The usual causes are silt and debris blocking inlets or drywell chambers, a drywell that has lost capacity, or soil that no longer absorbs water. Check the inlets and drywell covers first. If water still stands after 36 hours, a civil engineer needs to test the drainage rate and design the fix.
Start with what you can see
Blocked inlets and grates. Maricopa County says drainage facilities that are not maintained can back up in heavy storms, and tells owners to remove trash, overgrown vegetation and organic material like leaves, silt and dirt that restrict flow. Phoenix says the same about culverts, catch basins and grates: water has to flow through them during a storm, so keep them free of trash and debris.
Silt in the basin. Phoenix's maintenance hints for retention basins are to look for erosion on the slopes and to verify that the basin has not filled with silt or other material that stops it from retaining water. A basin partly filled with silt holds less and drains slower.
Then look at how the basin actually drains
Retention basins usually drain two ways: water soaks into the basin floor, or drywells carry it down.
- Floor infiltration. EPA's infiltration basin fact sheet says sedimentation can clog an infiltration basin and lead to its failure, and that a clogged basin that takes more than 3 days to drain can become a source for mosquitoes. Compacted or poorly infiltrating soil is the reason such basins are not suited to every site.
- Drywells. Sediment builds in the settling chamber, and the well itself can lose capacity. ADEQ's guidance says to inspect if water stands longer than 36 hours and to clean out the chamber at 10 percent full in paved areas. Read can a plugged drywell be cleaned or must it be replaced.
### A quick way to sort the cause
| What you see | Likely cause to check first |
|---|---|
| Water stops short of the inlet grates and trash is piled on them | Blocked inlets, grates or pipes |
| The basin floor is crusted with fine silt or a layer of mud | Sediment clogging the basin floor |
| The basin has drywells and the water drops slowly or not at all | Drywell chamber full of sediment, or a drywell that has lost capacity |
| The basin is wet even when it has not rained | A leak, an irrigation problem or another non-storm source |
| Nothing looks wrong but drainage is slower each year | Soil and design capacity: needs testing by an engineer |
These are starting points from the maintenance guidance, not a diagnosis. The engineer's test is what settles it.
### What EPA says keeps infiltration basins working
EPA's infiltration basin fact sheet, based on Minnesota's stormwater manual, lists routine work that keeps a basin from clogging: keep inlets clear of debris including sediment and oil or grease, repair eroded areas at inflow and outflow structures, inspect pretreatment devices and diversion structures for debris and integrity, scrape accumulated sediment from pretreatment areas and re-seed, and replace pea gravel or topsoil when it is clogged. For a basin that has failed, the fact sheet lists total rehabilitation and excavating the basin bottom to expose clean soil.
It also says vegetation matters. Thick turf and plant cover on the upland area reduce a basin's tendency to clog, and construction crews should stabilize upland soils so a new basin does not fill with sediment. That is why a basin next to bare dirt or an active construction site can clog fast.
### Can I fix it myself?
Some of it, yes:
- Clearing trash and leaves off grates and out of inlets is a job for grounds crews. Maricopa County's guidance names trash, overgrown vegetation and organic material as things to remove.
- Trimming overgrown plants that block flow is the same kind of task.
- Removing sediment from the basin, especially if it is thick, is better handled by a contractor with the right equipment and a place to dispose of the material.
What you should not do is dig into the basin floor, add rock or change the slopes without an engineer. Maricopa County says all drainage control structures, including retention, must be in platted tracts or easements, and the design was approved to hold a specific storm. Changing the shape can change what it holds.
When it is an engineering problem
If the inlets are clear, the silt is out and the drywell chambers are clean, and the water still lingers, the drainage rate itself has fallen. Maricopa County's plan review requires percolation testing and an engineer's certification that basins drain within 36 hours. Redoing that work is the same kind of job: testing, then a design for regrading, new drywells, or a rebuilt basin floor. That is civil engineering and site work, not plumbing repair.
Also ask a basic question: was anything other than stormwater going in? Irrigation runoff, wash water or leaks can keep a basin wet. If the water is wet all the time and no storm is involved, look for a source. A broken irrigation valve or a leaking water line can act like a permanent rainstorm. Finding a hidden leak explains how leaks get tracked down.
### What the engineer will look at
Maricopa County's drainage documents show the kind of evidence an engineer gathers to prove a basin drains. Expect some version of:
- Soil infiltration testing for the basin floor, and boring logs.
- A constant head percolation test for each drywell that drains the basin.
- Drywell percolation rates de-rated under the county's standard, so the numbers include a margin.
- Revised drain-down calculations showing whether the basin can clear within 36 hours.
- Proof of drywell registration if drywells are part of the system. See do drywells still need to be registered in Arizona.
In Phoenix, City Code 32C-103 also requires owners to keep written records of inspections, cleaning, repair and maintenance of basins and drywells for at least five years, so the engineer's results belong in that file. Ask for the results in writing and keep them with your drainage records, because the next owner or property manager will want them. If a fix is needed, the engineer's design can tell you whether it is cleaning, new drywells, a rebuilt basin floor or a change to how water gets in.
Keeping it from happening again
Phoenix's guidance for owners covers each part of the system, and it works as a maintenance calendar:
- 1Before monsoon season: clear culverts, catch basins and grates, check basin slopes for erosion and silt, and inspect drywells.
- 2After big storms: check that dry wells drain, with standing water gone by 36 hours.
- 3Through the year: remove debris, silt and plant material from drainage channels.
- 4Every year: repeat the drywell inspection ADEQ's guidance calls for, and keep the records.
Maricopa County's mosquito guidance says the same habits that prevent flooding also keep drainage facilities from becoming mosquito habitat. The easiest basin fix is the one done before the storm.
### Who to call, in order
The order below goes from least to most involved, so you do not pay for engineering when a clogged grate is the whole story.
- 1Whoever maintains the grounds, to clear inlets and grates.
- 2A drywell service company, if drywells are part of the system.
- 3A civil engineer, to test and redesign if the basin still will not drain.
What it means for you
Document how long the water stays after each storm, and act once it passes 36 hours. See how long water can stand in a retention basin for the rule, and how to inspect and maintain a commercial drywell for the yearly routine.