What is a catch basin and how often should it be cleaned?

A catch basin is a part of a storm drain system that collects runoff from paved surfaces like streets and parking lots. You see the grate at the surface. Below it is a box with an outlet pipe set above the bottom.

The design detail that matters is the sump. The sump is the space below the outlet pipe. Water enters through the grate, slows down inside the box and drops the heavy material it is carrying. Coarse sediment, gravel, trash and leaves settle in the sump. The cleaner water then leaves through the outlet pipe. The Stormwater Center describes the sump as a quiet storage area where solids can settle out of the water.

Because of that job, a catch basin does two things at once:

• It lets water off the pavement so the lot does not flood.
• It keeps part of the trash and sediment from moving into the pipes, retention basins and drywells downstream.

A catch basin is not the same as a sanitary sewer cleanout, and it is not connected to the drains inside your building. Chandler's stormwater page explains that the storm system is separate from the sanitary sewer and generally does not treat water before it is discharged. If you want the full picture of how the two systems differ, read do storm drains and sanitary sewers connect.

Every storm and every windy week adds material. Dust, sand, paper, cigarette butts, leaves, oil residue and plastic all wash toward the grate. The sump holds it until it fills.

The Stormwater Center reports that studies have shown that once a sump is more than 60 percent full, its ability to capture new sediment drops sharply. Past that point, fast-moving water can stir up what has already settled and carry it downstream in a single pulse. So a full basin does not only stop working. It can make things worse by releasing months of trapped material at once.

On a private lot, the downstream side matters. The pipe may lead to a retention basin or a drywell, and Phoenix code requires owners to keep those structures working to their design intent. Sediment that escapes a full catch basin ends up there. That is one reason a clean basin upstream protects the equipment downstream. See how to inspect and maintain a commercial drywell for the next step in the chain.

There is no single number that fits every site. The Stormwater Center gives a baseline for planning:

• Inspect every catch basin at least once a year.
• Clean before the sump is about 60 percent full.
• Many programs use a schedule of twice a year, for example before the wet season and after it.
• Places with heavy pollutant loads need more frequent cleaning. For industrial land uses, one study found that going from annual to monthly cleaning captured about six times as much sediment.

That source is a general national guide, not an Arizona rule. Use it as a starting point, then adjust to your lot. A few things push the schedule toward more frequent cleaning:

• Trees or landscaping that drop leaves and seed pods near the grates.
• Bare dirt or gravel next to the pavement.
• Heavy truck traffic, such as a loading dock or a distribution yard.
• A lot where oil, fluids or trash are common.
• A history of flooding at that inlet.

In the Phoenix area, the practical rhythm is a pre-monsoon check and a post-monsoon check. Phoenix's owner guidance for culverts, catch basins and drainage grates says water has to flow through them during a storm, so the structures need to be free of trash and debris. That is a simple visual check you can do before the season starts.

Phoenix City Code 32C-103 also requires people to maintain post-construction stormwater controls, and to keep written records of inspections, cleaning, repair and maintenance for five years. If your lot has catch basins that are part of an approved drainage plan, treat their cleaning as part of that record. Note the location, the date, how full the sump was and what you removed.

### How cleaning is done

The standard method is a vacuum truck, often called a vactor truck. The crew breaks up packed sediment with a high-pressure hose, then vacuums the water, sediment and debris out of the sump. This is different from scooping debris off a grate by hand. Hand clearing helps water get in, but it does nothing for the sump.

A few practical points come from the Stormwater Center's guidance:

• Disposal matters. The material pulled from a sump can be contaminated with oil and metals, especially at commercial and industrial sites. Rules for testing and disposal vary by jurisdiction, so ask the hauler how the spoils will be handled and get that in writing.
• Odor is normal. Sump material is often oxygen-poor. When it is disturbed it can release hydrogen sulfide, which smells like rotten eggs. Schedule cleaning away from customer entrances, outdoor dining and open windows when you can.
• Records help. Log each basin's location, the date, the volume removed and its condition. Over time those records show which basins fill fastest, so you can put your effort where it counts.
• Inserts are not a substitute. Filters and screens placed inside a basin can help with specific pollutants, but they need their own maintenance and do not replace cleaning.

Do not enter a catch basin yourself. The sump gas noted above is one reason, and a confined space can hold bad air. Leave the inside of the box to a trained crew. The grate and the ground around it are the parts an owner or property manager can safely inspect.

Start with a walk of the lot. Find every grate, note which ones are blocked, and check whether any are missing or damaged. Take a photo of each one. Then decide who owns the job.

In Phoenix, the city's definition of the public storm drain system includes catch basins in streets, curbs and rights of way, and the city looks after those. Basins inside your property line are usually private, and the owner is responsible unless a lease says otherwise. Lease language on who pays for drainage varies, so check yours. For a full breakdown, see who is responsible for parking lot drainage.

If a lot floods and the grate is clear, the problem may be further downstream. Look at the outlet, the pipe and the basin or drywell that receives the water. Why parking lots flood after monsoon storms walks through that order of checks, and why a retention basin is not draining covers the end of the line.

Also keep in mind what should never go into a catch basin. Only stormwater belongs there. Washing down a dumpster pad or pouring out mop water can send pollutants straight into the storm system. The rules are covered in can you hose a parking lot into a storm drain.

A catch basin keeps a lot from flooding, but only if the sump has room. Put a yearly inspection on the calendar before monsoon season, clean when the sump nears the 60 percent mark, and keep a simple log. If a basin fills again within weeks, look for the source of the sediment, such as bare ground or a failing edge, instead of cleaning it over and over.