Low Impact Development: Sizing Bioretention Systems for Peak Flow Reduction

Low Impact Development (LID) practices represent a paradigm shift in stormwater management, emphasizing distributed, small-scale controls that mimic natural hydrology. Bioretention systems are among the most effective LID techniques for urban applications.

Hydraulic Design Principles

Bioretention basin sizing requires careful consideration of infiltration rates, storage volume, and outlet control. The surface area should accommodate the design storm volume while maintaining ponding depths below 12 inches to prevent vegetation stress. Infiltration rates typically range from 0.5 to 3 inches per hour depending on soil media composition.

Soil Media Specifications

A well-designed media blend balances infiltration capacity with pollutant removal. Standard specifications call for 60-70% sand, 20-30% topsoil, and 5-10% organic matter. The media depth should be at least 18 inches, with 24-36 inches preferred for enhanced pollutant removal and plant root development.

Performance Monitoring

Post-construction monitoring is essential for verifying design assumptions. Key metrics include infiltration rate measurement, water quality sampling, and vegetation health assessment. Annual maintenance includes mulch replacement, vegetation management, and sediment removal from pretreatment areas.