The Nashville Metropolitan Statistical Area (MSA) has experienced significant growth during the past decade. With a population growth rate roughly three times that exhibited statewide, the demand for new housing has forced expansion of sewer treatment plants and collection systems in the region. Most sanitary sewers are gravity systems and follow regional topography, frequently flanking, and sometimes crossing, small tributary streams. Within the Karst dominated geologic setting of the MSA, conventional sewer line installation methods have sometimes resulted in the diminution or complete base flow interception of the impacted streams. Recognizing this problem, the Tennessee Division of Water Pollution Control tasked developers, as a component of the Aquatic Resource Alteration Permit program, to devise methods to prevent this phenomenon from occurring and perform pre- and post-construction stream base flow studies to confirm that the stream's hydrologic characteristics have not been altered by construction activities. An innovative sewer line installation method was designed whereby, in lieu of the classic anti-seep collars and aggregate stone bedding material, flowable fill encasement was utilized in conjunction with an anchoring system in settings where high risk of stream piracy exists. This sewer line construction method was applied in a project in Williamson County that encroached upon Dry Branch. A pre-construction flow study was performed in August 2005 at four locations in the stream and post-construction flow monitoring was performed in September 2006. These studies revealed that the alternative construction methods employed were successful in retaining the stream's base flow, with no sub-surface loss detected as compared to pre-project conditions.
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