🎓 Lesson 14
D5
Bioretention Sizing & Media Specification per Soil Type
Bioretention sizing and media specification means choosing the right size and soil mixture for a rain garden so it soaks up stormwater properly, based on the native soil underneath.
🎯 Learning Objectives
- ✓ Calculate required bioretention surface area using site-specific runoff volume and native soil infiltration rate
- ✓ Design an engineered media mix (sand, compost, clay) meeting minimum saturated hydraulic conductivity (Ksat) and porosity criteria per soil type
- ✓ Analyze native soil classification (USDA texture class) to select appropriate underdrain configuration and media depth
- ✓ Apply NRCS Curve Number and rainfall depth-duration-frequency data to estimate design storm runoff volume
📖 Why This Matters
In mining and infrastructure projects, stormwater runoff from disturbed areas can erode slopes, transport sediment and heavy metals, and contaminate downstream water bodies. Bioretention systems are among the most effective Low Impact Development (LID) practices—but they fail if sized or built without regard to local soil conditions. A rain garden installed over clay soil without underdrain or amended media will pond indefinitely; one over gravelly soil may infiltrate too fast to treat pollutants. Getting this right protects water quality, satisfies regulatory compliance (e.g., EPA NPDES), and reduces long-term maintenance costs.
📘 Core Principles
Bioretention performance hinges on three interdependent layers: (1) the engineered media (top 0.6–1.2 m), designed for infiltration, filtration, and biological uptake; (2) the transition layer (optional gravel reservoir), providing temporary storage and supporting underdrain function; and (3) the native soil subsoil, whose saturated hydraulic conductivity (Ksat) governs whether exfiltration is feasible—and at what rate. USDA soil texture classes (e.g., sandy loam vs. silty clay) dictate Ksat ranges and thus determine whether bioretention can be 'infiltrating' (no underdrain) or must be 'hybrid' (with underdrain and overflow). Key theory includes Darcy’s Law for vertical flow, Hortonian vs. Green-Ampt infiltration modeling, and the concept of 'design storm retention volume'—the volume that must be contained and treated onsite, typically derived from the 1-year, 24-hour storm event per local standards.
📐 Required Surface Area Calculation
The minimum surface area (A) of a bioretention cell is calculated by dividing the design storm runoff volume (V) by the effective storage depth (d_eff), which accounts for media void space, mulch, and ponding depth. When native soil Ksat < 0.1 cm/hr, underdrain-assisted storage is required, and d_eff includes both unsaturated media storage and gravel reservoir capacity.
Surface Area Sizing Equation
A = V / d_{eff}Calculates minimum bioretention surface area needed to retain and treat design storm runoff volume.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| A | Surface area | m² | Plan area of bioretention cell |
| V | Runoff volume | m³ | Volume of runoff to be retained, derived from site hydrology |
| d_{eff} | Effective storage depth | m | Depth of storage available in media + ponding + gravel reservoir (if present) |
Typical Ranges:
Sandy loam native soil (Ksat > 1 cm/hr): 0.3 – 0.6 m
Clay loam native soil (Ksat < 0.1 cm/hr): 0.4 – 1.0 m (with underdrain)
💡 Worked Example
Problem: Given: Site area = 0.8 ha; impervious cover = 75%; CN = 85 (post-construction); 1-year, 24-hr rainfall = 95 mm; native soil = silty clay (Ksat = 0.02 cm/hr); media = 0.8 m deep with 0.35 void ratio; target d_eff = 0.45 m (including 0.15 m ponding + 0.30 m media storage).
1.
Step 1: Calculate runoff volume V using NRCS TR-55: V = (P − Ia)² / (P − Ia + S), where P = 95 mm, S = (1000/CN) − 10 = (1000/85) − 10 ≈ 1.76, Ia = 0.2S ≈ 0.35 mm → V ≈ 68 mm depth over 0.8 ha = 0.068 m × 8000 m² = 544 m³.
2.
Step 2: Apply sizing formula: A = V / d_eff = 544 m³ / 0.45 m = 1209 m².
3.
Step 3: Verify feasibility: 1209 m² is ~15% of site area — acceptable per EPA LID guidance (< 20% for distributed systems). Also confirm underdrain required (Ksat < 0.1 cm/hr → yes).
Answer:
The required bioretention surface area is 1209 m², requiring an underdrain due to low native Ksat, and falls within acceptable spatial allocation limits.
🏗️ Real-World Application
At the Eagle Mountain Mine reclamation site (Utah), engineers faced highly variable soils—from volcanic ash (Ksat ≈ 15 cm/hr) to compacted glacial till (Ksat ≈ 0.01 cm/hr). For a 4.2-ha access road corridor, bioretention cells were zoned: over permeable ash, 0.6-m media (85% sand, 10% compost, 5% clay; Ksat = 12 cm/hr) was installed without underdrains; over till, cells used 1.0-m media (70% sand, 20% compost, 10% clay; Ksat = 5 cm/hr) with 10-cm perforated PVC underdrain on 15-cm gravel blanket, discharging to lined conveyance. Monitoring after 3 years showed >85% TSS removal and no standing water >24 hrs post-storm—validating soil-specific design.
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