11-Attachment F- Task 3.pdf

Post-Construction Maintenance Guide

7.7 Filtering Practice

December 2017

Prepared by:

Hazen and Sawyer

4035 Ridge Top Road, Suite 400

Fairfax, VA 22030

In consultation with:

Fairfax County Department of Public Works and Environmental Services (DPWES) Maintenance and Stormwater Management Division (MSMD)

10635 West Drive Fairfax, Virginia 22030 ii

Table of Contents

Form and Function

Functional Elements for Filtering Practices

Inflows and Pre-treatment

Pre-treatment Structures

Surface Layer

Filter Media

Underdrains and Observation Wells

Embankment or Berm

Outfall Structure

Non-Functional Elements for Filtering Practices

Access

Signage

Routine Maintenance

Non-Routine Maintenance

Inflows

Blockages

Erosion

Structural Deterioration (Cracks, Spalling, Rust, Etc.)

Separated Structure

Undermined or Settled Structure

Trees, Shrubs and Brush Maintenance

Pre-treatment Structure

Blockages

Structural Deterioration (Cracks, Spalling, Rust, Etc.)

Accessibility

Water Level Too Low

Bypass Pipe/Overflow Weir Missing or Non-Functional

Pre-treatment Structure Missing or Non-Functional

Surface Layer and Filter Media

Vegetation

Debris Removal iii

Sediment Removal

Surface Layer and Filter Media Remediation

Filter Media Short Circuiting

Exposed or Damaged Geotextile Fabric

Rodents

Filtering Practice Missing

Facility Not Sized to Plans

Structural Deterioration (Cracks, Spalling, Rust, Etc.)

Damaged Gabion Wall (Where Present)

Accessibility

Observation Wells, Cleanouts and Underdrains

Blocked or Silted-in Underdrain

Structural Deterioration (Cracks, Spalling, Rust, Etc.)

Deteriorated Underdrain Connection or Separation

Structural Integrity

Missing Underdrain

Missing Observation Well or Cleanout

Damaged/Missing Observation Well Cap

Accessibility

Control Structure and Principal Spillway Pipe (Not Typically Found)

Blockages of Low Flow Orifice, Weir (Where Present)

Blocked or Silted in PSP (Where Present)

Blockages of Overflow Orifice or Weir (Where Present)

Missing BMP (Water Quality) Plate/Device on Low Flow Orifice (Where Present)

Accessibility (Where Present)

Trees, Shrubs and Brush Maintenance (Where Present)

Structural Deterioration (Cracks, Spalling, Rust, Etc.) (Where Present)

Structural Integrity (Where Present)

Leaking Joints (Where Present)

Deteriorated PSP Connection (Where Present)

Undermined or Settled Control Structure (Where present)

Bypass Structure iv

Accessibility

Structural Deterioration (Cracks, Spalling, Rust, Etc.)

Blockage

Separated Structure

Bypass Pipe/Overflow Weir Missing or Non-Functional

Embankment or Berm and Vegetated Emergency Spillway

Turf Maintenance

Surface Erosion, Ruts, and Worn Paths Maintenance

Blockages of Emergency Spillway

Shrinkage Cracks Maintenance

Trees, Shrubs and Brush Maintenance

Rodents

Mosquitoes

Piping Failure (Internal Erosion)

Hydraulic Failure (Overtopping Erosion)

Depressed Areas and Settling

Fracturing

Embankment or Emergency Spillway Missing

Encroachments

Structural Deterioration (Cracks, Spalling, Rust, Etc.)

Outfall Structure and Channel

Accessibility

Dewatering Drain Valve Missing or Non-Functional

Trees, Shrubs and Brush Maintenance

Structural Deterioration (Cracks, Spalling, Rust, Etc.)

Separated Structure

Undermined or Settled Structure

Blockages

Erosion

Routine Maintenance Corrective Action and Annual Frequency Table

Non-Routine Maintenance Corrective Action and Annual Frequency Table

References v

Externally Sourced Figure References vi

List of Figures

Figure 1. Surface Filter (Austin Sand Filter) with Sand Filter Plan and Section View

Figure 2. Surface Filter (Austin Sand Filter) with Engineered Media Profile and Section View

Figure 3. Typical Surface Sand Filter Configuration

Figure 4. Underground Filter (DC Sand Filter) Schematic

Figure 5. Underground Filter (DC Sand Filter) with Bypass Pipe

Figure 6. Perimeter Sand Filter (Delaware Sand Filter) Schematic

Figure 7. Perimeter Sand Filter (Delaware Sand Filter)

Figure 8. Typical Underground Filter Chambers

Figure 9. Surface Filter with Piped Inflow

Figure 10. Piped Inflow to Underground Filter

Figure 11. Surface Filter Pretreatment Structure

Figure 13. Typical Surface Cover on Underground Filter Chambers

Figure 12. Typical Surface Cover on Surface Filters

Figure 14. Section View of Surface Sand Filter Media Layer

Figure 15. Cleanouts and Underdrains

Figure 16. Underground Filter Bypass Structures and Dewatering Valve

Figure 17. Embankment

Figure 18. Outfall from Clearwell Chamber

Figure 19. Blockage at Inflow Pipe

Figure 20. Inflow Pipe Structural Deterioration

Figure 21. Trash and Debris Accumulation in Sedimentation Chamber

Figure 22. Erosion of Surface Vegetation

Figure 23. Clogged Filter Chamber

Figure 24. Exposed Geotextile Fabric

Figure 25. Blocked Underdrain vii

Figure 26. Erosion on Embankment

Figure 27. Groundhog

Figure 28. Fracturing

Figure 29. Dewatering Valve (Partially Open)

