10-Attachment F-Task 2.pdf

Post-Construction Maintenance Guide

7.4 Bioretention Basin and Filter

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 Bioretention Basins and Filters

Inflows and Pre-treatment

Filter Media and Surface Cover

Vegetation

Underdrains, Stone Sumps and Observation Wells

Control & Bypass Structures

Embankment or Berm

Emergency or Overflow Spillway

Outfall Structure

Non-Functional Elements for Bioretention Basins and Filters

Access

Signage

Landscaping Plan

Routine Maintenance

Non-Routine Maintenance

Embankment/Berm and Vegetated Overflow Spillway

Turf Maintenance

Surface Erosion, Ruts, and Worn Paths Maintenance

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 Berm Missing

Vegetated Emergency or Overflow Spillway

Blockages and Encroachments

Absence of Overflow spillway or Adequately Sized/Constructed Overflow Spillway iii

Washed-out/Severely Eroded Overflow Spillway

Combined Spillway (Not Typically Found)

Blockages

Inadequately Sized or Constructed Overflow Spillway

Flat Top Trash Rack

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

Control Structure

Blockages of Low Flow Orifice

Blockages of Top Trash Rack (Where Present)

Missing BMP (Water Quality) Plate/Device (Where Present)

Accessibility

Trees, Shrubs and Brush Maintenance

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

Undermined or Settled Control Structure

Underdrain, Principal Spillway Pipe and Observation Well

Blocked or Silted in Underdrain or PSP

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

Deteriorated Underdrain or PSP Connection to the Control Structure or Outfall Structure

Structural Integrity

Leaking Joints

Missing Underdrain

Missing Observation Well

Damaged/Missing Observation Well Cap

Accessibility

Outfall Structure

Accessibility

Trees, Shrubs and Brush Maintenance

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

Separated Structure

Undermined or Settled Structure

Outfall Channel

Blockages iv

Erosion

Bioretention Ponding Area, Surface and Filter Media

Vegetation

Debris Removal

Sediment Removal

Surface Cover Remediation

Filter Media Remediation

Rodents

Bioretention Area Missing

Bioretention Area Not Sized to Plans

Bypass Structure

Accessibility

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

Blockage

Separated Structure

Upstream Inflows and Pre-treatment

Blockages

Erosion

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

Separated Structure

Undermined or Settled Structure

Trees, Shrubs and Brush Maintenance

Pre-treatment Structure Missing or Non-Functional

Routine Maintenance Corrective Action and Annual Frequency Table

Non-Routine Maintenance Corrective Action and Inspection Frequency Table

References

Externally Sourced Figure References v

List of Figures

Figure 1. Typical Bioretention Filter and Basin Plan View

Figure 2. Typical Bioretention Basin and Filter Section View (Level I and II)

Figure 3. Typical Rain Garden Section View (Level I and II)

Figure 4. Bioretention Configurations

Figure 5. Typical Bioretention Inflow and Pre-treatment

Figure 6. Common Inflow Types

Figure 7. Surface Cover

Figure 8. Vegetation

Figure 9. Observation Wells and Underdrains

Figure 10. Control Structures

Figure 11. Typical Bioretention Berm

Figure 12. Bioretention Berm

Figure 13. Emergency Spillway

Figure 14. Outfalls

Figure 15. Signage

Figure 16. Groundhog

Figure 17. Fracturing

Figure 18. Berm Missing

Figure 19. Blocked Underdrain

Figure 20. Deteriorated Connection

Figure 21. Vegetable Garden Planted in Place of Bioretention Plantings

Figure 22. Wood Pile Stacked on Bioretention Surface

Figure 23. Bioretention Surface Clogged with Sediment

Figure 24. Mulch Depth Not Adequate

Figure 25. Blockage at Inflow

Form and Function

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 1

Form and Function

Bioretention is a stormwater best management practice (BMP) that captures runoff from small drainage areas

(typically ≤ 2.5 acres) using shallow landscaped depressions that may provide water quality improvement along with runoff detention and retention. Pollutant removal mechanisms include adsorption, filtration, sedimentation (physical settling), volatilization, ion exchange, and biological decomposition. Bioretention facilities are commonly comprised of a pre-treatment overland flow grass filter strip, a temporary ponding area, a double shredded hardwood mulch layer with plantings, an engineered filter media, a stone reservoir layer and an underdrain (if underlying soils do not meet infiltration criteria), see Figures 1, 2, 3, and 4. During storm events, runoff flows through the grass filter strip and temporarily ponds above the mulch layer. The ponded water gradually passes through the filter media, into the stone reservoir and exits either through seepage into the underlying soils or through an underdrain connected to a downstream stormwater conveyance system. Moisture retained by the soil is used to sustain the vegetation and removed through evapotranspiration. An underdrain is typically a perforated Polyvinyl chloride (PVC) pipe located within the stone reservoir. Inflow to bioretention facilities is typically sheet flow. Concentrated flow is acceptable when coupled with energy dissipation mechanisms such as level spreaders, gravel diaphragms or rip-rap located at points of concentrated inflow. There are several different types of bioretention facilities: bioretention filters, bioretention basins, and rain gardens.

