is the depth of the fill. Live loads are distributed through the soil depth using a spreadsheet matrix, which reduces load intensity as fill depth increases. Bending Moments and Shear Forces
This determines the opening size needed to pass a specific design flow (e.g., a 50-year or 100-year flood) without causing excessive backwater or flooding upstream. Hydraulic design of box culverts is typically performed using Manning’s equation and the methods outlined in FHWA HDS-5 (Hydraulic Design of Highway Culverts). box culvert design calculations xls link