Figure 30. Dewatering Valve Missing from Clearwell Chamber

Form and Function

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Form and Function

Filtering practices are stormwater best management practices (BMPs) that filter runoff from small drainage areas

(typically 2 – 5 acres depending on the configuration) to provide water quality improvement. Filtering practices typically do not provide substantial water quantity or peak shaving benefits unless combined with other stormwater

BMPs. Pollutant removal mechanisms primarily include filtration, sedimentation (physical settling), and adsorption if specialized filter media is used. Filtering practices are commonly comprised of a pre-treatment structure, a perforated standpipe or orifice connecting the pre-treatment structure to the filter basin or chamber, a temporary ponding area with a 3 inch (in) top soil and sod or pea gravel surface layer, a filter media layer (sand or organic filter media), and a gravel choker layer with an underdrain system. A non-woven geotextile fabric is typically installed on all sides of the filter media to reduce the migration of outside sediment into the filter media. During storm events, runoff flows into the pre-treatment structure (typically a dry or wet sedimentation basin or chamber) where gravitational settling occurs to remove suspended sediment, debris and trash. The pre-treated runoff then flows through a perforated standpipe or orifice, which conveys the runoff into the filter basin or chamber. The runoff temporarily ponds within the filter basin or chamber above the surface layer of the filter. The ponded water gradually passes through the surface layer into the filter media and underlying gravel choker layer with underdrains. Underdrains convey the treated runoff to a clearwell chamber or directly to the downstream stormwater conveyance system. Inflow to filtering practices is typically through a piped inflow, often including a bypass structure to limit flows into the pre-treatment sedimentation basin or chamber during larger storm events. Sheet flow may also occur for certain types of filtering practices. Inflow structures may include energy dissipation mechanisms to ensure non-erosive velocities and limit the resuspension of settled materials within the pre-treatment structure. There are two categories of filtering practices: surface filters, and underground filters. Reference the Summary Table of Filtering Practices for an outline of the various filtering practice types. .

Surface Filters

Surface filters are located above ground, and include a pre-treatment structure, typically a dry or wet sedimentation basin, followed by a filter basin connected through a perforated standpipe. Reference Figures 1, 2 and 3. There are two types of surface filters: non-structural surface filters and structural surface filters.

Non-structural surface filters are structurally similar to bioretention basins and filters which are described in detail in

Post-Construction Maintenance Guide 7.4 Bioretention Basin and Filter, with some important distinctions:

• The bottom is lined with an impermeable filter fabric and there is always an underdrain.

• The surface cover is sand, grass or pea gravel.

• The filter media is 100% sand.

• The filter surface is not planted with trees, shrubs or herbaceous materials.

• The practice has two cells, with a dry or wet sedimentation basin preceding the sand filter bed.

Non-structural sand filters are installed where the contributing drainage area is 2 acres or less, and includes stormwater hotspots where the use of a bioretention practice would not be appropriate. Non-structural sand filters may use alternative pre-treatment structures such as a compost amended grass filter flow path, forebay, gravel diaphragm, check dam, level spreader, or a combination of the listed structures.

Structural surface filters, frequently referred to as Austin sand filters, consist of a dry or wet sedimentation basin, followed by a filter basin. The filter basin is created using pre-cast or cast-in-place concrete. Both the sedimentation

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Maintenance and Stormwater Management Division Page 2 basin and filter basin are located at the ground surface. Surface filters are normally off-line facilities, designed to receive the water quality treatment volume or flow rate for a contributing drainage area up to 5 acres, and bypassing higher flows. On-line facilities are sometimes constructed, typically on the bottom of an extended detention dry pond.

Runoff typically enters the sedimentation basin through an inflow bypass structure. Pre-treated runoff then flows to the filter basin through a perforated standpipe. Runoff temporarily ponds within the filter basin above the surface layer, a 3 in layer of top soil and sod, or pea gravel. Runoff gradually passes through the surface layer into the filter media and underlying gravel choker layer with underdrains. Underdrains convey the treated runoff to a downstream stormwater conveyance system. Surface filters have filter media comprised of either 100% sand or an engineered organic filter media. The organic filter media is typically specialized to remove specific pollutants such as heavy metals or hydrocarbons at stormwater hotspots. Often, the organic filter media is comprised of either a peat/sand filter media or a compost filter media.

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Figure 1. Surface Filter (Austin Sand Filter) with Sand Filter Plan and Section View

Source: Virginia Department of Environmental Quality (DEQ) BMP Clearinghouse

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Maintenance and Stormwater Management Division Page 4

Figure 2. Surface Filter (Austin Sand Filter) with Engineered Media Profile and Section View

Source: Virginia DEQ BMP Clearinghouse

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 5

Underground Filters

Underground filters are located below ground, and include a pre-treatment structure, typically a wet sedimentation chamber, followed by a filter chamber connected through a submerged orifice or elbow pipe. Please note that underground filters are classified as an underground confined space, and all Occupational Safety and Health

Administration (OSHA) rules and regulations must be followed when entry is required for maintenance purposes.

