The impact of bridge intrusion on the Mill River : hydraulic modelling and analysis using HEC-RAS

The Williamsburg Mill River Greenway Committee (WMRGC) is proposing the construction of three pedestrian bridges along the Mill River in the town of Williamsburg, MA. This honors extension to my Design Clinic team capstone project aims at understanding the hydraulic effects of introducing bridge abutments into a river channel. To achieve this, I used the Hydrologic Engineering Center River Analysis System (HEC-RAS) to hydraulically model the existing South Main Street Bridge in Williamsburg which displays abutments that intrude approximately twenty feet into the river channel. I analyzed the effect of abutment intrusion on velocities adjacently upstream and at the bridge by creating alternative bridge cross sections in HEC-RAS without abutment intrusion, and alternatively with a middle pier. I used these different channel geometries to also assess the effect of the latter on the erosive capacity of the river by comparing bed shear stress among different channel geometries. Finally, in anticipation of possible future work on the design of the proposed bridges in Williamsburg, I assessed the live and wind loads acting on one of the proposed bridges and conducted preliminary research on abutments and prefabricated bridges. Results of the above-described analysis show that at the existing conditions at South Main Street Bridge, velocities increase by 35% for bankful conditions and 63% for Storm Irene flow as the flow passes through the bridge constriction. Contrastingly for non-intruding abutments, flow undergoes a velocity increase of only 1% at bankful conditions, and a velocity drop of 0.5% for Storm Irene. Additionally, comparing bankful velocities at the bridge between intruding and non intruding conditions shows that intruding abutments increase velocities by around 24% for th