Big Tunnel Spill Erosion Mitigation Project

Big Tunnel Spill Erosion Mitigation Project

Maintenance repairs were made to the upper, improved section of the existing spillway structure in 2013. The repairs included filling voids in existing concrete, replacement of eroded rip-rap and soil nail stabilization. However, the channel below the improved section of spillway remains unimproved and prone to erosion, which could cause instability of the channel slopes and adjacent canal facility. This project will extend erosion mitigation improvements approximately 200 feet further downstream from the end of the existing spillway structure. The new improvements are defined by an upper and lower pool with a steep drop between them. This area of the channel has been impacted by previous erosion and landslides, and has a steeper average slope (approximately 22%) than the remainder of the spill channel (<10%). In addition, a naturally occurring rock knob in the right channel bank is obstructing flows and reduces channel width between the upper and lower pools which directs flow toward the left bank. The left channel bank is directly below the canal facility so will need to be stabilized and protected from further erosion. Erosion and bank sloughing in the unimproved spill channel immediately adjacent to the canal facility have led to concerns that further failure of the slopes will jeopardize canal integrity. The objectives of the project are to prevent further erosion by protecting the channel banks, redirect water flow away from critical slopes, and minimize the potential of channel down cutting.