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.
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.
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.
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.
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.
Storage facilities are often used in urban drainage systems to temporarily store or detain excess stormwater runoff and then release it at a regulated rate to downstream areas. Using this approach, runoff is stored in constructed or natural basins from which it is released continually until the water elevation in the facility reaches its design dry-weather stage. To function properly, the storage volume must be maintained at its design level and outlet facilities must be kept open and free from obstructions or clogging.
An important step in the design process is identifying whether special provision specifications are warranted to properly construct or maintain proposed storage facilities. To minimize long-term operational costs, storage facilities that require extensive maintenance are discouraged. The following maintenance problems are typical of urban detention facilities, and facilities shall be designed to minimize these problems:
Proper design should focus on the elimination or reduction of maintenance requirements by addressing the potential for problems to develop.
A good maintenance program should include preventive maintenance as well as corrective maintenance. The maintenance program should include:
The major prerequisite for a preventive and corrective type of program is an assured source of funds. The other component of a maintenance program, emergency repairs, is characterized by crisis-type responses to problems that go unrecognized, or are unattended, over a lengthy period of time. Often, such problems are treated as emergencies and, in some instances, little time is available for study, design, competitive bidding, etc.
Inspection of major detention facilities should be made as frequently as experience shows necessary, perhaps monthly as a minimum and more often in wet seasons. Where debris is a problem, inspections must be spaced according to debris generation. In any event, it is important to conduct inspections and cleanup work following major individual runoff events. It is sometimes necessary to make inspections during rainstorms when intense rainfall occurs.
Besides removing debris blockages during inspections, mechanical equipment such as generators, float valves, pumps, discharge controls, and other electrical and mechanical equipment should be checked and adjusted as necessary.
Inspect detention basins once a year, preferably during wet weather. Inspect after major flood events. Basins should be mowed as required (at least twice a year) and sediment should be removed every 5 to 10 years.
Infiltration facilities should be inspected at least once a year.
Observation of time to drain is an early indicator of plugging and need for maintenance. Maintenance would involve removal of silt material or debris that lead to reduction of infiltration capacity of the basin. Returning basin to design grade is critical to keep design storage volume.
Maintenance tasks can be grouped in three general categories:
Of these, the most important, from the standpoint of health and safety, is operation maintenance.
This category can be characterized as that level of maintenance required to ensure against failure of major structural components and/or flow controls, and to ensure that the facility continues to function as designed. Neglecting this level of maintenance could cause dam failure and subsequent property damage as well as possible loss of life. In addition, neglect often causes a facility to cease functioning as it was originally designed to do. A program of scheduled, periodic inspections of the facility is essential to recognize potential structural maintenance needs. The following is a partial list of items that should be checked periodically and corrective action taken as required:
In addition, the following actions should be taken on each facility, as required.
Also, observations should be made of any changes in topography, downstream drainage systems or land uses that may have a bearing on the operational effectiveness and safety of detention facilities.
Infiltration facilities should be promptly removed of any accumulated sediment. The pervious bed of the infiltration facility should be replaced if no longer functioning.
One of the most important variables in the design of a detention facility is the volume available for storage of runoff. If a detention facility is allowed to accumulate sediment and debris which will decrease the storage volume, the ability of the facility to function as designed can be greatly reduced. Thus it is essential to maintain the design volume. To facilitate the inspection of the facilities for volume control, it is recommended that some marker be installed in the detention facility to indicate the maximum level for silt buildup before the facility must be dredged or cleaned. This marker could be a small pipe with a stripe or suitable indicator at the cleanout level. A suitable indicator could also be place on the outlet device or in some location which can be easily identified during the inspection process.
Department of Great Lakes BMP Manual
Michigan Department of Transportation Stormwater Management