Impact of Extreme River Flows on Bridges An Application to River Lagan in Ljungby Municipality, Sweden Using HEC RAS

Abstract: The role of bridges in the transportation network has become increasingly important as they ensure the smooth and safe passage of people and goods across rivers and other bodies of water. Flowing water impacts bridges over rivers primarily in two ways: (1) Hydrodynamic forces that directly impact the structure and (2) scouring at the pier and abutment, including contraction scour. The design principles for bridges in Sweden are relatively simplistic, including both impacting forces and local scour. Therefore, the purpose of this thesis is to examine the impact of extreme river flows on bridges, particularly in circumstances beyond the design flow, using as a case study Ljungby Municipality, Skåne. Ten bridges are located across 6 km of the Lagan River within the municipality area. Although applications are made regarding Swedish conditions, the work is primarily based on international literature and experiences. An overview of the literature on the impacts of extreme river flows on bridges was conducted, as well as an analysis of the most common equations that have been used to model these impacts. Several countries, including the United States and Australia, have studied this topic extensively and recommended design and analysis procedures. Additionally, corresponding Swedish techniques were reviewed. This was followed by evaluation of HEC RAS (1D - steady flow) model for its ability to simulate the impact of extreme flows on bridges, including scour predictions using the Froehlich equation. This resulted in inundation and flood hazard maps for the study area (Ljungby Municipality), as well as the determination of overflowed bridges. A 100-year event resulted in the overflowing of Ljungsätersbron. During the 200-year event, Ljungsätersbron and Söderbron overflowed. Also, abutment scour analyses have been performed for all bridges. Overall, the model provided a satisfactory estimation of the potential threats posed by scouring at bridges abutments during extreme flow conditions.