There are two types of underground filters: underground sand filters and perimeter sand filters.

Underground sand filters, frequently referred to as DC sand filters, consist of a sedimentation chamber, a filter chamber and a clearwell chamber, reference Figures 4 and 5. Underground sand filters are designed to treat a contributing drainage area of 2 acres or less. The sedimentation chamber receives untreated runoff and may include an internal flow splitter, or overflow device that bypasses runoff from larger storm events around the filter chamber directly to the clearwell chamber. Alternatively, overflow may enter the filter chamber but bypass the filter media through an overflow weir connecting the downstream end of the filter chamber to the clearwell chamber. Runoff flows from the sedimentation chamber to the filter chamber through a submerged orifice. The orifice often includes a trash rack and elbow pipe to keep debris and floatables from entering the filter chamber. The filter chamber includes a 3 in gravel surface layer, the underlying sand filter layer with an underdrain to convey treated runoff to the clearwell.

Observation wells or cleanouts are installed in the filter media and connect to the underdrain. The clearwell provides access to the underdrain system, bypass pipe or overflow weir, outfall pipe and a dewatering valve set at an elevation to provide access to the filter media. Manholes or access ports should be provided to allow access to each chamber of the underground sand filter.

Sediment Forebay

Gabion Wall

Filter Bed

Underdrain Cleanout/Observation Well

Figure 3. Typical Surface Sand Filter Configuration

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Perimeter sand filters, frequently referred to as Delaware sand filters, consist of a sedimentation chamber, a filter chamber and a clearwell chamber, reference Figures 6 and 7. The main difference between the underground sand filter and perimeter sand filter is that runoff enters the perimeter sand filter through surface grates, typically at the edge of a parking lot instead of through an inflow pipe. Perimeter sand filters are designed to treat a contributing drainage area of 2 acres or less. The sedimentation chamber receives untreated runoff through the surface grates, typically at the edge of a parking lot. An overflow bypass pipe or weir connects the sedimentation chamber to the clearwell chamber or downstream stormwater conveyance network during large storm events. Runoff flows from the sedimentation chamber to the filter chamber through an orifice or weir. The orifice or weir may include a trash rack and elbow pipe to keep debris and floatables from entering the filter chamber. The filter chamber includes a 3 in pea gravel surface layer, and the underlying sand filter layer with an underdrain conveying treated runoff to the clearwell.

Observation wells or cleanouts are installed in the filter media and connect to the underdrain. The clearwell provides access to the underdrain system, bypass pipe or overflow weir, outfall pipe and a dewatering valve set at an elevation to provide access to the filter media. Manholes or access ports should be provided to allow access to each chamber of the underground sand filter. Reference Figure 8 for photos of inside of the three chambers for underground filters.

Summary Table of Filtering Practices

Non-Structural Surface

Sand Filter

Structural Surface

Sand Filter (Austin

Sand Filter)

Underground Sand

Filter (DC Sand

Filter)

Perimeter Sand

Filter (Delaware

Sand Filter)

Location Above Ground Above Ground Below Ground Below Ground

Maximum

Contributing

Drainage Area

2 acres 5 acres 2 acres 2 acres

Inflow Piped or Sheet Flow Piped or Sheet Flow Piped Surface Grates

Types of Pre-treatment

Structures

Compost amended grass filter flow path, Sediment forebay, Gravel diaphragm, Check dam, Level spreader

Dry or Wet

Sedimentation Basin

Wet Sedimentation

Chamber

Wet Sedimentation

Chamber

Filter Media Sand Sand or Engineered

Organic Filter Media

Sand or Engineered

Organic Filter Media

Sand or Engineered

Organic Filter Media

Underdrain

Connection

Underdrain Connects to

Downstream Stormwater

Conveyance Network

Underdrain Connects to

Downstream

Stormwater

Conveyance Network

Underdrain Connects to Clearwell Chamber

Underdrain Connects to Clearwell Chamber

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Figure 4. Underground Filter (DC Sand Filter) Schematic

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Figure 5. Underground Filter (DC Sand Filter) with Bypass Pipe

Maintenance and Stormwater Management Division Page 9

Figure 6. Perimeter Sand Filter (Delaware Sand Filter) Schematic

Source: Virginia DOT BMP Design Manual of Practice

Maintenance and Stormwater Management Division Page 10

Figure 7. Perimeter Sand Filter (Delaware Sand Filter)

Source: Virginia DOT BMP Design Manual of Practice

Maintenance and Stormwater Management Division Page 11

Sediment Chamber Filter Chamber

Clearwell

Figure 8. Typical Underground Filter Chambers

Functional Elements for Filtering Practices

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Functional Elements for Filtering Practices

Inflows and Pre-treatment

Inflows convey stormwater runoff from the drainage area into the filtering practice. Inflow to filtering practices is typically through a piped inflow, often including a bypass structure to limit flows into the pre-treatment sedimentation basin or chamber, reference Figures 9 and 10. Sheet flow and surface runoff may also occur for certain types of practices. Inflow structures may include energy dissipation mechanisms to ensure non-erosive velocities and limit the resuspension of settled materials within the pre-treatment structure. Filtering practices receive runoff in the following ways:

