Project Summary

The pre-project low-water crossing (ford) on Malibu Creek was identified as a partial barrier to southern steelhead trout, a Federally endangered species. The ford also retained sediment and required frequent repair following high flows, resulting in recurring disturbance to adjacent channel.

Numerous alternatives were considered before a preferred design was selected. Insufficient right-of-way precluded construction of suitable approaches to place a bridge above the 100-year flow. Bridge designs with two, three, and four piers were examined and cost versus impacts to the stream channel hydraulics were weighed.

The low-water crossing was replaced with a bridge that spans the bankfull channel. The bridge was designed to be overtopped by the 5-year peak flow of 2,500 cfs (70 cms) and withstand a 100-year peak flow which would submerge the bridge by more than 9 ft (2.7 m). The concrete bridge deck is supported by abutments and 3 pier walls that extend more than 13 ft (4 m) below the channel bed. The design depth for the pier walls was determined through a scour analysis. To maintain structural integrity of the bridge, the deck and pier walls were designed with a small cross-sectional area relative to the flow. This aids in passing debris and sediment and minimizes impacts on flood water elevation. A sloping face (nose) on each pier wall helps deflect debris over the bridge deck. To minimize debris capture, the pedestrian railings are collapsible, with a shear pin release on the downstream railing. Stream banks disturbed during construction were reconstructed using rock, erosion control mats, and native plants.

Post Project Observations, Challenges and Lessons Learned

An active water line was found under the low-water crossing after construction started. Rerouting the line to be supported under the bridge deck added substantial cost to the project.

Heavy rains during Winter 2004/2005 produced peak flows of about 13,000 cfs (368 cms), exceeding the 10 year recurrence interval. The deck was overtopped twice, and the upstream railing collapsed as designed. However, the flows never produced sufficient force to collapse the downstream railings, so the railing remained upright and caught debris. To ensure public safety, the shear pins are designed to release at forces greater than experienced during these storms. During subsequent storms local residents manually collapsed the railings prior to the bridge being overtopped. Overall, the bridge performed as designed and received only minor damage to the railings and deck runoff gutters. The bioengineering techniques of erosion mats and planting were insufficient for protecting banks from scour during high flows immediately following construction.

Project planning and design was extremely challenging. Lessons learned include the need to designate a project manager early in the process to serve as principal contact with agencies and landowners. Detailed work plans should be avoided until all design specifications have been identified and agreed upon by the involved agencies.

Published 02/23/07