Fournier Road Improvement Project
Bridge Replacement & Flood Resilience on Mariposa Creek
Mariposa, California
PEC Water served as the hydraulics and hydrology lead on the Fournier Road Improvement Project, a Mariposa County initiative to replace an undersized low-water crossing on Mariposa Creek that repeatedly overtopped during storm events. Working as a subconsultant to Mark Thomas, PEC Water rebuilt the HEC-RAS flood model from scratch to recreate FEMA baseline water surface elevations, evaluated 50-, 67-, and 90-foot bridge span alternatives against a 100-year design flow of approximately 2,000 cfs, and completed the no-rise analysis, scour calculations, and bank protection design needed to permit a new 50-foot bridge in a FEMA-regulated floodway.
Client
Prime
Subconsultant
Hydraulic & Hydrologic Analysis
PEC Water developed a new HEC-RAS hydraulic model for the Mariposa Creek reach at Fournier Road, rebuilding the floodplain analysis from the ground up to accurately represent current channel conditions. Working from available survey data, cross-section geometry, and the original FEMA Flood Insurance Study, PEC Water calibrated the model to match FEMA's published 100-year water surface elevations before using it as the basis for design. This step was essential to ensuring that any modifications to the crossing could be evaluated against a defensible regulatory baseline.
With the baseline model established, PEC Water evaluated the 100-year design flow of approximately 2,000 cubic feet per second through three bridge span alternatives — 50-foot, 67-foot, and 90-foot spans — combined with targeted channel improvements to assess their hydraulic performance. For each alternative, the model produced water surface profiles, velocity distributions, and freeboard comparisons that allowed the project team and Mariposa County to weigh hydraulic performance against constructability and cost. The 50-foot span emerged as the preferred alternative, providing adequate conveyance with manageable channel impacts and a realistic construction footprint given the site constraints.
Floodway Compliance and No-Rise Certification
Because the Fournier Road crossing sits within a FEMA-regulated floodway, any modification to the crossing requires a demonstration that the proposed structure does not raise the 100-year water surface elevation — a no-rise certification. For Mariposa County, this regulatory requirement shaped every aspect of the hydraulic design: alternative spans were evaluated not just for hydraulic adequacy but for their floodway impact, and the preferred 50-foot bridge was confirmed to meet the no-rise standard through careful comparison of modeled water surface profiles between existing and proposed conditions.
PEC Water prepared the full floodway impact documentation — water surface comparisons, freeboard evaluations, and narrative summaries — required to support the no-rise certification and satisfy Caltrans and California Department of Fish and Wildlife review. The package also addressed the encroachment from bridge approach fills, evaluating the lateral extent of grading within the floodway and confirming that the overall hydraulic footprint of the project remained within the no-rise standard. This regulatory documentation was a critical deliverable for moving the project into permitting and construction.
The Project
The Fournier Road Improvement Project replaces an undersized creek crossing that historically caused roadway overtopping and flood risk at a FEMA-regulated floodway. The project includes a new 50-foot bridge over Mariposa Creek and approximately 1,000 feet of roadway improvements, restoring reliable two-way access, adding a pedestrian sidewalk, and improving resilience to the extreme storms that are increasingly common in California's Sierra Nevada foothills. The project also supports broader county goals around climate adaptation and affordable housing access by ensuring a key road corridor remains open during and after storm events.
From a hydraulic engineering standpoint, the project presented a specific technical challenge: existing FEMA flood mapping for this reach of Mariposa Creek was based on a 2009 HEC-RAS model, and usable digital floodplain data were not available from that study. Before PEC Water could evaluate any bridge alternatives or demonstrate regulatory compliance, we had to reconstruct the floodplain model to current standards — recreating FEMA's published water surface elevations from the original study data and validating the model against observed conditions before proceeding with the design analysis.
HEC-RAS longitudinal profile — proposed 50-ft bridge on Mariposa Creek, showing water surface and energy grade line through the bridge opening under the 100-year design flow
Scour Analysis and Bank Protection
Replacing a bridge crossing on a regulated creek channel requires more than sizing the opening for flood flows — it requires understanding how the creek will respond to the new structure under design conditions and protecting the bridge foundations from the scour that results. PEC Water performed local scour analysis for the Fournier Road bridge using the methods described in the Federal Highway Administration's HEC-18 guidance, evaluating how stream velocities and flow patterns at the bridge opening would affect the streambed around the bridge abutments and piers during the design flood.
In addition to scour calculations, PEC Water assessed the need for bank protection along the channel reach adjacent to the new bridge, evaluating whether accelerated velocities through the bridge opening could cause erosive impacts on the channel banks downstream. Where protection was warranted, PEC Water provided riprap sizing and countermeasure concepts using a layered Caltrans design approach per the Bank and Shore Protection Manual. The scour and bank protection analysis was incorporated into the project's Location Hydraulic Report for Caltrans review.
What we Delivered
PEC Water served as the hydraulics and hydrology lead on the Fournier Road Improvement Project, delivering the full range of technical analysis and documentation needed to move a complex creek crossing through design, alternatives evaluation, and regulatory review. We rebuilt the HEC-RAS model from the ground up, evaluated three bridge span alternatives against the 100-year design flow, and produced the no-rise documentation required to permit a new bridge in a FEMA-regulated floodway. Our scour analysis and bank protection design protected the bridge foundations and channel against the hydraulic forces the creek will exert on the structure during major flood events.
Throughout the project, PEC Water coordinated directly with Mark Thomas, Mariposa County, and the project's permitting agencies — including the California Department of Fish and Wildlife for the streambed alteration permit. The Location Hydraulic Report we produced provides the hydraulic foundation not just for construction but for the FEMA map revision process that will update Mariposa County's flood mapping to reflect the improved crossing. When completed, the Fournier Road bridge will give residents, emergency responders, and freight a reliable connection across Mariposa Creek — one that stays open when the creek rises to levels that once shut the road down entirely.
From floodway constraints to finished structure — hydraulic engineering that turns a recurring hazard into a reliable crossing.
