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Pennsylvania Handbook of Best Management Practices for Developing Areas§7. BMP Maintenance
§7.1 Introduction Although the actual time that a BMP facility performs its function is relatively brief (during and immediately following a storm event), it must constantly be able to do so. The facility must be available at all times because of the random nature of rainfall events and the impracticality of inspecting the facility and maintaining them immediately before a storm event. In addition, pollutant-removal efficiencies will decline over time if the BMP is not adequately maintained. For a BMP to be operational, the BMP operator must establish and sustain a comprehensive, regularly scheduled maintenance program. BMP maintenance starts by ensuring thorough inspections during construction. Proper construction of the BMP will reduce the maintenance needs of the facility. The municipality needs to develop inspection checklists, and communicate to the inspectors the importance of scheduling and coordinating the BMP construction with other site activities. For more information on BMP maintenance requirements, refer to Section 8 for specific BMP descriptions. The following criteria will guide BMP designers with maintaining BMPs. For more information, refer to specific BMP descriptions. The criteria include access and maintenance easements, routine inspection of outlet structures, sediment disposal, maintenance agreements, and other maintenance aspects specific to wet ponds, extended detention dry ponds, and infiltration trenches. §7.2 Overview of BMP Maintenance Changes in downstream drainages may be too subtle or long in developing to provide adequate warning that the condition of a BMP is deteriorating. By the time problems are apparent, significant damage may have occurred. In many instances, impacts will not be experienced until an event approaching the design storm occurs. Failures triggered by large storm events may be as dramatic as washouts, flooding, and erosion of stream banks (NVPDC 1991). Therefore, preventative maintenance is essential. The components of a maintenance program are listed in Table 7-1. Although general maintenance tasks can be outlined, actual maintenance needs will vary according to specific site conditions, especially the following elements:
A BMP maintenance program also should consider the following:
§7.3 Routine Maintenance Needs As a minimum, BMPs should be inspected annually and after any storm larger than the design storm (i.e., peak detention storm, water quality storm, or runoff capture design storm, as appropriate). A sample inspection checklist is provided in Table 7-2. Not all of the checklist items will apply to every BMP. §7.3.2 Debris and Litter Removal Regularly removing debris and litter is well worth the effort and can be expected to help with the following:
Special attention should be given to removing floating debris, which can clog the outlet device or riser. §7.3.3 Maintenance of Mechanical Components Each type of BMP may have mechanical components that need periodic attention to ensure their continued performance. Valves, sluice gates, fence gates, locks, and access hatches should be functional at all times. Breeding grounds for mosquitoes and other insects can be created by ponded water. Though perceived as a significant nuisance, mosquitoes are not as big a problem as is often thought. The best control technique for wet ponds is to ensure that the permanent pool does not develop stagnant areas. Wet ponds and constructed treatment wetlands should include a source of steady dry-weather flow. Promptly removing of floatable debris helps eliminate still surface waters. In larger ponds fish, which feed on mosquito larvae, could be stocked. §7.3.5 Terrestrial Vegetation Maintenance Grasses and plants incorporated in vegetative BMPs, such as filter strips, grass swales, and bioretention facilities, require attention to ensure a robust stand of vegetation. The development of distressed vegetation, bare spots, and rills are an indication that a BMP is not functioning properly. Problems can have many sources, including:
The soil in vegetated areas should be tested biennially and adjustments made to sustain vigorous plant growth with deep, well-developed root systems. Aeration of soils is recommended for filter strips where high sediment accumulation rates exist. Ideally, vegetative covers should be mowed infrequently, allowing them to develop thick stands of tall grass and other plant vegetation. §7.3.6 Aquatic Vegetation Maintenance An important, yet often overlooked aspect of nonroutine maintenance of wet ponds and constructed treatment wetlands is the need to regularly monitor and manage conditions to promote a healthy aquatic environment. An indicator of excess nutrients (a common problem) is excessive algae growth in the permanent pool of a wet pond. In most cases, these problems can be addressed by encouraging the growth of more desirable aquatic and semi-aquatic vegetation in and around the permanent pool. Plants should be selected that are tolerant of varying water levels and have a high capacity to incorporate the specific nutrients that are associated with the problem (see Appendix H). If algae proliferation is not addressed, algae-laden water will be washed downstream during subsequent rain events where it may contribute to nuisance odors and introduce stresses to downstream aquatic habitat. §7.3.7 Bank Stabilization and Erosion Control The integrity of the banks and bottom of extended detention dry ponds and the visible banks of wet ponds and constructed treatment wetlands must be maintained. The routine task is maintaining a healthy ground cover on the embankments and bottoms of ponds. Areas of bare soil quickly will erode, clogging the facility with soil and threatening its integrity. Therefore, bare areas must be reseeded and stabilized as quickly as possible to avoid erosion. Newly seeded areas should be protected with an erosion mat that is securely staked to prevent flotation. Erosion in or around the inlet and outlet of the BMP facility needs to be repaired as soon as possible. Erosion control activities