8-Attachment F- Task 1.pdf

Post-Construction Maintenance Guide

7.1 Dry Pond (Peak Shaver) and Extended Detention Dry

Pond

December 2017 Revised July 2024

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 Dry Ponds

Control Structures

Dam Embankment

Emergency Spillway

Outfall Structure

Pond Floor

Inflows

Non-Functional Elements for Dry Ponds

Access

Signage

Landscaping Plan

Routine Maintenance

Routine Maintenance Instructions

Dewatering

Non-Routine Maintenance

Dam Embankment and Vegetated Emergency Spillway

Turf Maintenance

Surface Erosion, Ruts, and Worn Paths Maintenance

Shrinkage Cracks Maintenance

Trees, Shrubs and Brush Maintenance

Rodents

Waterfowl

Mosquitoes

Piping Failure (Internal Erosion)

Hydraulic Failure (Overtopping Erosion)

Depressed Areas and Settling

Slope Failure

Fracturing

Separate Emergency Spillway

Blockages and Encroachments iii

Absence of an Emergency spillway or an Adequately Sized/Constructed Emergency Spillway

Washed-out/Severely Eroded Emergency Spillway

Homes or Other Habitable Structures Located in the Discharge Zone of the Emergency Spillway

Combined Spillway

Blockages

Inadequately Sized or Constructed Emergency Spillway

Flat Top Trash Rack

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

Control Structure

Blockages of Low Flow Orifice

Blockages of Top Trash Rack

Missing BMP (Water Quality) Plate/Device

Accessibility

Trees, Shrubs and Brush Maintenance

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

Undermined or Settled Riser Structure

Principal Spillway Pipe

Blocked or Silted in PSP

Deteriorated PSP Connection to the Riser or Outfall Structure

Structural Integrity

Leaking Joints

Outfall Structure

Accessibility

Trees, Shrubs and Brush Maintenance

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

Separated Structure

Undermined or Settled Structure

Outfall Channel

Blockages

Erosion

Pond Floor

Vegetation iv

Debris Removal

Sediment Removal

Erosion

Structural Deterioration (Cracks, Spalling, Etc.)

Inflows

Blockages

Erosion

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

Separated Structure

Undermined or Settled Structure

Trees, Shrubs and Brush Maintenance

Routine Maintenance Corrective Action and Annual Frequency Table

Non-Routine Maintenance Corrective Action and Inspection Frequency Table

References

Externally Sourced Figure References

List of Figures

Figure 1. Typical Dry Pond and Extended Detention Dry Pond Plan View

Figure 2. Typical Dry Pond and Extended Detention Dry Pond Profile

Figure 3. Dry Pond and Extended Detention Dry Pond

Figure 4. Typical Circular Riser Control Structure

Figure 5. Circular Riser Control Structure

Figure 6. Rectangular Riser Structure

Figure 7. Typical Rectangular Riser Structure

Figure 8. Typical Covered Riser Structure

Figure 9. Typical Low Flow Orifice Trash Rack

Figure 10. Low Flow Orifice Trash Rack

Figure 11. Top Trash Rack with Anti-Vortex Plate

Figure 12. Dam Embankment

Figure 13. Typical Dam Embankment

Figure 14. Concrete Emergency Spillway

Figure 15. Typical Separate Emergency Spillway Cross Section

Figure 16. Rip-Rap Emergency Spillway

Figure 17. Grass Emergency Spillway

Figure 18. Outfall Structure

Figure 19. Typical Outlet Structure Protection

Figure 20. Typical Pond Floor Extended Detention Dry Pond

Figure 21. Typical Trickle Ditch Detail

Figure 22. Pond Floor and Trickle Ditch

Figure 23. Common Inflow Pipe Types

Figure 24. Sediment Forebay

Figure 25 Access – Example of Maintenance vi

Figure 26. Signage

Figure 27 Signage Maintenance Needs

Figure 28 Pond Maintenance Diagram

Figure 29 Dam Embankment Maintenance Example

Figure 30 Emergency Spillway Maintenance Example

Figure 31 Ponding Area Maintenance Example

Figure 32 Shoreline Maintenance Example

Figure 33 Channel Cleaning Example

Figure 34 Control Structure Maintenance Example

Figure 35 Control Structure Trash Rack Maintenance Example

Figure 36 Outfall Maintenance Example

Figure 37 Dewatering Example

Figure 38. Groundhog

Figure 39. Muskrat

Figure 40. Beaver

Figure 41. Piping Failure

Figure 42. Hydraulic Failure (Overtopping Erosion)

