20.2 BMP F110 - Detention Ponds
Detention ponds are depressions where stormwater is collected, stored, and slowly released.
20.2.1 Design Criteria Specific for Detention Ponds
Standard details for detention ponds are provided in Figure 4 - 49: Typical Detention Pond and . Control structure discussion and details are provided in A500 - Control Structures.
The Design Criteria below is in addition to the Design Criteria for All Detention Facilities.
Ponds must be designed as flow-through systems (however, parking lot storage may utilize a back-up system; see BMP F150 - Roofs for Detention). Developed flows must enter through a conveyance system separate from the control structure and outflow conveyance system. Maximizing distance between the inlet and outlet is encouraged to promote sedimentation.
Pond bottoms shall be level and be located a minimum of 0.5 feet below the inlet and outlet to provide sediment storage.
Design criteria for outflow control structures are specified in A500 - Control Structures.
The following setback requirements shall be met, along with those stipulated in Design Criteria for All Flow Control BMPs.
The 100-year water surface elevation shall be at least 10 feet from any building structure and at least 5 feet from any other structure or property line. If necessary, setbacks shall be increased from the minimum 10 feet in order to maintain a 1H:1V side slope for future excavation and maintenance. Vertical pond walls may necessitate an increase in setbacks.
Interior side slopes up to the emergency overflow water surface shall not be steeper than 3H:1V unless a fence is provided (see A700 - Fencing).
Exterior side slopes must not be steeper than 2H:1V unless analyzed for stability by a Washington State Licensed Professional Engineer or Washington State Licensed Professional Geologist.
Pond walls may be vertical retaining walls, provided:
They are constructed of minimum 3,000 psi structural reinforced concrete.
A fence is provided along the top of the wall.
Access for maintenance per this section shall be provided.
The design is stamped by a licensed Washington State Licensed Professional Engineer.
Ladders shall be provided on the walls for safety reason if all walls are vertical retaining walls.
Other retaining walls such as rockeries, concrete, masonry unit walls, and keystone type walls may be used if designed and stamped by a Washington State Licensed Professional Engineer.
Pond berm embankments higher than 6 feet must be designed by a Washington State Licensed Professional Engineer.
For berm embankments 6 feet or less in height, the minimum top width shall be 6 feet or as recommended by a Washington State Licensed Professional Engineer.
Pond berm embankments must be constructed on native consolidated soil (or adequately compacted and stable fill soils analyzed by a Washington State Licensed Professional Engineer) free of loose surface soil materials, roots, and other organic debris.
Pond berm embankments greater than 4 feet in height must be constructed by excavating a key equal to 50 percent of the berm embankment cross-sectional height and width unless specified otherwise by a Washington State Licensed Professional Engineer.
Embankment compaction should be accomplished in such a manner as to produce a dense, low permeability engineered fill that can tolerate post-construction settlements with a minimum of cracking. The embankment fill shall be placed on a stable subgrade and compacted to a minimum of 95% of the Standard Proctor Maximum Density, ASTM Procedure D698. Placement moisture content should lie within 1% dry to 3% wet of the optimum moisture content.
The berm embankment shall be constructed of soils with the following minimum characteristics per the United States Department of Agriculture’s Textural Triangle: a minimum of 20% silt and clay, a maximum of 60% sand, a maximum of 60% silt, with nominal gravel and cobble content. Soils outside this range may be used by stamped recommendation of a Washington State Licensed Professional Engineer.
Anti-seepage filter-drain diaphragms must be placed on all pipes in berm embankments impounding water with depths greater than 8 feet at the design water surface. See Dam Safety Guidelines, Part IV, Section 3.3.B. An electronic version of Dam Safety Guidelines is available in PDF format at https://ecology.wa.gov/Water-Shorelines/Water-supply/Dams.
Figure 4 - 49: Typical Detention Pond
Figure 4 - 50: Typical Detention Pond Sections
20.2.1.5 Overflow
The following overflow requirements shall be met along with those stipulated in Section 20.1.3.2 - Detention Facility Overflow.
A secondary inlet to the control structure shall be provided in ponds as additional protection against overtopping should the inlet pipe to the control structure become plugged. A grated opening in the control structure maintenance hole functions as a weir (see ) when used as a secondary inlet.
The maximum circumferential length of this opening must not exceed one-half the control structure circumference.
