Repaired Sanitary Sewer Channel Improves Neighborhood
During the heavy rains of August 2010, the City of Sioux Falls discovered a sanitary sewer that was exposed in a drainage channel in the Tomar Heights area of southeastern Sioux Falls. The drainage channel had eroded away over the years, leaving manholes and sewer pipes exposed. This situation represented a possible direct discharge of untreated sewage to the Big Sioux River. The City took action immediately to rectify the problem.
The Tomar Heights Sewer and Drainage Way Improvements project exemplifies an increasingly common problem in recently developed suburban areas. Prior to development, the natural drainage way carried relatively little flow after storm events. With adjacent lands now converted to pavement, rooftops and mowed lawns, a much greater portion of rainfall reaches the channel than ever before. Consequently, the flow is much greater than the natural stream gradient can handle. Essentially, a “mountain stream” quantity of water tries to negotiate a “woodland swale”, and it cannot do so without cutting the channel down to a steeper gradient and expanding to a wider channel section. All across the Midwest, this scenario is being repeated. In many cases, the erosion has become so severe that residential property is threatened.
The project used an innovative and customized biological approach to channel stabilization. Rather than simply armor the channel with unattractive rip-rap, HR Green engineers specified five different seed mixes across the 2,200 feet of 80-ft. wide channel cross-section, in combination with grouted Sioux quartzite drop structures at appropriate locations. HR Green engineers used XPSWMM 2D computer modeling to predict water velocity vectors and shear stress at critical points in the channel cross-section. Additionally, HR Green conducted a fluvial geomorphic assessment of the soils beneath the channel. This assessment, combined with the modeling information, was used to select the appropriate seed mixtures for various portions of the channel cross-section. The information gained from this assessment also helped HR Green determine the number, size and ideal locations for stone drop structures to be located at intervals along the channel.
No easements existed for the relocation of the sanitary sewer, or the construction of a new drainage way. Therefore, extensive negotiation with numerous property owners was required to complete the project. The result is a highly stable yet aesthetically pleasing stream that enhances the value of local property, protects City infrastructure, and provides habitat for wildlife.