• Non-structural surface sand filter: pipe inflow, sheet flow

• Structural surface sand filter: piped inflow, sheet flow

• Underground sand filter: piped inflow

• Perimeter sand filter: surface grates

Inflow Pipes

Figure 9. Surface Filter with Piped Inflow

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Pre-treatment Structures

Pre-treatment structures are required for all types of filtering practices. In general, dry or wet sedimentation basins or chambers are used to settle out sediment and debris from the runoff before it enters the filter basin or chamber. Sediment chambers must be sized to accommodate at least 25% of the water quality treatment volume and be designed to convey flows with non-erosive velocities to the receiving filter basin or chamber, reference the sediment chamber shown in Figure 8. Non-structural surface sand filters include alternative pre-treatment structures such as a compost amended grass filter flow paths, gravel diaphragms, check dams and level spreaders. Proper maintenance of the pre-treatment structures will enhance the functionality of the filtering practice, and result in lower cost and less frequent maintenance activities. With effective pre-treatment, restoring filtration rates for a filtering practice can typically be accomplished by replacing the surface layer of the filter media, as opposed to more substantial excavation and replacement of the underlying sand or engineered organic media filter.

Reference Figure 11 for a typical layout of a surface filter pretreatment structure.

Figure 10. Piped Inflow to Underground Filter

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Surface Layer

The filter basin or chamber typically includes a 3 in top soil and sod, or pea gravel surface layer. The surface layer is separated from the underlying sand or organic filter media by a non-woven filter fabric which serves to reduce the migration of sediment into the filter layer and facilitate removing the surface layer during maintenance operations, reference Figure 12. The filter fabric should be a needled, non-woven, polypropylene geotextile. In underground practices, the pea gravel helps to prevent bio-fouling or blinding of the sand or organic filter surface, reference Figure

13 and the filter chamber shown in Figure 8. Runoff received from the sediment basin or chamber is filtered through the surface layer and enters the sand or organic filter media.

Sediment Forebay

Inlet Pipe Gabion Wall

Figure 11. Surface Filter Pretreatment Structure

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Filter Media

Stormwater is pre-treated in the sedimentation chamber or basin, then filtered through the overlying surface layer and into the filter media. The filter media is either sand or an engineered organic media. Sand filter media consists of clean, washed ASSHTO M-6/ASTM C-33 medium aggregate concrete sand with particle size range of 0.02 – 0.04 in diameter. Engineered organic media generally conforms to the requirements contained within Section 6.5 Compost

Specifications of the Virginia Department of Environmental Quality Stormwater Design Specification No. 4 Soil

Compost Amendment. Often, the organic filter media is comprised of either a peat/sand filter media or a compost filter media, and meets the following criteria as reported by the U.S. Composing Council Seal of Testing (STA)

Compost Technical Data sheet:

• Derived from plant material in compliance with the U.S. Compost STA Assurance program.

• The material shall be well composted, free of viable weed seeds, and stable with regard to oxygen consumption and carbon dioxide generation.

Figure 13. Typical Surface Cover on Surface Filters

Stone Surface Cover Grass Surface Cover

Figure 12. Typical Surface Cover on Underground Filter Chambers

Pea Gravel Cover

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• 100% of the material must pass through a half inch screen.

• pH between 6 and 8.

• Inert material (plastic, concrete, ceramics, metal, etc.) less than 1% by weight.

• Organic matter content between 35% – 65%.

• Soluble salt content less than 6.0 mmhos/cm.

• Maturity index greater than 80%

• Stability of seven or less.

• Carbon/nitrogen ratio less than 25:1.

• Pass the trace metals test.

• Dry bulk density ranging from 40 lbs./ft3 – 50 lbs./ft3.

The organic filter media is typically specialized to remove specific pollutants such as heavy metals or hydrocarbons at stormwater hotspots. Care should be taken to use organic media that is low in soluble phosphorus and nitrogen to reduce the risk of nutrient leaching from the stormwater BMP. Sand and organic filter media depth typically ranges from 12 – 24 in. Greater depths are sometimes used to facilitate the removal of 1 – 3 in of sand during maintenance.

An underdrain is required to convey treated runoff from the filter media, or underlying gravel layer into the clearwell chamber or downstream stormwater conveyance network. Where present, the underlying gravel layer uses washed

Virginia Department of Transportation (DOT) #57 stone to surround the underdrain system. Reference Figure 14 for section view of surface sand filter.

Figure 14. Section View of Surface Sand Filter Media Layer

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Underdrains and Observation Wells

Underdrains are required for all filtering practices. Underdrain pipes are typically 6 in Schedule 40 Polyvinyl chloride

(PVC) or high density polyethylene (HDPE) smooth or corrugated flexible-wall pipe that complies with ASHTO M252 and ASTM F405. Cleanout risers should be installed at the end of all underdrains, with at least one cleanout pipe for every 2000 square feet (ft2) of filter surface area. Observation wells are used to observe the storage and flow conditions through the filtering practice, and can be used to help identify clogged layers. Observation wells are typically constructed of an anchored 6 in diameter Schedule 40 PVC pipe fitted with a lockable cap connected to the underdrain and extending above the filter media surface. The base of the observation well should be perforated and wrapped in geotextile as needed. Reference Figure 15 for photos of cleanouts and underdrains.

