About the author: Kathryn Grant is marketing manager for Propex Geosolutions. The Propex GeoSolutions engineering services team can be reached at [email protected] or 423.553.2465.
Last year, the water works board of the city of Auburn, Ala., funded a project to build a new spillway and weir at Lake Ogletree, the city’s primary water supply. The project aimed to replace an existing 75-year-old spillway and increase the lake’s water-holding capacity. The original spillway consistently required maintenance, and an engineering study determined the best course of action was to tear it down and build a new one to meet current standards.
Consider the Options
Because water continuously flows through the approach to the weir, a robust erosion control solution was needed. Originally, Schnabel Eng. planned to use rock riprap, but chose to move forward with Propex GeoSolutions Armormax engineered earth armoring solution, which is composed of high-performance turf reinforcement mats (HPTRMs) and engineered earth anchors that work together as an armoring system to lock soil in place and protect against hydraulic and non-hydraulic stresses, such as mowing, construction and debris loading.
Rock riprap is commonly used for erosion control because it is a standard practice to provide immediate protection upon installation. However, the Hydraulic Engineering Circular No. 15 (HEC-15) states that the permissible hydraulic shear stress of rock riprap depends on the mean size of the rock. When rock riprap is placed on a slope, it can become unstable, as the weight that typically holds it in place begins to drive failure. The erosion resistance and factory of safety of rock riprap declines as the slope gradient on which it is placed increases.
The earth armoring solution chosen provides equivalent hydraulic performance that is not dependent upon the side slope gradient. The hydraulic performance of HPTRMs has been tested and documented with more than 15 million sq yd of installations. The HPTRM component exhibits high reinforcement capacity with soil and root systems, demonstrates durability and long design life, and enhances deep rooted vegetation establishment in erosion control applications. The HPTRM protects and reinforces the subgrade soil, reducing the site's overall soil loss. The earth anchor component permanently secures the HPTRM, improving both the hydraulic and geotechnical factors of safety.
The U.S. EPA states in its Storm Water Technology Fact Sheet – Turf Reinforcement Mats, that while hard armor solutions can withstand great hydraulic forces, they are expensive and do not provide pollutant removal. EPA has recognized the use of HPTRMs as a best management practice (BMP) to allow for vegetated solutions where the hydraulic limits of unreinforced vegetation has been reached.
EPA further states that HPTRMs, “provide a cooler substrate than traditional hard armor techniques, reducing water temperature increases that could otherwise impact aquatic life.” HPTRMs also promote vegetation, which EPA has recognized as a BMP because it acts to slow water velocities, increases sedimentation and filtration of heavy metals, and encourages infiltration of water back into the groundwater table.
After six months, the lake was at full pool, the slope was fully vegetated and there was no erosion on the slopes leading to the approach.
Slope Support
The Lake Ogletree project's design phase began in June 2015, and installation was in August 2017. The slope at the spillway was 25 ft tall and had a 3:1 horizontal-to-vertical ratio. Approximately 8,000 sq yd of the earth armoring solution was installed along the bottom of the approach to the new weir to stabilize soils and along the slopes leading up to the approach for spillway protection, bank armoring and vegetation. Type B1 3-ft anchors were used to permanently secure the HPTRM below the water line. Transitions to rock and concrete were used to ensure that water will not get between the earth armorning solution and the spillway. Additionally, Geotex nonwoven geotextile was placed under it to serve as a filter between the HPTRM and the soil.
Installation took approximately 12 days to complete. After six months, the site was revisited for inspection. The lake was at full pool and the slope was fully vegetated. No erosion was present on the slopes leading to the approach.
