Storm Water Approaches

In class I outlined the main approaches to storm water management.  In the way I phrased things I may have been unclear, so I’m summarizing issues here.

Basic Requirement by Water Department

• The first inch of rain that falls on a site must be retained for at least 24 hours.

What This Means In Practice

• The portion of the site that is sufficiently pervious may be eliminated from calculations
• The designer must address only the portion that is impervious for calculation purposes
• The formula would be = Aimp * 1” / 12 = CF of water to address

The Primary Design Methods are

• Green roof
• essentially making that roof surface pervious for storm water purposes
• An overflow is provided for a large storm event
• Rain Garden (similar to Bio-Swale)
• Area with crushed rock below grade allowing water to be temporarily held in the voids. 
• An overflow is provided for a large storm event
• Detention Pond
• A depression in the earth that fills with water in a storm even and either drains off slowly over time and/or is absorbed into the earth
• An overflow is provided for a large storm event
• Cistern and Reuse
• A storage tank sufficient to hold the necessary volume.
• The retained water is used by a building for various purposes, often with treatment
• An overflow is provided for a large storm event