Bypass Structures

Filtering practices are typically designed as off-line facilities, receiving runoff up to a designed water quality design flow, and then bypassing the runoff in excess of the design flow around the facility. On-line filtering practices are sometimes installed as the pond floor of an extended detention dry pond.

Off-line surface filtering practices are sized to accept runoff up to the designed water quality treatment volume, and bypass larger flow volumes around the facility using a flow-splitter or diversion weir. This allows the facility to filter the water quality treatment volume while eliminating the need for control structures associated with on-line facilities. The flow splitter or diversion weir directs the first flush of stormwater runoff to the filtering practice until ponding within the bypass structure activates an overflow weir directing runoff around the practice through the bypass stormwater conveyance network. As runoff is treated and exits the filtering practice, additional flow will enter the practice and be treated. The bypass structure may be prefabricated, such as StormGateTM, or cast-in-place during construction.

Off-line underground filtering practices include a bypass pipe or weir connecting the sedimentation chamber directly to the clearwell chamber, or an overflow weir connecting the filter chamber and clearwell chamber allowing overflow to bypass the filter media, reference Figure 16

Cleanouts Underdrains

Figure 15. Cleanouts and Underdrains

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Embankment or Berm

Surface filtering practices include an embankment or berm to pond stormwater within the sedimentation basin, and another to pond pre-treated runoff within the filter basin, reference Figure 17. A perforated standpipe coveys pre-treated runoff from the sedimentation basin to the filter basin. Underdrains convey filtered runoff from the bottom of the filter media to the downstream stormwater conveyance network.

Outfall Structure

All filtering practices require underdrains to convey treated runoff from the BMP. Surface filtering practices typically connect the underdrains directly to a manhole structure for the downstream stormwater conveyance network. Often, the bypass pipe connects to the same manhole as the underdrains. Underground filtering practices include a clearwell chamber which receives flow from the underdrains and bypass pipe or weir, reference Figure 18. The clearwell provides access to the underdrain system, bypass pipe or overflow weir, outfall pipe and a dewatering drain valve. The clearwell chamber connects to the downstream stormwater conveyance network, typically a piped storm drain network accessible by manhole or access hatch. In other cases, the downstream stormwater conveyance network may be a channel.

Bypass Pipe

Dewatering

Valve

Figure 16. Underground Filter Bypass Structures and Dewatering Valve

Filter Basin

Control Structure

Embankment

Figure 17. Embankment

Maintenance and Stormwater Management Division Page 19

Figure 18. Outfall from Clearwell Chamber

Non-Functional Elements for Filtering Practices

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Non-Functional Elements for Filtering Practices

Access

Access must be provided for all filtering practices not located on individual buildable single family detached lots. For stormwater facilities with access roads, the road should be maintained to allow for BMP inspection and maintenance access. Access roads will either be constructed of rock (gravel road), asphalt or geotextile grid. Where access is not available, or no easement exists, county personnel should either obtain an easement or a letter of permission from the owner.

Areas designated for maintenance access should be kept free of plantings, fences, or other obstructions except for standard access road gates to prevent unauthorized vehicular access. Please note that underground filters are classified as an underground confined space, and all OSHA rules and regulations must be followed when entry is required for maintenance purposes.

Signage

Surface filtering practices with above ground ponding areas should be posted with signs designating the area as a water quality management area. Signs should state the facility is a water quality management area, water may pond after a storm, and the area is not to be disturbed except for required maintenance. Signs will be posted at approximately 150 foot (ft) intervals along the perimeter of the facility with a minimum of one sign for each facility.

Underground filtering practices should be stenciled, or have a plaque provided, on the inside of the BMP clearly visible upon removal of the manhole or access hatch designating the facility as a water quality management facility, water may pond after a storm, and the facility is not be disturbed except for required maintenance.

Routine Maintenance

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Routine Maintenance

There is no established routine maintenance program for filtering practices in Fairfax County, Virginia; however, regular inspections and maintenance are important to maintain BMP function. Routine inspections should occur once per year to check on the functionality of the BMP and schedule any required maintenance. Routine inspections should utilize the Fairfax County Maintenance and Stormwater Management Division (MSMD) Sand Filter Inspection

Form. Some general maintenance items include:

• Clean out accumulated sediment, debris and trash from the sedimentation basin or chamber, the filter basin or chamber, the bypass structure and the outfall structure. Sediment is typically removed every 2 – 3 years, or if impeding flow, or exceeds 6 in depth. If the chamber contains an oil skim, it should be removed by a firm specializing in oil recovery and recycling.

• Sand or gravel surface layers in the filter basin or chamber should be raked to a depth of 3 in.

• Mow any vegetated pre-treatment structures to 6 in.

• Mow the entire embankment or berm to 6 in. Mow the top and back of the berm, 10 ft from the toe of the berm, and down the face of the berm to the filtering practice surface.

• Mow the filtering practice surface cover to 6 in (where top soil and sod is present as surface cover).

Avoid the use of heavy equipment within the footprint of the filter basin.