Figure 43. Slope Failure

Figure 44. Fracturing

Figure 45. Washed Out/Severely Eroded Emergency Spillway

Figure 46. Structure Located in Emergency Spillway Discharge Zone

Figure 47. Blockage at Low Flow Orifice and Trash Rack

Figure 48. Blockage at Top Trash Rack

Figure 49. Undermined Structure

Figure 50. Sediment Buildup on Pond Floor

Figure 51. Blocked Inflow Pipe vii

7.1 Dry Pond (Peak Shaver) and Extended Detention Dry Pond

Form and Function

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 1

Form and Function

Dry ponds (also known as peak shavers or Detention Ponds) and extended detention dry ponds are stormwater management facilities designed to temporarily detain runoff and release it at a controlled rate. Dry Ponds (peak shavers) are often used to achieve reductions in peak discharge, channel protection and flood control requirements in the receiving water body. They are commonly known as peak shaver ponds because their primary function is to reduce the peak discharge during storm events and are designed to detain water for a short period of time.

Extended detention dry ponds provide the same benefits and function in the same way as dry ponds (peak shavers), but have a significantly smaller low flow orifice which allows for increased detention times and water quality benefits.

Extended detention dry ponds may have low flow orifices as small as 3 inches (in) in diameter. Compared to a dry pond (peak shaver) low flow orifice which is sized to control the maximum peak flow, the extended detention dry pond low flow orifice is sized to achieve a minimum drawdown time for a targeted water quality treatment volume.

The smaller low flow orifice reduces the peak discharge, allows for greater retention time within the pond, and achieves water quality benefits through gravitational settling of sediment and other particulates. Extended detention dry ponds are credited as a Best Management Practice (BMP) with 15% Total Phosphorus reduction, and 10% Total

Nitrogen reduction by the Virginia Department of Environmental Quality (DEQ).

Dry ponds (peak shavers) and extended detention dry ponds can be designed for different sized watersheds, ranging from small watersheds under 20 acres to larger watersheds in excess of 200 acres. Dry ponds (peak shavers) and extended detention dry ponds are constructed as open basins by excavating and re-grading below existing ground levels (where the dam embankment is referred to as a cut dam), or by constructing earth embankments across natural drainage courses (where the dam embankment is referred to as a fill dam), reference Figures 1, 2 and 3.

Although tree growth is typically discouraged if not banned on the dam embankment, landscape styles vary from well-manicured to more naturalistic and native plantings. Extended detention dry ponds (level 2) are discussed in

Post-Construction Maintenance Guide 7.2 Extended Detention (Level 2) Dry Pond and Constructed Wetland.

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 2

Figure 1. Typical Dry Pond and Extended Detention Dry Pond Plan View

Source: Fairfax County Public Facilities Manual

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 3

Figure 2. Typical Dry Pond and Extended Detention Dry Pond Profile

Source: Virginia DEQ BMP Clearinghouse

Maintenance and Stormwater Management Division Page 4

Maintenance and Stormwater Management Division Page 5

Figure 3. Dry Pond and Extended Detention Dry Pond

Functional Elements for Dry Ponds

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 6

Functional Elements for Dry Ponds

Control Structures

Control structures for dry ponds (peak shavers) and extended detention dry ponds control the flow rate of stormwater runoff exiting the facility. There are two types: separate (principal with emergency spillway) and combined. The control structure may be a riser structure, or a pipe end section, which is sized for a particular release rate given the drainage area and targeted storm events (typically the 1-year, 2-year, 10-year, 100-year storm events). The control structure reduces the peak discharge, provides channel protection and flood control, and may provide water quality benefits. As shown in Figures 4, 5, 6, 7 and 8, riser control structures include a low flow orifice which may require a trash rack. Dry pond (peak shaver) low flow orifices control the release rate of smaller storm events (typically the 1-year and 2-year storm event) and allows for the complete drawdown (draining) of the pond, reference Figures 9 and

