City of Lincoln
Lincoln Stormwater Regulations
The city of Lincoln is in Lancaster County, Nebraska. The county does not have a MS4 and the public works for the city administer stormwater inspections. The requirements for BMPs and post-construction mainenance meet the minimum requirements of the federal level, and the EPA has delegated Clean Water authority to the state of Nebraska. The state rules and regulations are in the Nebraska Environmental Protection Act.
Excerpt- Lincoln Drainage Criteria Manual Chapter 8 Stormwater best Management Practices
In order for stormwater BMPs to be effective, proper maintenance is essential. Maintenance includes both routinely scheduled activities, as well as non-routine repairs that may be required after large storms, or as a result of other unforeseen problems. BMP maintenance is the responsibility of the entity owning the BMP; however, the City per the National Pollutant Discharge Elimination System Permit (i.e. stormwater permit) is responsible for ensuring the maintenance of privately owned BMPs. BMPs should be designed with maintenance as a key design consideration. Planning-level design guidance pertaining to maintenance is included in the individual BMP information contained within this manual. This chapter focuses on maintenance of in-service BMPs and provides recommendations for private BMP owners, as well as City responsibilities for ensuring proper maintenance for both public and private facilities.
8.4.2 Defining Maintenance responsibility for Public and Private Facilities
Identifying who is responsible for maintenance of BMPs and ensuring that an adequate budget is allocated for maintenance is critical to the long-term success of BMPs. Maintenance responsibility may be assigned in different ways:
- Publicly owned BMPs are maintained by the City.
- Privately owned BMPs typically are maintained by the property owner, homeowner’s association, property manager or private company hired by them.
- Privately owned BMPs may be maintained bythe City under a written agreement with the owner, with appropriate fees a maintenance permit.
For public facilities, one of the key issues is ensuring that adequate staff and budget are provided to the department responsible for maintenance. Ponds, lakes, and wetland BMPs should be built only if assurances are provided that adequate maintenance staff and resources are identified in advance. For private facilities, such as those owned and maintained by homeowners’ associations, there can be a lack of understanding of maintenance required for BMPs. Maintenance plans should be prepared and submitted as part of the development review/approval process and be provided to the owner(s) upon sale of the development. It is also important to educate the general public on the purpose and function of stormwater BMPs. This is critical in cases where Low Impact Development (LID) or landscape-based BMPs are distributed throughout multiple parcels in developments. In addition to legally binding maintenance agreements, it is also helpful to have easy-to-understand informational brochures and signage that describe the functions and maintenance requirements for these facilities. Privately owned BMPs may be maintained by the City through a maintenance permit. Appendix 8.2 provides an example maintenance permit. ypical annual costs for a maintenance permit are approximately $150/year for City inspection plus approximately 5% of the construction cost annually for maintenance.
8.4.3 Developing a Maintenance Plan
A maintenance plan can be prepared as a stand-alonedocument, or be made part of a construction plan set. An example maintenance plan is attached in Appendix 8.3. The following outlines key components of a maintenance plan:
- A simple drawing or construction plan of the site development showing the locations of all stormwater quality BMPs at the site and key components such as forebays, inlets, outlets, low flow channels or other components that require inspections or maintenance. The drawing should be kept on-site at the property or the property management office. Any changes to the facility over time should be noted on the drawing.
- A brief description of the maintenance requirements and expected frequency of actions, which can be obtained from discussion within this chapter. Include instruction on how to access each component of each BMP and with what equipment.It is important to identify all maintenance requirements related directly to the water quality functions of the BMP and provide information concerning future site work that could potentially impact the integrity of the BMP. This is particularly true for landscaped BMPs. Additionally, the maintenance plans should identify constraints and considerations for future work that have the potential to affect the performance of the BMP.
- An inspection form or checklist specific to the facilitie(s) in place at the site. A log of inspection forms should be kept onsite or at the property management office to demonstrate that routine inspections and maintenance are occurring. Appendices 8.2 through 8.8 provide maintenance inspection forms for each of the BMPs in this Chapter.
- Contact information for the entity responsible for maintenance of the facility. For example, this could be a homeowner's association, municipality, or other entity. Stormwater Best Management Practices Drainage Criteria Manual.
- Copies of legally binding agreements associated with the facility that show that the facility owner is aware of, and will abide by, their maintenance responsibilities.
