Dry Creek Trunk Sewer Bank Stabilization
City of Roseville
During a storm event in December 2012, a section of the Dry Creek bank near a 39-inch trunk sewer became unstable and collapsed, causing a tree to fall into the creek. This event resulted in the new creek bank top being located within about 4 feet of the 39-inch sewer manhole. The roots of the fallen tree extend to the location of the sewer. This situation represented an emergency condition because another storm event could have floated the fallen tree and pulled the tree and roots downstream, causing the bank to collapse from under the sewer, and potentially causing the sewer to collapse into the creek.
This project required a rapid turnaround to assist the client in repairing the bank before the 2014/2015 wet season. The design team successfully maintained project milestones to meet that goal.
This project included planning and design of creek bank stabilization along the “at risk” segment of Dry Creek adjacent to the 39-inch sewer. West Yost modeled the creek using HEC-RAS to identify the maximum potential water velocities by evaluating flow rates in 1,000 cfs increments up to the design flow of 17,000 cfs. The maximum velocity was found to occur at about 6,000 to 7,000 cfs, which represented bank full flow just before water spilled onto the floodplains.
West Yost evaluated several alternatives for stabilizing the creek bank, including Vegetated Rock Slope Protection (VRSP riprap with vegetation), Ercon Mats, Vegetated Gabions, Concrete Block Plantable Retaining Wall, Rock Vane Weirs, Soldier Piles and Lagging, and Longitudinal Stone Toe Protection and Willow Planting. VRSP was found to be the most cost efficient and provided the required protection immediately upon construction.