need to also extend to areas immediately downstream of the BMP. The roots of woody growth, such as young trees and bushes, in embankments are destabilizing. Consistent mowing of the embankment will control stray seedlings that take root. Woody growth, such as trees and bushes, further away from the embankment should not pose a threat to the stability of the embankment and can provide important runoff filtering benefits. Trees and bushes should be planted outside maintenance and access areas. Animal burrows also will deteriorate the structural integrity of an embankment. Muskrats, in particular, will burrow tunnels up to 6 inches in diameter. Efforts should be made to control excessive animal burrowing. Burrows should be filled as soon as possible. §7.4 Nonroutine Maintenance -- Sediment Removal §7.4.1 Wet and Extended Detention Dry Ponds Sediment gradually will accumulate in many BMPs, including wet ponds, extended detention dry ponds, constructed treatment wetlands, bioretention facilities, and grass swales. Constructed treatment wetlands should be designed to accommodate sediment accumulation without the need for sediment removal during the life of the facility. To accommodate the sediment, constructed treatment wetlands have variable-height weirs and should have added embankment freeboard to anticipate sediment accumulations. For most other BMP applications, accumulated sediment will have to be removed eventually. However, facilities vary so dramatically that no "rules of thumb" exist to guide responsible parties about removing sediment. The specific setting of a BMP will be an important determinant in how often sediment must be removed. Important factors that determine rates of sedimentation are:
Removing sediment from swales and bioretention areas generally is not a significant maintenance concern. However, wet ponds and extended detention dry ponds may require a significant investment in sediment removal activities. Before installing a pond, good practice is to estimate the lifetime sediment accumulation that the pond will have to handle. Several time periods may be considered, representing expected changes in land use in the watershed. To estimate sediment accumulation, an estimate of pond efficiency will be required (see wet pond and dry pond BMP descriptions in Section 8). The analysis of watershed sediment loss and pond efficiency can be expedited by using a sediment delivery computer model, such as SEDCAD. In the absence of site-specific sediment loss computations, sediment removal from ponds should be anticipated as follows:
Sediment removal is usually the largest single cost of maintaining a BMP facility; therefore, it is best to plan ahead and set aside the necessary funds in advance. The sediment removed from a pond must be disposed of. The best solution is to have an onsite area or a site adjacent to the facility, but outside of the floodplain, set aside for the sediment. If such a disposal area is not set aside, transportation and landfill tipping fees can greatly increase the cost. Disposal of wet sediment is not allowed in many landfills, so the material often must be dried (dewatered) before being disposed of. This extra step adds to the cost and requires a place where wet material can be temporarily placed to dry. A worksheet for estimating sediment removal and disposal requirements is provided as Table 7.3. The additional cost of sediment removal for a wet pond is partially offset by the longer interval between dredging cycles. Wet sediment is more difficult and expensive to remove than dry sediment. Ideally, the entire facility can be drained and allowed to dry sufficiently so that heavy equipment can operate on the bottom. Provisions for draining permanent pools should be incorporated in the design of wet ponds where feasible. Also, low flow channels and outlets should be included in all ponds to bypass stormwater flow during maintenance. However, in many wet ponds and extended detention dry ponds, periodic rainfall will maintain the sediment in a soft condition, preventing access by heavy equipment. In these cases, sediment may need to be removed from the shoreline by using backhoes, gradalls, or similar equipment. Infiltration devices include infiltration trenches, dry wells, and seepage beds beneath permeable pavements. Infiltration facilities are prone to losing function from clogging by sediment. Therefore, these facilities should be inspected two to four times a year. One purpose of regular inspection is to determine if the sediment-trapping measures, such as filter fabric or graded sand filter, require routine maintenance. Keeping the sediment filter clean is vital to ensuring the long-term performance of the infiltration trench. Although maintenance must be undertaken more often than with surface facilities, the costs are significantly less. For trenches or dry wells, periodic maintenance requirements usually include removing the top 6 to 12 inches of filter gravel and replacing the filter fabric sediment filter covering the aggregate reservoir. A layer of clean filter gravel replaces the gravel removed. The maintenance of permeable pavement systems requires the routine sweeping of surfaces. Specific maintenance requirements for infiltration BMPs are discussed further in Section 8. A clogged sediment filter is indicated when water cannot flow into the device and instead surcharges. However, sometimes suspended sediment will clog the interface of the seepage reservoir and the native soil to which the stored water must eventually exfiltrate. All infiltration devices should be provided with standpipes to observe water levels. If an overflow condition exists, the observation standpipe should be checked to determine the cause. If the device continues to overflow after the sediment filter is repaired and stays filled with water after a rain, then the aggregate stone must be excavated and the facility rebuilt. §7.5 Maintenance Responsibility When a maintenance program is designed, safety, cost, and effectiveness of the maintenance need to be balanced. Some maintenance can be cost-effectively undertaken by facility owners, if desired. Minor landscaping tasks, litter removal, and mowing