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When field measurements cannot be obtained, accurate estimation of bankfull channel width is critical for hydraulic modeling, culvert sizing, and stream crossing planning, particularly in small, steep watersheds where field data is often limited. Structures, including culverts, are sized for the width of the channel at full flow to maintain natural sediment and habitat processes, and avoid frequent clogging, scour, or upstream flooding. In Vermont, regional hydraulic geometry curves developed in 2006 have been widely used for this purpose but are not well constrained for drainage areas below 3 square miles, often resulting in oversized culverts that require additional Agency resources. Structures Data Analyst Jorge Lindley updated bankfull width regression relationships for small drainage areas based on an expanded dataset of 314 field-measured stream reaches collected statewide between 2022 and 2025 by Keith Friedland and others in the Hydraulics Unit.
The resulting design curve exhibits substantially narrower confidence intervals compared with the 2006 statewide relationship, providing improved predictive reliability for small mountainous basins. Models for several Hydrologic Unit Code (HUC-8) watersheds were also devised, allowing for an even more refined approach. The updated curves are recommended for planning-level applications in Vermont streams with local channel slopes ≥1%, including preliminary culvert span estimation, replacement cost analyses, and delineating river corridors.
These findings demonstrate the value of large, practitioner-collected datasets in improving hydraulic geometry tools. This effort has been reviewed by the Rivers Program of the Agency of Natural Resources and is included in a just-released Hydraulic Engineering Instruction (HEI). Many thanks to our team and their efforts.
View the Hydraulic Engineering Instruction: HEI 25-004
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