Bioretention filters are bioretention facilities with an underdrain located at the bottom of the stone reservoir layer.

Underdrains are utilized when the underlying soils have low infiltration rates or the facility is surrounded by an impermeable liner due to high groundwater or soil contamination risks. Underdrains allow treated runoff to freely discharge from the facility into a downstream stormwater conveyance system.

Bioretention basins are intended to maximize runoff retention by either not including an active underdrain or containing a storage layer below the underdrain. Bioretention basins are utilized when the underlying soils are highly permeable, there is a low groundwater table and a low risk of groundwater contamination. Bioretention basins rely on seepage to underlying soils to drain the facility between storm events. Bioretention basins, as utilized in Fairfax

County, generally include capped or restricted underdrains to allow conversion of the facility to a bioretention filter if the infiltration capacity of underlying soils reduces over time.

Bioretention basins and filters have maximum drainage areas of 2.5 acres. The filter media depth ranges from 24 inches (in) to 6 feet (ft). Pre-treatment is required by a combination of a pre-treatment cell (similar to a sediment forebay), grass filter strip, gravel diaphragm, gravel flow spreader or another approved (manufactured) pre-treatment device. The bioretention floor is vegetated with a combination of grass and forb cover, shrubs and trees with a vegetative surface coverage of 75 – 90% within 2 years. Fairfax County has decided to avoid the use of trees in bioretention facilities for all future planting plans.

Rain gardens are small-scale or micro-bioretention practices which are used on an individual residential lot. Rain gardens are small, distributed stormwater management practices designed to treat runoff from small areas, such as individual rooftops, driveways and other on-lot features in single-family residential developments. The maximum drainage area for a rain garden is 0.5 acres with only 0.125 acres coming from impervious surfaces. Pre-treatment devices include leaf screens, grass filter strips and energy dissipaters. The filter media depth ranges from 18 – 36 in.

The bioretention floor is vegetated with grass and forb cover, shrubs and trees depending on the design of the

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 2 facility. Fairfax County has decided to avoid the use of trees in rain gardens for all future planting plans. Rain gardens may include an underdrain depending on the infiltration characteristics of the underlying soils.

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 3

Figure 1. Typical Bioretention Filter and Basin Plan View

Source: Fairfax County Public Facilities Manual

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 4

Figure 2. Typical Bioretention Basin and Filter Section View (Level I and II)

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

Maintenance and Stormwater Management Division Page 5

Figure 3. Typical Rain Garden Section View (Level I and II)

Source: Virginia DEQ BMP Clearinghouse

Maintenance and Stormwater Management Division Page 6

Ponding Area

Plantings

Mulch Layer

Berm

Figure 4. Bioretention Configurations

Functional Elements for Bioretention Basins and Filters

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 7

Functional Elements for Bioretention Basins and Filters

Inflows and Pre-treatment

Inflows convey stormwater runoff from the drainage area into the bioretention facility, see Figures 5 and 6. Pre-treatment is required for all bioretention facility inflows. Pre-treatment structures are used to stop coarse sediment and debris from entering and ultimately clogging the bioretention facility. Proper maintenance of the pre-treatment structures will enhance the functionality of the bioretention facility, and result in lower cost and less frequent maintenance activities. With effective pre-treatment, restoring infiltration rates for a bioretention facility can typically be accomplished by replacing the mulch layer and top few inches of filter media, as opposed to more substantial excavation and soil replacement. Pre-treatment devices may include leaf screens (for rooftop discharges), grass filter strips, energy dissipaters (rip rap inflow channel, concrete splash pad, etc.), gravel/stone diaphragms, gravel/stone flow spreaders, pre-treatment cells (similar to a sediment forebay), and other proprietary devices. Pre-treatment cells may be formed by a wooden or stone check dam, or an earthen or stone berm. Where present, pre-treatment cells should be designed with a storage volume of at least 15% of the water quality treatment volume.