- Other items as appropriate for specific conditions, which may include any of the following:
- A permanent control point and other critical elevations for ponds, (i.e. bottom of pond, 100-year Water Surface Elevation, overflow, etc).
- The estimated baseflow used for the design and other hydrologic information for larger watersheds.
- Information pertaining to materials testing for any contaminant testing requirements for removed sediment.
- Post-maintenance considerations, (e.g. restoration of flow paths).
- Long-term monitoring requirements, (e.g. 404 permit reports).
It is also important to note that the guidelines included in this manual should always be combined with common sense and good judgment based on field observations and practical experience. Often, there will be maintenance requirements that are specific to a given site in addition to the general maintenance guidance provided in this manual. On a general note with regard to BMPs that have a vegetation component or involve weed and pest control, it is strongly advocated to use Integrated Pest Management (IPM) practices that help to reduce the level of chemical applications. Although water quality monitoring is not typically required as part of maintenance agreements, it is encouraged as an effective tool for determining if the BMP is functioning effectively. Stormwater quality monitoring guidelines can be downloaded from the International Stormwater BMP Database website (www.bmpdatabase.org).
8.4.4 Bioretention Maintenance
The primary maintenance objective for bioretention, also known as porous landscape detention, is to keep vegetation healthy, remove sediment and trash, and ensure that the facility is draining properly. The growing medium may need to be replaced eventually to maintain performance. This Section summarizes key maintenance considerations for bioretention.
Inspection: Inspect the infiltrating surface at least annually and following major precipitation events to determine if the bioretention area is providing acceptable infiltration. Bioretention facilities are designed with a maximum depth for the Water Quality Control Volume of one foot and soils that will typically drain the Water Quality Control Volume over approximately 24 hours. If standing water persists for more than 40 hours after runoff has ceased, clogging should be further investigated and remedied. Additionally, check for erosion and repair as necessary (See Appendix 8.4 for sample Inspection Form).
Debris and Litter Removal: Remove debris and litter from the infiltrating surface to minimize clogging of the media. Remove debris and litter from the overflow structure.
Mowing and Plant Care: Maintain healthy, weed-free vegetation. Weeds should be removed before they flower. The frequency of weeding will depend on the planting scheme and cover. When the growing media is covered with mulch or densely vegetated, less frequent weeding will be required.For grasses, when started from seed, allow time for germination and establishment of grass prior to mowing. If mowing is required during this period for weed control, it should be accomplished with hand-held string trimmers to minimize disturbance to the seedbed. After vegetation is established, mow as desired or as needed for weedcontrol. Remove cut vegetation from the BMP and dispose of properly. Following this period, mowing of native/drought tolerant grasses may stop or be reduced to maintain a length of no less than 6 inches. Mowing of manicured grasses may vary from as frequently as weekly during the summer, to no mowing during the winter. See Section 4.4 for additional guidance on mowing.
Irrigation Scheduling and Maintenance: Adjust irrigation throughout the growing season to provide the proper irrigation application rate to maintain healthy vegetation. Less irrigation is typically needed in early summer and fall, while more irrigation is needed during the peak summer months. Native grasses and other drought tolerant plantings should not typically require routine irrigation after establishment, except during prolonged dry periods. Check for broken sprinkler heads and repair them, as needed. Completely drain the irrigation system before the first winter freeze each year. Upon reactivation of the irrigation system in the spring, inspect all components and replace damaged parts, as needed.
Replacement of Wood Mulch: Replace wood mulch only when needed to maintain a mulch depth of up to approximately 3 inches. Excess mulch will reduce the volume available for storage of stormwater.
Sediment Removal and Growing Media Replacement: If ponded water is observed in a bioretention cell more than 40 hours after the end of a runoff event, check underdrain outfall locations and clean-outs for blockages. Maintenance activities to restore infiltration capacity of bioretention facilities will vary with the degree and nature of the clogging. If clogging is primarily related to sediment accumulation on the filter surface, infiltration may be improved by removing excess accumulated sediment and scarifying the surface of the filter with a rake. If the clogging is due to migration of sediments deeper into the pore spaces of the media, removal and replacement of all or a portion of the media may be required. The frequency of media replacement will depend on site-specific pollutant loading characteristics. Although surface clogging of the media is expected over time, established root systems promote infiltration. This means that mature vegetation that covers the filter surface should increase the life span of the growing media, serving to promote infiltration even as the media surface clogs.
