Catch basin
A catch basin is a buried box with a grate on top and a pipe leaving the side. Water enters through the grate, sediment sinks into the sump below the pipe, and the cleaner water exits. One rule decides whether it works: the outlet pipe must be lower than the inlet. Miss that, and you have built a pond with a lid.
Anatomy
- Grate — takes surface flow; the first filter, and the first thing leaves defeat.
- Barrel — the box, usually plastic or concrete; carries the connections.
- Sump — the space below the outlet pipe invert, where grit drops out. A real sump is the difference between a catch basin and a pipe with a window.
- Outlet — the pipe that leaves, toward daylight, a dry well, or a storm line where allowed.
The numbers
| Dimension | Value | Note |
|---|---|---|
| Sump depth | ≥ 2 ft below outlet invert | Shallower and the sediment storage is decoration |
| Outlet grade | ≥ 1% | Same rule as every pipe on this desk — grade |
| Grate capacity | matches catchment | One 12-in grate is not a parking-lot drain |
| Cleanout | annual, before wet season | Or more often under trees |
| Placement | ≥ 10 ft from foundation | Common minimum; check your code — the one-hour job |
The buried-pond trap
Every year someone sets a basin in a low spot with no lower place to pipe to, and calls it done. Water arrives and stays. The sump breeds mosquitoes through the grate, and the “drain” becomes a liability with a brand name. A catch basin is a junction, not a destination. If the outlet question has no answer, the basin is the wrong object — the honest fixes are a swale to a legal outfall, or a pump, or re-grading the low spot. There is no passive fix at the bottom of the yard; that is physics, not pessimism.
Basin vs dry well
Both are buried boxes. The basin passes water along; the dry well ends water in the ground. The basin wants an outlet; the well wants a perk rate. Confuse them and you either soak what should have been piped, or pipe what should have been soaked.
Where to go next
- Dry well — the other buried box, and when it fits.
- Diagnose standing water — basin or swale or regrade: the check that picks.
- French drain — what usually feeds the basin.
Sources
- US EPA — Stormwater discharges from construction activities — supports construction-phase inlet and sediment controls.
- Minnesota Stormwater Manual — Sediment control practices: check dams, ditch checks — supports check dams and ditch checks at inlets.
- 40 CFR 122.26 — Storm water discharges — supports the discharge rule inlets are permitted under.