Población y muestra

Required Elements

• The outlet control structure shall be located within the embankment for maintenance access, safety and aesthetics.

• The outlet control structure shall be sized and designed for CPv and Qp, as required.

• For discharges beyond 200 ft from jurisdictional waters in cold-water fisheries, the underdrained gravel trench shall be designed to meet the following requirements:

− Shall be sized to release the CPv over at least 12 hrs and not more than 24 hrs to

provide adequate cooling of stormwater runoff discharging from the basin;

− Shall be four feet wide, located at least 2 feet from the permanent pool, and

located at the furthest location opposite from the principal inflow location to the facility;

− The trench shall have a length of 3 feet per 1,000 ft3 of CPv storage volume, have

a depth of at least 3 feet, and maintain 2 feet of gravel cover over a 6-inch diameter perforated pipe outlet (Rigid Sch. 40 PVC or SDR35);

− Shall utilize geotextile fabric placed between gravel trench and adjacent soil; and − Shall utilize clean poorly-graded (uniform size material) gravel (refer to Figure

5-4). 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).

7.2.10 Basin Drain

Required Elements

• Except where local slopes prohibit this design, each wet basin shall have a drain pipe that can completely or partially drain the permanent pool. The drain pipe shall have an elbow or protected intake within the basin to prevent sediment deposition, and a diameter capable of draining the basin 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.

7.2.11 Safety Features

Required Elements

• Side slopes to the basin shall not exceed 3:1 (h:v) and, for wet basins, shall terminate on a 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

7.0 STORAGE PRACTICES FOR STORMWATER QUANTITY CONTROL 7-9

prevent a hazard. Design Guidance

• Both the safety bench and aquatic bench may be landscaped to prevent access to the pool.

• Warning signs prohibiting swimming and ice skating may be posted.

• Basin fencing is generally not encouraged, but is often appropriate and may be required by the owner or by local ordinance in residential neighborhoods. A preferred method is to manage the contours of the basin to eliminate dropoffs or other safety hazards.

Figure 7-1 Dry Extended Detention Basin

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Figure 7-2 Wet Extended Detention Basin

Adapted from MDE, 2000 7.3 UNDERGROUND STORAGE DEVICES

Underground stormwater detention practices capture and store stormwater from a site, slowly releasing it back to a natural channel or receiving waters at pre-development peak flows. These underground storage vaults provide minimal, if any, stormwater quality benefits; however, they can be used to meet channel protection and peak flow attenuation standards.

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Subsurface storage structures are typically made of concrete (vaults) or large diameter, rigid pipes or arches with capped ends and made of plastic, steel or aluminum. Storage structures and appurtenances (inlet and outlet pipes, maintenance access/manholes) are constructed in a predetermined excavated area sized for the required criteria (CPv

and Qp). The entire area is then back-filled with gravel to surrounding grades and

surfaced. Due to on-going maintenance requirements and the potential need for

repairs, underground storage facilities should not be built over and should be located in areas where large-sized maintenance vehicles can easily operate and excavate, if required.

Underground storage is most often used at sites where land availability, shape, and/or land costs preclude or discourage the development of surface stormwater storage. Underground storage is ideal for use under parking lots, roadways and paved areas associated with commercial, industrial and residential developments. The advantages of an underground storage facility are (1) rapid installation using prefabricated modular systems; (2) systems are durable with a long life (50 years plus for most systems); (3) increased level of public safety vs. open, deep storage basins; (4) efficient use of space in urban areas and for retrofits; and (5) ground provides insulation from freezing.

Limitations of underground systems include that they often require extensive, costly excavation; material costs are high compared to surface methods; and maintenance costs are higher. In addition, routine maintenance is often overlooked because the practice is not easily inspected via casual observation.

7.3.1 Feasibility

Required Elements

• Designers shall check with local authorities regarding design requirements and necessary permits for construction of underground storage. There is great variability between localities in requirements and permissible construction materials.

Design Guidance

• Placement of underground stormwater storage is site specific. During early site inspections, special note should be made of site size, shape, and physical

characteristics of the landscape. These factors will help determine basic structure of the detention system and what materials are best used in construction.

• The suggested maximum area of stormwater drainage to be collected for one underground storage system is 25 acres.

• Underground storage devices have confined entry limitations per Occupational Safety and Health Administration (OSHA) regulations.

7.3.2 Material Selection

Design Guidance

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• Depth and area of excavation: deeper and larger excavated areas require more fill for maintaining the integrity of plastic or metal pipe;

• Shape of space: continuous space allows for the use of concrete, while angular spaces favor the use of pipe systems. However:

- Pipes store less water than square concrete vaults per unit of land surface. - Pipes require more fill than concrete structures, thus using more excavated area. - Use the largest pipe diameter possible. Doubling pipe diameter quadruples

capacity and only doubles cost.

7.3.3 Conveyance

Required Elements

• Outfalls to the ground surface, where needed, shall be constructed such that they do not increase erosion by discharging near the stream water surface elevation or into an energy dissipating step-pool arrangement.

• An emergency overflow system shall be designed to convey flows larger than the 100-year storm or to divert water in case system fails for any reason.

Design Guidance

• If system discharges to a watercourse with dry weather flow, care should be taken to minimize tree clearing along the downstream channel, and to reestablish a forested riparian zone in the shortest possible distance.

7.3.4 Design

Required Elements

• Capacity and discharge rate shall depend on the CPv and Qp requirements. Storage

is a function of geometry of the structure, which shall be provided by the manufacturer.

• Sufficient maintenance access points (manholes) shall be incorporated in design to facilitate easy maintenance. Placement shall, at a minimum, occur near the intake and another at the outlet end of the system. The number of manholes depends on maintenance methods used.

• The design shall address implications of the depth to groundwater at the site. A high water table can cause structures to displace due to uplift forces if not designed correctly. Anti-floatation calculations are required when system designed below the water table.

Design Guidance

• If system outfalls to ground surface, rip-rap may be required and should be sized properly to reduce erosion.

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