8.4.5 Constructed Wetland Maintenance
Inspection: Inspect the pond at least annually. Note the amount of sediment in any forebays and look for debris at the outlet structure (See Appendix 8.5 for sample Inspection Form).
Debris and Litter Removal: Remove debris and litter from the pond as needed. This includes floating debris that could clog the outlet or overflow structure.
Aquatic Plant Harvesting: Harvesting entire plants including roots will permanently remove nutrients from the system although removal of vegetation can also resuspend sediment and leave areas susceptible to erosion. For this reason, it is not recommend to harvest vegetation as a routine maintenance item. However, aquatic plant harvesting can be performed if desired to maintain volume or eliminate nuisances related to overgrowth of vegetation. When this is the case, perform this activity during the dry season (November to February). This can be performed manually or with specialized machinery. If a reduction in cattails is desired, harvest them annually, especially in areas of new growth. Cut them at the base of the plant just below the waterline, or slowly pull the shoot out from the base. Cattail removal should be done during late summer to deprive the roots of food and reduce their ability to survive winter.
Mosquito Control: Mosquito control may be necessary if mosquitoes are found to be breeding in the BMP. The most effective mosquito control programs include weekly inspection for signs of mosquito breeding with treatment provided when breeding is found. These inspections and treatment can be performed by a mosquito control service and typically start in mid-May and extend to mid-September. The use of larvicidal briquettes or "dunks" is not recommended for ponds due to their relative large size and configuration.
Sediment Removal from the Forebay: Remove sediment from the forebay before it becomes a significant source of pollutants for the remainder of the pond. More frequent removal will benefit long-term maintenance practices. For dry forebays, sediment removal should occur once a year. Sediment removal in wet forebays should occur approximately once every four years or when build up of sediment results in excessive algae growth or mosquito production. Ensure that the sediment is disposed of properly and not placed elsewhere in the pond. Sediment Removal from the Pond Bottom Removal of sediment from the bottom of the pond may be required every 10 to 20 years to maintain volume and deter algae growth. This typically requires heavy equipment, designated corridors, and considerable expense. Harvesting of vegetation may also be desirable for nutrient removal. When removing vegetation from the pond, take care not to create or leave areas of disturbed soil susceptible to erosion. If removal of vegetation results in disturbed soils, implement proper erosion and sediment control BMPs until vegetative cover is reestablished. For constructed wetland ponds, reestablish growth zone depths and replant if necessary.
8.4.6 Extended Detention Basin Maintenance
Extended Detention Basins have low to moderate maintenance requirements on a routine basis, but may require significant maintenance once every 15 to 25 years. Maintenance frequency depends on the amount of construction activity within the tributary watershed, the erosion control measures implemented, the size of the watershed, and the design of the facility.
Inspection: Inspect the Extended Detention Basin at least annually, observing the amount of sediment in the forebay and checking for debris at the outlet structure (See Appendix 8.6 for sample Inspection Form).
Debris and Litter Removal: Remove debris and litter from the detention area as required to minimize clogging of the outlet.
Mowing and Plant Care: When starting from seed, mow native/drought tolerant grasses only when required to deter weeds during the first three years. Following this period, mowing of native/drought tolerant grass may stop or be reduced to maintain a height of no less than 6 inches (higher mowing heights are associated with deeper roots and greater drought tolerance). In general, mowing should be done as needed to maintain appropriate height and control weeds. Mowing of manicured grasses may vary from as frequently as weekly during the summer, to no mowing during the winter.
Aeration: For Extended Detention Basins with manicured grass, aeration will supply the soil and roots with air and increase infiltration. It reduces soil compaction and helps control thatch while helping water move into the root zone. Aeration is done by punching holes in the ground using an aerator with hollow punches that pull the soil cores or "plugs" from the ground. Holes should be at least 2 inches deep and no more than 4 inches apart. Aeration should be performed at least once per year when the ground is not frozen. Water the turf thoroughly prior to aeration. Mark sprinkler heads and shallow utilities such as irrigation lines and cable TV lines to ensure those lines will not be damaged. Avoid aerating in extremely hot and dry conditions. Heavy traffic areas may require aeration more frequently.
