Impact of extreme flows on bridges: A case study from Lagan river, Ljungby

Flowing water impacts bridges over rivers primarily in two ways: (1) hydrodynamic forces that directly impact the structure and (2) scouring at piers and abutments, including contraction scour. The design prin-ciples for bridges in Sweden, including both impacting forces and local scour, are relatively simplistic. The purpose of this paper 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. Although the study was made with focus on Swedish conditions, the work is to a large extent based on international literature and experiences. First, an overview of the literature on the impacts of extreme river flows on bridges was conducted. This was followed by a numerical study using the HEC RAS (1D – steady flow) hydraulic model to simulate the impact of extreme flows on bridges in Ljungby. This resulted in inundation and flood hazard maps for the study area, besides the determination of overflowed bridges. A 100-year event resulted in the overflowing of the Ljungsätersbron. During the 200-year event, Ljungsätersbron and Söderbron overflowed. Also, abut-ment scour analyses were performed for all bridges. Overall, the modeling approach provided a satisfactory estimation of the potential threats posed by scouring at bridges abutments during extreme flow conditions.