10. Extended detention dry pond low flow orifices are designed to achieve a target drawdown time (typically between

12 – 24 hours) for a particular water quality treatment volume to achieve water quality benefits through gravitational settling of sediment and particulates, in additional to controlling the peak flow. The low flow trash rack prevents litter and debris from entering and clogging the low flow orifice and riser structure. Riser structures may also require a top trash rack and anti-vortex plate. Similar to the low flow trash rack, the top trash rack prevents large floatable debris

(large logs, sticks, garbage) from entering and clogging the riser structure during larger storm events (typically storms larger than 10-year event). 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. The anti-vortex plate prevents swirling in the pond and riser structure during large storm events, reference

Figures 4 and 11. There may be additional orifices or weirs in the riser structure to allow discharge during other storm events such as the 2-year and 10-year storm events. The riser structure controls the flow rate into the principal spillway pipe (PSP) which conveys the runoff through the dam and outfall. Pipe end section control structures, found in dry ponds (peak shavers), are directly connected to the PSP (no riser structure) and are typically sized to control medium to large storm events such as the 2-year, 10-year and 100-year storm events.

Maintenance and Stormwater Management Division Page 7

Figure 4. Typical Circular Riser Control Structure

Maintenance and Stormwater Management Division Page 8

Figure 5. Circular Riser Control Structure

Combined Emergency Spillway (100-year Orifice) and Top Trash Rack

Anti-Vortex Plate

10-year Orifice and Trash Rack

2-year Orifice and Trash Rack

Low Flow Orifice and Trash Rack

Figure 6. Rectangular Riser Structure

Combined Emergency Spillway

Low Flow Orifice and Trash Rack

Trickle Ditch

Maintenance and Stormwater Management Division Page 9

Figure 7. Typical Rectangular Riser Structure

Maintenance and Stormwater Management Division Page 10

Figure 8. Typical Covered Riser Structure

Maintenance and Stormwater Management Division Page 11

Figure 9. Typical Low Flow Orifice Trash Rack

Maintenance and Stormwater Management Division Page 12

Emergency Spillway

High Flow Orifice

Reverse Slope Trash Rack

Figure 10. Low Flow Orifice Trash Rack

Figure 11. Top Trash Rack with Anti-Vortex Plate

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 13

Dam Embankment

The dam embankment is constructed to pond stormwater inflows upstream of a control structure and allow for the control structure to release flows at a controlled rate. Essentially, the dam embankment holds the water back. Ponds are constructed as open basins by excavating and re-grading below existing ground levels (where the dam embankment is referred to as a cut dam), or by constructing earth embankments across natural drainage courses

(where the dam embankment is referred to as a fill dam). The PSP conveys water (sometimes preceded by a riser control structure) through the dam embankment to discharge via the outfall. Depending on the size of the pond, there may be a single PSP or multiple pipes traversing the dam embankment. Reference Figures 12 and 13 below for examples of dam embankments.

Figure 12. Dam Embankment

Maintenance and Stormwater Management Division Page 14

Figure 13. Typical Dam Embankment

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 15

Emergency Spillway

The emergency spillway allows major storm events (typically storms over the 10-year event) to pass through the facility without causing damage to the dam embankment. Without an emergency spillway, the dam embankment could be overtopped during large storm events causing erosion, scouring and ultimately dam failure. Emergency spillways may be designed as a combined emergency spillway, or a separate or auxiliary emergency spillway, reference Figures 14, 15, 16 and 17. A combined emergency spillway is incorporated into the riser structure, where the major storm events are designed to pass through the top of the riser (including the top trash rack and anti-vortex plate), through the PSP to discharge via the outfall. A separate or auxiliary emergency spillway functions as a reinforced weir in the dam embankment located away from the PSP which allows for controlled release of stormwater during major storm events without subjecting non-reinforced portions of the dam embankment to damaging high flows.

