Why does my parking lot flood after a monsoon storm?

The most common cause is also the cheapest to fix. Trash, leaves, silt and landscape debris pile up on grates and inside catch basins, and the water has nowhere to go. Chandler's stormwater page says trash, leaves and other debris can clog the drain system and cause flooding, and that yard clippings swept into a storm drain can cause flooding, especially during a storm. Phoenix's owner guidance tells owners that water needs to flow through culverts, catch basins and drainage grates during a storm, so they must be kept free of trash and debris.

Look at the grates right after a storm, while water is still standing. Debris pressed flat against a grate, or a pool that stops rising at a low inlet, points to a blockage. Our page on what a catch basin is and how often to clean it covers what to do next.

Most newer commercial lots do not drain to the street. They drain to an on-site retention basin, often with drywells to help empty it. Maricopa County's plan review rules ask sites to retain the volume of the 100-year, 2-hour storm plus one foot of freeboard, and to drain the basin within 36 hours. If the basin never fully emptied after the last storm, it starts the next one partly full.

So ask two questions: was the basin already wet before this storm, and how long did it take to clear afterward? Maricopa County's drainage guidance warns that facilities that are not maintained can back up in heavy storms. See why is my retention basin not draining and how to inspect and maintain a commercial drywell.

Even a perfectly maintained system can be overrun by a big enough storm. EPA's CADDIS resource lists parking lots among common impervious surfaces and notes a large increase in stormwater runoff as imperviousness increases, at the expense of infiltration. It adds that impervious cover generally leads to more frequent, larger and shorter peaks in flow.

A parking lot is also designed for a specific storm, not for every storm. The county's 100-year, 2-hour design storm is a big one, yet a rare, intense cell can still produce more than the design assumed. Short-lived ponding in a low spot during a heavy downpour that clears within hours is different from water that hangs around for a day or more. The first may be the system working as designed. The second is a sign something is wrong.

### Grading and construction changes

Sometimes the cause is not maintenance at all. Repaving, adding a curb island or changing a landscape bed can change where water goes. Retention areas that were filled in, regraded or built over for extra parking also lose the storage the design counted on. Maricopa County requires drainage control structures, including retention, to be in platted tracts or easements, so changes to them are not a casual site decision. If flooding started after a construction project, tell the engineer what changed.

### Reading the clues

How the flooding behaves tells you where to look. These are starting points, not a diagnosis:

• Water piles up at one inlet and nowhere else. Check that inlet and its pipe first.
• Every inlet is clear but the whole lot stays wet for a day or more. Look at the basin and drywells the lot drains to.
• It floods after small storms, not just big ones. Something is restricting flow: a blocked pipe, a full sump or a basin that never drained from the last storm.
• It only floods in the largest storms and clears within hours. The system may be working, and the design storm was simply exceeded.
• The problem started after paving, landscaping or a construction project. Compare the site to the approved plans.

Write down which pattern you see for each storm. A few entries turn a vague complaint into something an engineer can act on.

### Why it matters beyond the puddle

Standing water is more than an inconvenience. Maricopa County's mosquito guidance says the primary breeding habitat for mosquitoes is stagnant or shallow water that lasts at least 72 hours, and that maintained drainage facilities help prevent both flooding and mosquito outbreaks. Phoenix's guidance says standing water at dry wells should be gone within 36 hours, which prevents damage to structures and helps prevent mosquito breeding. Water against a building foundation, in a doorway or across an accessible route also creates its own problems for customers and tenants, so flooding is worth fixing even when no one is complaining yet.

### Before the next monsoon season

Phoenix's owner guidance doubles as a pre-season checklist:

• Culverts, catch basins and grates: clear trash and debris.
• Retention basins: look for erosion on the slopes and for silt or other material that keeps the basin from holding water.
• Dry wells: confirm that stormwater dissipates after a storm, with no standing water after 36 hours.
• Drainage channels: remove debris, silt and plant material that keep them from carrying water.

Do the walk-through before storm season, not during it. A dry lot makes it easier to see low spots, clogged grates and damaged edges. Write down what you find and the date, so the next person who manages the property can see what was checked and what was fixed.

1. 1Photograph the flooding while it is happening, with a fixed landmark in the frame.
2. 2Check and clear inlets and grates when it is safe. Do not open a manhole cover or wade into moving water.
3. 3Clean catch basins and note how full the sumps were.
4. 4Time the basin. If it is still wet after 36 hours, see how long water can stand in a retention basin.
5. 5Get an engineer's assessment if the flooding repeats after the easy fixes. Maricopa County's guidance suggests consulting a professional to assess the drainage system when a community has flooding or maintenance issues.

### Is a trench drain part of the problem?

A trench drain across a driveway or entrance is another inlet that can clog with sand and trash. Lift the grates and look at the channel. See what a trench drain is and where one is required.

### Is it a drain problem inside the building instead?

Water in a parking lot and sewage or water coming up through floor drains inside are different problems. Storm drainage and sanitary sewers are separate systems. If floor drains inside your building back up during monsoon storms, read why drains back up during monsoon and how to prevent a sewer backup before monsoon.

Treat a flooded lot as a symptom with a short list of causes. Clear the inlets, check the basin and drywells, and keep a log of each storm. Property managers who do that before the season starts rarely get surprised. If the lot still floods with clean inlets and a basin that drains in time, the design is being overrun, and the fix belongs with a civil engineer. Owners and tenants can sort out who pays in who is responsible for parking lot drainage.