The beehive grate overflow structure as shown in Figure 4 - 73: Overflow Structure may also be used as a secondary inlet.
In addition to the above overflow provisions, ponds shall have an emergency overflow spillway. Design the Emergency Overflow Spillway per A600 - Emergency Overflow Spillway.
Access shall be provided to the inlet, outlet, and control structure. Access roads and ramps shall conform to A3000 - Access Ramps and Roads
20.2.1.7 Signage
Provide a stormwater facility sign. Sign shall conform to A800 - Signage.
20.2.1.8 Planting Requirements
Exposed earth on the pond bottom and interior side slopes shall be sodded or seeded with an appropriate seed mixture or landscaped. See A1000 - Vegetation for seed mixes and plant selection recommendations. All remaining areas of the tract shall be planted with grass or be landscaped and mulched with a 4-inch cover of hog fuel or shredded wood mulch. Shredded wood mulch is made from shredded tree trimmings, usually from trees cleared on site. The mulch should be free of garbage and weeds and should not contain excessive resin, tannin, or other material detrimental to plant growth. Do not use construction materials, wood debris, or wood treated with preservatives for producing shredded wood mulch. The seed mix and coverage shall be specified on the plan set.
20.2.1.9 Landscaping
Landscaping is encouraged for most stormwater tract areas (see below for areas not to be landscaped). However, if provided, landscaping should adhere to the criteria that follow so as not to hinder maintenance operations. Landscaped stormwater tracts may, in some instances, provide a recreational space. In other instances, “naturalistic” stormwater facilities may be placed in open space tracts.
The following guidelines shall be followed if landscaping is proposed for facilities.
Provide a planting plan that clearly shows the species of plants and the spacing for each plant species.
No trees or shrubs shall be planted on berms meeting the criteria for dams regulated for safety.
No trees or shrubs shall be planted within 10 feet of inlet or outlet pipes or artificial stormwater and surface water structures such as spillways or flow spreaders.
Species of trees with roots that seek water shall be avoided within 50 feet of pipes or artificial structures.
Planting shall be restricted on berms that impound water either permanently or temporarily during storms. This restriction does not apply to cut slopes that form pond banks, only to berms.
Trees or shrubs may not be planted on portions of water-impounding berms taller than four feet high. Only grasses may be planted on berms taller than four feet.
Trees planted on portions of water-impounding berms less than 4 feet high must be small, not higher than 20 feet mature height, and must have a fibrous root system. A1000 - Vegetation gives some examples of trees with these characteristics developed for the Central Puget Sound.
Note: The internal berm in a detention pond is not subject to this planting restriction since the failure of an internal berm would be unlikely to create a safety problem.
All landscape material, including grass, shall be planted in topsoil. Topsoil shall be tilled in or otherwise integrated into the detention pond bottom or sides to ensure proper plant growth. If a liner is present, this may require additional consideration to ensure liner depth is maintained. Native underlying soils may be made suitable for planting if amended with 4 inches of compost tilled into the subgrade. Compost shall comply with A900 - Compost.
For a naturalistic effect as well as ease of maintenance, trees or shrubs shall be planted in clumps to form “landscape islands” rather than planting evenly spaced.
The landscaped islands shall be a minimum of six feet apart, and if set back from fences or other barriers, the setback distance should also be a minimum of 6 feet. Where tree foliage extends low to the ground, the 6 feet setback should be counted from the outer drip line of the trees (estimated at maturity).
This setback allows a 6-foot wide mower to pass around and between clumps.
Evergreen or columnar deciduous trees along the west and south sides of ponds are recommended to reduce thermal heating. Evergreen trees and trees which produce relatively little leaf-fall are preferred in areas contributing to the pond.
Trees should be set back so that branches do not extend over the pond (to prevent deposition of leaves into the pond).
Drought tolerant species are recommended.
The method of construction of soil/landscape systems can cause natural selection of specific plant species. Use soils that encourage desired species and discourage undesired species.
20.2.1.10 Detention Ponds and Infiltration
Detention ponds may occasionally be sited on soils that allow for infiltration. These detention ponds have a surface discharge and may also utilize infiltration as a second pond outflow. Detention ponds sized with infiltration as a second outflow must meet all the requirements of BMP 710 - Infiltration Basins, including a soils report, testing, groundwater protection, pre-settling, and construction techniques.