Emergency Spillway

Dam

Figure 14. Concrete Emergency Spillway

Maintenance and Stormwater Management Division Page 16

Figure 15. Typical Separate Emergency Spillway Cross Section

Maintenance and Stormwater Management Division Page 17

Figure 16. Rip-Rap Emergency Spillway

Figure 17. Grass 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 PSP with the downstream receiving water body (typically a stream, channel or piped storm drain network). Outfall structures may be designed with an end wall and wing walls, or as a simple flared end section, pipe end or underground outfall. For above ground outfalls, some type of protective material such as a plunge pool, concrete apron, rip-rap or gabion baskets may be present to reduce flows to non-erosive velocities in the receiving downstream waterbody, reference

Figures 18 and 19.

Figure 18. Outfall Structure

Maintenance and Stormwater Management Division Page 19

Figure 19. Typical Outlet Structure Protection

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 20

Pond Floor

The pond floor is where the stormwater runoff ponds upstream of the dam embankment until it passes through the control structure, reference Figures 20, 21 and 22. Vegetation is allowed on the pond floor except within 10 feet (ft) of the dam heel. Vegetation will not be permitted to block access to structures or flow in the low flow channel or trickle ditch. The landscaping plan is very useful in determining if and where vegetation is permitted. The pond floor may have a low flow channel or trickle ditch with vegetative cover, rip-rap or concrete cover. The low flow channel or trickle ditch conveys water from the pond floor to the control structure to allow for efficient drawdown of the pond facility.

Figure 20. Typical Pond Floor Extended Detention Dry Pond

Source: Virginia DEQ BMP Clearinghouse

Maintenance and Stormwater Management Division Page 21

Figure 21. Typical Trickle Ditch Detail

Maintenance and Stormwater Management Division Page 22

Inflows

Pond inflows convey stormwater runoff from the drainage area into the pond. Inflows may be piped or an open channel. Reference Figure 23 for common pipe end types. Some types of protective material such as a plunge pool, concrete apron, rip-rap or gabion baskets may be present to reduce flows to non-erosive velocities as the inflow pipe discharges into the pond area. While more commonly observed in extended detention dry ponds (level 2) and wet ponds, sediment forebays may be located at major inflows of dry ponds depending on the design and size of the contributing drainage area. Trapping the coarse heavy sediment in the forebay also makes maintenance easier and less expensive. Sediment forebays are separated from the pond floor using a structure such as an earthen or rip-rap berm, a concrete wall with underdrain, or a gabion system. An example of a sediment forebay is shown in Figure 24.

Figure 22. Pond Floor and Trickle Ditch

Pond Floor

Trickle Ditch

Control Structure

Inflow Structure

Maintenance and Stormwater Management Division Page 23

Flared End Pipe End

End Wall

Figure 23. Common Inflow Pipe Types

Sediment Forebay

Figure 24. Sediment Forebay

Non-Functional Elements for Dry Ponds

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 24

Non-Functional Elements for Dry Ponds

Access

Access for maintenance should be designated on plans; however, many older facilities may not have permanent access. 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.

Clear obstructions along the access road or entryway, including a 3-foot-wide strip on either side of paved or gravel pathways. A 10-foot vertical clearance along the access road should be maintained. Utilize hand tools to remove obstacles that hinder access, such as limbs, small trees, and brush.

Figure 25 Access – Example of Maintenance

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 25

Signage

Posted signs must be provided to inform the public about the locations of detention ponds, reference Figure 25.

Signs should be located such that they are clearly visible from the adjoining lots and roadways. The number of signs posted will depend on the size of the facility.

Ensure that signs are present, upright, and legible. Repair or replace as needed during routine or nonroutine maintenance.

Figure 26. Signage

Maintenance and Stormwater Management Division Page 26

Figure 27 Signage Maintenance Needs

Landscaping Plan

A landscaping plan may be provided in the site plan for the pond. Where available, landscaping plan elements may include:

• Delineation of landscaping zones in the pond.

• Planting plan for each zone in the pond including the size, list of planting stock and sources of plant species.

