Stormwater Drainage Design for South Florida Sites: Sizing, Permits, and Common Mistakes
How South Florida engineers size drainage for commercial sites — exfiltration trenches, dry retention, weirs, and the SFWMD permit thresholds that drive every design.
Why South Florida drainage is different
Coastal South Florida sits on highly permeable limestone with a water table 1–5 feet below grade — too high for traditional dry detention. The dominant systems are exfiltration trenches and shallow dry retention. See related dewatering considerations.
Design storms are aggressive: the 25-year/3-day event is the standard regulatory benchmark, with critical infrastructure designed to the 100-year storm.
Permit thresholds that drive design
Sites over 1 acre: NPDES construction permit + erosion control plan required.
Sites over 10 acres or 40% impervious: SFWMD ERP (Environmental Resource Permit) typically required.
Discharge to a canal, lake, or coastal water: water-quality treatment volume (first 1 inch of runoff) must be retained on-site.
Common, expensive mistakes
Undersized exfiltration trench: the K-factor (hydraulic conductivity) varies 10x across South Florida. Always pull a site-specific percolation test.
Trench above seasonal high water table: the trench can't drain into saturated rock. Design must allow minimum 1 foot separation.
Inlet grates too small for paved areas: undersized inlets flood the parking lot 3x per year. Bedrock self-performs stormwater from inlet structure to outfall.
Frequently asked questions
Rule of thumb: 1 cubic foot of exfiltration trench per 12 sq ft of impervious area, tuned by site K-factor.
Sometimes — most South Florida municipalities require a connection permit and proof your discharge won't overload the receiving system.
Standard ERP: 60–120 days. Major modification: 6+ months. Plan ahead.
Get a free quote from Bedrock.
Residential and commercial. Licensed, bonded, insured.