Graduation Date

Fall 2020

Document Type



Master of Science degree with a major in Environmental Systems, option Environmental Resources Engineering

Committee Chair Name

Margaret Lang

Committee Chair Affiliation

HSU Faculty or Staff

Second Committee Member Name

Michael Love

Second Committee Member Affiliation

Community Member or Outside Professional

Third Committee Member Name

Eileen Cashman

Third Committee Member Affiliation

HSU Faculty or Staff

Subject Categories

Environmental Resources Engineering


This project compared design decisions and hydraulic analyses of full-span stream crossings using one- and two-dimensional (1-D and 2-D) hydraulic models. The project was initiated by the California Department of Transportation’s interest in moving from 1-D to 2-D hydraulic modeling and by the Federal Highways Administration’s support for adopting SRH-2D (Sedimentation and River Hydraulics-2D developed by the US Bureau of Reclamation) in Aquaveo, LLC’s Surface-water Modeling System as their standard design model. Two-dimensional hydraulic models calculate more detailed water depths and velocities than 1-D models, which can better identify fish passage conditions, areas of potential scour or deposition, and aquatic organism habitat characteristics.

The project evaluated two recently constructed full-span (channel spanning) crossings that were designed based on HEC-RAS 1-D model analysis and constructed in 2017. The 1-D hydraulic models were not available for either of the projects, so the 1-D model results within the final project reports were used for comparison with 2-D model results completed for this project. Little Mill Creek crossing is a bridge with five rock weirs installed in the channel below located in Del Norte County, California. North Fork Ryan Creek is located in Mendocino County and is a box culvert with inlet and outlet headwalls and rock weirs installed both upstream and downstream of the crossing. The sites were re-surveyed in 2019 and 2020, and current conditions were modeled using SRH-2D. Current site conditions and 2-D model depth and velocity results were used to identify design elements that may have been designed differently using a 2-D model analysis in an effort to inform future full-span crossing design processes.

Using local 2-D model velocities for bank rock slope protection or riprap (RSP) sizing and abutment scour calculations resulted in differences in RSP size recommendations and abutment scour depth estimations. For Little Mill Creek Bridge, the RSP was estimated to be currently undersized, while at North Fork Ryan Creek crossing the RSP was oversized compared to the 2-D analysis based calculations. The local velocities and water depths available from 2-D model results provide greater spatial detail of the estimated forces experienced at the banks and abutments and account for local turbulence.

In terms of practicality, model efficiency and computing power continue to increase, making 2-D modeling more accessible. Computer processing time was found to increase linearly with the number of mesh elements so model run times are not likely to limit 2-D modeling for stream crossing sites. Sites with expansive floodplains could experience longer run times if detailed results, and therefore more mesh elements, are needed on the floodplain.

Citation Style


Virgil-LittleMillDesignDrawings.pdf (240 kB)
Little Mill Design Drawings

Virgil-NorthForkRyanDesignDrawings.pdf (9032 kB)
North Fork Ryan Design Drawings


Thesis/Project Location