13.4 LID Principles, Water Quality Volume & Bioretention Design

Core Focus: Modern landscape architecture prioritizes green infrastructure and Low Impact Development (LID) to treat stormwater as an ecological asset. LARE Section 4 rigorously tests the mathematical sizing of the Water Quality Volume (WQV), structural layer detailing for bioretention cells, hydraulic parameters for bioswales, open-graded aggregate reservoir design for permeable pavements, and green roof specifications.

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1. Low Impact Development (LID) Philosophy & Core Principles

Traditional civil engineering relied on centralized "end-of-pipe" systems designed to collect runoff in curbs, accelerate conveyance through smooth pipes, and dump untreated stormwater into distant detention ponds. In contrast, Low Impact Development (LID) is a decentralized, micro-scale design approach that manages stormwater at its source.

Core LID Principles

1. Mimic Pre-Development Hydrology: Preserve pre-settlement runoff volumes, peak discharge rates, infiltration, and baseflow duration.
2. Manage Runoff at the Source: Treat stormwater in small, dispersed, micro-scale features distributed across the site rather than conveying it to a single massive retention basin.
3. Preserve Natural Assets: Protect high-permeability native soils (HSG A and B), mature woodland canopies, wetlands, and natural riparian swales from grading and vehicular compaction.
4. Disconnect Impervious Surfaces: Break continuous paths of pavement and roof planes by discharging downspouts and sheet runoff across vegetated filter strips, bioretention basins, and permeable pavers.
5. Slow the Flow (Elongate Flow Paths): Maximize surface roughness and incorporate check dams to extend the Time of Concentration (Tc), mitigating downstream peak discharge surges.
6. Multifunctional Landscapes: Integrate stormwater management directly into civic open spaces, streetscapes, parking islands, and recreational courtyards.

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2. Water Quality Volume (WQV) & First Flush Sizing

Stormwater regulations distinguish between water quantity control (attenuating peak flood stages for 10- or 100-year storms) and water quality treatment (filtering pollutants wash-off from everyday storms).

The First Flush Phenomenon

The initial 0.5 to 1.0 inch of surface runoff carries the vast majority (up to 85%-90%) of the annual wash-off pollutant load—including suspended solids (TSS), heavy metals (copper, zinc, lead from tires and brake pads), automotive hydrocarbons, petroleum oils, fertilizers (nitrogen and phosphorus), and bacterial pathogens. Best Management Practices (BMPs) are sized to capture and filter this critical initial volume, termed the Water Quality Volume (WQV).

Sizing the WQV: Schueler's Simple Method

The universal standard for calculating the Water Quality Volume is the Simple Method developed by Thomas Schueler:

WQV = [(P * Rv * A) / 12] * 43,560 (cubic feet)

Where:

• WQV = Water Quality Volume in cubic feet
• P = Design storm rainfall depth in inches (typically the 90th-percentile 24-hour storm, established regionally at 1.0 inch in most US jurisdictions, or 0.75" to 1.2")
• A = Contributing drainage area in acres
• Rv = Volumetric runoff coefficient (dimensionless), calculated directly from the percentage of site imperviousness:

Rv = 0.05 + 0.009 * I

Where:

• I = Percent of site imperviousness expressed as a whole number (e.g., if a site is 65% impervious, I = 65, giving Rv = 0.05 + (0.009 * 65) = 0.635).

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3. Bioretention Cells & Rain Gardens: Anatomy & Detailing

A bioretention cell (or engineered rain garden) is a shallow landscaped depression designed to treat stormwater through physical filtration, chemical adsorption, biological uptake, and microbial decomposition.

+-------------------------------------------------------------------------+
|               DETAILED CROSS-SECTION OF A BIORETENTION CELL             |
+-------------------------------------------------------------------------+
| Inflow Curb Cut                                                         |
|        v                                                                |
| +-------------+ Maximum Ponding Depth: 6" to 12" (Drains in 24-48 hrs)  |
| | PONDING     | Elevated Overflow Grate set at Max Ponding Level        |
| | ZONE        |=========================                                |
| +-------------+ Surface Mulch: 2" to 3" Double-Shredded Hardwood        |
| | FILTER      |                                                         |
| | MEDIA       | Engineered Soil: 24" to 36" Depth                       |
| | (SAND/SOIL) | (80-85% ASTM C33 Sand, 10-15% Fines, 3-5% Organics)    |
| +-------------+                                                         |
| | PEA GRAVEL  | Choker Layer: 2" to 4" ASTM No. 8 Pea Gravel (NO FABRIC)|
| +-------------+                                                         |
| | STONE BED & | Storage Bed: 8" to 12" Clean ASTM No. 57 Crushed Stone |
| | UNDERDRAIN  | Perforated PVC Pipe (Perforations DOWN)                 |
| +-------------+                                                         |
|   Native Subgrade (Uncompacted)                                         |
+-------------------------------------------------------------------------+