are tasks appropriate for owners to handle. However, it is usually worth the cost to have a professional do the more difficult work. Mowing and handling a wheelbarrow can be dangerous on the sloping embankments of a extended detention dry pond. Filling eroded areas and soil-disturbing activities, such as resodding or replanting vegetation, also are tasks that a professional landscaping firm might best manage. If not performed properly the first time, not only will the effort have been wasted, but damage may be done to the facility by creating excessive erosion. Grading and sediment removal are best left to professional contractors. In addition, trained personnel will be able to identify potential problems in their early stages of development when repairs or alterations can be made cost-effectively. The routine costs of maintaining a BMP will be highly site-specific. Factors that influence costs include the type of development on the site and the landscape of the site. Routine maintenance includes inspections, debris and litter control, mechanical components maintenance, vegetation management, and other routine tasks as determined for the specific facility. Quotations should be obtained from firms experienced with the tasks that are relevant for selected BMPs. If high costs are projected, then modifying the design or using alternative BMPs with lower maintenance costs should be considered. §7.7 Estimating Nonroutine Costs Costs for nonroutine maintenance of BMPs also is highly site-specific and will vary greatly depending on the size and depth of the facility, the volume of sediment trapped in the BMP, the accessibility of the BMP, and whether or not onsite disposal of the dredged sediment is possible. In general, maintenance costs for both wet and dry pond are similar unless otherwise noted. Table 7.4 shows the range of costs for removing sediment from various size BMPs. The last column is blank and can be used with the data below to estimate the costs of a particular facility. The cost figures used here are based on an informal survey of firms that provide such services. One of the larger fixed costs in dredging a BMP facility is the mobilization and demobilization of the machinery and personnel needed to dredge a BMP. Large wet ponds or flood control dams often will require a waterborne operation during which an excavator or a crane must be mounted to a floating barge and moved into position. The cost of such an operation readily approaches $30,000. For smaller ponds, which can be drained or dredged readily from the banks, the cost of mobilizing and demobilizing for this type of operation will range from between $5,000 to $7,000 (NVPDC, 1992 and 1991). Table 7-4 The costs of physically dredging the sediment from a BMP once mobilization has taken place depends on the total volume (in cubic yards) of sediment removed. The cost per cubic yard is largely influenced by the depth of the water and the distance between the excavation area and the "staging area" where sediment is transferred to trucks for removal. A further consideration is whether the equipment can easily access the BMP bottom. The cost range for dredging is $6 to $15 per cubic yard. The costs of maintaining a BMP over the long run can be considerable, particularly if dredging or other nonroutine maintenance is required. To lessen the immediate financial impact of the nonroutine costs, the party responsible for BMP maintenance should create a sinking fund for this eventuality. For dry ponds, from which sediment must be removed every 2 to 10 years, 10 percent to 50 percent of the anticipated dredging costs should be collected each year. For wet ponds, which need to be dredged every 5 to 15 years, approximately 6 percent to 20 percent of the anticipated costs should be accrued per year. Present value of the assessment can include anticipated interest. Access for inspections, maintenance personnel, and equipment must be provided to all areas of a facility that must be observed or maintained. The location and configuration of easements must be established during the design phase, built to the design standards during the initial construction of the facility, and maintained regularly. The areas requiring access include the dam embankment, emergency spillway, side slopes, inlets, sediment forebays, riser structures, BMP devices, and pond outlets. To provide access for heavy equipment, a suitable 10-foot-wide roadway in a 20-foot-wide cleared access easement must be provided to the BMP facility. At large or regional facilities, additional easements to both upstream and downstream areas should be provided for maintenance access, and additional improvements, such as all-weather roads, access restrictions, and vandalism deterrents should be considered. An agreement providing for long-term maintenance should accompany the installation of any BMP, including ponds, constructed treatment wetlands, bioretention areas, and grass swales. In many cases the agreements will be incorporated in conventional grounds maintenance contracts. Maintenance agreements should be specific regarding schedules and required tasks of such as inspections, routine and nonroutine maintenance obligations, and emergency response measures. In addition, the agreement should include clauses to allow the municipality to conduct the maintenance, if the owner/operator fails to inspect and maintain the facility in accordance to an established maintenance schedule. Typical agreements also include indemnification and hold harmless clauses, and are recorded in the land records of the municipality. For some facilities, exploring the possibility of obtaining the participation of the local municipality in maintaining the facility may be worthwhile. Easements for BMPs that are not publicly maintained should include provisions to permit public inspection and maintenance (including reimbursement to the public agency for incurred costs) if a private organization fails in its maintenance responsibility and creates a public nuisance. Facilities for commercial, industrial, and rental residential developments typically are maintained by the owner. Table 7.4
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