9-Attachment F- Tasks 1 and 2.pdf

Post-Construction Maintenance Guide

7.2 Extended Detention Dry Pond (Level 2) and Constructed

Wetland

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 Extended Detention Dry Ponds (Level2) and Constructed Wetlands

Control Structures

Dam Embankment

Emergency Spillway

Outfall Structure

Inflows

Non-Functional Elements for Extended Detention Dry Ponds (Level 2) and Constructed Wetlands

Access

Signage

Landscaping Plan

Routine Maintenance

Routine Maintenance Packages

Routine Maintenance Instructions

Dewatering

Invasive Plant Species Management

Wetland Vegetation Management

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)

Cloudy/Muddy Foundation/Toe Drain Discharges

Depressed Areas and Settling

Slope Failure iii

Fracturing

Separate Emergency Spillway

Blockages and Encroachments

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.)

Low-level Outlet Gate (Pond Drain)

Undermined or Settled Riser Structure

Principal Spillway Pipe

Blocked or Silted in PSP

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

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 iv

Outfall Channel

Blockages

Erosion

Extended Detention Dry Pond (Level 2) and Constructed Wetland Floor

Wetland Vegetation

Debris Removal

Sediment Removal

Erosion

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 v vi

List of Figures

Figure 1. Typical Extended Detention Pond (Emergent and Forested Basin)

Figure 2. Typical Constructed Wetland Plan and Profile

Figure 3. Extended Detention Dry Pond (Tallow Tree - 0232DP)

Figure 4. Typical Circular Riser Control Structure

Figure 5. Typical Rectangular Riser Structure

Figure 6. Typical Covered Riser Structure

Figure 7. Typical Low Flow Orifice Trash Rack

Figure 8. Circular Riser Control Structure with Low Flow Orifice, Combined Emergency Spillway, Anti-vortex Plate and Trash Racks

Figure 9. Alternative Control Structure Configurations

Figure 10. Dam Embankment

Figure 11. Typical Dam Embankment

Figure 12. Concrete Emergency Spillway

Figure 13. Typical Separate Emergency Spillway Cross Section

Figure 14. Rip-Rap Emergency Spillway

Figure 15. Grass Emergency Spillway

Figure 16. Outfall Structure

Figure 17. Typical Outlet Structure Protection

Figure 18. Pond Floor

Figure 19. Common Inflow Pipe Types

Figure 20. Sediment Forebay

Figure 21 Access – Example of Maintenance

Figure 22 Signage

Figure 23 Signage Maintenance Needs

Figure 28 HOA Packages (1 Mow/Year) vii

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

Figure 24 Pond Maintenance Diagram

Figure 25 Dam Embankment Maintenance Example

Figure 26 Emergency Spillway Maintenance Example

Figure 27 Shoreline Maintenance Example

Figure 28 Channel Cleaning Example

Figure 29 Control Structure Maintenance Example

Figure 30 Control Structure Trash Rack Maintenance Example

Figure 31 Outfall Maintenance Example

Figure 32 Dewatering Example

Figure 33. Site Specific Maintenance Card Front (Village Park – 0090DP)

Figure 34. Site Specific Maintenance Card Back (Village Park – 0090DP)

Figure 35. Groundhog

Figure 36. Muskrat

Figure 37. Beaver

Figure 38. Piping Failure

Figure 39. Hydraulic Failure (Overtopping Erosion)

Figure 40. Slope Failure

Figure 41. Fracturing

Figure 42. Washed Out/Severely Eroded Emergency Spillway

Figure 43. Structure Located in Emergency Spillway Discharge Zone

Figure 44. Blockage at Low Flow Orifice and Trash Rack

Figure 45. Blockage at Top Trash Rack

Figure 46. Undermined Structure

Figure 47. Sediment Buildup on Pond Floor

Figure 48. Blocked Inflow Pipe viii

7.2 Extended Detention Dry Pond (Level 2) and Constructed Wetland

Form and Function

Fairfax County Department of Public Works and Environmental Services

Maintenance and Stormwater Management Division Page 1

Extended detention dry ponds (level 2) and constructed wetlands (wetlands) are stormwater management facilities designed to store runoff, provide water quality benefits to a water quality treatment volume (Tv) and ultimately release runoff which exceeds the Tv at a controlled rate. Extended detention dry ponds (level 2) and constructed wetlands include a series of micropools, channels and wetland vegetation, reference Figures 1, 2 and 3. These features provide permanent pools of water which help to increase sediment and nutrient removal, reductions in peak discharge, channel protection and flood control. The stormwater runoff which remains in the permanent pools undergoes multiple pollutant removal processes including gravitational settling, biological uptake, and microbial transformation. Incoming stormwater displaces the Tv in the facility during each storm event. In general, the longer the time between storm events, the longer the runoff is treated by the best management practice (BMP) facility

(residence time) which results in better water quality.