Structural Profile Layers (Top to Bottom)

1. Surface Ponding Zone:
   • Sized for a maximum depth of 6 to 12 inches. Ponding deeper than 12 inches drowns plant crowns, compacts the filter bed under hydrostatic pressure, and generates anaerobic conditions.
   • Drain-Down Time: Must drain fully within 24 to 48 hours (optimally <= 24 hours). Because Culex mosquitoes require 72 hours of stagnant water to complete their larval cycle, a 24-to-48-hour drain time eliminates insect breeding vectors.
2. Surface Mulch Layer:
   • Depth: 2 to 3 inches of aged, double-shredded hardwood mulch.
   • Why double-shredded hardwood? Fibrous hardwood strands interlock into a dense mat that resists floating during ponding events.
   • Prohibited Materials: Pine bark nuggets, wood chips, and light triple-shredded mulch float freely during storm events, clogging overflow grates and depositing debris along cell perimeters.
3. Engineered Soil Media (Filter Bed):
   • Media Depth: 24 to 36 inches (minimum 18-24 inches for herbaceous perennials and turf; 36 inches for woody shrubs and trees).
   • Strict Material Composition:
     • 80% to 85% Coarse Sand (washed concrete sand per ASTM C33)
     • 10% to 15% Soil Fines (silt and clay to provide cation exchange capacity for pollutant binding)
     • 3% to 5% Organic Matter (well-aged, leaf compost)
   • Design Infiltration Rate: 1.0 to 5.0 inches per hour (typically designed at 2.0 to 3.0 in/hr).
   • The Critical Nutrient Leaching Rule: Organic matter content must NEVER exceed 5% by weight. While intuitive gardening logic suggests adding rich organic compost, studies prove that bioretention mixes with > 5% organic matter leach dissolved phosphorus and nitrogen directly into the underdrain, transforming the BMP from a nutrient sink into a severe water pollution source!
4. Choker / Transition Layer:
   • Depth: 2 to 4 inches of washed ASTM No. 8 pea gravel (1/8" to 3/8") or coarse sand.
   • The Geotextile Fabric Failure Trap: Landscape architects must NEVER specify geotextile filter fabric horizontally between the bioretention soil media and the underdrain stone reservoir. Over time, fine silts and clays migrate to the fabric boundary, creating an impermeable layer of "blinding" that permanently chokes the system. The ASTM No. 8 pea gravel acts as a graded aggregate filter, preventing soil migration into the stone void space while ensuring lifelong permeability.
5. Underdrain & Reservoir Stone Bed:
   • Sized with a minimum of 8 to 12 inches of washed, open-graded ASTM No. 57 crushed limestone or granite (35% to 40% void space).
   • Underdrain pipe: 4-inch to 6-inch diameter slotted or perforated Schedule 40 or SDR 35 PVC pipe.
   • Orientation: Pipe perforations must face DOWNWARD. Placing holes facing up allows gravel to fall into the pipe and prevents the lower portion of the stone bed from draining.
   • Internal Water Storage (IWS): Elevating the underdrain outlet with an upturned elbow creates an anaerobic saturation zone in the bottom stone reservoir, promoting microbial denitrification (converting nitrates to harmless nitrogen gas).
6. Overflow Structure:
   • An elevated beehive grated catch basin, drop inlet, or broad-crested stone weir set exactly at the maximum ponding elevation (6" to 12" above mulch) to safely bypass storms exceeding the WQV without scouring the planting bed.

Bioretention Planting Zonation

Bioretention vegetation must tolerate severe hydrological extremes—ranging from 48 hours of complete water submergence during cloudbursts to weeks of severe drought during hot summer dry spells:

• Zone 1: Deep Ponding / Basin Floor (Inundation Zone): Subject to frequent, repeated flooding. Plants must be Obligate Wetland (OBL) or Facultative Wetland (FACW) species (e.g., Iris versicolor [Blue Flag Iris], Carex stricta [Tussock Sedge], Cornus sericea [Red Osier Dogwood], Cephalanthus occidentalis [Buttonbush]).
• Zone 2: Fluctuation Slope (Intermediate Inundation): Subject to temporary ponding and moving water. Plants should be Facultative (FAC) species (e.g., Panicum virgatum [Switchgrass], Lobelia cardinalis [Cardinal Flower], Clethra alnifolia [Summersweet], Viburnum dentatum [Arrowwood]).
• Zone 3: Upper Perimeter (Transition / Upland Zone): Seldom or never flooded; acts as an aesthetic transition. Plants should be Facultative Upland (FACU) or Upland (UPL) species (e.g., Echinacea purpurea [Purple Coneflower], Rudbeckia fulgida [Black-eyed Susan], Amelanchier canadensis [Serviceberry]).

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