Caltrans District 12 On-Call Environmental Analysis Services

Hydraulic Studies and Modeling to Increase Safety in Flood-Prone Canyon

Challenge

Due to the topography of Laguna Canyon and its upstream watershed, the area surrounding the City of Laguna Beach, CA is prone to seasonal short-term flooding as well as debris-carrying catastrophic flooding, which often closes Laguna Canyon Road to traffic between the beach and SR-73. The road is a critical access point for first responders and serves as a local evacuation route. To increase public safety and improve safety operations during emergencies, Caltrans District 12 undertook the SR-133 Widening and Drainage Improvement Project in the City of Laguna Beach, requiring hydraulic studies and modeling to evaluate the floodplain and assess the risks for project development.

Solution

Based on findings from a Location Hydraulics Study, a Flood Risk Assessment, and hydraulic modeling, TRC prepared a Floodplain Evaluation Report that outlined alternatives to encroachment, risk assessment, impacts of the project on incompatible development, impacts of the project on natural and beneficial floodplain values and measures to minimize floodplain impacts. We overlaid the results of research and data collection, FEMA floodplain data, and other available floodplain data on project base maps to determine the locations of longitudinal encroachment at Laguna Canyon.

Results

In close coordination with Caltrans District 12, our hydraulics studies and models helped minimize encroachments and impacts during the redevelopment of this flood-prone canyon road. To improve safety, standard shoulders and class II bicycle lanes were provided, a second travel lane northbound from El Toro Road was extended by 1,200 feet, the acceleration lane southbound was extended by 900 feet and the exclusive right-turn lane was reassigned to an optional right or left turn lane from westbound El Toro Road onto SR-133.