MDEQ Nonpoint Source Best Management Practices Manual
The preferred method of material removal from catch basins is using a sewer vacuum, referred to as a vactor truck.
Some references recommend catch basin cleaning when the sediment height in the sump gets up to 0.3 meters (one foot) below the outlet pipe invert. Other references recommend clean-out when the sediment depth reaches anywhere from 30 percent to 50 percent of total sump depth. Too-infrequent cleaning can result in sumps filling up to a point where any sediment entering catch basins simply passes on through the system. Sumps that are too full of material are also more prone, at lower flows, to re-suspension and passage of any previously settled material.
One study (Herrera Environmental Consultants, Inc., 2006) suggests the following clean-out frequencies, based on the land use draining to a given catch basin:
- Industrial: quarterly to semi-annually
- Commercial: semi-annually
- Residential: annually
This study also states that, based on a review of previous monitoring studies, there is a point of diminishing returns. That is, above a certain clean-out frequency, no more water quality benefit is gained, because so little material has accumulated since the previous clean-out.
Therefore, the clean-out frequencies cited above should only be used as a guideline or a starting point. Rather than relying completely on arbitrary, fixed maintenance schedules, operators should monitor and track their storm sewer systems to determine how quickly catch basins sumps fill up. They should then base their clean-out frequency on that monitoring data, to maximize the amount of residue obtained, and water quality benefit provided.
The combined solid and liquid residue generated from cleaning out catch basins is legally defined as "liquid industrial waste" in Part 121, Liquid Industrial Wastes, of the Natural Resources and Environmental Protection Act, 1994 PA 451, as amended, (NREPA)and is subject to all the requirements of that legislation. Refer to MDEQ (2007) guidance for proper handling, transportation, and disposal of material generated by cleaning catch basins.
A very high failure rate occurs with infiltration trenches if they are not maintained. The most critical maintenance item for this BMP is the periodic removal of accumulated sediment. If sediment is allowed to accumulate, the storage volume of the trench for wastewater will become reduced as the space between rocks is filled with sediment. Surface soils can become clogged and the trench will cease to operate as designed. Normally, total rehabilitation of the trench will be needed if it becomes clogged.
Total rehabilitation can be avoided if filter fabric is placed 6 to 12 inches below the surface of the trench. If failure occurs, only the portion of the trench above the filter fabric will require replacement. This is most useful where systems to remove course sediment have a high probability of failure.
The observation well should be checked several times within the first few months of operation to be sure the trench is operating correctly. The well should be checked annually thereafter to determine when rehabilitation is needed.
Where Catch Basins or Oil/Grit Separators are used, the sediment and oil accumulated within them must be periodically removed. Follow the maintenance schedule in the BMP. Debris which can clog the inlets or outlets must also be removed.
Other maintenance items include mowing Buffer/Filter Strips. Follow specifications in the Buffer/Filter Strip BMP.
Eroding or barren areas must be revegetated as soon as possible.
Wet Detention Basin
Regular maintenance includes mowing the buffer/filter strip and removing debris from the basin. The side banks must be mowed regularly to prevent woody plant growth. If maintained as a lawn, mowing is much more frequent. If maintained as a meadow, mowing can be reduced to twice a year.
The basin should be inspected regularly during wet weather. Particular attention should be given to the inlet and outlet structures.
If properly designed, sediment removal from the basin will only be necessary every five to ten years. Excessive algae must be removed to prevent odors and to maintain nutrient removal capacity.
Any eroded banks must be stabilized as soon as possible.
Extended Detention Basin
Regular maintenance includes mowing the riparian buffer, and removing debris from the basin. Follow mowing specifications in the Riparian Buffer BMP. A properly-designed extended detention basin will require sediment removal only every five to ten years. Provide convenient access for sediment removal. Considering including a benchmark, such as a staff gage or permanent physical feature, to help determine the depth of accumulated sediment.
Inspect the basin regularly during wet weather, paying particular attention to the outlet structure and low-flow channel, and any seepage through berms. Inspect berms annually for structural integrity.
Porous Asphalt Pavement
All porous pavement should be inspected several times in the first few months after construction, and at least annually thereafter. Inspections should be conducted after large storms to check for surface ponding that might indicate local or widespread clogging. If severe clogging occurs, the entire structure may have to be replaced.
The porous pavement surface should be vacuum swept at least four times per year, followed by high pressure jet hosing to keep the asphalt pores open.
Spot clogging of the porous pavement layer can be relieved by drilling half-inch holes through the porous asphalt layer every few feet. In cases where clogging occurs in a low spot in the pavement, it may be advisable to install a drop inlet to route water into the stone reservoir.
Potholes and cracks can be repaired using conventional, non-porous patching mixes as long as the cumulative area repaired does not exceed 10% of the parking lot area.
Stormwater Conveyance Channel
At a minimum, check all constructed channels after each storm which meets or exceeds the design storm. On riprap-lined waterways, check for scouring below the riprap layer, and be sure the stones have not been dislodged by the flow.
Particular attention should be paid to the outlet of the channel. If erosion is occurring at the outlet, appropriate energy dissipation measures should be taken.
Sediment should be removed from riprap-lined channels if it reduces the capacity of the channel.