ENGEO provided geotechnical engineering services for the 6,700-acre Yuba Goldfields flood breach and inundation evaluation that resulted in ENGEO designing a new 200-year setback levee to contain floodwaters. The Yuba Goldfields are characterized by several generations of successive dredge mining that has produced a complex labyrinth of mine tailing mounds and interior waterways. A breach of the Goldfields would have resulted in extensive flooding within the levee-protected areas of the Three Rivers Levee Improvement Authority (TRLIA).
ENGEO received the 2020-21 CalGeo Outstanding Project Award for the work done on the 200-year levee plan.
Timing of the evaluation and mitigation design was critical to TRLIA for both maintaining FEMA 100-year authorization and obtaining a California 200-year level of protection. ENGEO achieved successful completion for both these objectives by (1) developing a safe and stable interim 100-year embankment constructed by economically reshaping the dredge tailings (a mixture of cobble, gravel, and sand) to meet FEMA certification criteria, and (2) designing and providing oversight of the construction of an approximately 2.6-mile-long setback levee capable of protecting TRLIA from a 200-year flood event breaching through the Goldfields.
Completion of the project required analyzing over 15 different alternatives within the Goldfields to restrain floodwaters and provide an expedient, robust, and cost-effective line of protection. The final 100-year embankment was completed for less than $1 million, far less than the other alternatives. In addition, ENGEO’s 200-year levee design had to accommodate future aggregate mining areas, multiple landowners, environmental considerations, and an accelerated timeframe that was critical to securing State funding for the project.