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

Routine Maintenance

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 27

Routine Maintenance

Routine maintenance refers to the regular, scheduled tasks performed to ensure the proper functioning, cleanliness, and upkeep of facilities. These activities are conducted to prevent costlier repairs, extend the lifespan of assets, and maintain operational efficiency. Routine maintenance typically includes inspections, cleaning, minor repairs, and adjustments. The specific tasks may vary depending on the type of facility or equipment, but the overarching goal is to address potential issues before they become major problems, ensuring the continuous and optimal operation of the assets.

Routine maintenance occurs twice per year for most ponds. Routine Maintenance services may generally include:

• Mowing, trimming, and clearing of the dam embankment, pond floor, and side slopes (Peak Shavers and Extended Detention Ponds only), emergency spillway, and outfall area.

• Pond dewatering1.

• Channel cleaning (minimum 25ft radius2).

• Control Structure (in and around), low-flow trash rack cleaning (inside and outside), and top rack cleaning.

• Sign repair or replacement.

• Lock installation or replacement.

• Displaced or removed material (channel clearing, control structure, low-flow trash rack cleaning, top rack cleaning) shall be disposed of offsite

Routine maintenance occurs monthly during the growing season for regulated facilities. Routine maintenance includes:

• Mowing on steep slopes

• String trimming

• Nutrient Management Plan Development

Routine Maintenance Packages

There are two primary types of maintenance packages for non-regulated facilities:

• HOA Packages (1 Mow per Year):

These packages are distributed across eight (8) geographically defined maintenance areas, as shown in the color-coded graphics below. Each site is mowed once annually, scheduled between March and

October.

• Non-HOA Packages (2 Mows per Year):

These are divided into four (4) geographic maintenance areas, also color-coded in the graphics below.

1 Pond dewatering should be initiated when a blockage is observed by the contractor using hand tools. Alert county team if a pump-down is needed.

2 Channel cleaning beyond 25ft, if possible while onsite. Invoice and documentation should reflect additional efforts.

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 28

Each site is mowed twice annually, with schedules falling into one of the following pairs:

March/July, April/August, May/September, or June/October.

Figure 28 HOA Packages (1 Mow/Year)

Maintenance and Stormwater Management Division Page 29

Figure 29 Non-HOA Packages (2 Mows/Year)

Routine Maintenance Instructions

Generally, mowing and clearing entail the removal of all grasses, weeds, woody stocks, brush, and tree saplings up to 2 inches in diameter, including clusters of saplings up to 2 inches in diameter originating from the same base.

Grass should be trimmed to a finished height between 5.5 and 6 inches (not shorter than 5 inches), while woody stocks, brush, and saplings should be cut near ground level. Mowing should follow a horizontal pattern; mowing up and down the slope is not permitted, ensuring an even grass cut length. Grass cuttings should not be left in rows. A mulching mow pass may be required to sufficiently break down cuttings.

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 30

String or blade trimming should address areas inaccessible to standard mowing equipment, such as access gates, embankment perimeters, toe areas, steep slopes, emergency spillways, shorelines, riprap areas, around signs, and fence lines.

Equipment required for this task may include, but is not limited to, a string or blade trimmer, rough terrain or field mower (such as Ventrac or equivalent), chainsaws, and a small brush hog or skid steer with a brush cutting attachment. Equipment used for routine maintenance must have the capability to traverse steep slopes horizontally without slipping or causing earth impact (rutting). Contractors are responsible for ensuring all equipment is maintained in good working order, which includes routine tasks such as sharpening blades, greasing moving parts, and replacing worn components. Hand tools can include, but are not limited to the following: shovels, rakes, pruning shears, loppers, hand saws, hoes, wheelbarrows, poking bars, manhole hooks, etcetera.

Routine Maintenance services may generally include:

• Clear obstructions along the access road or entryway, including a 3-foot-wide strip on either side of paved or gravel pathways. Utilize hand tools to remove obstacles that hinder access, such as limbs, small trees, and brush.

• Non-Regulated Facilities: Mow or trim the entire dam embankment (front, top, back) plus 10 feet from front toe and 10 feet from back toe. Clear obstructions with hand tools that impede mowing, including limbs, small trees, and brush.

