PWD Stormwater Management Guidance Manual v3.4 F.14 Subsurface Detention
Appendices

F.14 Subsurface Detention

  Print / Download

F.14.1 Subsurface detention plan standards

  1. Verify that the plans include an appropriate sequence of construction that is specific to the construction of the subsurface detention SMP. Refer to Section 4.8.5 for guidance. [Section 2.3.1]
  2. Verify that the plans include an appropriate cross-sectional detail for the subsurface detention SMP. [Section 2.3.1]

F.14.2 Subsurface detention design standards

  1. Verify that the SMP drains within the acceptable 72‑hour period after the 24‑hour storm event. [Section 4.8.3, 1]
  2. Verify that positive overflow is provided for large storm events, up to and including the 100‑year, 24‑hour storm event, or, if the project is exempt from Flood Control, the ten‑year, 24‑hour storm. [Section 4.8.3, 2]
  3. Verify that overflow structures and pipes are designed to convey at least the ten‑year, 24‑hour storm event. [Section 4.8.3, 2]
  4. Verify that the distance between subsurface detention basins and any adjacent private property line is at least ten feet. This includes lined basins. Exceptions can be made for water-tight vaults with their own structural integrity, such as concrete or fiberglass vaults. It is acceptable for SMPs to be located directly adjacent to the public right-of-way (ROW) (unless a deed restriction is put in place extending at least ten feet from the perimeter of the infiltrating SMP). [Section 4.8.3, 4]
  5. Verify that the distance between subsurface detention basins and any building or retaining wall is at least ten feet. This includes lined basins. The following requirements and exceptions apply: [Section 4.8.3, 5]
    1. For existing and proposed buildings with basements, the setback is measured from the basement wall and may be waived if the basin is a water-tight vault with its own structural integrity, such as a concrete or fiberglass vault.
    2. For existing buildings without basements and existing retaining walls, the setback is measured from the foundation and may be waived if a signed and sealed geotechnical analysis is submitted that evaluates the impacts of residual infiltration and excavation on the existing foundation and determines it to be feasible.
    3. For proposed buildings without basements and proposed retaining walls, the setback is measured from the foundation and may be waived if the foundation is proposed to be designed with the basin’s proximity in mind.
  6. Verify that pretreatment is provided for all runoff entering the subsurface detention SMP, including pretreatment of runoff from all inlets. At a minimum, this can be achieved through the use of sumps and traps for inlets, sump boxes with traps downstream of trench drains, and filter strips for overland flow. [Section 4.8.3, 6]
  7. Verify that the storage area provides adequate storage to control release rates to meet all applicable Stormwater Regulations. All permanent pool areas must be excluded from the SMP’s storage volume estimation. [Section 4.8.3, 9]
  8. Verify that pipe, vault, grid and chamber storage areas are adequately bedded with stone to prevent settling or subsidence. [Section 4.8.3, 10a]
  9. Verify that bedding thickness is not less than six inches. [Section 4.8.3, 10b]
  10. Verify that foundations/footers are provided as warranted by system loading, geotechnical conditions, and manufacturer’s recommendations. Foundation designs must be performed by an appropriate design professional. [Section 4.8.3, 10d]
  11. Verify that the storage design accounts for potential loading from vehicles, as appropriate, based on expected maximum active loading, including consideration for emergency vehicles. [Section 4.8.3, 11]
  12. Verify that the porosity values used for storage volume calculations are as follows: [Section 4.8.3, 13]
    1. Soil media: 0.20
    2. Sand: 0.30
    3. Stone 0.40
    4. Void space provided by linear chamber systems, plastic grids, or other related structures must be as specified by the manufacturer and noted in supporting documentation.
  13. Verify that the stone storage layer, if proposed, is separated from soil media by a geotextile or pea gravel filter to prevent sand, silt, and sediment from entering the system. [Section 4.8.3, 14]
  14. Verify that any impervious liner, if necessary, is not interrupted by structures within the basin footprint. The plans must indicate that the impervious liner is to be continuous and extend completely up the sides of any structures that are located within the lined basin footprint to the ground surface. If additional liner material must be added to extend up the structures, the additional liner sections are to be joined to the rest of the liner with an impervious seam per the manufacturers’ recommendations. [Section 4.8.3, 16]
  15. Verify that an adequate number of appropriately placed cleanouts, manholes, access panels, and other access features are provided to allow unobstructed and safe access to the subsurface detention SMP for routine maintenance and inspection of inflow, outflow, underdrains, and storage systems. [Section 4.8.3, 17]
  16. Verify that observation wells are provided for SMPs that include stone storage and that they meet the following requirements:
    1. The observation well must be placed at the invert of the stone bed. [Section 4.8.3, 18a]
    2. The observation well must be located near the center of the stone bed system to monitor the level and duration of water stored within the system (drain down time). A minimum of six inches of clearance must be provided between the edge of the observation well’s concrete ring or frame and any nearby site features (curbing, tree pits, sidewalk edge, etc.). Plan view dimensioning for the proposed location of the observation well, relative to the boundaries of the SMP it lies within, must be provided on the plans. [Section 4.8.3, 18b]
    3. Adequate inspection and maintenance access to the observation well must be provided. [Section 4.8.3, 18c]
    4. An observation well located in a paved area must be outfitted with a cover and frame. The cover plate must be installed flush with the surrounding surface at finished grade, and grading should drain away from the observation well wherever possible. The solid section of the observation well must extend into the cover frame enough such that a bentonite/cement seal can be placed around the well within the frame, and a solid slip-on cap can be fitted onto the pipe end. [Section 4.8.3, 18d]
    5. An observation well located in an unpaved area must have a concrete ring or frame poured for support, with three lengths of rebar evenly spaced around the ring and three inches from its outer edge. The observation well should be located outside of the ponding area of any surface SMPs. If, due to space limitations, an observation well must be placed in the ponding area of a surface SMP, the concrete ring and well cover must extend three inches above the SMP’s maximum ponding depth. [Section 4.8.3, 18e]
    6. The observation well must include a slotted section within the storage layer of the SMP. The top of the slotted well section must extend to a minimum of six inches below the top of the SMP storage layer, and the slotted section must extend down the entire depth of the SMP. [Section 4.8.3, 18f]
  17. Verify that access features are provided for any subsurface detention SMP that is not comprised of a stone storage bed. [Section 4.8.3, 19a]
  18. Verify that a sufficient number of access points in the SMP are provided to efficiently inspect and maintain the storage area. [Section 4.8.3, 19b]
  19. For cast-in-place vault systems, verify that access features consist of manholes or grated access panels or doors. Grated access panels are preferred to maintain airflow. A minimum of 50 square feet of grate area is recommended for permanent pool designs. [Section 4.8.3, 19c]
  20. For grid storage or other manufactured systems, verify that the manufacturer’s recommendations are followed. [Section 4.8.3, 19d]
  21. Verify that ladder access is provided for vaults greater than four feet in height. [Section 4.8.3, 19e]
  22. Verify that header pipes, at minimum 36‑inch in diameter, connected to manholes at each corner of the subsurface detention SMP are provided. Alternatively, smaller header pipes may be used if cleanouts are provided on the manifold/header pipe junction for each distribution pipe. The cleanouts must be on alternating sides of the SMP. [Section 4.8.3, 19f]