• Remove all cuttings from facility.

• Repair any structural components of the practices where evidence of deterioration is found.

• Exercise dewatering drain valve where present.

Routine inspection of filtering practices should be integrated with the routine maintenance tasks of the surrounding area and scheduled mowing activities. Where maintenance is scheduled, additional inspections on publicly maintained filtering practices should be scheduled within a week of the maintenance, so that the maintenance contractor’s work can be verified.

Inspections are important to ascertain the operational condition and safety of the facility, particularly the condition of all functional elements. Inspections also present an opportunity to determine if there are issues such as encroachments, modifications, or evidence of illicit discharge to the facility. If evidence of illicit discharge is found, please report to the MSMD at 703-877-2800. Previous inspection reports should be used as a reference whenever evaluating the health of a stormwater management facility. Additional information may be found in the comments section of Infor-EAMTM and in the facility’s correspondence folder. The non-routine maintenance section of this guide details the potential issues and corrective solutions for some common maintenance items which may be called out during routine inspections.

Non-Routine Maintenance

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Non-Routine Maintenance

Some common non-routine maintenance issues which may need to be addressed after routine inspections include:

Inflows

Inflows convey stormwater runoff from the drainage area into the filtering practice. Inflow to filtering practices is typically through a piped inflow, often including a bypass structure to limit flows through the stormwater BMP. Sheet flow and surface runoff may also occur for certain types practices. Inflow structures may include energy dissipation mechanisms to ensure non-erosive velocities and limit the resuspension of settled materials within the pre-treatment structure. Filtering practices receive runoff in the following ways:

• Non-structural surface sand filter: pipe inflow, sheet flow

• Structural surface sand filter: piped inflow, sheet flow

• Underground sand filter: piped inflow

• Perimeter sand filter: surface grates

Blockages

Keep inflows to the facility open and in good working condition. Remove deposited sediment, sand, debris and trash at inflow points that could redirect flow away from the filtering practice, reference

Figure 19. Inspect curb cuts, gravel diaphragms and pavement edges which may be susceptible to sediment build up and clogging, causing runoff to bypass the facility during smaller storm events.

Inspect contributing drainage area for sources of sediment or sand and stabilize immediately.

Open up a blocked inflow pipe as soon as practicable. Follow the Debris Removal and Sediment

Removal instructions under Surface Layer and Filter Media on page 27. Remove excess vegetation which blocks access to the inflow. Follow the vegetation removal instructions under Trees, Shrubs and Brush Maintenance on page 44.

Figure 19. Blockage at Inflow Pipe

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Erosion

Repair bare spots or eroded inflow channels as soon as practicable with additional rip-rap armoring or alternative erosion resistant materials. Energy dissipaters and flow spreaders should be checked for structural integrity along with missing or displaced rocks. Inspections should look for exposure of native soils or mono-layer of rocks above native soils. These areas should be immediately stabilized and restored to original design of the energy dissipater.

Repair grass cover within the stormwater facility embankment and grass filter strip where bare spots or erosion are present. Areas with evidence of rill or gully erosion should be re-graded. If necessary, backfill with topsoil and stabilize immediately. Problem areas should be promptly seeded and protected with mulch, or otherwise stabilized, to eliminate the potential for continued erosion and sediment loads entering the facility until grass cover is re-established.

Structural Deterioration (Cracks, Spalling, Rust, Etc.)

Depending on the significance of the deterioration, the structure may need to be replaced, reference

Figure 20. However, if the overall integrity of the structure is reasonably sound, the following repairs are recommended:

Cracked Sections (Concrete)

• Thoroughly clean the surface prior to performing maintenance.

• Seal cracks with an epoxy non-shrink grout.

• Paint the surfaces of the structure with two coats of an epoxy paint.

Cracked Sections (PVC)

• Thoroughly clean the surface prior to performing maintenance.

• Apply sandpaper to repair area to roughen the plastic (allows sealant to seep into pipe).

• Apply sealant designed for plastic or PVC piping following the manufacture recommendations.

• Alternatively, replace the cracked section of the PVC pipe using a hacksaw to remove damaged section, coupling joints and sealant to install new section.

Spalled Sections (Concrete)

• Thoroughly clean the surface prior to performing maintenance.

• Repair spalled areas with a two-part cement bonding agent.

• Paint the affected surfaces of the structure with two coats of an epoxy paint.

Rusted Sections (Metal)

• Scrape rusted surfaces and clean thoroughly.

• Prime all metal surfaces with one coat of rust penetrating primer and paint with two coats of paint.

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Maintenance and Stormwater Management Division Page 24

Separated Structure

If the pipe connection has deteriorated, remove the old connection parging, clean the joint, and seal with an epoxy non-shrink grout. Grout does not provide structural strength to a joint, therefore if the pipe joints have separated or become misaligned, cast in place piping (CIPP) or segmental CIPP may be considered if conditions permit. Generally, separated pipe joints may be CIPP lined as long as no obstructions are protruding into the pipe from the surrounding soil. Any voids which surround the separated pipe section should be filled with grout in conjunction with CIPP lining. In general, misaligned pipes may be CIPP lined as long as the misalignment is no greater than 5 – 10% of the pipe diameter, and the misalignment is within the thickness of the pipe (cannot see outside edge of adjacent pipe). If the misalignment is beyond the acceptable range for CIPP lining, excavation and replacement of the inflow pipe should be considered. Alternative techniques for pipe repair and replacement include Folded Liners, Ferro-cement, pipe bursting, pipe eating, pipe pulling, slip-lining, modified slip-lining and spray lining. Please note that the above discussion is to provide general guidance and all separation and misalignment pipe repairs should be analyzed on a case-by-case basis by a qualified professional.

