Learn what a catch basin sump captures, what passes through it, how deep it should be, and how often to clean it. Includes how to measure an existing basin and when a trap is needed.
A catch basin sump is the part of the structure below the outlet pipe, where coarse sediment settles and, with a hooded outlet, trash and floating oil collect on a permanent pool. A sump cannot effectively remove fine particles or dissolved pollutants such as metals, nutrients and dissolved hydrocarbons.
The parts of a catch basin
A typical catch basin has an inlet grate or curb opening, a chamber, a sump, the outlet pipe, and often a hood or baffle. The grate keeps large objects out while letting water and sediment in. The chamber carries flow down to the pipes. Water sits in the sump between storms, so debris and suspended material settle out before stormwater exits (Michigan DOT construction wiki).
A hood is an inverted pipe fitted to the outlet. Some sumped inlet chambers also carry an internal baffle designed to trap debris and floating oil and grease (EPA technology verification report).
To tell what you have, look for a floor that sits below the outlet invert. A structure whose floor is flush with the pipe sends solids straight downstream.
How does the sump trap sediment and floatables?
Velocity drops in the permanent pool, so coarse sediment sinks to the bottom. A hooded or baffled outlet draws water from below the surface, which keeps trash and oil on top of the pool.
The aim is to capture coarse sediments, debris and oils in a confined location where a crew can collect and remove them (Iowa Stormwater Management Manual). That confinement gives crews one cleanout point and keeps deposits from spreading along the pipe.
If your sump has an open outlet with no hood, floatables can ride out with the flow. A hood is usually a worthwhile addition.
What does a sump let through?
A sump handles coarse material, and finer or dissolved pollutants pass through it. The target is primarily coarse sediment, according to Michigan EGLE's catch basin guidance. That means particles of 0.04 inch (1 millimeter) diameter and larger, such as medium or coarse sand and gravel. Anything too fine to settle while water sits in the pool stays in the water column and leaves through the outlet.
Coarse sediment, trash and floating oil are sump jobs. Fine particles, dissolved metals, phosphorus and dissolved hydrocarbons need media in the flow path or treatment downstream. A catch basin acts as pretreatment for other practices by capturing large sediments (Mississippi DEQ's stormwater manual). If your exceedance is dissolved zinc or copper, the sump is not treating it.
How deep should the sump be?
Two depth figures apply to a catch basin, and they measure different things. Under Connecticut's design guidance for deep sump catch basins, the bottom of the outlet pipe should be at least 4 feet below the inlet grate. The basin should also be at least 4 feet in diameter. Those figures size the structure down to the outlet.
Sump depth runs from the basin floor to the outlet invert. Most catch basin designs allow at least a foot of that liquid depth, according to a plumbing training text on sumps and catch basins. Local standards vary, so confirm the figure that applies to your system.
To compare an existing basin, measure from the grate down to the outlet invert, then from the invert to the sump floor. A first reading under 4 feet puts the outlet shallower than the deep sump figure. A second reading under about 1 foot falls short of the usual liquid depth. That sump has less storage and needs cleaning sooner.
Sumps and traps on combined sewers
Where a catch basin connects to a combined sewer, expect to need a trap as well as a sump. Cuyahoga County's standard drawing for a catch basin with sump and trap requires sumps and traps for combination storm/sanitary sewer systems.
The trap is a water seal on the outlet, and it keeps sewer gas from venting through the grate. Check which system each basin discharges to before you specify a replacement or retrofit. The local standard drawing governs what the structure must include.
How often should a sump be cleaned, and how is it done?
Clean a sump as often as it takes to keep settled sediment below the outlet invert. Set a trigger for each basin: a sediment depth, measured up from the sump floor, that still leaves clear water below the invert. That ties the schedule to a measurement you take at each inspection.
Basins that hit the trigger between routine inspections need a shorter interval. Basins that stay well under it can keep the current one.
A cleaning visit starts with a measurement and ends with disposal. Crews lower a rod or tape to the sediment surface and compare the deposit depth with the trigger. If the basin is due, they remove sediment, debris and floating oil, typically with a vacuum truck. They haul the material to an approved disposal site.
Before leaving, they check the hood, the outlet and the grate for damage. A basin fitted with a filter insert needs the insert serviced on the same visit. This catch basin insert maintenance checklist for after heavy rain covers those steps.
Problems from a neglected sump
A full sump loses settling depth, and the basin stops doing its job. During large storms, flow can resuspend trapped sediment and carry floatables downstream. Without frequent maintenance, the basin can become a source of pollutants. Cleaning also protects capacity, because a working sump keeps solids from clogging the storm sewer and contributing to flooding.
Between storms, the standing pool collects leaves and other organic debris. As that material decays it can produce odors. Catch basins hold standing water by design, and debris that builds up in the grate or the storage area allows mosquito breeding (EPA guidance on stormwater structures and mosquitoes). For that reason, catch basins need cleaning throughout the year.
Next step for your basins
We make Smart Sponge® media and Ultra Urban® Filter catch basin inserts. The standard hydrophobic media targets oil, grease and hydrocarbons. This guide explains how a catch basin insert filter and Smart Sponge® media work together. Results depend on your site's water, so the next step is a site assessment for the basins you are concerned about. You can start one through the site assessment form.