By Kurt Chirbas
The Yuba Water Agency in Yuba County, Calif., was established in 1959 to reduce flood risk and provide a sustainable water supply to the people of the region. Yuba County has historically endured devastating floods, due in part to Gold Rush-era hydraulic mining practices that washed millions of cubic yards of debris into the Yuba River, raising the riverbed and increasing the flood risk.
Lake Francis Dam is one of several dams that is owned and maintained by the Yuba Water Agency. The dam, rebuilt in 2000, is in the foothills of the Sierra Nevada Mountains, in Dobbins, Calif., on the edge of the Tahoe National Forest. The California Division of Safety of Dams (DSOD) is the regulator associated with the dam and classifies Lake Francis Dam as a jurisdictional dam.
Due to a recent standard inspection as part of the dam maintenance program, the Yuba Water Agency sought to improve the drainage ditch located at the toe of the dam to correct an ongoing erosion-related depression and to provide further erosion protection of the ditch and dam embankment. The subject erosional feature appeared to be at a location where the ditch grade steepens. The project improvements consisted of replacing the existing riprap protection and underlying geotextile liner with a geosynthetic cementitious composite mat (GCCM), along with regrading and recompacting the subsurface. GCCM is a flexible, concrete-impregnated fabric that hardens on hydration to form a thin, durable, waterproof and fire-resistant concrete layer. Essentially, it is concrete on a roll. GCCM enhances erosion protection by increasing hydraulic capacity (flow, shear and velocity), minimizing the seepage path for water infiltration to saturate the subsurface soils and transport surficial fines, and overall provides a reduction in long-term maintenance costs.
Gannett Fleming Inc., from Roseville, Calif., was retained by the Yuba Water Agency as the engineering design firm and prepared construction drawings for the improvement of the drainage ditch. The drawings were reviewed and approved by DSOD. The design was to ensure minimal disturbance to the existing dam and surrounding area, augment the accumulated flow conditions of the runoff water (from the dam slope, maintenance access road and upper surface runoff water basin), increase the flow capacity to provide a larger safety factor, and achieve an aesthetic appearance for the visitors at Lake Francis. Concrete Cloth GCCM from GeoTree Solutions was specified as the material of choice.
RCI General Engineering from Oroville, Calif., was contracted to regrade and compact the drainage ditch and installed the GCCM per the construction drawings by Gannett Fleming. The material was supplied through Spec-West of Chico, Calif., and a GeoTree associate was on-site to provide training and support during the installation. Gannett Fleming, RCI General Engineering, the Yuba Water Agency and their hired quality assurance of construction team of SR Diversified were in attendance to oversee the process.
The installation includes deployment of the GCCM across the drainage ditch width in a shingle-like manner, proceeding perpendicular to the flow direction, seaming the overlaps by applying a bead of caulk under the overlap and then securing screws to create a mechanical fasten between panels, securing the GCCM to the subsurface with metal spikes, and then hydrating the GCCM for the curing period.
The installation of the GCCM, a 300-foot (91-m) section in length, was completed in three days with a small crew. The upgraded drainage ditch at Lake Francis Dam is now ready for the winter storms that occur annually in California.
Kurt Chirbas is the western regional manager for GeoTree, a division of ClockSpring|NRI, specializing in the technical sales of Concrete Cloth.