Extended detention dry ponds (Level 2 design) have a sediment forebay and a series of micropools separated by low marsh channels and high marsh berms. Up to 60% of the Tv may be in the form of extended detention above the permanent pool. Extended detention dry ponds (level 2) detain stormwater runoff (in excess of the permanent pools) for 36 hours on average. The vegetation within the facility is primarily composed of emergent and forested wetland vegetation. The function (physical and bio-chemical processes) within the facility are described in the opening paragraph of this section.

Constructed wetlands have three main configurations which are classified by Virginia Department of Environmental

Quality (DEQ):

1. Wetland basin (Level 1 design) consists of a sediment forebay followed by a single micropool/wetland cell with a uniform permanent pool elevation. The average depth within the wetland is more than 1 foot (ft), with greater depths in the sediment forebay and wetland pool at the control structure. Up to 50% of the Tv may be in the form of extended detention above the permanent pool of the wetland for the 1-year storm event which has a drawdown time of 24 hours.

Emergent plants (rooted under water, flower at or above water surface) are the dominant vegetation types. The function (physical and bio-chemical processes) within the facility are described in the opening paragraph of this section.

2. Multi-cell wetlands (Level 2 design) consist of a sediment forebay followed by multiple micropool/wetland cells which are separated by low marsh channels and high marsh berms. Each micropool may have a unique permanent pool elevation. The average depth within the wetland is less than 1 ft, with greater depths in the sediment forebay and micropools. The Tv equals the permanent pool volume. No detention or extended detention is allowed within the wetland cell;

limited water surface fluctuations will occur during the 1-year storm. This prevents more frequent water level fluctuations which can reduce the diversity and function of wetland plantings. Emergent and forested wetland plantings are present. The function (physical and bio-chemical processes) within the facility are described in the opening paragraph of this section.

3. Multi-cell pond/wetland combination systems are Level 2 wetlands which are off-line systems. The combination systems are designed to convey small storms, such as the 1-year storm event, through

Maintenance and Stormwater Management Division Page 2 the multi-cell wetland portion, while diverting larger storms through the wet pond. The flow from the wet pond to the multi-cell wetland is typically accomplished using a reverse slope pipe, or a low flow diversion device sized for the Tv flow rate. The sediment forebay is typically part of the wet pond design upstream from the multi-cell wetland inflow. This eliminates the need for a sediment forebay within the multi-cell wetland portion. The function (physical and bio-chemical processes) within the facility are described in the opening paragraph of this section.

Figure 1. Typical Extended Detention Pond (Emergent and Forested Basin)

Source: Virginia DEQ BMP Clearinghouse

Maintenance and Stormwater Management Division Page 3

Figure 2. Typical Constructed Wetland Plan and Profile

Source: Virginia DEQ BMP Clearinghouse

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Control Structure

Micropool

High Marsh

Low Marsh

Figure 3. Extended Detention Dry Pond (Tallow Tree - 0232DP)

Functional Elements for Extended Detention Dry Ponds (Level2) and Constructed Wetlands

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Functional Elements for Extended Detention Dry Ponds (Level2) and Constructed Wetlands

Control Structures

Control structures for extended detention dry ponds (Level 2) and constructed wetlands store and control the flow rate of stormwater runoff exiting the facility, reference Figures 4, 5, 6, 7, 8, and 9. There are two types: separate

(principal with separate 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 stores stormwater runoff in the facilities up to the elevation of the low flow orifice, which equals permanent pool or normal pool elevation. Once the low flow orifice is activated, the control structure reduces the peak discharge, and provides channel protection and flood control. Riser control structures include a low flow orifice which may require a trash rack. 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. 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 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.

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Figure 4. Typical Circular Riser Control Structure

Source: Fairfax County Public Facilities Manual

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Figure 5. Typical Rectangular Riser Structure

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Figure 6. Typical Covered Riser Structure

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Figure 7. Typical Low Flow Orifice Trash Rack

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Figure 8. Circular Riser Control Structure with Low Flow Orifice, Combined Emergency Spillway, Anti-vortex Plate and Trash Racks

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

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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, reference Figures 10 and 11. Essentially, the dam embankment holds the water back. Extended detention dry ponds (level 2) and wetlands 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 extended detention dry ponds (level 2) or wetland, there may be a single PSP or multiple pipes traversing the dam embankment.

