NTM performed a hydrologic and hydraulic (H&H) analysis of the SR 2004-A01 Back Maitland Road over Jacks Creek. The project involved replacing a state-owned, structurally deficient steel I-beam bridge. The site is located in a detailed FEMA study area with a floodway, northeast of Lewistown in Derry Township, Mifflin County, PA.
Due to the flood-prone area and complex hydraulic conditions, both one-dimensional (1D) and two-dimensional (2D) hydraulic models were utilized to ensure no impacts to the numerous homes in the FEMA floodplain. The 1D analysis was performed using the U.S. Army Corps of Engineers HEC-RAS River Analysis System program and the 2D finite difference analysis was performed using the TUFLOW program in the SMS interface. The site’s hydraulics were complicated due to flood flows overtopping SR 2004 in the eastern floodplain, which bypass the structure.
There is a home in the in the northeast quadrant of the project site that is frequently flooded, and video obtained from a 2014 highwater event was used to validate the 1D hydraulic model. Along with this local flooding history, NTM performed a 2D hydraulic analysis in TUFLOW to more accurately determine the flooding pattern in the overbank areas and the percentage of overtopping flow. The 2D results compared well with the local testimony and assisted in making determinations such as hydraulic cross section orientation and lateral weir implementation in the 1D model.
As an adaptive design approach, NTM provided the results of the H&H analysis and was in continuous coordination with District 2-0 in-house design personnel for their design of the integral abutment bridge, roadway design, and waterway permit.