Separated or misaligned PVC pipe is difficult to repair without digging the pipe up. Separated or misaligned PVC pipe should be evaluated to determine if complete replacement is warranted.

Undermined or Settled Structure

For surface sand filters, remove an undermined/settled structure, reconstruct the subbase, and reset the structure. To preclude subsequent undermining, a concrete curtain wall (attached to the end of the structure) is recommended. Extend this curtain wall a minimum of 6 in below the bottom of the adjacent rip-rap inflow channel. The placement of additional rip-rap in the inflow channel at this location may also be necessary.

Figure 20. Inflow Pipe Structural Deterioration

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Trees, Shrubs and Brush Maintenance

If woody vegetation threatens the structural integrity of the inflow structure, it should be removed and the area stabilized. Check landscaping plans for approved planting locations. Follow the vegetation removal instructions under Trees, Shrubs and Brush Maintenance on page 44. Dispose of all vegetation clippings appropriately.

Pre-treatment Structure

Pre-treatment structures are required for all types of filtering practices. In general, dry or wet sedimentation basins or chambers are used to settle out sediment and debris from the runoff before it enters the filter basin or chamber. Non-structural surface sand filters include alternative pre-treatment structures such as a compost amended grass filter flow paths, gravel diaphragms, check dams and level spreaders.

Blockages

Pre-treatment structures should be monitored for sediment, debris and trash accumulation and cleaned out at regular intervals to maintain functionality. Maintenance plans should schedule cleanouts of the sedimentation basin or chamber every 2 – 3 years, or when inspections indicate that the depth of sediment exceeds 6 in. If the chamber contains an oil skim, it should be removed by a firm specializing in oil recovery and recycling. The accumulated sediment, mud, sand, and debris should be cleaned out from the sedimentation basin or chamber with earth-moving, dredging, or vactoring equipment and disposed of properly. Other types of pre-treatment structures should be cleaned out once per year, or as recommended by the manufacturer instructions.

Open up a blocked pre-treatment structure as soon as practicable. Follow the Debris Removal and

Sediment Removal instructions under Surface Layer and Filter Media on page 27, reference trash and debris accumulation shown in sediment chamber in Figure 21.

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Structural Deterioration (Cracks, Spalling, Rust, Etc.)

Depending on the significance of the deterioration, the structures may need to be replaced. However, if the overall integrity of the structure is reasonably sound, the following repairs are recommended:

Cracked Sections (Concrete)

• Thoroughly clean the surface prior to performing maintenance.

• Seal cracks with an epoxy non-shrink grout.

• Paint the surfaces of the structure with two coats of an epoxy paint.

Cracked Sections (PVC)

• Thoroughly clean the surface prior to performing maintenance.

• Apply sandpaper to repair area to roughen the plastic (allows sealant to seep into pipe).

• Apply sealant designed for plastic or PVC piping following the manufacture recommendations.

• Alternatively, replace the cracked section of the PVC pipe using a hacksaw to remove damaged section, coupling joints and sealant to install new section.

Spalled Sections (Concrete)

• Thoroughly clean the surface prior to performing maintenance.

• Repair spalled areas with a two-part cement bonding agent.

• Paint the affected surfaces of the structure with two coats of an epoxy paint.

Figure 21. Trash and Debris Accumulation in Sedimentation Chamber

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Rusted Sections (Metal)

• Scrape rusted surfaces and clean thoroughly.

• Prime all metal surfaces with one coat of rust penetrating primer and paint with two coats of paint.

Accessibility

Accessibility to the sedimentation basin or chamber is a necessity for maintenance personnel. For underground sand filters, use a lockable access hatch or manhole through the top of the structure and steps or ladder down the inside of the structure. Locking mechanisms should be visually inspected once per year to ensure they are intact and operative. Repair or replace as necessary. If present, the ladder or steps into the sedimentation chamber should be inspected and repaired or replaced as needed to maintain safe access to inside the structure. Any manholes covering the structure should be inspected and repaired or replaced as needed to maintain safe access to inside the structure.

Water Level Too Low

If designed with a wet sedimentation basin or chamber, the water level should be compared with the design plans during inspections. If the water level is lower than plans show, there could be a leak in the sedimentation basin or chamber. Review weather log to determine if lack of rain may be a reason behind low water level. If absence of rain is confirmed, continue to monitor water level after several rain events to determine if low water level is due to lack of rain events, or potentially due to leak. If a leak is suspected, dewater the sedimentation basin or chamber, and remove and dispose of sediment and debris as appropriate. Inspect the sedimentation basin or chamber for cracks, or other evidence of leakage and repair as appropriate.

Bypass Pipe/Overflow Weir Missing or Non-Functional

For underground filtering practices, if the bypass pipe, or overflow weir is missing, blocked, non-functional, inconsistent with plants, or otherwise damaged, repair or install in accordance with approved plans.