Figure 9. Alternative Control Structure Configurations

End Wall Pipe Control Structure

Reverse Slope Combined Control Structure

Custom Shaped Control Structure

Pipe End and Overflow Control

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Figure 10. Dam Embankment

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Figure 11. Typical Dam Embankment

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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, reference Figures 12, 13, 14, and 15. 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. 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.

Dam

Figure 12. Concrete Emergency Spillway

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Figure 13. Typical Separate Emergency Spillway Cross Section

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Figure 14. Rip-Rap Emergency Spillway

Figure 15. Grass Emergency Spillway

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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), reference Figures 16 and 17. 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.

Figure 16. Outfall Structure

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Figure 17. Typical Outlet Structure Protection

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Extended Detention Dry Pond (Level 2) and Constructed Wetland Floor

Extended detention dry ponds (level 2) and constructed wetlands have a sediment forebay followed by a single or multiple micropool/wetland cells which are separated by low marsh channels and high marsh berms, reference

Figure 18. Extended detention dry ponds (level 2) and multi-cell wetlands have internal structures which create micro-topographic features within the facility to promote dense and diverse emergent and forested wetland vegetative cover. Micro-topography is achieved using:

• Tree peninsulas, high marsh wedges or rock filter cells configured perpendicular to the flow path

• Tree islands above the normal pool elevation and maximum extended detention zone, formed by coir fiber logs

• Inverted root wads or large woody debris

• Gravel diaphragm layers within high marsh zones

• Cobble sand weirs

• Additional, deeper pools

Wetland vegetation (emergent and forested wetland plants) is planted within the micropools, the low marsh and high marsh areas, and on tree peninsulas. Micropools are typically planted with emergent vegetation plugs grouped by species approximately 18 inches (in) apart from one another on triangular spacing. This type of planting helps the emergent vegetation take hold in the first growing season and reduces the likelihood of invasive plants taking over.

The low marsh and high marsh areas are planted with tolerant seed mixes. Tree peninsulas are planted with water and drought tolerant trees and shrubs. With proper initial planting, reinforcement planting, and maintenance, the entire wetland should be colonized within 3 years from establishment.

The landscape plan or planting plan depicts where each wetland vegetation boundary lies (micropool plugs, low marsh seed mix, etc.), and where turf grass should be managed on the embankment and access points.

Micropool Control

Low Marsh

High Marsh

Figure 18. Pond Floor

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Inflows

Inflows convey stormwater runoff from the drainage area into the dry pond or wetland, reference Figure 19. Inflows may be piped or an open channel. 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 discharges into the sediment forebay. Sediment forebays are required at major inflows of extended detention dry ponds (level 2) and constructed wetlands. A major inflow is defined as conveying runoff from at least 10% 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, reference Figure 20.

Flared End Pipe End

End Wall

Figure 19. Common Inflow Pipe Types

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Sediment Forebay

Figure 20. Sediment Forebay

Non-Functional Elements for Extended Detention Dry Ponds (Level 2) and Constructed Wetlands

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Non-Functional Elements for Extended Detention Dry Ponds (Level 2) and Constructed Wetlands

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 21 Access – Example of Maintenance

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Signage

Posted signs must be provided to inform the public about the locations of extended detention dry ponds (level 2) and constructed wetlands, reference Figure 21. 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 22 Signage

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Figure 23 Signage Maintenance Needs

Landscaping Plan

A landscaping plan is required for all extended detention dry ponds (level 2) and constructed wetlands. The plan should outline a detailed schedule for the care, maintenance and possible reinforcement of the wetland vegetation for up to 10 years after the original planting. Wetland vegetation refers to the emergent vegetation, the low marsh/high marsh seed mixes and any trees or shrubs which are planted within regions of the facility which periodically become inundated with water. The landscaping plan should specify the locations, quantities, and planting instructions for all emergent, perennial, shrub and tree species within the extended detention dry ponds (level 2) or constructed wetlands. The plan depicts where each wetland vegetation boundary lies (micropool plugs, low marsh seed mix, etc.), and where turf grass should be managed on the embankment and access points. Reinforcement plantings should be added to the wetland in any areas that remain unvegetated.

Significant changes in wetland species composition should be expected to occur over time within the wetland vegetation. Inspections should carefully track changes in wetland plant species distribution over time. Invasive plants should be removed as soon as they begin to colonize the wetland or extended detention dry pond (level 2). Invasive species should not be allowed to exceed 15% coverage of a wetland cell area. Thinning or harvesting of woody

Maintenance and Stormwater Management Division Page 25 growth may be periodically required to guide the forested wetland areas into a mature state, or if the vegetation becomes overgrown. Thinning or harvesting of woody growth should be scheduled to occur approximately 5 – 10 years after the initial construction.

