by Bliss, Daniel James

Abstract (Summary)
A green roof was constructed in Pittsburgh, PA to help reduce combined sewer overflows (CSOs) events. They occur because the sewer infrastructure of Pittsburgh, and many older cities, is inadequately designed to handle the large amounts of stormwater runoff created by the sprawling areas of impervious surfaces in todays modern cities. Raw sewage is frequently dumped into Pittsburghs three rivers when the sewer system becomes overloaded. A 12, 300 square foot extensive green roof was constructed at a commercial and residential site in Pittsburgh to measure the stormwater benefits. The green roof sits atop the new portion of an expanded supermarket, with a 21,000 square foot conventionally ballasted roof covering the existing portion. The green roof consists of a 5 1/2 inch thick layer of soilless mix and a plant layer consisting mainly of sedum varieties. Using the conventional roof as a control, an extensive monitoring system was constructed to measure parameters including rainwater, soil moisture and runoff. The absorption of rainwater by the substrate and plants reduces the total volume of runoff that reaches the sewer system by between 5 and 70 percent. At least a 20 percent reduction occurs for a rain storm of 0.6 inches or less. Storm duration, total rainfall amount and soil moisture prior to the storm affect performance. The peak flow rate of runoff leaving the roof is also significantly reduced, by 5 to 70 percent. The flow rate is lower at all stages of storm. The green roof also delayed the start of runoff and, at times, the peak flow. At the conclusion of a storm, runoff would continue to flow from the green roof for up to several hours longer than the control roof at a very low rate. Water quality tests on runoff and rainfall samples indicate that a first flush effect is not present in the green roof runoff. The concentration of phosphorus was elevated in the green roof runoff, likely from the use of fertilizers. COD was also elevated, while turbidity levels were lower than the control roof. Runoff from both roofs neutralized the slightly acidic rainwater.
Bibliographical Information:

Advisor:Dr. David Dzombak; Dr. Robert Ries; Dr. Ronald Neufeld

School:University of Pittsburgh

School Location:USA - Pennsylvania

Source Type:Master's Thesis

Keywords:civil and environmental engineering


Date of Publication:06/12/2007

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