F.14.3 Subsurface detention material standards

  1. Verify that stone designed for stormwater storage, if proposed, is specified on the plans as being uniformly graded, crushed, clean-washed stone, and that it is noted that PWD defines “clean-washed” as having less than 0.5% wash loss, by mass, when tested per the AASHTO T-11 wash loss test. AASHTO No. 3 and AASHTO No. 57 stones can meet this specification. [Section 4.8.4, 3]
  2. Verify that sand, if proposed, is specified on the plans to be AASHTO M‑6 or ASTM C‑33 sand and to have a grain size of 0.02 inches to 0.04 inches. [Section 4.8.4, 4]
  3. Verify that storage pipe, if proposed, is specified on the plans as meeting the following specifications:
    1. Pipe used within the subsurface detention SMP must have a minimum inner diameter of four inches. [Section 4.8.4, 5a]
    2. High-density polyethylene (HDPE) pipe must meet the specifications of AASHTO M252, Type S or AASHTO M294, Type S. [Section 4.8.4, 5b]
    3. Any pipe materials outside the SMP are to meet the City Plumbing Code Standards. [Section 4.8.4, 5c]
  4. Verify that geotextile, if proposed, is specified on the plans to consist of polypropylene fibers and to meet the following specifications (AASHTO Class 1 or Class 2 geotextile is recommended): [Section 4.8.4, 6]
    1. Grab Tensile Strength (ASTM-D4632): ≥ 205 lbs
    2. CBR Puncture Strength (ASTM-D6241): ≥ 500 lbs
    3. Flow Rate (ASTM-D4491): ≥ 95 gal/min/ft2
    4. UV Resistance after 500 hrs (ASTM-D4355): ≥ 70%
    5. Tear Resistance (ASTM-D4533): ≥ 80 lbs
    6. Heat-set or heat-calendared fabrics are not permitted
  5. Verify that the observation well is specified on the plans as meeting the following specifications:
    1. The observation well must consist of slotted plastic pipe with a minimum inner diameter of four inches and a solid cap at the surface. The slotted section of the well must consist of PVC slotted well screen with 0.01‑inch slot size and an attached perforated PVC cap at the bottom. [Section 4.8.4, 8a]
    2. The observation well’s cover must be ductile iron in a gray iron frame. [Section 4.8.4, 8b]
    3. For an observation well located in an unpaved area, the well’s concrete ring must be a minimum of six inches wider in any dimension than the cover frame, of equal depth as the cover frame itself, and poured atop a minimum of six inches of AASHTO #57 stone bedding. A frame conforming to these minimum dimensions may be circular or square. [Section 4.8.4, 8c]
  6. Verify that cleanouts are made of material with a smooth interior having an inner diameter that is no less than six inches. If the connecting pipe is eight inches in diameter or larger, verify that the cleanout is eight inches in diameter. [Section 4.8.4, 9]
Jump to Top ↑