Routine Maintenance

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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 (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.

• Removal of accumulated trash.

• 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 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.

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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 24 HOA Packages (1 Mow/Year)

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Figure 25 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

Maintenance and Stormwater Management Division Page 29 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.

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.

• (Second Routine Maintenance) The impoundment area, encompassing the entire pond floor and interior slopes up to the 100-year flood elevation is considered selective mowing and should be completed in accordance with the description below.

• 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.

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Figure 26 Pond Maintenance Diagram

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Figure 27 Dam Embankment Maintenance Example

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Figure 28 Emergency Spillway Maintenance Example

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Figure 29 Shoreline Maintenance Example

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Figure 30 Channel Cleaning Example

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Figure 31 Control Structure Maintenance Example

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Figure 32 Control Structure Trash Rack Maintenance Example

Figure 33 Outfall Maintenance Example

Dewatering

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

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2. Check for water flow inside the control structure.

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 34 Dewatering Example

Routine maintenance for extended detention dry ponds (level 2) and constructed wetlands is broken into two types:

invasive plant species management, and trash, turf grass and wetland vegetation management. Routine maintenance should commence after one full growing season (July/August of the next calendar year). The facility should be closely monitored during the first 6 months after initial planting and inspected after storm events which exceed 0.5 in of rain. During this initial period, spot repairs of bare vegetation and supplemental watering should occur within the wetland facility where needed. Trash removal and turf grass management on the embankment may also be required.

Invasive Plant Species Management

This consists of trash removal, management of turf grass, cataloging the wetland plant species, and identification and removal of invasive plant species within the facility. If target area is small, hand removal of invasive species may be appropriate. Herbicidal treatment requires coordination with Fairfax County, and the contractor must provide the

Maintenance and Stormwater Management Division Page 38 following information: target invasive species, optimal treatment timing, treatment area, type3 of herbicide or method, and expected duration of treatment.

This task includes the annual identification and documentation of invasive plant species within the stormwater facility, along with the development of an Invasive Species Management Plan to be implemented within the same 8-month growing season. Cataloging must be performed by trained professionals and should reference the Virginia DCR

Invasives Plant List. Invasive plant species removal should be performed by qualified personnel, typically in July and

August, after plant species cataloging is complete and prior to the seed set of invasive species. This activity helps determine the level of non-routine (corrective) maintenance required for effective invasive species management.

Invasive plant species removal should be performed by qualified personnel, typically in July and August, after plant species cataloging is complete and prior to the seed set of invasive species.

• Removal of accumulated trash

• Turf Grass Management:

• Mow all access areas, the top and back of the dam embankment, 10 ft from the toe of the dam, and down the face of the embankment in accordance with the landscaping plan

(typically down to the control structure)

• In cases where no landscaping plan exists, the wetland vegetation should be evaluated and mapped by a qualified professional. A post-construction landscaping plan should then be developed

• Catalogue invasive plant species in May and June

• Remove invasive species in July and August

Wetland Vegetation Management

This consists of trash removal and management of turf grass and wetland vegetation. This maintenance should occur in September through December, after invasive plant species management has been completed. This maintenance includes:

• Removal of accumulated trash

• Turf Grass Management:

• Mow all access areas, the top and back of the dam embankment, 10 ft from the toe of the dam, and down the face of the embankment in accordance with the landscaping plan

(typically down to the control structure)

• In cases where no landscaping plan exists, the wetland vegetation should be evaluated and mapped by a qualified professional. A post-construction landscaping plan should then be developed

• Wetland Vegetation Management in October through December at a frequency of once every two years. This frequency will be achieved by numerically dividing the inventory in half, maintaining half the facilities a year on a rotating scheduling:

3 The following active-ingredient chemicals are generally considered acceptable within Fairfax County: Aminopyralid, Clopyralid, Glyphosate, Imazamox, Imazapic, Imazapyr, Pendimethalim, Triclopyr Amine, Triclopyr Ester, Or Owner approved equal.

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• Use a rotary mower (Bush Hog or equivalent) to harvest vegetation (trim to 6 in) in all high marsh areas.

• Dependent on conditions, balance the removal of clippings from site while minimizing the disturbance done by equipment for harvesting.. Do not mow or trim trees and shrubs. While maintenance personnel should always use the landscaping plan to determine where the high marsh zones are, a general rule of thumb is that if personnel can access the area with a rotary mower, it is a high marsh zone.

• Use a string trimmer (weed-whacker) to harvest vegetation (trim to 6 in) in all low marsh areas.

