Excerpt from Chapter 8 Stormwater Storage facilities
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:
- Grass and vegetation maintenance,
- Sedimentation control,
- Bank deterioration,
- Standing water or soggy surfaces,
- Mosquito control,
- Blockage of outlet structures,
- Litter accumulation, and
- Maintenance of fences and perimeter plantings.
Proper design should focus on the elimination or reduction of maintenance requirements by addressing the potential for problems to develop.
- Both weed growth and grass maintenance may be addressed by constructing side slopes that can be maintained using available power-driven equipment, such as tractor mowers.
- Constructing traps to remove sediment may control sedimentation. Sediment traps also reduce erosion and sediment transport in low-flow channels.
- Bank deterioration can be controlled with protective lining or by limiting bank slopes.
- Standing water or soggy surfaces may be eliminated by sloping basin bottoms toward the outlet, constructing low-flow pilot channels across basin bottoms from the inlet to the outlet, or by constructing underdrain facilities to lower water tables.
- In general, when the above problems are addressed, mosquito control will not be a major problem.
- Outlet structures should be selected to minimize the possibility of blockage (i.e., very small pipes tend to block quite easily and should be avoided). Weirs are recommended with either sheet piling or earth embankment weirs.
- Finally, one way to deal with the maintenance associated with litter and damage to fences and perimeter plantings is to locate the facility for easy access where this maintenance can be conducted on a regular basis.
8.5.2 Maintenance Program
A good maintenance program should include preventive maintenance as well as corrective maintenance. The maintenance program should include:
- periodic inspection, adjustment, replacement;
- preventive maintenance - doing maintenance to prevent problems from occurring, such as removal of debris at inlets; and
- corrective maintenance - making changes to the system so that system functions as intended at the lowest annualized cost, such as resetting an inlet to reduce ponding.
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.
8.5.3 Inspection Intervals
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.
8.5.4 Maintenance Tasks
Maintenance tasks can be grouped in three general categories:
- aesthetic maintenance,
- nuisance maintenance, and
- operation maintenance.
Of these, the most important, from the standpoint of health and safety, is operation maintenance.
8.5.5 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:
- settling of embankment,
- woody growth on the embankment (roots can create channels for dam leakage and eventual failure),
- signs of piping (leakage) through embankment,
- signs of seepage or wet spots on the downstream face of an embankment (may require toe drains or chimney drains to solve problems),
- riprap failures,
- deterioration of primary and auxiliary spillways,
- various stage/outlet controls,
- effectiveness of debris racks,
- outlet channel conditions,
- safety features (access controls to hazardous areas),
- mechanical and electrical equipment (pumps, generators, automatic controls, etc.),
- access for maintenance equipment,
- availability of manufacturer's mechanical and electrical information manuals, and availability of design information such as rating curves and tables for spillway flow, bypass flow, total flow, and storage and pump-out calculations.
In addition, the following actions should be taken on each facility, as required.
- Replace soil removed by rodent burrows.
- Inspect drainage systems and relief wells annually for proper functioning and clean out or replacement as necessary.
- Maintain riprap or other wave-protective measures and replace as needed.
- Remove and/or stabilize slide material as soon as practical. It may be necessary to construct a berm or flatten the slope.
- Replace eroded material and establish vegetation in eroded areas in emergency spillways, swales and other areas.
- Repair any unusual seepages, boils, or settlements in fill areas, or sinkholes in pool areas.
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.
8.5.6 Volume Maintenance
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.