Mosquito Control: Although the design provided in this manual implements practices specifically developed to deter mosquito breeding, some level of mosquito control may be necessary. The most effective mosquito control programs include weekly inspection for signs of mosquito breeding with treatment provided when breeding is found. These inspections can be performed by a mosquito control service and typically start in mid-May and extend to mid-September. Treatment should be targeted toward mosquito larvae. Mosquitoes are more difficult to control when they are adults. This typically requires neighborhood fogging with an insecticide. The use of larvicidal briquettes or "dunks" may be appropriate. These are typically effective for about one month and perform best when the basin has a hard bottom (e.g., concrete lined micropool).
Irrigation Scheduling and Maintenance: Adjust irrigation throughout the growing season to provide the proper irrigation application rate to maintain healthy vegetation. Less irrigation is typically needed in early summer and fall, with more irrigation needed during July and August. Native grass and other drought tolerant plantings should not require irrigation after establishment. Check for broken sprinkler heads and repair them, as needed. Completely drain the irrigation system before the first winter freeze each year. Upon reactivation of the irrigation system in the spring, inspect all components and replace damaged parts, as needed
Sediment Removal from Forebay, Trickle Channel and Micropool: Remove sediment from the forebay and trickle channel annually. If portions of the watershed are not developed or if roadway or landscaping projects are taking place in the watershed, the required frequency of sediment removal in the forebay may be as often as after each storm event. The forebay should be maintained in such a way that it does not provide a significant source of resuspended sediment in the stormwater runoff. Sediment removal from the micropool is required about once every one to four years, and should occur when the depth of the pool has been reduced to approximately 18 inches. Small micropools may be vacuumed and larger pools may need to be pumped in order to remove all sediment from the micropool bottom. Removing sediment from the micropool will benefit mosquito control. Ensure that the sediment is disposed of properly and not placed elsewhere in the basin.
Sediment Removal from the Basin Bottom: Remove sediment from the bottom of the basin when accumulated sediment occupies about 20% of the water quality design volume or when sediment accumulation results in poor drainage within the basin. The required frequency may be every 15 to 25 years or more frequently in basins where construction activities are occurring.
Erosion and Structural Repairs: Repair basin inlets, outlets, trickle channels, and all other structural components required for the basin to operate as intended. Repair and vegetate eroded areas as needed following inspection.
8.4.7 Green Roof Maintenance
A five-year maintenance plan should be established prior to the completion of all new green roofs. Both plant maintenance and inspection of various roof structural elements will be required regularly. Additionally, green roof plants requireregular attention and care including irrigation, weeding, fertilizing, pruning, and replanting. While the first several years following green roof construction are critical for establishing vegetation, controlling weeds, and detecting problems such as leaks, a long-term maintenance plan will also be necessary. During the first five years, the maintenance plan should be refined and adjusted based on experience to develop an effective long-term plan.
Inspection: Green roof inspection should be conducted at least three times per year (See Appendix 8.7 for sample Inspection Form). At a minimum, the following areas require inspection:
- Inspect joints, borders or other features that pass through the roof to remove roots and identify damage that could lead to leaks. For example, inspect abutting vertical walls, roof vent pipes, outlets, air conditioning units, and perimeter areas. Joints with facades must provide open access for inspection, maintenance, and upkeep.
- A vegetation-free zone of approximately one foot should be maintained at the border of roof edges and at drain openings on the roof. Vegetation-free zones should be lined with pavers, stones, or gravel. Drains must remain free of vegetation and foreign objects. In order to allow for regular inspections and maintenance, drains on a green roof must remain permanently accessible.
- Because of the severe consequences of drain backups, inspection of drainage flow paths is crucial. Remove the inlet cover and visually inspect drainage pipes for roots or other material that could impede the flow of water. Plants are susceptible to poor drainage in the soil. If too much water is present and unable to drain, the plants will drown or rot. Routine inspections of drains should take place approximately three times per year as well as after precipitation events of 0.6 inches or more.
- Inspect the irrigation system for leaks or malfunctions. Uneven vegetative growth or dying plants should serve as indicators of potential irrigation system problems.