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 8

Figure 5. Typical Bioretention Inflow and Pre-treatment

Maintenance and Stormwater Management Division Page 9

Figure 6. Common Inflow Types

Gravel Diaphragm

Curb Inflow

Rip-Rap Inflow

Grassed Swale Inflow

Piped Inflow

River Rock Inflow

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 10

Filter Media and Surface Cover

Bioretention facilities route surface runoff into subsurface soils or media where it is temporarily held in pore spaces before evaporating, transpiring from vegetation, seeping into surrounding subsoils and groundwater, or exiting through underdrains. The filter media and surface cover provide the primary mechanism for pollutant removal through filtration which captures sediment and particle-bound pollutants. Some practices include specialized media and/or internal water storage zones to promote the removal of dissolved constituents such as orthophosphate phosphorus and oxidized nitrogen through sorption and other biogeochemical interactions.

Filter media depths in bioretention basins and filters range from a minimum of 24 in (Level 1) to 36 in (Level 2) and can extend to a maximum depth of 6 ft. If trees are included in the planting plan, the filter bed must be at least 4 ft deep. For rain gardens, the filter media minimum depth ranges from a minimum of 18 in (Level 1) to 24 in (Level 2) and can extend to a maximum depth of 36 in.

Surface cover is often achieved through a 2 – 3 in layer of mulch with the expectation that 70-95% of the area will be covered by mulch and vegetation. The mulch layer enhances plant survival, suppresses weed growth, and filters out a portion of the sediment and debris which enters the facility before it reaches the filter media. Double shredded, aged hardwood bark mulch is recommended for surface cover in order to minimize the potential for mulch to float and migrate. Alternative surface cover includes turf grass, native groundcover, erosion control matting (coir or jute matting), river stone, or pea gravel, reference Figure 7.

Maintenance and Stormwater Management Division Page 11

Figure 7. Surface Cover

Mulch Cover Forb Cover

Grass Cover

Stone Cover

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 12

Vegetation

A variety of vegetation types, including grasses, shrubs and small trees may be used to promote evapotranspiration, maintain soil porosity and encourage biological uptake and/or degradation of some pollutants, see Figure 8. Pollutant removal may be maximized by selectively harvesting and disposing of bioretention vegetation. Turf grass (fine fescue and tall fescue) cover is typically used only in residential rain garden applications. Bioretention basins and filters are typically vegetated with a combination of native grasses and forbs, shrubs and trees. Fairfax County promotes the use of native vegetation and has decided to avoid the use of trees in bioretention facilities for all future planting plans.

The landscaping plan includes a delineation of planting areas, the planting plan, post-nursery care and initial maintenance requirements.

Figure 8. Vegetation

Shrubs

Grasses and Forbs

Shrubs and Trees

Grasses and Forbs

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 13

Underdrains, Stone Sumps and Observation Wells

Unless subsoils are highly permeable, underdrains are required in bioretention (typically 6 in schedule 40 PVC). A rain garden may only be installed without an underdrain if the infiltration rate of subsoils exceeds 1 in per hour. A bioretention basin may only be installed without an underdrain if the infiltration rate of the subsoil exceeds 1/2 in per hour. Bioretention facilities may also be constructed with a capped or plugged underdrain. A capped or plugged underdrain is often utilized by Fairfax County in bioretention basins allow conversion of the facility to a bioretention filter by removing the cap or plug if the infiltration capacity of underlying soils reduces over time.

Stone sumps, or underground storage layers, are required for Level 2 bioretention facilities. The stone sumps are comprised of clean, washed #57 stone or an approved infiltration module such as a high-density polyethylene

(HDPE) chamber or concrete vault. The stone sumps are located underneath the filter media and underdrain, and are typically 6 – 12 in. in depth. The stone sump provides an anoxic environment where denitrification may occur given enough time and a suitable carbon source.

Observation wells are required for most bioretention facilities. Observation wells are used to observe the flow conditions through a bioretention facility, and can be used to help identify clogged layers. Cleanout pipes may be provided and connected to the observation well and underdrain (where present) when drainage areas exceed 1 acre.

Reference Figure 9 for photos of observation wells and underdrains.

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 14

Control & Bypass Structures

Bioretention facilities may either be designed as on-line or off-line facilities. On-line facilities receive the entire runoff volume from the contributing drainage area during all storm events. Off-line facilities receive the runoff up to a designed volume, and then bypass the flows around the facility after the target storage volume has been reached within the facility.