• Regulated Facilities: Mow or trim the entire dam embankment (front, top, back) plus 20 feet from front toe and 20 feet from back toe. Clear obstructions with hand tools that impede mowing, including limbs, small trees, and brush.

• Clear the impoundment area, encompassing the entire pond floor and interior slopes up to the 100-year flood elevation, where accessible and mowable with standard equipment. Excludes wet ponds and extended detention (level 2) ponds.

o When encountering trees larger than 2 inches in diameter, work around them whenever feasible.

o Charge is based on the percentage of completed area.

• Non-Regulated Facilities: Clear a minimum of 10 feet around the control structure. Remove grassy and woody vegetation growing against or on the structure as well as on the trash racks.

• Regulated Facilities: Clear a minimum of 20 feet around the control structure. Remove grassy and woody vegetation growing against or on the structure as well as on the trash racks.

• Clear vegetation from within the emergency spillway.

• Where possible, remove trash rack and clear inside area of woody debris, sediment and trash to ensure orifices are clear.

• Non-Regulated Facilities: Clear vegetation within a minimum of 10 feet around and on each side of the outfall structure and the outfall channel.

• Regulated Facilities: Clear vegetation within a minimum of 20 feet around and on each side of the outfall structure and the outfall channel.

• Ensure that cut vegetation is removed from the channel and from around the control structure. Properly dispose of it off-site.

Maintenance and Stormwater Management Division Page 31

Figure 30 Pond Maintenance Diagram

Maintenance and Stormwater Management Division Page 32

Figure 31 Dam Embankment Maintenance Example

Maintenance and Stormwater Management Division Page 33

Figure 32 Emergency Spillway Maintenance Example

Maintenance and Stormwater Management Division Page 34

Figure 33 Ponding Area Maintenance Example

Maintenance and Stormwater Management Division Page 35

Figure 34 Shoreline Maintenance Example

Maintenance and Stormwater Management Division Page 36

Figure 35 Channel Cleaning Example

Maintenance and Stormwater Management Division Page 37

Figure 36 Control Structure Maintenance Example

Maintenance and Stormwater Management Division Page 38

Figure 37 Control Structure Trash Rack Maintenance Example

Figure 38 Outfall Maintenance Example

Dewatering

1. Where safe and practical, poke the front of the trash rack with hand tools to loosen/dislodge debris.

2. Check for water flow inside the control structure.

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 39

3. Return the next day to assess conditions. Repeat step 1 if needed. (If unsuccessful after 2 attempts, contact MSMD.)

4. Wait for water to drain (usually 2-3 days) before continuing with routine maintenance.

Figure 39 Dewatering Example

Routine inspection of pond facilities should be integrated with the routine maintenance tasks of the surrounding area and scheduled mowing activities. Inspections on publicly maintained pond 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 the Maintenance and Stormwater

Management Division (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

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 40

Non-Routine Maintenance

Non-Routine Maintenance, also known as corrective maintenance, refers to unscheduled repairs and interventions carried out to fix or restore the functionality of facilities following a failure, breakdown, or deterioration. This type of maintenance is reactive and occurs when regular operations are disrupted due to unforeseen issues. Corrective maintenance involves diagnosing the problem, determining the cause, and performing the necessary repairs or replacements to return the system or asset to its normal operational state. The goal is to address and resolve issues promptly to minimize downtime and prevent further damage.

Non-Routine Maintenance Services may encompass various tasks, including but not limited to:

• Removal of accumulated sediments.

• Repair of critical structures.

• Installation of maintenance features (forebay, plunge pools, step pools).

• Erosion and sediment control plan development, control installation, and inspection.

• Rehabilitation and stabilization of eroded or denuded areas.

• Vegetation removal and/or management.

• Any other services not covered by the routine maintenance services.

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

Dam Embankment and Vegetated Emergency Spillway

The dam embankment is constructed to pond stormwater inflows upstream of a control structure and allow for the control structure to release flows at a controlled rate. Essentially, the dam embankment holds the water back.