Plant Care and Media Replacement: As with any garden, plant replacement will be required periodically throughout the life of a green roof. For green roofs serving stormwater functions, heat-tolerant plants with shallow, spreading and fibrous root systems are recommended. Plant selection is crucial on roofs with intense wind and light such as roofs of skyscrapers or roofs that receive reflected solar radiation from other structures. Additionally, certain portions of the roof may experience more intense sunlight and or reflected heat, requiring additional care or irrigation system adjustments. Care of the plants on a green roof will require the most attention during the critical establishment phase. A horticultural professional should work with individuals caring for the new roof to organize schedules and routines for hand weeding, thinning, pruning, fertilizing, irrigation system scheduling and adjustments, and plant replacement. Watering and weeding are particularly important for the first two years of the green roof. For overall health of the green roof, weeds should be identified and removed early and often. If the growing medium needs to be replaced, it should be replaced in accordance with the original design specifications, unless these specifications have been identified as a cause of poor plant growth or green roof performance. Any substitutions or adjustments to the original green roof media must be balanced carefully to meet loading limits, drainage requirements, and characteristics conducive to healthy plant growth. When caring for plants or adjusting growing media, care should be taken to avoid use of materials likely to result in nutrient export from the green roof. For example, growing media and compost should have a low phosphorus index (P index). Appropriate plants with low fertilization requirements should be chosen. If used, fertilizerapplication should be minimized to levels necessary only for plant health.
Irrigation Scheduling and Maintenance: Green roofs should be equipped with irrigation systems, even if the ultimate goal is for the plants to rely primarily on natural precipitation. Irrigation schedules should be based on the evapotranspiration requirements of the plants, the type of irrigation system used (e.g., drip or spray), and changing evapotranspiration over the growing season. Irrigation systems equipped with advanced irrigation controllers based on soil moisture can help facilitate watering according to the changing water needs of the plants. If advanced systems are not used, irrigation should be manually adjusted during the growing season to replace water lost through evapotranspiration. During the first two years of plant establishment, regular irrigation will likely be needed. After plant establishment, it may be possible to reduce supplemental irrigation during non-drought conditions. Completely drain the irrigation system before the first winter freeze each year. Upon reactivation of the irrigation system in the spring, inspect all components and replace damaged parts, as needed
8.4.8 Permeable Pavement Maintenance
The key maintenance objective for any permeable pavement system is to know when runoff is no longer rapidly infiltrating into the surface, which is typically due to void spaces becoming clogged and requiring sediment removal. This Section identifies key maintenance considerations for various types of permeable pavement BMPs.
Inspection: Inspect pavement condition and observe infiltration at least annually, either during a rain event or with a garden hose to ensure that water infiltrates into the surface (See Appendix 8.8 for sample Inspection Form). Video, photographs, or notes can be helpful in measuring loss of infiltration over time. Systematic measurement of surface infiltration of pervious concrete, Permeable Interlocking Concrete Pavers, concrete grid pavement, and porous asphalt can be accomplished using ASTM C1701 Standard Test Method for Infiltration Rate of In Place Pervious Concrete.
Debris Removal, Sweeping and Vacuuming: Debris should be removed, routinely, as a source control measure. Typically, sites that require frequent sweeping already plan for this activity as part of their ongoing maintenance program. For example, a grocery store may sweep weekly or monthly. Although this type of sweeper can be effective at removing solids and debris from the surface, it will not remove solids from the void space of a permeable pavement. Use a vacuum or regenerative air sweeper to help maintain or restore infiltration. If the pavement has not been properly maintained, a vacuum sweeper will likely be needed. Permeable Interlocking Concrete Pavers, Concrete Grid Pavements (with aggregate infill), Pervious Concrete, and Porous Asphalt should use a regenerative air or vacuum sweeper after any significant site work (e.g., landscaping) and approximately twice per year to maintain infiltration rates. This should be done on a warm dry day for best results. Do not use water with the sweeper. The frequency is site specific and inspections of the pavement may show that biannual vacuuming is more frequent than necessary. After vacuuming Permeable Interlocking Concrete Pavers and Concrete Grid Pavers, replace infill aggregate as needed.
Snow Removal: In general, permeable pavements do not form ice to the same extent as conventional pavements. Additionally, conventional liquid treatments (deicers) will not stay at the surface of a permeable pavement as needed for the treatment to be effective. Sand should not be applied to a permeable pavement as it can reduce infiltration. Plowing is the recommended snow removal process. Conventional plowing operations should not cause damage to the pavements.