Pre-treatment Structure Missing or Non-Functional

If pre-treatment structure is found to be missing, non-functional, inconsistent with plans, or otherwise damaged, repair or re-install in accordance with approved plans.

Surface Layer and Filter Media

The filter basin or chamber includes a 3 in top soil and sod, or pea gravel surface layer. The surface layer is separated from the underlying filter media by a non-woven filter fabric which serves to reduce the migration of sediment into the filter layer and facilitate removing the surface layer during maintenance operations. The filter fabric should be a needled, non-woven, polypropylene geotextile.

In underground practices, the pea gravel helps to prevent bio-fouling or blinding of the sand or

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Maintenance and Stormwater Management Division Page 28 organic filter surface. Runoff received from the sediment basin or chamber is filtered through the surface layer and enters the sand or organic filter media.

Vegetation

Surface sand filters may include a 3 in thick topsoil and sod layer placed on top of the filter media. All sediment, trash and debris should be removed from the top soil and sod layer. Where top soil and sod layer is on approved plans, the grass cover should be inspected for signs of erosion or bare soil and be maintained with at least 95% vegetative cover, reference Figure 22. Problem areas should be promptly seeded. Freshly seeded areas should be protected with a biodegradable erosion mat that has been secured in place to prevent flotation, or otherwise stabilized, to eliminate the potential for continued erosion until grass cover is re-established. Stakes should be avoided to eliminate the risk of puncturing the underlying geotextile fabric. The following outlines maintenance procedures for grass maintenance:

Repairs

• Vigorously rake any bare areas and cover with enough top soil to bring back to grade.

• Areas with evidence of rill or gully erosion should be re-graded if necessary, backfilled with topsoil and stabilized immediately.

• Over-seed these areas (and any other disturbed areas) and cover with straw (mulched) for re-stabilization. Check the design plans for approved grass seed mix for the filtering practice.

• If vegetation fails to establish, test soil for proper liming and fertilization rates of the top soil.

Whenever possible, fertilizer additions should be kept to a minimum.

• For significant repairs, use a biodegradable erosion mat and securely fasten.

• Arrange for watering if large enough area.

Mowing

• Mow grass during both growing seasons to discourage woody growth.

• Mow the facility surface cover to 6 in (where top soil and sod is present as surface cover).

• If the grass height is maintained lower than 4 in height, more frequent mowing (ten to fifteen times per year) will be required to avoid cutting too much plant per mowing pass.

• Remove all clippings from facility.

Seeding

• Apply seed during the beginning months of both growing seasons (April and September).

• Apply lime any time from March through November.

• Apply fertilizer sparsely and only in accordance with nutrient management plan and manufacturer instructions in October/November. Whenever possible, fertilizer additions should be avoided.

If the original approved vegetation is winter- or salt-killed, consider replacing the original approved sod layer with a hardier native grass species appropriate for the stormwater BMP environment. Note that snow, leaves, mulch or any other material should never be piled or stored on the surface of the filtering practice.

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Fertilizer and pesticide use should not be required, will degrade the pollutant removal capability of the facility and will likely introduce pollutant loads to the downstream environment. If absolutely necessary, soil tests can be performed to establish need for a one-time, spot fertilization after the first growing season to increase vegetative density.

If unauthorized plantings are found within the facility, coordinate with the owner, Homeowners’

Association (HOA), or other local organizations to provide education and outreach on what the facility is and proper maintenance of it. Return the facility to the functionality by removing the unauthorized plantings and re-establishing the approved surface cover shown on design plans. Follow the vegetation removal instructions under Trees, Shrubs and Brush Maintenance on page 44. Dispose of all vegetation clippings appropriately.

Invasive species and weeds should be removed by hand without the use of pesticides. Established weeds that have already seeded may take multiple years to eradicate. Do not allow weeds to go to seed and remove as soon as possible. Invasive species should be identified and monitored to prevent complete takeover.

Debris Removal

Debris removal is an integral part of facility maintenance. Trash and debris cleanup should be performed by hand at least once per year to remove trash and floatables that tend to accumulate in the facility. Regularly removing debris and litter is well worth the effort and can be expected to help reduce the chance of clogging and failure in the surface layer, filter media, underdrain, observation wells and cleanout risers, outfall structures, and other facility components. This prevents damage to vegetated areas and stagnant pool formation. Ensuring proper functionality also helps maintain facility aesthetics and reduce conditions for mosquito habitat. Ponding water should not be present in the facility beyond the designed drawdown time for a particular facility and design storm. If there is still ponded water on the surface or in the observation wells 48 hours after a storm event, the facility is considered to be clogged and should be diagnosed and rehabilitated as discussed under

Sediment Removal below. Please note that some filtration practices are designed with wet

Figure 22. Erosion of Surface Vegetation

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Maintenance and Stormwater Management Division Page 30 sedimentation basins or chambers which include a permanent pool of water. The design plans should always be reviewed prior to inspection and requests for maintenance.

Sediment Removal

Sediment accumulation within the filter basin or chamber will clog the surface layer and underlying filter media, reference Figure 23. Clogging will result in lost efficiency of the filtering practice and may necessitate costly remediation.