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Maintenance Inspections
How to Maintain Drainage Structures
Maintaining your drainage structures keeps them working so they don’t back up and flood your property or release pollutants into the stormwater system and reach creeks, lakes and Puget Sound.
Drainage Structures Found in Most Systems
Storm grates allow stormwater runoff to enter the underground drainage system. Grates are located over a catch basin (an area for collecting sediment) or over an inlet structure that leads directly to a storm drainage pipe.
Catch basins are underground boxes that receive stormwater through a grate at ground level and/or through an inlet pipe. Water discharges through an outlet pipe, which is usually at least three feet above the bottom of the catch basin. If the water isn’t too turbulent, heavy pollutants and sediment settle to the bottom and collect there.
Outlet traps prevent automotive fluids and floating debris from leaving the catch basin and entering creeks and Puget Sound. They are inexpensive and make it much easier and cheaper to clean up motor oil or other floatable pollutants that spill into your catch basin.
Flow control structures store water in a large underground pipe and release it slowly into the municipal stormwater system to keep from overwhelming the City’s system during a storm. They also help to store sediment and keep it from entering streams.
Check Your Drainage Structures Regularly
- • To see if your system is functioning properly, inspect it at least every year - every season is recommended. Look for:
- • Oil sheen, stains, dirt or other signs of pollutants around the storm grate.
- • Poor drainage (indicating a plugged outlet or line).
- • Sagging asphalt around catch basins.
- • Excessive sediment in the catch basin. Insert a yard stick or other probe to see how much sediment has collected. As a general rule, clean it when there’s less than 18 inches between the top of the accumulated sediment and the outlet pipe (where the water level usually is). Some catch basins fill up in three months. Others take years.
- • Damaged or missing outlet traps.
- • Flow control structures that are completely full during the dry season.
Perform Maintenance as Needed
- • Remove leaves and trash from storm grates to prevent clogging.
- • Eliminate sources of pollutants if you notice an oil sheen, stains, or dirt around the grate.
- • Stencil the message “Dump No Waste, Drains to Lake” next to your grates. Call SPU’s Carlton Stinson (206) 684-7624, to borrow the materials to do this.
- • Sweep your lot regularly to reduce the need for catch basin cleaning.
- • Repair or replace damaged outlet traps and reattach after cleaning the catch basin.
- • Clean the catch basin. You can hire a contractor (see list of contractors below) or clean it yourself by lifting the lid and using a bucket and a shovel.
1. Take care in lifting the lid to the catch basin, as it may be heavy.
2. Use a bucket to scoop out excess water.
3. Shovel out the sediment and place it in a pile next to the catch basin until the water drains out.
4. When dry, put the sediment in plastic bags (up to 20 pounds each).
5. Dispose of the sediment in a dumpster, unless it has a significant amount of oil or other contaminants. In that case, the sediment should be disposed of at an appropriate site. Call the King County Waste Characterization Line at (206) 296-4633 for more information.
6. Don’t leave an open catch basin unattended; someone could trip or fall in.
7. Never enter a catch basin, flow control or other drainage structure unless you are properly trained. Confined space safety requirements may apply. For information, call the Department of Labor and Industries, (206) 281-5470, and ask to speak with a safety consultant.
It’s a good idea to keep inspection and maintenance records to help you plan future maintenance.
Related Links
Questions and Answers About Maintenance Inspections - For detail on what to expect during an inspection.
Using a Contractor for Maintenance
List of Drainage System Contractors (PDF)
Pollution Control Inspections - To help businesses keep pollutants out of their stormwater runoff.
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