On-line bioretention facilities require a control structure to safely convey larger storm events through the facility, reference Figure 10. The control structure associated with the 2-year and 10-year design storms should be sized to release flows at non-erosive velocities at the outfall. Bioretention overflow structures are control structures with an invert at the maximum ponding depth. Control structures for bioretention basins and filters allow stormwater to pond within the facility, typically 6 – 12 in, to increase the amount of water which is infiltrated into the facility before an overflow weir or pipe is activated. Bioretention basins and filters are typically sized to provide water quality treatment for smaller storm events (1-year and 2-year), but may be designed to safely convey storms up to the 10-year event without damage to the facility. Ideally, larger events should be diverted around the bioretention facility, or through an emergency or overflow spillway. Once the overflow pipe or weir is activated, the control structure reduces the peak

Figure 9. Observation Wells and Underdrains

Observation Well

Observation Well Looking Down

Underdrain

Underdrain

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 15 discharge, and may provide some level of channel protection and flood control, depending upon the design. A trash rack may be required on the overflow pipe or weir to prevent litter and debris from entering and clogging the overflow device and underdrain (where present). The overflow pipe or weir controls the flow rate into the underdrain (where present) which conveys the runoff through the embankment and outfall.

Off-line bioretention facilities 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 bioretention facility to infiltrate the water quality treatment volume while eliminating the need for control structures associated with on-line facilities. In general, a flow splitter or diversion weir directs the first flush of stormwater runoff to the bioretention facility until ponding within the bypass structure sends runoff around the facility through the bypass stormwater conveyance network. As runoff infiltrates into and through the facility, additional flow will be able to enter the facility and be treated. The bypass structure may be prefabricated, such as StormGateTM, or cast-in-place during construction.

Embankment or Berm

The embankment is constructed to pond stormwater inflows upstream of a control structure and allow for the infiltration and controlled release of stormwater flows through an overflow device or emergency or overflow spillway, reference Figures 11 and 12. Essentially, the embankment holds the water back. The underdrain (where present) conveys water, and precedes an overflow pipe or weir control structure, through the embankment to discharge via the outfall.

Figure 10. Control Structures

Control

Structure

Control Structure

Maintenance and Stormwater Management Division Page 16

Figure 11. Typical Bioretention Berm

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 17

Emergency or Overflow Spillway

For on-line facilities that do not bypass larger storms around the facility, an emergency or overflow spillway allows major storm events (typically storms over the 10-year event) to pass through the facility without causing damage to the embankment, reference Figure 13. Without an overflow spillway, the embankment could be overtopped during large storm events causing erosion, scouring and ultimately embankment failure. A separate or overflow spillway functions as a reinforced weir in the embankment located away from the underdrain which allows for controlled release of stormwater during major storm events without subjecting non-reinforced portions of the embankment to damaging high flows.

Berm

Emergency Spillway

Figure 12. Bioretention Berm

Emergency Spillway

Berm

Figure 13. Emergency Spillway

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 18

Outfall Structure

The outfall structure is located on the downstream face of the dam embankment and connects the underdrain or principal spillway pipe (PSP) with the downstream receiving water body (typically a stream, channel or piped storm drain network), reference Figure 14.

Figure 14. Outfalls

Outfall to piped storm drain network

Outfall to concrete swale Outfall to rip rap swale

Outfall to natural channel

Non-Functional Elements for Bioretention Basins and Filters

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 19

Non-Functional Elements for Bioretention Basins and Filters

Access

Access must be provided for all bioretention facilities not located on individual buildable single family detached lots.

For stormwater facilities with access roads, the road should be maintained for 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.

Signage

Posted signs should be provided designating the area as a water quality management area, reference Figure 15.

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 ft intervals along the perimeter of the bioretention facility with a minimum of one sign for each facility.

Landscaping Plan

A landscaping plan is required for all bioretention facilities. The landscaping plan should include:

• Bioretention template used.

• Delineation of planting areas.

• Planting plan including the size, list of planting stock and sources of plant species.

• Planting sequence including post-nursery care and initial maintenance requirements.

Figure 15. Signage

Routine Maintenance

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 20

Routine Maintenance

Routine maintenance occurs four times per year for bioretention facilities, and includes three maintenance periods during the growing season and one over the winter. Summer maintenance should include:

• Removal of accumulated trash.

• Clearing and removing accumulated sediment from any inlets and pre-treatment areas.

• Mowing the grass filter strip to 6 in

• Maintaining the bioretention vegetation. Herbaceous plants should be pruned to remove dead stems and branches. Any missing plants should be noted for replacement.

• Remove all cuttings from the facility and dispose of correctly.

• Maintain 2 – 3 in of mulch.

• Remove all weeds (plants not specified on the planting plan) from the facility and dispose of properly.

Winter maintenance includes all of the summer maintenance activities. Additionally, senesced or wilted/dying above ground vegetation should be pruned or removed and disposed of properly. Mowing will not be necessary during the winter maintenance.

