13.5 Bioswales, Permeable Pavement, Green Roofs & Harvesting

1. Bioswales & Vegetative Filter Strips

Bioswales (Enhanced Vegetated Swales)

A bioswale is an engineered, gently sloped linear drainage course combining conveyance with infiltration and filtration. Unlike traditional turf grass ditches, bioswales incorporate engineered soil media and underdrains beneath a vegetated channel.

+-------------------------------------------------------------------------+
|              BIOSWALE CROSS-SECTION & LONGITUDINAL PROFILE              |
+-------------------------------------------------------------------------+
| Trapezoidal Channel (3:1 side slopes)                                   |
| +------------------------------------+ Water Quality Depth <= 0.5'      |
| | Dense Vegetation (n = 0.20-0.24)   | Conveyance Depth <= 1.5'         |
| +------------------------------------+                                  |
| | Engineered Soil Media (24"-30")    |                                  |
| +------------------------------------+                                  |
| | ASTM No. 57 Stone & Underdrain Pipe|                                  |
| +------------------------------------+                                  |
|                                                                         |
| LONGITUDINAL PROFILE WITH CHECK DAMS (Slopes > 2%)                      |
|             Check Dam                                                   |
|                 |              Water Quality Ponding Pool               |
| Ground Slope    v                   v                                   |
| ===============[#]-----------------[#]================>                 |
|                 ^                   ^                                   |
|                 Stone or Timber Weir with Center Notch                  |
+-------------------------------------------------------------------------+

• Cross-Section Geometry: Trapezoidal or parabolic cross-section. Bottom width must measure 2 to 8 feet (widths > 8 ft encourage braided, meandering rills; widths < 2 ft concentrate velocities, causing erosion). Side slopes should be 3:1 (33.3%) or flatter (4:1 ideal) for bank stability and maintenance safety.
• Hydraulic Roughness Mechanics:
  • Under shallow water quality flow depths (<= 0.5 foot), grass blades remain erect, creating immense friction: Manning's n = 0.20 to 0.24.
  • Under deep storm conveyance depths (> 1.5 feet), high velocities push vegetation flat, causing roughness to plunge to Manning's n = 0.030 to 0.050.
• Velocity Ceilings: Flow velocity must remain <= 1.0 ft/s during the water quality storm to allow suspended particulates to settle. For the 10-year conveyance storm, peak velocity must not exceed 3.0 to 4.0 ft/s to prevent soil scour.
• Check Dams: On longitudinal swale slopes steeper than 2.0%, check dams (constructed of riprap stone, concrete, or rot-resistant timbers) must be placed at intervals to step the grade down. Each check dam must incorporate a central weir notch that is at least 6 inches lower than the outer edges to prevent water from cutting around the abutments and scouring the banks.
• Hydraulic Residence Time (HRT): The swale length must ensure that runoff takes a minimum of 5 to 10 minutes to travel through the system during the water quality design storm, allowing adequate biological contact and sediment settling.

Vegetative Filter Strips

A vegetative filter strip is a uniformly graded, planar band of dense turf or herbaceous vegetation designed to accept sheet flow from an adjacent impervious surface (such as a parking lot or roadway):

• Longitudinal slopes should be 1% to 5% (maximum 8% to prevent rill formation).
• The maximum allowable flow length of the contributing impervious surface discharging to a filter strip is 75 feet (150 feet across pervious surfaces).
• Level Spreader / Gravel Diaphragm: Runoff must enter the filter strip as uniform sheet flow. A pea gravel diaphragm (a trench filled with ASTM No. 8 stone) or a notched concrete curb flush with the pavement must be installed at the pavement edge to dissipate energy and prevent concentrated rivulets from gouging the turf.

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2. Permeable Pavement Systems

Permeable pavements replace impervious asphalt and concrete, allowing storm runoff to infiltrate directly through the surface into an underlying open-graded aggregate reservoir where it is temporarily stored until it infiltrates into the native subgrade or exits through an underdrain.

+-------------------------------------------------------------------------+
|          PERMEABLE INTERLOCKING CONCRETE PAVER (PICP) PROFILE           |
+-------------------------------------------------------------------------+
| +------------------------------------+ Pavers: 3-1/8" (80mm) with Stone |
| | PICP Concrete Pavers (ASTM C936)   | Joints filled with No. 8/89 Stone|
| +------------------------------------+ Bedding: 2" of ASTM No. 8 Stone  |
| | Bedding Layer (ASTM No. 8 Stone)   |                                  |
| +------------------------------------+ Base: 4" of Open ASTM No. 57     |
| | Base Reservoir (ASTM No. 57 Stone) |                                  |
| +------------------------------------+ Subbase Reservoir: 6" to 24"+    |
| | Subbase Reservoir (ASTM No. 2 Stone| Clean, crushed, angular stone   |
| | 30% to 40% Void Space)             | (ASTM No. 2 or No. 3)            |
| +------------------------------------+                                  |
|   Native Subgrade (Uncompacted!)                                        |
+-------------------------------------------------------------------------+

Primary Permeable Pavement Types

1. Porous Asphalt: Standard asphalt cement combined with open-graded coarse aggregate without fine sand, creating 15% to 25% interconnected voids.
2. Pervious Concrete: Portland cement, coarse aggregate, water, and specialized admixtures without fine sand, creating 15% to 25% void space.
3. Permeable Interlocking Concrete Pavers (PICP): Solid, high-strength impervious concrete blocks manufactured with engineered spacer lugs that establish 1/4" to 3/8" wide joints filled with clean, open-graded aggregate (ASTM No. 8 or No. 89 stone).

