Catch Basin Maintenance: Solving Standing Water and Mosquitoes

Managing Water in Catch Basins

Catch basins (storm drain inlets) are designed with a sump—a basin below the outlet pipe—to capture sediment. Standing water in this sump is normal but breeds mosquitoes and causes odors.

Why the Sump Exists

The 3-inch gap between basin floor and outlet pipe allows sediment to settle, preventing pipe clogs. Removing this sump or adding a bottom drain would defeat its purpose and cause downstream blockages.

Effective Mosquito & Odor Control

• Mosquito dunks (Bti): Biological larvicide lasting 30+ days; safe for pets/wildlife.
• Bleach: 1 cup per basin weekly; less persistent than dunks.
• Regular flushing: Hose out basins every 2-3 weeks to disrupt larval cycle.

What NOT to Do

• Don't install filter fabric inserts—they clog rapidly with sediment.
• Don't drill bottom drains—they bypass sediment capture.
• Don't use oil or pesticides—they contaminate waterways.

Design Verification

Your system appears correctly designed. The standing water is a maintenance issue, not a design flaw. Implement a seasonal dunk/flush schedule for a mosquito-free, odorless system.