15.4 Detention, Retention & Proprietary BMPs: Dry/Wet Basins, Wetlands, Inline Storage, Catch Basin Inserts, Vaults, and Manufactured Devices

Multipurpose basins and permanent operation

A multipurpose basin serves more than one stated objective, such as temporary construction-sediment control, permanent water-quality treatment, peak-flow detention, and flood storage. Its functions can occupy different elevations and change between construction phases. The label does not mean that the same outlet or storage volume is suitable for every phase. During construction, inspect the temporary sediment-storage zone, surface withdrawal, stabilized inflow, embankment, and spillway against the construction detail. At conversion, verify sediment removal, restoration of the design elevations, installation of the permanent outlet, and completion of vegetation and access provisions before accepting the facility for permanent service.

A useful closeout record identifies each phase and responsible owner. For example, a development’s basin may begin with a temporary skimmer and later use a multistage permanent outlet. Leaving construction sediment in the basin reduces the permanent water-quality and flood-storage volumes even when its slopes look green. Record sediment elevations and compare the as-built survey with the permanent plan; the authorized designer or owner determines whether capacity is acceptable. Photograph the outlet before and after conversion and keep the maintenance access clear.

Long-term inspection checks sediment and litter, eroding inlets, outlet blockage, vegetation survival, embankment settlement, and evidence of overtopping. The O&M agreement assigns responsibility and access, while the approved plan supplies cleanout levels and inspection intervals. Multipurpose use may add public access or recreation, which also requires the owner’s safety provisions. A construction NOT ends that construction permit coverage; it does not extinguish post-construction maintenance duties or establish a basin’s hydraulic adequacy by itself.

Surface Detention vs. Retention Basins: Core Hydraulic & Water-Quality Distinctions

On the CESSWI exam, candidates must clearly distinguish between Detention and Retention surface impoundments:

• Detention Basin (Dry Basin): Temporarily stores stormwater runoff during a storm event and releases $100\%$ of the stored water through a calibrated outlet structure so the basin floor is normally DRY between storm events.
• Retention Basin (Wet Detention Basin / Wet Pond): Maintains a permanent pool of water ($4\text{ to }8\text{ feet}$ deep) year-round and temporarily detains additional live storm runoff storage above the normal pool elevation.

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1. Multi-Purpose Detention Basin (12.7.4) and Dry Extended Detention Basin (12.7.5)

Multi-Purpose Detention Basin (12.7.4)

A Multi-Purpose Detention Basin is a dry flood-control detention basin specifically graded and drained (often incorporating subsurface turf underdrains and sandy root-zone soils) so its flat vegetated floor serves a secondary community recreational or civic function during dry weather—such as municipal soccer and baseball fields, open park lawn, walking trails, or an outdoor amphitheater—while detaining 10-year to 100-year flood peaks during major storms. After floodwaters recede, maintenance crews clear any floatable debris and mud film before reopening the recreational facility.

Dry Extended Detention (ED) Basin (12.7.5)

Whereas a conventional flood-control dry detention basin drains rapidly (often within $6\text{ to }12\text{ hours}$) solely to shave peak flood discharge ($Q_p$) and achieves poor fine-sediment removal ($< 30\%\text{ to }40\%$ TSS), a Dry Extended Detention (ED) Basin is engineered for both peak-flow attenuation and water quality treatment:

• Extended Drawdown Time ($24\text{ to }72\text{ Hours}$): A small, calibrated low-flow water-quality orifice (protected against clogging by an external perforated trash rack/hood or an internal reverse-slope pipe) at the invert of the multi-stage outlet riser holds back and slowly releases the Water Quality Volume ($WQ_v$) over $24\text{ to }72\text{ hours}$ ($40\text{ to }48\text{ hours}$ typical design target), giving fine silt particles quiescent residence time to settle out ($\sim 60\%$ TSS removal).
• Forebay, Low-Flow Channel & Micropool: Includes an inlet sediment forebay to trap coarse sand, a stabilized low-flow pilot channel or shallow wet outlet micropool (preventing bottom scour and orifice clogging as the last few inches drain), and an earthen embankment dam with an armored emergency spillway.
• Critical Earthen Dam Inspection Rule: CESSWI inspectors must ensure earthen embankment dams, side slopes, and emergency spillways are kept completely free of trees and woody brush (whose decaying roots create internal seepage piping channels that can breach the dam) as well as burrowing animals (groundhogs, muskrats, beavers).