Routine inspection of bioretention facilities should be integrated with the routine maintenance tasks of the surrounding area and scheduled mowing activities. Inspections on publicly maintained bioretention facilities are scheduled within a week of routine maintenance when possible, so that the maintenance contractor’s work can be verified and to ensure overgrown vegetation does not hamper the facility’s assessment. 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 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

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 21

Non-Routine Maintenance

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

Embankment/Berm and Vegetated Overflow Spillway

An embankment is constructed to pond 6 – 12 in of stormwater within the facility and allow for the infiltration and controlled release of stormwater flows through an overflow device or spillway.

Essentially, the embankment holds the water back. For on-line facilities which do not bypass larger storms around the facility, an overflow spillway allows major storm events (typically storms over the

10-year event) to pass through the facility without causing damage to the embankment. A vegetative overflow spillway functions as a reinforced weir in the embankment located away from the underdrain which allows for controlled release of stormwater during major storm events without subjecting non-reinforced portions of the embankment to damaging high flows. The embankment is referred to as a cut dam when the facility is constructed as an open basin by excavating and re-grading below existing ground levels. The embankment is referred to as a fill dam when the facility is constructed by building an earthen embankment across a natural drain course. Most bioretention facility embankments are constructed as cut dams.

Turf Maintenance

All sediment, trash and debris should be removed from the embankment. The grass cover of the embankment should be inspected for signs of erosion or bare soil and appropriate vegetative cover.

Problem areas should be promptly seeded. Freshly seeded areas should be protected with a biodegradable erosion mat that has been securely staked in place with biodegradable stakes to prevent flotation, or otherwise stabilized, to eliminate the potential for continued erosion until grass cover is re-established. The following outlines maintenance procedures for turf maintenance:

Repairs

• Vigorously rake any bare areas on the embankment and cover with a 2 in layer of topsoil.

• Over-seed these areas (and any other disturbed areas) and cover with straw (mulched) for re-stabilization. A Kentucky-31 (K-31) tall fescue grass seed is recommended. During dormant/semi-dormant periods, a K-31 mixture with 10% annual rye may be used to establish quick ground cover.

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

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 22

• Mow grass during both growing seasons to discourage woody growth. Do not leave rows of cut grass and avoid rutting by tires.

• Mow the grass filter strip.

• Mow the embankment, 10 ft from the heel of the embankment and down the face of the embankment to the bioretention surface cover.

• Cut grass to 6 in height.

• If the grass height is maintained lower than 4 in height, more frequent mowing (10 – 15 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 in accordance with nutrient management plan and manufacturer instructions in October/November. Whenever possible, fertilizer additions should be avoided.

Surface Erosion, Ruts, and Worn Paths Maintenance

The greatest threat to an earthen embankment is erosion. Erosion results from surface runoff, pedestrian, and vehicular or animal traffic. Areas with evidence of rill or gully erosion should be re-graded if necessary, backfilled with topsoil and stabilized immediately. Problem areas should be promptly seeded. Freshly seeded areas should be protected with a biodegradable erosion mat that has been securely staked in place to prevent flotation, or otherwise stabilized, to eliminate the potential for continued erosion until grass cover is re-established. Special attention should be given towards solving the source of the erosion, including:

• Maintaining a healthy stand of grass.

• Providing paved trails for pedestrian access.

• Providing convenient access points and travel surfaces for maintenance vehicles.

• Providing lockable gates/barricades and signage to prohibit unauthorized vehicular access.

• Use alternate mowing directions, along privately maintained embankments, to avoid rutting.

Shrinkage Cracks Maintenance

During dry seasons, the clay in embankments may shrink, resulting in “alligatoring”, or small cracking.

Though typically not a significant threat, shrinkage cracks can progressively worsen through rainfall infiltration and freeze-thaw cycles. Provide close monitoring for such situations. Repair cracks wider than 1 in and/or deeper than 1 ft, cover with a 2 in layer of topsoil, and plant with grass.

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 23

Trees, Shrubs and Brush Maintenance

In Fairfax County, earthen embankments and areas within 10 ft of the embankment’s upstream heel and downstream toe are typically to be kept free of trees, shrubs, ornamental plantings, and scrub brush. Tree roots can be extensive, extending into the bioretention facility filter media. These roots can provide paths for seepage and may even weaken the embankment compacted soil. Check landscaping plans for approved planting locations on the embankment. Dispose of all vegetation clippings appropriately.

