Green Infrastructure for the Central Corridor Light Rail Transit Project

The construction of the Central Corridor Light Rail Transit, a two-way light rail line down University Avenue connecting the downtown areas of Minneapolis and St. Paul (Minnesota), presented a unique opportunity to improve the quality of stormwater runoff through the implementation of best management practices (BMPs) incorporating green infrastructure. Project BMPs were required to improve stormwater quality by retaining one inch rainfall events on site and removing 90% of total suspended sediment and 60% of total phosphorus. Four categories of green infrastructure practices were constructed to achieve the runoff reduction and water quality goals of the project: Integrated tree trench systems, infiltration trenches, stormwater planters, and rain gardens. An assessment of the treatment effectiveness of each BMP was performed. Eleven (11) infiltration trench systems, ranging in length from 100 to 300 feet, were selected and evaluated to confirm discharge into the washed rock below the pipe. The duration of time and volume of water to fill the trench and provide between 1-2 feet of head at the downstream catch basin was measured and the permeability of the soils (hydraulic conductivity) was assessed using a falling head test. The trench systems appear to meet or exceed stormwater volume reduction requirements, and infiltration rates exceed the design infiltration calculations. Eight (8) infiltration BMPs and three (3) filtration BMPs, consisting of stormwater planters and rain gardens, were modeled to determine their treatment effectiveness. The modeling shows that the sites are estimated to capture between 10 and 98% of storms treating 20 to 93% of the runoff with eight of the eleven sites treating greater than 50% of the runoff volume.