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Bioretention Cell Ponding & Media Sizing Calculator

Bioretention cells and rain gardens capture first-flush stormwater runoff, filtering particulate pollutants and promoting biological nutrient uptake.

Total impervious watershed catchment surface area (roofs, parking lots, roads).

Design storm 90th percentile rainfall depth (typically 20 to 30 mm).

Maximum allowable temporary ponding bowl depth above soil media (typically 150 to 300 mm).

Thickness of the sandy bioretention soil mix bed (typically 0.6 to 1.2 m).

Design saturated hydraulic infiltration rate of media (typically 25 to 75 mm/hr).

Maximum allowable time for complete drainage to avoid mosquito breeding (typically 24 to 48 hours).

Calculated Result
82.5 m²

Required Bioretention Cell Surface Area

Filter Bed Footprint Area

82.5 m² (888 sq ft)

Water Quality Volume (WQv)

118.8 m³

Ponding Surface Pool Storage

12.4 m³

Soil Media Void Storage

15.5 m³

Footprint % of Impervious Catchment

1.65%

Ponding Pool Emptying Time

3 hours (≤ 24h standard)

Calculation Breakdown

  1. Runoff Water Quality Volume (WQv)WQv = 5000 m² × (25/1000 m) × 0.95 = 118.8 m³
  2. Darcy Filter Bed Area SizingA_f = (WQv × d_m) / [k × (d_p + d_m) × t] = (118.8 × 0.75) / [0.05 × (0.15 + 0.75) × 24] = 82.5 m²

Bioretention Sizing Metrics

Interactive visualization based on your current inputs

Value
0.0305989119WQv Volume (m³)Filter Area (m²)Catchment Ratio (%)Ponding Vol (m³)Drain Time (h)ParameterValue

What Is the Bioretention Cell Ponding & Media Sizing Calculator?

The Bioretention Cell Ponding & Media Sizing Calculator determines optimal surface footprint and storage volumes for urban stormwater bioretention beds.

How Does the Bioretention Cell Ponding & Media Sizing Calculator Work?

It computes water quality capture volume and determines required filter area using Darcy porous media equation under varying hydraulic heads.

Bioretention Cell Ponding & Media Sizing Calculator Formula & Variables

The core mathematical equation utilized by this calculator is expressed as:

A_f = \frac{\text{WQ}_v \cdot d_m}{k_m (d_p + d_m) t_d}, \quad \text{WQ}_v = A_{imp} \cdot P \cdot 0.95

Darcy law filtration rate applied across the hydraulic gradient of ponding head plus media bed thickness.

How to Use the Bioretention Cell Ponding & Media Sizing Calculator

  1. Enter contributing impervious area and local water quality storm depth.
  2. Specify surface ponding depth and media bed thickness.
  3. Input saturated media infiltration rate and allowable drain-down duration.

Step-by-Step Example Calculation

5,000 m² Commercial Parking Lot Infiltration

Input Values:

contributingImperviousAreaM2:5000
designRainfallDepthMm:25
pondingDepthMm:150
filterMediaDepthM:0.75
mediaHydraulicConductivityMmHr:50
targetDrainDownHours:24
Worked Steps: Requires 82.3 m² bioretention surface footprint (1.65% of catchment) draining within 24 hours.

Understanding Your Result

Filter Bed Surface Area: Minimum surface area required to treat runoff volume.

Catchment Ratio: Bioretention area expressed as a percentage of drainage area.

Ponding Storage: Volume stored above the soil surface during peak inflow.

Factors That Affect the Result

  • Higher media hydraulic conductivity directly reduces the required filter area.
  • Deeper surface ponding provides more driving head, speeding filtration.

When Should You Use This Calculator?

  • LID stormwater designs, Green Infrastructure streetscapes, parking lot bioretention, and urban drainage permits.

Assumptions & Limitations

  • Assumes uniform soil mix conductivity and functional underdrain or permeable native subgrade.

Frequently Asked Questions

Calculation Accuracy & Reference Note

Adheres to US EPA Low Impact Development and Georgia Stormwater Management guidelines.

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