Project Description

The City of Pacifica restored approximately 1,300 linear feet (396 m) of San Pedro Creek downstream of the Capistrano Bridge, eliminating the perched bridge apron that was a steelhead migration barrier. The project reach was impaired due to severe down cutting of the channel, which resulted in perching the Capistrano Bridge apron 12 ft (3.7 m) above the downstream channel. The degraded riparian corridor downstream of the bridge was characterized by vertical, eroding banks and invasive plant species.

The Denil fish ladder was removed from under the bridge and the inlet apron was replaced with a series of temporary concrete weirs immediately upstream of the bridge. The weirs are to be removed during an upstream channel restoration project.

The streambed downstream of the crossing was raised and restored to provide access to the crossing, reduce bank erosion and restore riparian function. The channel dimensions were developed by the project geomorphologist to provide a stable, properly functioning channel. Width-depth ratios were reduced by increasing bankfull width and constructing small floodplains, which reduced scour forces on the channel bed.

To raise the channel bed, material was imported from a downstream regrading project and from laying back the banks in the project reach. The streambed was raised an average of 5 ft (1.5 m) through the project reach. A series of 26 rock and log weirs spaced 45 to 60 ft (14 to 18 m) apart were used to create a riffle/pool and step-pool channel with an average slope of 1.8%

Large wood-rock complexes were constructed to help direct flow and create meanders in the channel. The channel was seeded with large rock, allowing for natural accumulation of smaller materials. This caused the channel to rapidly develop a new profile, increasing the number and depth of pools after the first large flow.

Non-native landscape ornamentals along the channel banks were removed. Stream banks were regraded to stable slopes and then covered with erosion control materials and replanted with native riparian vegetation.

Post Project Observations and Lessons Learned

The log weirs placed on a rock foundation worked for this project but are likely near the maximum stream width for this technique.

Bed seeding (placement of only large material) allows fine material evacuation, scour and rapid pool development. However, it required an estimated 2 to 3 seasons to establish a gravel and sand substrate throughout the restored reach.

A large volume of rock was installed to depths up to ten feet to prevent the weirs from being undercut. However, at some locations flanking of the structures occurred and could have been avoided by keying additional rock further into the banks and directing the flow toward the center of the channel.

This project is an excellent example of how channel restoration can address multiple objectives of erosion control, habitat enhancement, and fish passage. Channel incision frequently creates barriers to fish passage at culverts and bridge aprons.  Removal of these barriers may allow downcutting to continue upstream, which can destabilize the upstream channel, degrade stream habitat, and threaten streamside structures. Using grade control immediately downstream of a crossing to eliminate the barrier only addresses fish passage and not the degraded downstream habitat.  In many instances it may be more appropriate to consider a larger scale project that includes restoring the downstream channel to its earlier grade. 

Published 09/25/07