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2. Wet Detention Basin / Retention Pond (12.7.6) and Constructed Wetlands (12.7.7)

Wet Detention Basin / Retention Pond (12.7.6)

A Wet Detention Basin (Retention Pond) maintains a permanent pool of water ($4\text{ to }8\text{ feet}$ deep) throughout the year, overlaid by temporary live extended-detention and flood-control storage zones.

• Superior Water Quality via Plug-Flow Displacement: When a storm occurs, incoming runoff enters the inlet and pushes out (displaces) the quiescent, already-treated water that has been sitting in the permanent pool since the previous storm. Simultaneously, the deep permanent water cushion absorbs incoming kinetic energy so settled bottom muck cannot be scoured and resuspended. Consequently, wet detention basins achieve $80\%\text{ to }90\%+$ TSS removal, along with substantial biological removal of dissolved nitrogen and phosphorus (consumed by aquatic plants and phytoplankton) and trace metals.
• Four Mandatory Structural & Safety Features:
  1. Inlet Sediment Forebay: Sized to hold $10\%\text{ to }20\%$ of the total permanent pool volume and separated from the main pool by a submerged rock berm or gabion weir. Equipped with a permanent vertical depth staff gauge so inspectors know to trigger maintenance dredging when forebay capacity is reduced by $50\%$ (or when the main pool loses $25\%$ of its volume).
  2. Vegetated Aquatic Bench (Safety Shelf): A shallow $6\text{- to }12\text{-inch}$ deep, $10\text{- to }15\text{-foot}$ wide shelf graded around the inside shoreline perimeter and planted with dense emergent wetland plants. The aquatic bench serves two vital functions: life safety (preventing children or maintenance workers who slip on the bank from plunging into $6\text{ to }8\text{ feet}$ of deep water) and biological polishing / wave-erosion protection.
  3. Non-Clogging Reverse-Slope or Hooded Outlet Riser: Withdraws water from approximately $1\text{ foot}$ below the permanent pool surface so floating oil, grease, and trash cannot escape downstream.
  4. Anti-Seep Collars / Filter Diaphragms & Bottom Drain Valve: Prevents piping along the outlet barrel through the earthen dam and provides a lockable bottom drain valve to dewater the pond for periodic sediment dredging.

Constructed Wetlands (12.7.7)

Constructed Stormwater Wetlands are engineered shallow marsh systems that mimic natural wetland ecosystems, delivering the highest biological, biochemical, and physical pollutant removal of any surface basin (removing fine TSS, dissolved nitrate via anaerobic denitrification, dissolved phosphorus, fecal bacteria, hydrocarbons, and trace metals through dense stem filtration, microbial biofilms/periphyton coating submerged stems, and root-zone uptake).

• Multi-Zone Bathymetry (Depth Profile):
  • Deep Inlet Forebay & Outlet Micropool ($3\text{ to }6\text{ feet}$ deep): Settle coarse grit at the entrance, keep the outlet orifice free of marsh vegetation, and provide year-round deep-water habitat for predaceous mosquito-eating fish (such as Gambusia affinis mosquitofish) and dragonfly nymphs, naturally controlling mosquito larvae.
  • Low-Marsh Zones ($6\text{ to }18\text{ inches}$ deep) & High-Marsh Benches ($0\text{ to }6\text{ inches}$ deep): Cover $70\%\text{ to }80\%$ of the wetland footprint. Internal earthen berms and islands force water along a long, sinuous, meandering flow path (maximizing length-to-width ratio and residence time) through diverse native emergent aquatic macrophytes (bulrushes, pickerelweed, arrowhead, sedges, rushes). Inspectors check for invasive plant monocultures (such as invasive Phragmites or purple loosestrife) and forebay sediment depth.