Shrubs, Brush, and Small Trees Less than 4 in Diameter

Small trees, shrubs, brush, and other woody vegetation may prevent observation of embankment surfaces and may provide a haven for burrowing animals. Inspect for any poisonous or nuisance vegetation which may constitute a hazard to maintenance personnel or the public. Carefully remove any noxious weeds such as Poison Ivy, Poison Oak, Poison Sumac and Giant Hogweed. Ensure exposure to these noxious weeds is kept to an absolute minimum while removing.

Existing ornamental trees, shrubs, and other landscape plantings may be transplanted from the embankment, heel, and toe to other more appropriate locations. Fall is the optimum season for transplanting. Clear all other small trees and dense vegetation as soon as practicable. Grind down the stumps of the small trees and grub-out to approximately 6 in below the ground surface. Repair all disturbed areas, as discussed above under Turf Maintenance on page 21.

Trees Greater Than 4 in Diameter

Large trees can uproot, leaving large voids and depressions which may weaken the embankment or lead to erosion. Shrubs, brush, and small woody stock obscure embankment surfaces and provide a haven for burrowing animals. Remove trees greater than 4 in diameter and grub-out all roots greater than 1 in diameter. Due to the significant coordination required to complete tree clearing operations, the following time schedule is recommended:

First Fall Season

• Coordinate with all affected/surrounding property owners.

• Remove all trees, leaving a 2 – 3 ft tall stump for future identification and stump removal leverage.

Following Fall

• Coordinate with all affected/surrounding property owners.

• Grub-out the stumps and all roots greater than 1 in diameter, filling the voids with a soil-cement slurry.

• Repair and stabilize all disturbed areas.

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 24

Following Spring

• Provide additional grass stabilization as necessary.

A suggested alternative to the grubbing operation above is the careful removal of roots by attaching a chain to individual roots and systematically pulling them out with machinery. “Mud-pack” the voids left

(fill with a soil-cement slurry) and cover with a 6 in layer of topsoil. Timing and coordination should follow the description in this section.

Rodents

The embankment should be checked for animal burrows. Animal burrows must be backfilled and stabilized. Burrowing animals should be humanely removed from the area. It should be noted that a well-maintained (mowed) embankment discourages rodent infestation and habitation.

The burrowing habits of groundhogs make them the most destructive rodents in bioretention facilities.

Their burrows will have many tunnels, chambers, and multiple entrances which threaten the integrity of earthen embankments. Occupied burrows can be identified by piles of fresh dirt on the downstream face of an embankment. Commence controlling groundhog infestation in early spring when burrows are active and easier to find. If control measures are delayed, other animals may seek refuge in a groundhog burrow, so quickly addressing the burrow is important. Reference Figure 16 for photo of groundhog.

After removing animals, "Mud-pack" (fill with a soil-cement slurry) burrow holes and cover with a 6 in layer of topsoil. The following slurry mix is recommended: one-part cement, nine-parts soil (primarily sand), and enough water to enable the slurry to flow.

Figure 16. Groundhog

Source: U.S. National Oceanic and Atmospheric Administration

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 25

Mosquitoes

Mosquitoes reproduce by laying eggs in stagnant pools of water. A few inches of standing water can become a mosquito breeding habitat. Stagnant water for 4 – 5 days is enough to span a single generation of mosquitoes. Any facility which does not drain within 48 hours is considered clogged. All clogged structures, or poorly graded areas which are causing the facility to have a stagnant water should be repaired immediately.

Piping Failure (Internal Erosion)

The embankment should be inspected for signs of piping or seepage. Piping is the progressive loss of soil from an earth embankment (usually around the underdrain or PSP) as a result of concentrated seepage through the embankment. The repair/mitigation of piping may be fairly complex, as well as costly. Proper repair may include replacement of the outlet works and partial reconstruction of the embankment. To avoid cave-ins, areas with evidence of piping or seepage should be addressed immediately.

Wet areas, or soft spots, on the downstream face or toe of the embankment may also be indicative of seepage through the embankment. Monitor these areas periodically for any changes in the nature of the wet areas and/or the development of slope failure or slippage areas. If such a condition exists, identify the cause and repair as needed.

Hydraulic Failure (Overtopping Erosion)

Erosion head-cuts and debris on the downstream face and/or at the downstream toe of an embankment are evidence that the embankment has overtopped. Immediately repair eroded areas and investigate the causes of the overtopping. Overtopping conditions are usually the result of very intense rain events, severe loss of impoundment volume, such as due to sediment, and/or an inadequate or blocked overflow spillway.

