Streambank Improvement and Stormwater Management

Project Overview

In 2012, the City of St. Peters, Missouri, created its Stormwater Master Plan (SMP), which laid out a blueprint for projects and defined best practices for stormwater management, pollutant loading reductions and water quality improvements. This blueprint included the Spencer Creek streambank improvement project. The highly developed project area encompassed the main channel of Spencer Creek, which runs through Spencer Creek Park and is a tributary to Dardenne Creek. The site’s four-cell 12’ by 12’ box culvert was classified as undersized in the Dardenne Creek hydraulic model. The reach from the park to the culvert showed evidence of significant bank erosion and debris jams. Street flooding was also present adjacent to the park.

GBA’s Water Environment team evaluated the entire project extent, which includes a 3.5-square-mile drainage area. GBA utilized fluvial geomorphology techniques to identify three distinct energy regimes along the creek. The stable threshold sections with low over banks and healthy riparian properties were left alone. Sections of the channel exhibiting high streambank erosion or near trails, perched storm sewer outfalls or sanitary sewers were modified to create flood benching and rock toe revetments. Rock vanes were used in the lower section at an acute bend on the creek to nudge high-shear stress flows away from the eroding outside bend. Deep-rooting native seeds and plants were planted in disturbed areas for long-term soil protection. Additionally, the city established an invasive species management plan for the entire project corridor for three years following construction.

During these evaluations, the project team also reviewed the box culvert originally classified as undersized by the city’s 2012 SMP. However, a revised model created by GBA showed that the current culvert would have the capacity for such an event and would not need to be replaced. This discovery changed the direction of many of the project’s improvements, including using the project budget that would have been used to design the culvert expansion for an elevated boardwalk.

The Spencer Creek streambank improvements included leaving the culvert in place, approximately 3,600 feet of streambank stabilization and riparian renovation improvements, upsized storm sewers and an improved trail system. Additionally, two aerial sanitary sewer crossings that were close in proximity were included in the project to address debris accumulation and deterioration of wooden support structures.