Required Elements
• Maintenance responsibility for a WVTS and its setback shall be vested with a responsible authority by means of a legally binding and enforceable maintenance agreement that is executed as a condition of plan approval.
• General inspections shall be conducted on an annual basis and after storm events greater than or equal to the 1-year, 24-hour Type III precipitation event.
• The principal spillway shall be equipped with a removable trash rack, and generally accessible from dry land. See the maintenance checklist in Appendix E for more detail on maintaining trash racks.
• A maintenance and operation plan must specify that sediment removal in the forebay shall occur every 5 years or after 50% of total forebay capacity has been lost, whichever occurs first.
• An operation and maintenance plan shall specify that if a minimum vegetative coverage of 50% is not achieved in the planted areas after the second growing season, a reinforcement planting is required.
• Sediment and organic build-up shall be removed from a Gravel WVTS every 2 years, as needed.
• In a Gravel WVTS, vertical cleanouts must be constructed that are connected to the distribution and collection subdrains at each end.
• For discharges beyond 200 ft from streams (and any contiguous natural or vegetated wetlands) in cold-water fisheries, the gravel trench outlet shall be inspected after every storm in the first 3 months of operation to ensure proper function. Thereafter, the trench shall be inspected at least once annually.
Inspection shall consist of verifying that the WVTS is draining to the permanent pool elevation within the 24-hour design requirement and that potentially clogging
material, such as accumulation of decaying leaves or debris, does not prevent the discharge through the gravel. When clogging occurs, at least the top 8 inches of gravel shall be replaced over with new material. Sediments shall be disposed of in an acceptable manner.
Design Guidance
• Areas with a permanent pool should be inspected on an annual basis. The maintenance objectives for these practices include preserving the hydraulic and removal efficiency of the WVTS and maintaining the structural integrity.
5.0 STRUCTURAL STORMWATER TREATMENT PRACTICES FOR MEETING
WATER QUALITY CRITERIA 5-16
• Sediments excavated from a WVTS that do not receive runoff from designated
LUHPPL are generally not considered toxic or hazardous material, and can be safely disposed by either land application or land filling. Sediment testing may be required prior to sediment disposal when a LUHPPL is present. Sediment removed from forebays shall be treated similarly to street sweepings as provided in Appendix G.4.1.
• Sediment removed from a WVTS should be disposed of according to an approved comprehensive operation and maintenance plan.
• The slopes of the basin or WVTS should be inspected for erosion and gullying. Reinforce existing riprap if riprap is found to be deficient, erosion is present at the outfalls of any control structures, or the existing riprap has been compromised. Re- vegetate slopes as necessary for stabilization.
• All structural components, which include, but are not limited to, trash racks, access gates, valves, pipes, weir walls, orifice structures, and spillway structures, should be inspected and any deficiencies should be reported. This includes a visual inspection of all stormwater control structures for damage and/or accumulation of sediment.
• All dead or dying vegetation within the extents of the WVTS should be removed, as well as all herbaceous vegetation rootstock when overcrowding is observed and any vegetation that has a negative impact on stormwater flowage through the facility. Any invasive vegetation encroaching upon the perimeter of the facility should be pruned or removed if it is prohibiting access, compromising sight visibility and/or compromising original design vegetation.
Maintenance Access
Required Elements
• A maintenance right of way or easement shall extend to a WVTS from a public or private road.
Design Guidance
• Maintenance access should be at least 10 feet wide, have a maximum slope of no more than 15%, and be appropriately stabilized to withstand maintenance equipment and vehicles.
• The maintenance access should extend to the forebay(s), safety bench, emergency spillway, outlet control structure, and outlet and be designed to allow vehicles to turn around.
• The grass around the perimeter of the WVTS should be mowed at least 4 times annually.
Non-clogging Low-flow Orifice/Weir
Required Elements
5.0 STRUCTURAL STORMWATER TREATMENT PRACTICES FOR MEETING
WATER QUALITY CRITERIA 5-17
low-flow orifice or weir shall be designed to ensure that no clogging shall occur. Design Guidance
• The low-flow orifice should be adequately protected from clogging by either an acceptable external trash rack (recommended minimum orifice of 3”) or by internal orifice protection that may allow for smaller diameters (recommended minimum orifice of 1”). See Appendix H for sample schematics of low-flow orifice protection alternatives.
• The preferred method is a submerged reverse-slope pipe that extends downward from the outlet control structure to an inflow point one foot below the normal pool elevation (see Figure 5-1 for schematic profile).
• Alternative methods are to employ a broad-crested rectangular, V-notch, or
proportional weir, protected by a half-round pipe or “hood” that extends at least 12 inches below the normal pool.
• The use of horizontally extended perforated pipe protected by geotextile fabric and gravel is not recommended. Vertical pipes may be used as an alternative where a permanent pool of sufficient depth is present.
Outlet Control Structure in Embankment
Required Elements
• The outlet control structure shall be located within the embankment for maintenance access, safety and aesthetics.
Design Guidance
• Access to the outlet control structure should be provided by lockable manhole covers, and manhole steps within easy reach of valves and other controls. The principal spillway opening should be "fenced" with pipe or rebar at 8-inch intervals (for safety purposes).
WVTS Drain
Required Elements
• Except where local slopes (e.g., coastal areas) prohibit this design, each WVTS shall have a drain pipe that can completely or partially drain the practice. The drain pipe shall have an elbow or protected intake within the WVTS to prevent sediment deposition, and a diameter capable of draining the permanent pool within 24 hours.
• Access to the drain pipe shall be secured by a lockable structure to prevent
vandalism and/or accidental draining of the pond, which could pose a safety hazard due to high drainage velocities.
5.0 STRUCTURAL STORMWATER TREATMENT PRACTICES FOR MEETING
WATER QUALITY CRITERIA 5-18
Safety Features
Required Elements
• Proposed graded side slopes to the WVTS shall not exceed 3:1 (h:v), and shall terminate on the safety bench.
• The principal spillway opening shall not permit access by small children, and endwalls above pipe outfalls greater than 48 inches in diameter shall be fenced to prevent a hazard.
• “Token” or emergency spillways (those placed above the water elevation of the largest managed storm) are required if not already provided as part of the conveyance of the 100-year storm event and must be a minimum 8 ft wide, 1 ft deep, with 2:1 channel side slopes.
Design Guidance
• Both the safety bench and the aquatic bench may be landscaped to prevent access to the permanent pool.
• Warning signs prohibiting swimming and skating may be posted.
• Fencing is generally not encouraged, but may be required by some owners and/or agencies. A preferred method is to manage the contours of the WVTS to eliminate dropoffs or other safety hazards.
5.0 STRUCTURAL STORMWATER TREATMENT PRACTICES FOR MEETING
WATER QUALITY CRITERIA 5-19