Available at: https://digitalcommons.calpoly.edu/theses/3222
Date of Award
12-2025
Degree Name
MS in Civil and Environmental Engineering
Department/Program
Civil and Environmental Engineering
College
College of Engineering
Advisor
Stefan Talke
Advisor Department
Civil and Environmental Engineering
Advisor College
College of Engineering
Abstract
ABSTRACT
This study evaluates bridge scour at the Toro Creek crossing of Highway 1 in Morro Bay, California, under present conditions and future climate conditions with more frequent extreme rainfall events. California infrastructure is becoming increasingly vulnerable to extreme storm events and increased intense precipitation events brought by climate change. This analysis focuses on the calculation of scour for both the northbound and southbound spans. Highway 1 plays a vital role in California’s coastal transportation infrastructure by carrying more than 20,000 vehicles daily (Caltrans, 2017). In this thesis, a hydrological model (HEC-HMS) is used to simulate runoff under present-day and future climate conditions, using different annual exceedance probability precipitation intensities and three antecedent moisture conditions. The simulated runoff values are used to drive an unsteady 1D hydraulic model (HEC-RAS), which is used to simulate velocities and scour underneath the Toro Creek Bridge. The three antecedent moisture conditions (AMC I, II, III) are dry, average, and wet with design storm events with 2-, 10-, 50-, and 100-year return periods. Water depth parameters were tuned based on the March 10, 2023, storm event with a modeled peak discharge of 4,740 cfs.
Future rainfall scenarios were derived from Cal-Adapt’s LOCA downscaled projections using three Global Climate Models (HadGEM2-ES, CNRM-CM5, and CanESM2) under RCP 4.5 and RCP 8.5 emission pathways. The CSU and Froehlich equations were applied to compute pier scour, while contraction scour was estimated using Laursen’s live-bed method. Results indicate that scour depth increases with flow velocity, with velocity being the dominant factor in the CSU equation. For the northbound bridge, total scour under present and possible future conditions ranged from 3.6 ft to 8.46 ft, dominated by pier scour. At the southbound bridge, total scour under present and possible future conditions ranged from 4.2 ft to 7.25 ft, dominated by pier scour. The river cross-section at the southbound bridge exhibited a broader distribution of contraction and pier scour, with maximum combined depths of 7.25 ft. The highest velocities modeled under climate change conditions reached 9.1 ft/s. Scour and flow rate increased relative to present day conditions for RCP 4.5 midcentury and RCP 8.5 midcentury, and end of century. Because RCP 4.5 assumes that greenhouse gas emissions peak around 2040 and subsequently decline, the HadGEM2-ES and CNRM-CM5 climate models project reduced precipitation intensities later in the century. As a result, both models produce lower flow rates, and therefore lower predicted scour depths, than present-day AMC III conditions for the 10-, 50-, and 100-year precipitation events.
The analysis demonstrates that projected mid-to-late-century increases in extreme precipitation could substantially enhance erosion at both Highway 1 bridges. These findings emphasize the need for adaptive bridge design and maintenance strategies that account for hydrologic variability and future climate uncertainty along California’s Central Coast.