Depressed Areas and Settling

Depressed areas may (or may not) be indicative of a problem. Depressed areas can often be the result of poor final grading at original construction or subsequent maintenance activities (i.e., the removal of a tree, etc.). If the cause is indeterminate based on a lack of institutional history regarding the embankment, monitor the depressed areas. Establish control points in these areas and survey periodically to chart any additional settlement/movement. Settling can be an indication of more severe problems within the embankment. A qualified professional should be consulted to determine the source of the settlement and if the embankment should be built back to the design elevation and grade.

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Maintenance and Stormwater Management Division Page 26

Fracturing

Fractures are defined as cracks wider than 1 in and deeper than 3 ft. Typically, fractures will have very defined directions (i.e., parallel to the axis of the embankment, perpendicular to the axis of the embankment, etc.). Such a condition is indicative of differential settlement in the embankment and/or foundation support problems. Fractures can seriously threaten the structural integrity of an embankment. If such a condition exists, repair as needed as soon as possible. Reference Figure 17 for photo of fracturing.

Embankment or Berm Missing

Where the embankment or berm is found to be missing, contact the Maintenance and Stormwater

Management Division (MSMD), reference Figure 18. The facility will be researched using the Fairfax

County Land Development Services library to determine if any facility modifications have been

Figure 17. Fracturing

Source U.S. Natural Resources Conservation Service

Figure 18. Berm Missing

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 27 approved. The embankment or berm will need to be re-established if no supporting documentation is identified. During re-establishment activities ensure heavy equipment or vehicles are not allowed to drive on the filter bed.

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 28

Vegetated Emergency or Overflow Spillway

For on-line facilities which do not bypass larger storms around the facility, an overflow spillway allows major storm events (typically storms over the 10-year event) to pass through the facility without causing damage to the embankment. A vegetative overflow spillway functions as a reinforced weir in the embankment located away from the underdrain which allows for controlled release of stormwater during major storm events without subjecting non-reinforced portions of the embankment to damaging high flows.

Blockages and Encroachments

Remove all sediment, trash and debris blocking the entry of storm flows into or the passage of storm flows through the overflow spillway. Coordinate with property owner to remove all structures not shown on approved plans (encroachments).

Absence of Overflow spillway or Adequately Sized/Constructed Overflow Spillway

The best assurance against embankment overtopping is an adequate overflow spillway. Provide an adequate overflow spillway if one does not currently exist as soon as possible.

Washed-out/Severely Eroded Overflow Spillway

In the case of an overflow spillway in hydraulic failure, repair as soon as possible.

Combined Spillway (Not Typically Found)

Bioretention facilities do not typically have a combined spillway. However, the Fairfax County

Bioretention Inspection Form includes a Top Trash Rack and Anti-Vortex Plate section within the

Control Structure section. Top trash racks and anti-vortex plates are typically associated with combined spillways. For that reason, some common maintenance items for combined spillways have been included in this guide.

Blockages

Remove all sediment, trash and debris blocking the entry of storm flows into or the passage of storm flows through the combined spillway.

Inadequately Sized or Constructed Overflow Spillway

The best assurance against embankment overtopping is an adequate combined spillway designed to pass the larger storms (greater than the 10-year storm event). Where required, provide an adequate spillway if one does not currently exist.

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Maintenance and Stormwater Management Division Page 29

Flat Top Trash Rack

Flat top trash racks are very susceptible to blockages from floating/settling debris. A peak roofed retrofit trash rack is recommended as it will grant a larger factor of safety against debris blockage.

Typical non-proprietary details for such racks can be obtained from MSMD. All drop inlet spillways designed for pressure flow should have adequate anti-vortex plate or device. An anti-vortex plate or device is not required if weir control is maintained in the control structure through all flow stages.

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

Depending on the significance of the deterioration, the combined spillway structure (control structure) may need to be replaced. However, if the overall integrity of the structure is reasonably sound, perform the repairs outlined under Control Structure on page 28.

Control Structure

On-line bioretention facilities require a control structure to safely convey larger storm events through the facility. The control structure associated with the 2-year and 10-year design storms should be sized to release flows at non-erosive velocities at the outfall. Bioretention overflow structures are control structures with an invert at the maximum ponding depth. This may occur in the form of a vertical control structure or horizontal pipe similar to what you might find in a wet or dry pond. Control structures for bioretention basins and filters allow stormwater to pond within the facility, typically 6 –

12 in, to increase the amount of water which is infiltrated into the facility before an overflow weir or pipe is activated. Bioretention basins and filters are typically sized to provide water quality treatment for smaller storm events (1-year and 2-year), but may be designed to safely convey storms up to the

10-year event without damage to the facility. Once the overflow pipe or weir is activated, the control structure reduces the peak discharge. A trash rack may be required on the overflow pipe or weir to prevent litter and debris from entering and clogging the overflow device and underdrain.