Remove all clippings from facility. While maintenance personnel should always use the landscaping plan to determine where the low marsh zones are, a general rule of thumb is that if personnel can walk through the area without standing or sinking in mud, then it is a low marsh area.

• Do not harvest the emergent vegetation. Emergent vegetation is typically found growing in the micropools. While maintenance personnel should always use the landscaping plan to determine where the micropools and emergent vegetation are, a general rule of thumb is that if personnel are standing or sinking in mud then they should not harvest the vegetation in that area. Buildup of decomposing plant material within the micropools will decrease stormwater capacity overtime, and ultimately impede the stormwater management function of the facility.

To balance the mutual goals of wetland vegetative health and maintaining stormwater capacity, removal of detritus and harvesting a portion of the emergent vegetation may be considered on a case-by-case basis. Removal should not be considered until the micropool has 90% vegetated surface coverage and then only if the facility appears to be non-functional due to buildup of materials within the micropools. The emergent vegetation should be harvested, and detritus removed, working from the center of the micropool outwards until the vegetative surface coverage is reduced to 50%. Decreases in stormwater capacity may be measured and calculated using depth surveys.

Site specific maintenance cards have been developed for numerous extended detention dry pond (level 2) facilities within Fairfax County, reference the Figures Table. Check for the maintenance card within a facility fixed references folder for additional information on maintenance items, known invasive species present, and overview of the facility.

Routine inspection of extended detention dry ponds (level 2) and constructed wetlands should be integrated with the routine maintenance tasks of the surrounding area and scheduled vegetation management activities. Inspections on publicly maintained extended detention dry pond (level 2) and wetland facilities are scheduled within a week of annual routine maintenance for turf grass and wetland vegetation, 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 present an opportunity to determine if there are issues such as encroachments, modifications and evidence of illicit discharge in the extended detention dry pond (level 2) or wetland. If evidence of illicit discharge is found, please report to 703-324-5500 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

Maintenance and Stormwater Management Division Page 40 details the potential issues and corrective solutions for some common maintenance items which may be called out during routine inspections.

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Figure 35. Site Specific Maintenance Card Front (Village Park – 0090DP)

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Figure 36. Site Specific Maintenance Card Back (Village Park – 0090DP)

Non-Routine Maintenance

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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. Use the landscaping plan to determine where turf grass should be maintained. 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:

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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.

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Mowing

• Each fall after invasive plant species management has been completed, mow all access areas, the top and back of the dam embankment, 10 ft from the toe of the dam, and down the face of the embankment in accordance with the landscaping plan (typically down to the control structure).

• In cases where no landscaping plan exists, the wetland vegetation should be evaluated and mapped by a professional qualified in wetland design and vegetation management. A post-construction landscaping plan should then be developed.

• 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.

• Dispose of all vegetation clippings appropriately.

• The wetland vegetation located on the extended detention dry pond (level 2) and wetland floor should be maintained as described under Extended Detention Dry Pond (Level 2) and

Constructed Wetland Floor on page 51.

Seeding

• Apply seed during the beginning months of both growing seasons.

• 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 dam is erosion. Erosion results from surface runoff, pedestrian, vehicular or animal traffic, and changes in pool levels. Areas with evidence of rill or gully erosion should be re-graded if necessary, backfilled with topsoil and stabilized 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.

• Armoring groin areas with rip-rap, articulated concrete block, or a permanent erosion mat.

• 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 dam embankments, to avoid rutting.

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Shrinkage Cracks Maintenance

During dry seasons, the clay in dam 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. If such a condition exists, consult with a qualified professional to identify the cause and repair as needed.

Repair cracks wider than 1 in and/or deeper than 1 ft, cover with a 2 in layer of topsoil, and plant with grass.

Trees, Shrubs and Brush Maintenance

In Fairfax County, earthen dams and areas within 10 ft of the dam’s upstream heel and downstream toe, groins, and abutments 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 dam compacted soil. Check landscaping plans when available for approved planting locations on the dam. 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 dam 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 dam, heel, toe, groin, and abutment clearance areas 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

28.

Trees Greater Than 4 in Diameter

Large trees can uproot, leaving large voids and depressions which may weaken the dam or lead to erosion. Shrubs, brush, and small woody stock obscure dam 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:

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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

(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 24 for photo of a groundhog.

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Muskrats

Muskrats construct dens by burrowing into the banks of lakes and streams, and may be found in the embankment of extended detention dry ponds (level 2) and constructed wetlands adjacent to the final micropool at the control structure. Muskrat burrows start from 6 – 8 in below the normal pool (water surface) and then tunnel upwards to dry ground (above the phreatic surface of the dam). Burrowing provides a path for water to 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.