The emergency spillway allows major storm events (typically storms over the 10-year event) to pass through the facility without causing damage to the dam embankment. Emergency spillways may be designed as a combined emergency spillway, or a separate or auxiliary emergency spillway. A separate or auxiliary emergency spillway functions as a reinforced weir in the dam embankment located away from the PSP which allows for controlled release of stormwater during major storm events without subjecting non-reinforced portions of the dam embankment to damaging high flows.

Turf Maintenance

All sediment, trash and debris should be removed from the embankment. The grass cover of the dam 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

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 41 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

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

• Mow the dam embankment, 10 ft from heel and toe of embankment, and within 25 ft radius of channels, (allow the remaining pond floor to grow naturally).

• 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, backfill with topsoil and stabilize immediately. Areas with severe erosion should be inspected by a qualified professional. 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 42

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 earthen embankment. 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 25.

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.

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

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 43

(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. The three rodents most destructive to an earthen dam are the groundhog, the muskrat, and the beaver. These animals are attracted to the environment and habitat provided by a dam and reservoir. Beavers may attempt to block the principal and emergency spillways, potentially raising the elevation of the reservoir water surface. The burrowing habits of the groundhog and the muskrat may threaten the integrity of an earthen structure. It should be noted that a well-maintained (mowed) dam discourages rodent infestation and habitation.

Groundhogs

A groundhog typically burrows into the dam on the downstream face. The burrow will have many tunnels and chambers, and will always have multiple entrances. Occupied burrows can be easily identified by the piles of fresh dirt on the downstream face of the dam. 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 26 for photo of a groundhog.

Muskrats

Muskrats, which are not typically found in dry ponds unless clogged and filled with water, construct dens by burrowing into the banks of lakes and streams. All clogged structures which are causing the dry pond to have a permanent pool of water should be cleared immediately. Muskrat burrows start from 6 – 8 in below the normal pool (water surface) and then tunnel upwards to dry ground (above

Figure 40. Groundhog

Source: U.S. National Oceanic and Atmospheric Administration

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 44 the phreatic surface of the dam). Burrowing provides a path for water flow through the dam, which can seriously weaken the structural integrity of the dam. An even more serious condition occurs when a muskrat den on the upstream side connects with a groundhog den on the downstream side, resulting in an unlined conduit through the dam. Further investigation of potential remedies is needed if clogging persists after repeated attempts to remedy the source of the clogging and remove the muskrat population. In dry ponds where conditions exist which are likely to cause clogging again in the future, a successful method for eliminating a muskrat burrow problem is the placement of rip-rap on the upstream side of the dam. Extend rip-rap 3 ft below the water surface. Consider trapping of the rodent. Reference Figure 27 for photo of muskrat.

Beavers

Beavers are not typically found in dry ponds unless clogged and filled with water for an extended period of time. All clogged structures which are causing the dry pond to have a permanent pool of water should be cleared immediately. Beavers block spillways and have even been known to try to raise reservoir water levels. Such activities can lead to the overtopping of a dam. In dry ponds where conditions exist which are likely to cause clogging again in the future, removal of the woody cuttings of a beaver dam may only temporarily eliminate the problem. More aggressive measures may be necessary, including professional trapping. Even after removal of beavers from a pond, if a favorable habitat remains then the continued presence of beavers may be expected. Monthly site checks are recommended to ensure that dams and lodges are not rebuilt in the weeks after removing them.

Refer to the ‘Standard Operating Procedures for Beaver Management’ prepared by the Fairfax

County Park Authority. Reference Figure 28 for photo of beaver.

Figure 41. Muskrat

Source: U.S. Fish and Wildlife Service

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 45

Repair

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.

Waterfowl

Waterfowl are not typically a concern in dry ponds unless clogged and filled with water for an extended period of time. Waterfowl damage typically occurs from over grazing or high fecal coliform counts. All clogged structures which are causing the dry pond to have a permanent pool of water should be cleared immediately.

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 to 5 days is enough to span a single generation of mosquitoes. Any facility which does not drain within 72 hours is considered clogged. All clogged structures, or poorly graded areas which are causing the dry pond to have a stagnant water should be repaired immediately.

