Hydraulic design often tends to be on a conservative side for safety reasons. Hydraulic structures are typically oversized with the goal being reduced future maintenance costs, and to reduce the risk of property owner complaints. This approach leads to a conservative design with higher construction costs. Therefore, there is a need to quantify the cost-benefit aspect of this conservative approach. Accordingly, the overall objective of this project is to compare hydraulic design policies of Indiana Department of Transportation (INDOT) with that of other states, and perform cost-benefit analysis of large versus smaller hydraulic structures in terms of capital and maintenance costs. Comparison of INDOT’s culvert design is similar to that of Michigan, and is most updated compared to Ohio, Illinois and Kentucky. INDOT uses Q100 as the design discharge, which is conservative compared to other neighboring states that use Q50 as the design discharge for designing culverts. By using the data from 16 culvert design examples including both new-alignment and replacement structures, cost benefit analysis is performed in the light of suggested revision in culvert hydraulics policy. Results show that an increase in backwater limit to 1’ will result in 44% reduction in culvert size (represented as culvert area) with an average backwater of 0.79’. Increase in backwater limit will also increase the outlet velocity by 72% that may result into extra cost in outlet protection structures. Depending on the type and the size of the culvert, a change in hydraulic policy may result in saving from 12 -58% of the original cost associated with the current conservative design.

Report Number



hydraulic policy; culvert; bridge; design discharge; SPR-3405

SPR Number


Performing Organization

Joint Transportation Research Program

Publisher Place

West Lafayette, Indiana

Date of this Version


Technical Summary.pdf (610 kB)
Technical Summary

Culvert_Ana_Rev2.xlsx (39 kB)
Excel File for Culvert Analysis