Maricopa County Zoning 1205: Retention Basins and the 36-Hour Rule

The ordinance sets the size first: "The retention system shall be designed to receive and retain the volume generated from the 2-hour, 100-year runoff event falling over the entire development site including all rights-of-way, excluding off-site flows." That's the whole parcel plus the adjacent street right-of-way, not just the roof and parking lot.

Then the clock: "On-site runoff that has been retained shall be disposed of within 36 hours either by percolation, drywells or draining into an approved drainage way." The county's Drainage Policies and Standards explain why: "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." County Environmental Services says mosquitoes breed in standing water that lasts "at least 72 hours or 3 days," so a basin that drains on time never gets there.

The parts, in order:

• Volume. The 100-year, 2-hour storm over the entire site. The county's plan review form adds "one (1) foot of freeboard."
• Drywells don't count as storage. "Drywell volume shall not be used as part of the retention volume."
• Drain-down. 36 hours, by soaking in, drywells or an approved drainage way.
• Parking lots. No more than 25 percent of public parking can hold water, no deeper than 12 inches, and "Private parking areas shall not be used for runoff retention."
• Drywells. Stormwater only, and "Proof of drywell registration with the Arizona State Department of Environmental Quality shall be required prior to final as-built approval or issuance of any drainage clearance."
• Setbacks. Basins stay 25 feet from septic systems. Drywells sit at least 10 feet above saturated soil and 100 feet from any water supply well.
• Paperwork. Commercial projects need a drainage report and plan sealed by an Arizona registered civil engineer, and a final drainage clearance before the final Certificate of Occupancy.

• 100-year, 2-hour storm. A two-hour storm with a 1 percent chance of happening in any year. The engineer turns it into a volume with the manual's formula, V = C × (P/12) × A: runoff coefficient times rainfall depth times area.
• Retention versus detention. Retention holds the water on site until it soaks in or drains away. It's not released to the street during the storm.
• Drywell. A shaft that drains the basin into deeper soil. ADEQ now treats it as a Class V injection well.
• Constant-head percolation test. The field test run on each finished drywell to set its real drain rate.
• De-rating factor. The safety divisor applied to that test: 2 for sand and gravel, 3 for silts and loams, 5 for clay. The result can't be under 0.1 cubic feet per second per well.
• Freeboard. Extra depth above the design water level, one foot here.

Commercial, industrial, multi-family and institutional projects in unincorporated Maricopa County, such as parts of New River, Rio Verde, Sun City West and county islands. The Planning and Development Department's Drainage Administrator enforces it.

This does not apply inside a city or town. The ordinance says it "shall not pertain to or otherwise regulate cities, towns or other incorporated municipalities." A site in Phoenix, Mesa or Surprise follows that city's drainage rules instead.

A contractor yard near New River. The engineer sizes a basin for the whole parcel plus half the road right-of-way, adds a foot of freeboard, and uses two drywells. Each drywell gets a constant-head test before the as-built, and the owner uploads proof of ADEQ registration.

A church in a county island. The design calls for ponding in the staff lot behind the building. Review kicks it back: private parking can't hold retention, so the basin moves to landscaping.

The same church gets annexed. Here the answer flips. Once the parcel is inside a city, Section 1205 no longer applies and the city's drainage manual controls any new work.

A basin that won't drain. Five years after opening, water sits for three days after each monsoon. The manual puts the fix on the owner: "Drywells that cease to drain a retention basin with 36-hours shall be replaced or refurbished by the owner or his representative."

1. 1Confirm the parcel is unincorporated on the county's parcel viewer or with Planning and Development before using these rules.
2. 2Hire an Arizona registered civil engineer to prepare the sealed drainage report and plan.
3. 3Test each drywell with a constant-head percolation test after it's built, and each basin with the ASTM D3385 double-ring test (two tests minimum per basin).
4. 4Inventory the drywells with ADEQ in myDEQ and keep the proof for the as-built.
5. 5Submit the certified as-built stating the system "will drain within 36 hours," then request the final drainage clearance before the Certificate of Occupancy.
6. 6Put drywell cleaning on a schedule no longer than every three years, and keep the annual maintenance certification the landscaping and maintenance agreement calls for.

• "County rules apply everywhere in the county." Only unincorporated land. Cities write their own.
• "The drywell adds storage." Its volume doesn't count toward retention.
• "Any parking lot can double as a basin." Only public parking, only 25 percent of it, and only 12 inches deep.
• "Maintenance is the city's or county's job." The standards say basins "are to be privately maintained," and drywell cleaning is the owner's responsibility.
• "The 36-hour rule is just paperwork." It's the county's mosquito control line.

No as-built approval and no final drainage clearance means no final Certificate of Occupancy for a commercial building. After that, Chapter 15 of the zoning ordinance makes a violation a class 2 misdemeanor, or a civil penalty capped at the class 2 misdemeanor fine.

Retention and drywells are civil engineering work. When the water backing up is inside the building, from floor drains, restroom lines or the sewer, a licensed plumber is the right call.