Piping Failure (Internal Erosion)

The dam embankment should be inspected for signs of piping or seepage. Piping is the progressive loss of soil from an earth dam (usually around the PSP) resulting from concentrated seepage through the dam, reference Figure 29. 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 dam

Figure 42. Beaver

Source: Fairfax County Park Authority

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 46 embankment. Areas with evidence of piping or seepage should be addressed immediately. Cave-in may occur if not corrected.

Wet areas, soft spots, or areas of inconsistent grass growth on the downstream face or toe of the embankment may also be indicative of seepage through the dam. 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, consult with a qualified professional to 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 a dam are evidence that the dam has overtopped, reference Figure 30. 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 due to sediment, and/or an inadequate or blocked emergency spillway. Consult a qualified professional to ensure structural integrity of the embankment dam.

Figure 43. Piping Failure

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 47

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 knowledge regarding the dam, 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 dam. A qualified professional should be consulted to determine the source of the settlement and the embankment should be built back to the design elevation and grade.

Slope Failure

Areas of slope slippage may be indicative of a concentrated seep exiting through the downstream face of the dam or slope stability problems on the upstream face of the dam (usually resulting from rapid drawdown conditions), reference Figure 31. Slope failure can seriously threaten the integrity of a dam. If such a condition exists, consult a qualified professional to evaluate and repair as needed.

Figure 44. Hydraulic Failure (Overtopping Erosion)

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 48

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 dam, perpendicular to the axis of the dam, etc.).

Such a condition is indicative of differential settlement in the dam and/or foundation support problems, reference Figure 32. Fractures can seriously threaten the structural integrity of a dam. If such a condition exists, consult a qualified professional to evaluate and repair as needed as soon as possible.

Figure 45. Slope Failure

Figure 46. Fracturing

Source: U.S. Natural Resources Conservation Service

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 49

Separate Emergency Spillway

The emergency spillway allows major storm events (typically storms over the 10-year event) to pass through the facility without causing damage to the dam embankment. Emergency spillways may be designed as a combined emergency spillway, or a separate or auxiliary emergency spillway. A separate or auxiliary emergency spillway functions as a reinforced weir in the dam embankment located away from the PSP which allows for controlled release of stormwater during major storm events without subjecting non-reinforced portions of the dam embankment to damaging high flows.

Blockages and Encroachments

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

Absence of an Emergency spillway or an Adequately Sized/Constructed Emergency Spillway

The best assurance against dam overtopping is an adequate emergency spillway. Consult with a qualified professional to evaluate and provide an adequate emergency spillway if one does not currently exist as soon as possible.

Washed-out/Severely Eroded Emergency Spillway

In the case of an emergency spillway in hydraulic failure, consult with a qualified professional to evaluation and repair as soon as possible, reference Figure 33.

Figure 47. Washed Out/Severely Eroded Emergency Spillway

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 50

Homes or Other Habitable Structures Located in the Discharge Zone of the Emergency

Spillway

An owner of a dam with homes or other structures located in the discharge zone of the emergency spillway should study the situation carefully, and consult with a qualified professional to determine if an emergency action plan is needed, including notification of downstream owners on major storm events, reference Figure 34.

Combined Spillway

The emergency spillway allows major storm events (typically storms over the 10-year event) to pass through the facility without causing damage to the dam embankment. Emergency spillways may be designed as a combined emergency spillway, or a separate or auxiliary emergency spillway. A combined emergency spillway is incorporated into the riser structure, where the major storm events are designed to pass through the top of the riser (including the top trash rack and anti-vortex plate), through the PSP to discharge via the outfall.

Blockages

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

Figure 48. Structure Located in Emergency Spillway Discharge Zone

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 51

Inadequately Sized or Constructed Emergency Spillway

The best assurance against dam overtopping is an adequate combined spillway which is typically designed to pass the larger storms (greater than the 10-year storm event). Provide an adequate spillway if one does not currently exist as soon as possible.

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 peaked roofed trash racks can be obtained from the MSMD. All drop inlet spillways designed for pressure flow should have adequate anti-vortex plate or device.