk of flooding. A cistern can hold significantly more water than the soil around it, allowing for more flood water to be contained within the ground before it floods onto the surface. Modern underground cisterns are modular and can be built from plastic, metal, or concrete, and can be sized to the needs of the project. They can be located underneath parks, parking lots, or even occupied buildings, although the cistern may affect the load-bearing capacity of the soil.

Benefits

  • Rainwater capture reduces stormwater runoff and non-point source pollution during storm events.
  • Reuse of rainwater can reduce building water use and associated costs.
  • Captured rainwater used for irrigation preserves the hydrological cycle by allowing for groundwater recharge.
  • Size and shape variable to the needs of the project.

Reinfiltration systems:

  • Enhances groundwater infiltration while reducing stormwater runoff volume, rate, and pollutants.
  • Retains water onsite instead of producing runoff that overloads sewers.
  • Increases stability of the ground if the area is prone to flooding.
  • Underground tanks can have infrastructure built on top of them.

Drawbacks

  • High capital cost; due to initial infrastructure investment, operations and maintenance, and pumping costs, life cycle cost and net present value benefits assessments have shown that rainwater harvesting is not economically viable given present conditions.
  • Cisterns placed above ground must be winterized in cold climates, increasing capital cost. Cisterns placed below ground may require excavation, increasing costs.
  • Underground cisterns are not an efficient retrofit, and require a good deal of space for new construction.

Regulatory Impacts and Requirements

A summary of potential regulatory touchpoints follows below.

rainwater

Financing Options, Incentives, And Rebates

Additional Resources

PROJECT EXAMPLES

SAMPLE OF SUPPLIERS

SOURCES

Photo Credit: User Pengo, CC by 2.5 [http://creativecommons.org/licenses/by/2.5/], Wikimedia Commons