The Open-Graded Aggregate Structural Profile

• Bedding Layer: 1.5 to 2.0 inches of clean, crushed ASTM No. 8 stone (provides a smooth, level bedding surface for pavers).
• Base Reservoir Layer: 4.0 inches of clean, crushed ASTM No. 57 stone (chokes the open voids of the subbase while providing structural stability).
• Subbase Reservoir Layer: 6 to 24+ inches of clean, uncrushed, open-graded ASTM No. 2 or No. 3 aggregate (1.5" to 3" crushed stone). This layer provides a structural void ratio of 30% to 40%, functioning as an underground detention reservoir.

Subgrade Infiltration Regimes

• Full Exfiltration (Unlined): Native subgrade possesses an infiltration rate > 0.5 in/hr (HSG A or B). All stored runoff infiltrates into the underlying aquifer. No underdrains are needed.
• Partial Exfiltration: Subgrade infiltration rates range from 0.1 to 0.5 in/hr (HSG C). A perforated underdrain is installed raised 4 to 6 inches above the subgrade, forcing water stored below the pipe to infiltrate while draining excess volume to the municipal sewer.
• No Exfiltration (Lined / Underdrained): Subgrade infiltration is < 0.1 in/hr (HSG D), or the installation is situated over contaminated brownfield soils, high water tables, or sensitive karst limestone prone to sinkholes. An impermeable ethylene propylene diene monomer (EPDM) geomembrane lines the excavation, and perforated underdrains collect 100% of filtered effluent.

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3. Green Roofs & Rainwater Harvesting Systems

Extensive vs. Intensive Green Roofs

Rainwater Harvesting & Cisterns

Rainwater harvesting intercepts rooftop runoff for non-potable reuse (such as landscape irrigation, decorative water features, and toilet flushing):

• First-Flush Diverter: A mechanical bypass chamber that diverts the initial 0.02 to 0.05 inch of roof wash (carrying bird droppings, soot, airborne dust, and pollen) away from the storage reservoir.
• Cistern Sizing: Sized by balancing cumulative monthly rainfall supply against projected landscape irrigation demand, accounting for dry-season deficits.

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4. BMP Performance & Sizing Summary Matrix

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5. Real-World Case Scenario: Designing an LID Campus Retrofit

Scenario: An urban university campus is retrofitting an existing 3.0-acre asphalt parking lot (I = 100%) into a high-performance landscape incorporating bioretention cells and permeable interlocking concrete pavers (PICP). Local municipal regulations require capturing and treating the Water Quality Volume (WQV) for a 1.0-inch storm (P = 1.0"). The site soils belong to Hydrologic Soil Group C (sandy clay loam with measured infiltration of 0.20 in/hr).

Design Resolution:

1. Calculate Water Quality Volume (WQV):
   • Site area: A = 3.0 acres, I = 100
   • Volumetric coefficient: Rv = 0.05 + (0.009 * 100) = 0.95
   • Compute volume: WQV = [(1.0" * 0.95 * 3.0 ac) / 12] * 43,560 = 0.2375 * 43,560 = 10,345.5 cubic feet
2. Allocate Treatment between BMPs:
   • The landscape architect designs parking stalls (1.2 acres) using PICP with a 12-inch ASTM No. 2 stone reservoir subbase (40% void space). Because native soil is HSG C (0.20 in/hr), an underdrain is installed raised 4 inches above the subgrade (Partial Exfiltration).
   • Drive aisles and pathways (1.8 acres) sheet-drain into two central bioretention cells totaling 4,500 sq ft of surface filter bed.
3. Detail the Bioretention Cells:
   • Filter bed: 30 inches of engineered mix (85% concrete sand, 10% fines, 5% leaf compost).
   • Surface ponding: 9 inches, providing 4,500 * 0.75' = 3,375 cu ft of immediate surface storage.
   • Drain-down time check: 9" / 2.5 in/hr media infiltration = 3.6 hours <= 24 hours (compliant).
   • Choker layer: 3 inches of clean ASTM No. 8 pea gravel (no geotextile fabric).
   • Mulch: 3 inches of aged, double-shredded hardwood.
   • Underdrain: 6-inch slotted PVC pipe bedded in 10 inches of ASTM No. 57 aggregate, tied into the municipal storm sewer.

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6. Exam Traps & Pitfalls

1. The Geotextile Fabric Filter Trap: Never specify geotextile filter fabric horizontally between bioretention engineered soil media and the stone reservoir. Fine particles blind the fabric, causing premature hydraulic failure. Always specify a choked stone layer of clean ASTM No. 8 pea gravel.
2. The Excess Organic Matter Trap: Never specify > 5% organic matter by weight in bioretention soil mixes. High organic compost content leaches dissolved nitrogen and phosphorus into groundwater and underdrains.
3. Exceeding 12-Inch Bioretention Ponding: Sizing bioretention ponding deeper than 12 inches drowns plant crowns, compacts the filter bed under excessive hydrostatic head, and risks mosquito breeding if drain-down exceeds 48 hours.
4. Unlined Permeable Pavements on Brownfields: Never specify full subgrade exfiltration over contaminated brownfield sites or active karst terrain. Stormwater will mobilize chemical contamination plumes into aquifers or trigger structural sinkholes. Use an impermeable geomembrane liner and perforated underdrain.
5. Light Mulch Selection: Never specify pine bark chips, wood chips, or light triple-shredded mulch in bioretention basins. These materials float during storm events, suffocating plants and clogging overflow weirs. Specify dense, interlocking double-shredded hardwood mulch.