It is almost pointless to talk to someone in July when the temperature is 110 degrees about what happens in Phoenix when it rains. You want to point out how planned communities have drainage plans and retention basins, etc. The situation in the summer may be that no precipitation has been seen for months, so they wonder why you’re even bringing it up. Well, the last couple of day we’ve been drenched and now those plans and preparation are very much needed and appreciated. Meteorologists are even saying that we may see as much rainfall in this 5 day period as we saw all of last year! Yesterday there was even a tornado warning, and many roads have been closed because of flooding. The picture above is not a lake water feature for a subdivision. It may be for the time being, but actually it is a retention basin with grass in the community common area that accumualtes the runoff from storms such as the one we are having that has dumped over 3″ inches of rain in a 24 hour period in some parts of they valley.
Drainage Plans, Dry Wells and Retention Basins
This is not the most flashy or sexy subject, but very few residents know about the planning that goes into a subdivision before the houses go up. Each city has engineering standards for drainage that the developer much follow. I called and spoke to a Town of Gilbert planning engineer to get the details. The standards and guidelines can also be found on line.
How much water needs to be retained? For the Town of Gilbert subdivisions must provide retention for the run-off generated from a fifty year, 24 hour storm which equals three inches. House main level floors must be built higher than the water level that would be produced by a 100 year, 24 hour flood. Many of the retention basins are also used as play areas or grassy parks within the subdivision. However, the maximum allowable dept of a retention area is 2.5 feet as measured from the top of the side walk next to the basin.
How fast must the retained water drain from the basin? A retention area must drain within 36 hours to prevent “ponding”. Dry wells can be used to assist with the draining of the area and must penetrate at least 10′ into highly permeable soil.
Because of the drainage plans that must be in place for a subdivision, it is highly unlikely that homes would require flood insurance. However, it is best to consult a flood plain map.
This planning for the most part goes unnoticed until you have a significant rainfall that shows how important it is to manage run-off from a storm even in Arizona.