Blockages of Low Flow Orifice

Bioretention facilities are typically designed to drain within 48 hours after storm events. Depending on the design of the bioretention facility, the drawdown time of ponding water above the control structure

(riser/pipe end/weir) will vary. The low flow orifice activates during storm events where the rate of inflow exceeds the rate of infiltration and ponding occurs. The invert of the low flow orifice is set at the maximum ponding depth for the facility (typically 6 – 12 in above the facility floor). Once ponding exceeds the maximum depth, the low flow orifice controls the flow of runoff out of the facility to the downstream stormwater conveyance network. Refer to the plans of the facility to determine the designed drawdown time after a storm event. If ponding water is present for more than 48 hours, or evidence of prolonged standing water is present, determine the cause of the standing water and rehabilitate the facility. Evidence of a clogged or slow draining facility includes mulch floatation, ponding beyond the draw-down time, dead plants, offensive odors, and algae or presence of wetland vegetation.

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Maintenance and Stormwater Management Division Page 30

To determine why the facility has standing water, check for flow through the control structure and underdrain. Bioretention facilities may clog in several locations including the control structure, underdrain, filter bed surface and within the filter media. The low flow orifice/weir and trash rack should be visually inspected at least once a year:

• The low flow orifice, weir or riser structure may be clogged due to excessive debris or sediment buildup in the riser or underdrain. Clean and repair as necessary.

• Where not installed, a low flow trash rack (or a more adequate cage/debris management design) may be needed to enable the facility to function as intended. Typical non-proprietary details can be obtained from the MSMD.

• Where installed, the low flow trash rack may be clogged with floating debris or sediment, blocking flow from reaching the low flow orifice. Clean and repair as necessary.

Blockages of Top Trash Rack (Where Present)

On-line bioretention facilities may include a control structure with multiple weir levels to restrict the 2-year and 10-year storm runoff events runoff using different hydraulic controls. Some facilities may include an open top control structure which requires a top trash rack. Where present, flat top trash racks are very susceptible to blockages from floating/settling debris. A peak roofed retrofit trash rack is recommended as it will grant a larger factor of safety against debris blockage. Typical non-proprietary details for such racks can be obtained from the MSMD. All drop inlet spillways designed for pressure flow should have adequate anti-vortex plate or device. An anti-vortex plate or device is not required if weir control is maintained in the riser through all flow stages.

Missing BMP (Water Quality) Plate/Device (Where Present)

Where a BMP plate is on the approved plans but found to be missing, install a BMP plate/device with an approved orifice size/configuration. It should be noted that most designs shown in construction plans are relatively difficult to keep operational (e.g., they block-up far too easily, resulting in facilities that require frequent "baby-sitting") and a retrofit is suggested. Typical non-proprietary details of

"upgraded" BMP plates and low flow debris cages can be obtained from MSMD.

Accessibility

Accessibility to the control structure is a necessity for maintenance personnel. Use a lockable access hatch through the top of the structure and steps or ladder down the inside of the structure. Trash racks and 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 control structure should be inspected and repaired or replaced as needed to maintain safe access to inside the control structure. Any manhole covering the control structure should be inspected and repaired or replaced as needed to maintain safe access to inside the control structure.

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Maintenance and Stormwater Management Division Page 31

Trees, Shrubs and Brush Maintenance

Vegetation surrounding the control structure should be maintained during routine maintenance as described under Routine Maintenance on page 20. Where non-routine maintenance is required, follow the landscaping plan for vegetation management. Portions of the embankment maintained as turf grass should be maintained along with the embankment vegetation as described under

Embankment/Berm and Vegetated Overflow Spillway on page 21. Dispose of all vegetation clippings appropriately.

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

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

However, if the overall integrity of the structure is reasonably sound, perform the following repairs:

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. Inspect filter media to insure over compression does not occur. Verify media meets design standards.

Spalled Sections (Concrete)

• Thoroughly clean the surface prior to performing maintenance.

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

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

Rusted Sections (Metal)

• Scrape and clean thoroughly rusted surfaces.

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

Any repairs made to the control structure other than routine maintenance (e.g. cleaning, parging, etc.) should be inspected by a qualified professional.

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Maintenance and Stormwater Management Division Page 32

Undermined or Settled Control Structure

An undermined and/or settled structure can be an indication of foundation movement or piping problems in the vicinity of the control structure. Any repairs made to the control structure other than routine maintenance (e.g. cleaning, parging, etc.) should be inspected by a professional engineer.

Undermining

Fill undermined areas with the approved filter media and stabilize immediately with a biodegradable erosion control mat and…