October 2025: Scour Countermeasure Complete
The City of Portola has completed the scour countermeasure project (riprap installation) on the Gulling Street Bridge at pier 3.
This project is an engineered FEMA approved countermeasure to protect the area from further erosion while simultaneously re-enforcing the area overall. In addition to FEMA approval, the permitting process was extremely strict due to regulations, both state and federal, associated with the designation of the area as a “Wild and Scenic River”. The permit needed to be reviewed as well as approved by the following agencies and required that a biologist continually examine the work area for the presence of the Sierra Nevada Yellow-Legged frogs:
United States Army Corp of Engineers
Department of Fish & Wildlife
Regional Water Quality Control Board
Locally affected tribes
Completed Project Details
What is “riprap” and how does this help? Riprap is the placement of large and small materials around the base of a bridge pier. This
material and placement was designed and specified by our engineering team (see below). FEMA also approved this project as a countermeasure to the scour condition of the pier. Lastly, the material is approved by Caltrans, which confirms the appropriate material is
utilized to be able to withstand the physical forces applied by the river on the material and the use as a viable scour countermeasure.
Riprap acts like armor. It breaks up the force of the water, helping prevent it from washing away the ground underneath the bridge pier. By slowing the water down and holding the soil in place, riprap helps protect the bridge pier from further erosion.
How do we know the project was successful?
Throughout the course of the installation, the Public Works team monitored the placement of the materials utilizing an underwater drone. This allowed the team to confirm that the materials were being placed appropriately to shore up the condition of the pier.
Utilizing a custom extended reach excavator, the materials were first placed along the pier then moved into position underwater with the excavator. During each phase, a specific sized material was placed along the pier, moved into position, then confirmed for
appropriate placement. As the project continued, larger materials were utilized to further support the area and
protect against further erosion. At the completion of the project, our engineering team reviewed the work to confirm adherence to design and specifications.
How do we know the bridge is safe?
1. The bridge is not at risk of collapse. The structural engineers determined possible failure modes for this bridge structure and their analyses indicate that the bridge would not suddenly and catastrophically collapse without a proceeding catastrophic event such as an earthquake.
2. In addition, in January of 2025, a continuous monitoring system was installed on the bridge. This system was designed by our engineers. Similar systems are utilized in the same fashion in other locations on bridges that have similar conditions. The purpose of the continuous monitoring equipment is to continuously monitor the bridge for settlement and deflections. The instruments on the bridge can measure settlement and deflections in the bridge structure to one-thousandths of an inch. The instrumentation continuously monitors the structure for any movement and threshold alarms will alert the City so that actions can be taken as deemed necessary. As of the date of this update there have been zero (0) alarm conditions. The data is reviewed frequently by both city staff and our engineering team.
Next Step: Caltrans to Inspect the Bridge
The City has made the request to Caltrans to perform an underwater dive inspection and any other prudent inspections to examine the condition of the bridge post the installation of the riprap. We are awaiting confirmation when this inspection may occur.
Additional Action: Secure Funding for Structural Rehabilitation
In August of 2025, the City submitted an application to Caltrans under the “Highway Bridge Program” for funding for the structural rehabilitation of the bridge. The purpose of this program as described by Caltrans is as follows:
“The purpose of the Program is to replace or rehabilitate public highway bridges over waterways, other topographical barriers, other highways, or railroads when the State and the Federal Highway Administration determine
that a bridge is significantly important and qualifies under the HBP program Guidelines. Reimbursable scopes of work include replacement, rehabilitation, painting, scour countermeasure, and preventative maintenance activities.”
Source:
https://dot.ca.gov/programs/local-assistance/fed-and-state-programs/highway-bridge-program
Once the application is approved, the bridge will be assigned to a priority level by Caltrans for funding allocation. We are hopeful that an appropriate priority designation is provided so that we can move forward with this work in a timely manner.
In addition to this funding source, the City and our engineering team are exploring other funding streams for this project. We have targeted at least one additional potential source as of this correspondence and will be requesting local assistance to provide the funding.
A Message of Gratitude
We sincerely thank you for your patience and understanding as we continue to navigate this complex process.
If you have any questions about this project, please reach out to City Manager Ryan Bonk directly at [email protected] or 530-832-6800.
