Will new bridge allow cars to avoid floods?
Last updated 12/15/2022 at 7:42am
I read recently in the Village News about the proposed bridge over Sandia Creek Drive with both interest and puzzlement. The South Coast Chapter of Trout Unlimited collaborated with the California Department of Fish and Wildlife on the Steelhead Habitat Assessment and Enhancement Plan (2013) for the Santa Margarita River and we were further engaged on the design of fish passage alternatives for the Sandia Creek Drive bridge until 2019, when different perspectives with the project team lead us to conclude that effort.
Although I am not commenting here in my official capacity of the South Coast Chapter of Trout Unlimited, I share a unique understanding of the Santa Margarita River and Sandia Creek Drive with the local residents and wanted to take this opportunity to share information that will further inform the proposed bridge project on the river.
A proposal is at hand that would address the fish passage problem by constructing a new bridge to replace the existing structure. The proposed bridge is designed to allow both fish passage (during low flows) and vehicular passage (during high flows). Our concern is that although the bridge is designed to allow vehicular passage during flood events, it will not perform that function. It can’t.
The limitations of the proposed bridge are best understood in a landscape-scale view of the river canyon. We know from engineering studies that the entire canyon from De Luz Road to Rock Mountain Road floods; the only area that escapes flooding is the rock outcropping adjacent to the Wildlands Conservancy property.
When the flooded canyon fills with water, the proposed bridge will be inaccessible from the south, rendering its construction moot. The greatest flood risk will be to vehicles crossing on the new bridge, from the north side of the river at Rock Mountain Road. If those vehicles cross the proposed bridge, they will be out of the floodway until they descend further south on Sandia Creek Drive. In that location, they will be within the floodway, paralleling the river which overtops the road to a depth of approximately six feet. Thus, the new bridge will not offer safe vehicle passage during floods, it will only change the location at which south bound traffic encounters the flooded road.
Planning for the proposed bridge can consider different mitigation options to prevent southbound traffic from being swept off the flooded road south of the project site. One option is to vertically realign Sandia Creek Drive from Rock Mountain Road to De Luz Road. That effort would entail an elevated roadway along the entire river bottom.
Another option would be to barricade Sandia Creek Drive from vehicular traffic on the north side of the river at Sandia Creek Drive and Rock Mountain Road, preventing south bound traffic from blindly entering the canyon when rain is forecast. That option would appear to undermine the entire premise of the bridge, however, since the structure is designed to allow vehicular passage during floods.
A third option is to modify the existing bridge to allow fish passage while maintaining its counter intuitive safety characteristics. We formally submitted these thoughts to the County of San Diego during the public comment period of the CEQA process and encouraged a robust discussion on the options for fish passage.
Locals from the greater Fallbrook area granted us their confidence in a collaborative spirit during our efforts and we want to honor that trust with open, authentic discussion on issues that will impact the community for decades to come. Thanks for the read…..to read the comments that we submitted to SD County during the CEQA comment period, go to https://issuu.com/villagenewsinc/docs/smr_flood_review_-_ceqa_comments.