- Permeable Interlocking Concrete Pavers and Concrete Grid: Deicers may be used on Permeable Interlocking Concrete Pavers and grid pavers; however, it may not be effective for the reason stated above. Sand should not be used. If sand is accidentally used, use a vacuum sweeper to remove the sand. Mechanical snow and ice removal should be used.
- Pervious Concrete: Do not use liquid or solid deicers or sand on pervious concrete. Deicers can damage the concrete and sand will reduce infiltration. Mechanical snow and ice removal should be used.
- Porous Asphalt: Use liquid or solid deicers sparingly; mechanical snow and ice removal is preferred. Do not apply sand to porous asphalt. Full and Partial Replacement of the Pavement or Infill Material
- Permeable Interlocking Concrete Pavers and Concrete Grid: Concrete pavers, when installed correctly, should have a long service life. If a repair is required, it is frequently due to poor placement of the paver blocks. Follow industry guidelines for installation and replacement after underground repairs. If surface is completely clogged and rendering a minimal surface infiltration rate, restoration of surface infiltration can be achieved by removing the first ½ to 1 inch of soiled aggregate infill material with a vacuum sweeper. After cleaning, the openings in the Permeable Interlocking Concrete Pavers will need to be refilled with clean aggregate infill materials. Replacement of the infill is best accomplished with push brooms.
- Porous Gravel: Remove and replace areas of excessive wear or reduced infiltration as needed. The frequency is dependent on-site characteristics including site uses, vegetation, and materials.
- Pervious Concrete: Partial replacement of pervious concrete should be avoided. If clogged, power washing or power blowing should be attempted prior to partial replacement. Any patches should extend to existing isolated joints. Conventional concrete may be used in patches, provided that 90 percent of the original pervious surface is maintained.
- Reinforced Grass: Remove and replace the sod cover as needed to maintain a healthy vegetative cover or when the sod layer accumulates significant amount of sediment (i.e., >1.5 inches). Maintenance and routine repairs should be performed annually, with sod replacement approximately every 10 to 25 years. When replacing sod, use a high infiltration variety such as sod grown in sandy loam.
8.4.9 Retention Pond Maintenance
See Constructed Wetland Ponds Section 8.4.5. For sample Inspection Form see Appendix 8.9.
8.4.10 Underground BMP Maintenance
Maintenance requirements of underground BMPs canvary greatly depending on the type of BMP. Frequent inspections (approximately every three months) are recommended in the first two years in order to determine the appropriate interval of maintenance for a given BMP. This Section provides general recommendations for assorted underground BMPs. For proprietary devices, the manufacturer should provide detailed maintenance requirements specific for the BMP (See Appendix 8.10 for sample Inspection Form).
Inspection: Inspect underground BMPs at least quarterly for the first two years of operation and then twice a year for the life of the BMP, if a reduced inspection schedule is warranted based on the initial two years. Specifically look for debris that could cause the structure to bypass water quality flows. Strong odors may also indicate that the facility is not draining properly. Inspection should be performed by a person who is familiar with the operation and configuration of the BMP. If applicable, inspect inlet inserts frequently; at a minimum, inspect after every storm event exceeding 0.6 inches. Removal of flow blocking debris is critical for flood control. Debris Removal, Cartridge
Replacement and Vacuuming: All Underground BMPs should follow the manufacturer's recommended maintenance requirements and remove any flow blocking debris as soon as possible following inspection. Replacement of filter cartridges is anticipated on an annual basis. Depending on site characteristics, the replacement frequency may be extended to no less than once every three years. However, semi-annual inspection should continue to ensure that proper function of the system is maintained. Maintenance is required when any of the following conditions exist:
- If there is more than 4 inches of accumulated sediment on the vault floor.
- If there is more than ¼ inch of accumulation on the top of the cartridge.
- If there is more than 4 inches of standing water in the cartridge bay for more than 24 hours after the end of a rain event.
- If the pore space between media granules is full.
- If inspection is conducted during an average rainfall event and the system remains in bypass condition (water over the internal outlet baffle wall or submerged cartridges).
- If hazardous material release (automotive fluids or other) is reported.
- If pronounced scum line (≥1/4" thick) is present above top cap.
- If system has not been maintained for three years.
- Hydrodynamic Separators: Vacuum sediment and debris from units at least once annually and more frequently as needed, based on inspections.
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Lincoln Drainage Criteria Manual Chapter 8