Retention Basin and Drywell Rules by Valley City

Maricopa County's Drainage Policies and Standards set the pattern most cities follow. Basins must "completely drain within 36-hours after the runoff event has ended," and the reason is spelled out: "The purpose for the 36-hour drain time is vector control and to allow for the probability of a second severe storm following the previous storm." Mosquitoes breed in water that stands "at least 72 hours or 3 days," according to county Environmental Services, so a basin that empties in 36 hours never gets there.

The county also sets the order of preference: "The preferred method for draining retention basins is by infiltration/percolation. The next preferred method is the use of dry wells, or a combination of infiltration and dry wells." Each city then writes its own version into its engineering standards or city code:

• Design storm. Most use the 100-year, 2-hour storm. Gilbert uses the 50-year, 24-hour storm (3 inches). Tempe's own method uses the 100-year, 1-hour storm.
• Drain time. 36 hours everywhere on this page. Scottsdale targets 12 to 24 hours, and Mesa wants bleed-off lines to take at least 24.
• Depth. Usually 3 feet. Gilbert allows only 2 feet on arterial frontage; Glendale caps depth at 1.5 feet within 10 feet of the right-of-way.
• Drywells. From "least preferred" (Mesa) to required in every basin (Avondale).
• Upkeep. Always the owner's. Inspection schedules range from none written to twice a year.

• Retention basin. A low area that holds runoff on the property until it soaks in or drains out through a drywell or bleed-off pipe.
• Detention. Holding water briefly and releasing it at a controlled rate. Scottsdale prefers detention; most others require retention.
• Drywell. A shaft that drains the basin into deeper soil. Since October 15, 2025, ADEQ regulates it as a Class V injection well.
• Dual-chamber drywell. A drywell with a settling chamber that catches silt and oil before the water drains down. Mesa, Tempe, Surprise and Avondale commercial sites require them.
• Design rate. The drain rate the engineer may count per drywell, usually 0.1 cubic feet per second unless a field test supports more.
• Freeboard. Extra depth above the design water line.

Commercial, industrial and multi-family sites, whoever owns them. Owners, property managers and associations inherit the upkeep; developers and engineers handle the design. Every city here puts maintenance on the property owner or association.

These rules don't cover the plumbing inside the building. Roof drains and interior storm piping follow the city's plumbing code, not its drainage manual.

A national tenant builds the same pad in Chandler and Gilbert. In Chandler the civil engineer sizes the basin for the 100-year, 2-hour storm plus 10 percent. In Gilbert the same building's basin is sized for the 50-year, 24-hour storm. Same footprint, different volume.

A Tempe owner with drywells and underground storage. The site gets a recorded "Declaration for Inspection and Maintenance of Drainage Structures." The owner commits to inspecting "no less often than annually" and keeps the records on site.

An Avondale site where the soil drains well. Here the answer flips from most cities. Even with good percolation, Avondale says "Drywells must be installed in all retention basins."

A Chandler basin that holds water for two days. City code says "It shall be unlawful for any person owning or controlling a stormwater retention system or facility to permit water to stand therein longer than thirty-six (36) hours."

1. 1Find the city that has jurisdiction over the parcel. County islands follow Maricopa County.
2. 2Pull that city's engineering or drainage standards from the table above and give them to your civil engineer.
3. 3Check the approved grading and drainage plan for the basin volume, drywell count and any recorded maintenance declaration.
4. 4Inventory every drywell with ADEQ in myDEQ. Chandler wants registration in the owner's name; Peoria and Glendale want proof before acceptance.
5. 5Set an inspection schedule at least as strict as your city's: yearly in Chandler and Tempe, after every 0.6 inch storm plus twice a year for underground storage in Scottsdale and Peoria.
6. 6Replace or rebuild drywells that stop emptying the basin within 36 hours. Every city here puts that on the owner.

• "It's 100-year, 2-hour everywhere." Not in Gilbert or under Tempe's own method.
• "Drywells are the standard fix." Several cities treat them as a last resort, and Scottsdale calls them "less acceptable."
• "Old drywell registration still counts." The state drywell statute was repealed in 2022. Drywells now go into ADEQ's Class V inventory.
• "Parking lots can hold as much as the basin." Depth caps run from six inches (Phoenix, Avondale) to one foot (Gilbert).
• "The city maintains private basins." None of these cities do. Phoenix even requires a stamped "No City Maintenance" sign.

In Chandler, standing water over 36 hours is unlawful under City Code 45-4, and failing to maintain the system is a violation of Chapter 45. In Phoenix, Chapter 32C allows civil penalties (sanctions) of up to $2,500 per violation per day. In Maricopa County, drainage clearance, and with it the final Certificate of Occupancy, waits on the as-built and drywell proof.

Basin and drywell design is civil engineering. When the backup is inside the building, from floor drains, restroom lines or the sewer, that's plumbing, and a licensed plumber can camera and clear it.