Strengthening Orange County's Flood Channels through resilient and innovative design
At a glance
The project embodies Orange County Public Works' (OCPW) mission statement – "Protect and enrich Orange County communities through sustainable delivery of projects and services." The County will secure Federal Emergency Management Agency (FEMA) accreditation by strengthening the channel walls. Flood insurance will remain in place and the community will be protected against floods and future sea-level rise.
The challenge
The existing channel walls were constructed circa 1990 and are deteriorating faster than initially anticipated, portions of the walls have reached less than 10-years of their remaining service life. This deterioration has reduced the lifecycle expectancy of the sheets where the structural capacity of the steel sheet pile walls is considered inadequate for static loading and the levee stability requirements necessary to meet a 10-year levee certification from FEMA.
Construction and accreditation of the levee system before the current FEMA accreditation expires is critical. Without it, over 23,442 structures neighboring the channel would lose FEMA flood insurance. The design team had less than 18 months to complete the design and develop an approach to reduce construction times.
The channel severs a residential zone. The adjacent right of way is fully developed and regulatory permitting times to work within the channel are prohibiting to complete construction activities within the time necessary. This reduces the working area to an existing 15 to 18-foot-wide maintenance roads atop the levees.
An existing timber pedestrian bridge provides the neighborhood easy access to the local school by avoiding vehicle traffic along the main roads. The bridge has to be temporarily relocated to repair the sheet piles and its abutments reconstructed to facilitate the installation of the new piles.
Additionally, two major utility corridor crossings where large diameter pipes run across the channel, beneath the invert, needed to be re-designed.
Our response
The Orange County Flood Control District (OCFCD) has engaged GHD to design repairs for existing corroded steel sheet pile (SSP) wall segments of the Huntington Beach Channel and the Talbert Channel (OCFCFD Facilities D01 and D02, respectively).
We designed repairs for approximately 17,000 linear feet of the SSP lining the channel walls by installing new sheet piles (large section modulus) to depths varying from 30 to 35 feet below the channel invert. The new SSP will be installed using the state-of-the-art technology available for installation to minimize noise and vibrations in the project's vicinity. In addition, the new SSP will be installed within 4-inches on the landside of the existing unrestrained SSP to reduce otherwise lengthy permitting times required for construction within the channel (waterside). This approach addresses the requirement to expedite the construction schedule for the accreditation process and minimizes any potential impacts on the sensitive ecosystem.
The pedestrian bridge will be removed from service until after the SSPs are installed. The new abutments are designed to work with the new SSPs providing a more resilient foundation system.
The new SSPs also include additional sacrificial thickness to account for the anticipated corrosion during the design life, eliminating the need for marine coatings that typically get damaged during installation. This method also avoids the need for other means of corrosion protection such as impressed current, passive anodes or cathodic protection that require continuous maintenance and upkeep, adding to the overall lifecycle cost.
The project uses the Design-Build method of delivery, thereby benefitting from a collaborative effort between OCFCD, Contractor and Engineer to develop a robust, economical solution for a complex site with several environmental constraints.
The impact
The new sheet pile walls will strengthen the channels and protect the community against floods and future sea-level rise for another 50 years.
The project allows the County to continue with the FEMA accreditation process while the repairs are under construction and allows the residents to continue with the current FEMA flood insurance plan instead of seeking additional expensive insurance independently.
After completion, this project will provide for a more resilient and stable infrastructure for the County as they continue strengthening the flood channels in the region.
The project achieved recognition from the APWA SoCal Chapter for the BEST Project of the Year Award 2023. This deserving acknowledgment underscores the significance of our role in fortifying Orange County's flood channels through resilient and innovative design solutions.