📋 Case Study

Highway Interchange Reconstruction in Seattle, WA

Maintaining service during construction while meeting WA Dept. of Ecology’s dissolved copper and zinc TMDL limits

🏗️ Project Overview

I-5/SR-520 interchange upgrade with 3.2 miles of new storm conveyance and 11 major culverts

🎯 Challenge

Maintaining service during construction while meeting WA Dept. of Ecology’s dissolved copper and zinc TMDL limits

🔧 Design Approach

Oil-water separators + filtration swales + extended detention with sediment forebays and vegetated filter strips

📐 Design Diagram

SeparatorC_in → C_outFiltration SwaleE = 89%Extended Detention
with Sediment ForebayY_s = 4.2 ftVegetated Filter StripChallenge: Maintain Service
& Meet WA Ecology TMDL LimitsCu Reduction: 94% • Zn Reduction: 87%Outflow to Receiving Water

AI-generated project design illustration

📐 Key Calculations

Culvert Scour Depth

Y_s = K_s × Y_1 × (Q/Q_1)^0.5

Result: 4.2 ft

Guided riprap sizing per FHWA HEC-18

Swale Filtration Efficiency

E = 1 − e^(−k × t)

Result: 89%

Based on 15-min residence time and measured k=0.8 min⁻¹

TMDL Load Reduction

Mass Removed = Σ(C_in × Q_in − C_out × Q_out)

Result: 94% Cu, 87% Zn

Exceeded Ecology’s 80% target

📊 Results

Zero permit violations during 28-month construction; 100% compliance with TMDL reporting; 2.1M gal/year oil capture verified via quarterly sampling

💡 Lessons Learned

•Filtration swales require 6-month commissioning period before calibration
•Scour modeling must include debris accumulation factor
•TMDL compliance hinges on influent concentration variability—not just flow

✅ Key Takeaways

1Filtration swales require 6-month commissioning period before calibration
2Scour modeling must include debris accumulation factor
3TMDL compliance hinges on influent concentration variability—not just flow