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3. Inline Storage (12.7.14), Catch Basin Inserts (12.7.15), Underground Vaults (12.7.16), and Manufactured Devices (12.7.18)

Inline Storage (12.7.14)

Inline Storage utilizes oversized storm sewer pipes, reinforced concrete box culverts, or manifolded large-diameter corrugated metal/HDPE pipes beneath roadways and parking lots—throttled at the downstream end by a vortex flow-control valve (hydro-brake), calibrated orifice plate, or weir wall inside a control manhole—to back up and detain stormwater runoff inside the storm pipe conveyance network itself.

Catch Basin Inserts (12.7.15)

Catch Basin Inserts are permanent drop-in stainless-steel, fiberglass, or polymer filter baskets, curb-throat hood trays, or multi-stage geotextile media pouches mounted inside storm drain inlets just below the grate or curb opening. They capture trash, floatable plastics, autumn leaves, coarse sand, and free petroleum hydrocarbons (using replaceable hydrophobic oil-absorbent pillows or booms suspended inside the basket). Because their holding volume is very small, catch basin inserts require frequent routine inspection and vacuum or manual cleanout—typically quarterly or whenever sediment and debris reach $50\%$ of basket capacity—to prevent inlet clogging and roadway flooding.

Underground Vaults (12.7.16)

Underground Vaults are large subterranean precast or cast-in-place reinforced concrete vaults, multi-barrel corrugated metal/HDPE pipe manifolds, or modular structural plastic arches buried beneath high-density urban parking lots, plazas, and buildings where zero surface land is available. They provide either underground flood detention (dry) or multi-chamber wet sedimentation and oil/grit separation.

Manufactured / Proprietary Devices (12.7.18)

Manufactured (Proprietary) Treatment Devices are factory-engineered subterranean stormwater treatment units verified under standardized third-party testing protocols—most notably the New Jersey Corporation for Advanced Technology (NJCAT) and the Washington State Department of Ecology TAPE (Technology Assessment Protocol – Ecology) program. They fall into two primary engineering families:

1. Hydrodynamic Separators (Swirl Concentrators / Vortex Separators):
   • Compact cylindrical precast concrete manhole structures with no moving parts that direct incoming stormwater tangentially (or through an angled internal deflection cone/louver) into a circular separation chamber.
   • The resulting cyclonic vortex (swirl) uses centrifugal and gravitational forces to spin coarse sand, road grit, and settleable solids inward and down into an isolated bottom storage sump (shielded from high-flow scouring and resuspension by an internal horizontal skirt/weir), while trapping floating oil, grease, and buoyant trash behind a submerged outlet baffle wall.
   • Because hydrodynamic separators reliably remove $50\%\text{ to }80\%$ of coarse sand/grit and floatables at high flow rates inside a single manhole footprint, they are widely installed as pretreatment units immediately upstream of underground infiltration trenches, underground detention vaults, or bioretention cells.
2. Proprietary Media Filtration Cartridge Systems:
   • Subterranean concrete vaults housing manifolded arrays of replaceable vertical filter cartridges filled with engineered filtration and sorptive media—such as expanded perlite, zeolite, granular activated carbon (GAC), or iron/alumina-amended media.
   • As stormwater rises in the vault, it flows radially inward through the media cartridges into an underdrain manifold, achieving $80\%+$ fine TSS removal along with chemical capture of dissolved heavy metals, phosphorus, and emulsified hydrocarbons.
   • Maintained by pumping out the pretreatment sump with a vacuum truck (Vactor truck) when sediment reaches the manufacturer's depth limit (typically $6\text{ to }12\text{ inches}$) and replacing spent media cartridges every $1\text{ to }3\text{ years}$.

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Decision & Inspection Flowchart: All 18 Post-Construction Stormwater BMPs

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