Sunday, August 5, 2012

The City Without Storm Drains

Gilbert, Arizona, USA is a new town that grew from a population of 5,000 in 1980 to 208,000 in 2010.

In 1980, much of Gilbert's residential population lived on flood irrigated lots that rarely if ever shed any storm water onto adjacent streets.  And in the outlying rural areas of town, the streets or roads in front of houses, farms, and businesses had no curbs, gutters, and sidewalks, so their storm water runoff was shed onto their fronting properties as is the typical case in rural areas.  But unlike many rural areas, the Gilbert area terrain was nearly level without exception, lacking any hills, valleys, creeks, and rivers, and it was fully developed as farmland, with no natural watercourses having been preserved as it developed.  The pre-growth condition of Gilbert's eventual limits existed without any regional storm drainage system already.

Also, by the time Gilbert's explosive growth began in 1980, the northerly adjoining City of Mesa had already established a policy that all newly developed areas and projects were required to provide local storm water retention basins large enough to store 100% of the runoff from the biggest storm likely to occur in 2 hours with a 1% chance every year (nicknamed "the 100-year, 2-hour rainfall"), which was about 2 1/2" (625 mm) of rain.  Before Gilbert's growth took off, it went one better than Mesa by requiring new developments to store 100% of the 24-hour, 2% chance (50-year) storm, which was about 3" (750 mm).

Also significantly, the Town of Gilbert made a different decision than the City of Mesa about how to empty storm water retention basins after storms had passed.  Both municipalities decided to allow basins to be emptied no faster than a relative trickle (around 1 cfs, 450 gpm, or 30 l/s), and the main alternatives for drainage were either into a public storm sewer or into a deep groundwater aquifer by way of a recharge dry well. At that time, there was a valid concern that someday the United States Environmental Protection Agency might outlaw dry well flow into underground aquifers and even require expensive reparative construction. But significantly, where the City of Mesa had decided to discourage dry wells, in Gilbert, dry wells ended up being the default means of emptying basins.  By the time 30 years had passed, basic dry well storm water treatment had become standard practice, and USEPA had not yet stepped in to stop the construction of drywells.  My cursory search of the Arizona Department of Environmental Quality dry well database for dry wells in Gilbert revealed 35 pages of registrations with 120 drywells on page 1 and 150 drywells on page 24, probably indicating around 4000 drywells in the Town of Gilbert.

So today the Town of Gilbert stands as an example of a city without storm drains aside from the short storm drains (less than 1/2 mile long) that feed many of the retention basins.  When it rains, there is no storm water flow from one neighborhood or business to another.  All storm water is directed into landscaped areas and parks typically ponding no more than 3 feet deep or occasional underground pipes/tanks, and the storm water all goes promptly into the ground after each storm.  The 11-year groundwater level change at many Gilbert-area monitoring wells in the 1990s was greater than +60 feet (ADWR, page 130, pdf p 30).  And there doesn't seem to be any reason why that condition will change in the future.  Sometimes you lose, and sometimes you win.

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