Catch Basins, Trench Drains and Parking Lot Lines: A Commercial Property Checklist Before El Niño
A one-acre parking lot takes about 27,000 gallons of water for every inch of rain that falls on it. Almost none of it soaks in. It runs across the asphalt to a few catch basins and trench drains, drops into a buried line, and heads for the street storm drain. When one of those inlets is plugged, that water has only one place to go, and on a lot of Valley strip centers that place is the front door of a store.
NOAA's winter outlook, released October 15, calls this year's El Niño very strong and favors wetter than normal conditions across Southern California from December through February. For anyone who manages a retail center on Roscoe Boulevard, an industrial building off Nordhoff Street in Chatsworth, an apartment complex in Panorama City or a restaurant pad on Ventura Boulevard, November is the month to get the drainage cleaned and filmed.
What is actually in a commercial drain system
- Catch basins. Precast concrete boxes with cast iron grates in the low spots of the lot. Most have a sump below the outlet pipe that is meant to trap sand and trash. Once the sump is full, everything goes down the pipe.
- Trench drains. Long grated channels across driveway entries, at loading docks and at the bottom of garage ramps. Their narrow channels fill with grit and leaves fast.
- Storm drain lines. The buried pipes connecting the inlets to each other and to the public storm drain at the street. On older Valley properties these are often 8 to 18-inch reinforced concrete pipe or corrugated metal pipe, and the corrugated metal rusts out along the invert, the bottom of the pipe where the water runs. Newer sites use PVC or HDPE.
- Roof drains. On a flat-roofed commercial building, interior roof drains run down through the building and out to the site storm system. A clog anywhere along that run can back water up onto the roof.
- Stormwater treatment devices. Many sites built or remodeled in the last 20 years have filter inserts, separators or infiltration systems required by their stormwater permits. They have their own cleaning schedules and get overwhelmed in a wet year.
Where it goes wrong in a big storm
After a long dry stretch the Valley gets its rain in a few heavy storms, and the first one carries a season's worth of dust, tire grit, landscape mulch and trash into the inlets. A sump that was half full is now full. The line below it gets a slug of sediment and settles out at the first low spot or bend. By the second storm the basin is holding water at the grate.
Trees make it worse. Ficus, eucalyptus and pine planted in parking lot islands decades ago send roots into the joints of the storm drain lines under them, the same way street trees get into residential sewers. A root mass catches leaves and silt until the line is closed.
Restaurants add grease. If kitchen mats, hood filters or equipment get washed out on the back lot, the grease goes into the nearest storm inlet. That is illegal in Los Angeles, since the storm drain system goes to the river untreated, and it also hardens in the line like it does in a sewer. Mats and filters should be cleaned where the water drains to the sanitary sewer through the grease interceptor. Our post on grease line maintenance schedules covers the kitchen side.
A pre-storm checklist for property managers
- Clean every catch basin sump and trench drain channel. Get the sediment out of the structure so it cannot go down the line.
- Jet the lines between structures. A trailer jetter with a 3/8-inch hose and a sewer nozzle scours the full diameter of the pipe and pulls the sediment back to a basin where it can be removed. Our jetting and line cleaning page describes how we work structure to structure.
- Run a camera behind the jetter. This confirms the line is clear and shows cracks, root entry, offset joints and rusted-out pipe bottoms before they become a sinkhole in the lot.
- Check the roof drains. Clear the strainers on the roof and make sure the leaders inside the building run free.
- Look at the sanitary sewer too. Rain that gets into a cracked commercial sewer lateral can back up restrooms and kitchen drains during a storm. The same crew and camera can check both systems on one visit, going in at the building cleanout or the nearest sewer manhole on the property.
- Write down what was done. Keep a dated record of what was cleaned and filmed. If your property has a stormwater maintenance plan, inspectors will ask for it.
Working on an occupied site
Commercial drain cleaning rarely needs the whole lot closed. We block off a few stalls at a time, clean and jet one run, film it, and move to the next. On retail centers we can start early so the work is done before stores open. For HOAs and apartment buildings, we coordinate around garage access and resident parking.
BBC Rooter & Plumbing handles the buried pipe on commercial and multi-family properties across the San Fernando Valley, both the storm drain lines and the sanitary sewer. More about that work is on our commercial sewer contractor page. For homeowners, start with El Niño 2026-27 and your sewer line. See also our FAQ.
Call BBC Rooter & Plumbing at 818-280-9135, Northridge, CA. CSLB #720343. Open 7 days, 6am to 6pm.
Frequently asked questions
How often should a parking lot catch basin be cleaned?
At least once a year, before the rainy season, and again after any heavy storm on a lot with landscaping, trees or nearby slopes. Many Los Angeles area properties also have stormwater permits or maintenance plans that set their own schedule, so check those first.
Can a restaurant's grease get into the storm drain?
It can, and it is a problem when it does. Grease that is washed off a back patio, dumpster pad or kitchen mat area into a parking lot inlet goes to the storm drain system, which runs to the river and the ocean without treatment. Los Angeles bans putting sewage or other waste into the storm drain system. Mats and equipment should be washed where the water goes to the sanitary sewer through the grease interceptor.
What is the difference between a catch basin and a trench drain?
A catch basin is a box with a grate, usually in a low spot of the parking lot, with a sump below the outlet pipe that lets sand and trash settle. A trench drain is a long narrow channel with a grate, often across a driveway entry or a loading dock, that feeds a pipe at one end. Both fill with sediment and both need cleaning.
Do you need to shut down a parking lot to clean storm drains?
Usually only a few stalls at a time. Basins are vacuumed or cleaned out, lines are jetted from one structure to the next, and the camera goes in behind the jetter to confirm the line is clear. On retail sites we can schedule early morning work before stores open.
Is storm drain work on a commercial lot something a sewer contractor handles?
Yes. The buried storm drain lines on a commercial property use the same cameras, jetters and excavation methods as sewer lines. We clean, inspect and repair those lines and the sanitary sewer on the same site. Repaving and regrading the lot surface is a paving contractor's job.