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Henry, Nebraska

The Wyoming-Nebraska stateline flow measurement structure on the North Platte River southwest of Henry, Nebraska, is a crucial flow measurement location for water users in the two states. Severe erosion and bank degradation had occurred upstream of the structure, changing the stage discharge relationships for the weir. WRS was contracted in 2011 to design a project to stabilize sedimentation in the approach to this critical weir. The initial suggested project approach was to use 1-D HEC-RAS analysis to set water levels and boundary conditions that would then support more advanced 2-D numeric modeling of strategically placed bendway weirs, estimated to cost $450,000 to construct. However, in practice, it was found that building a finite element mesh and validating a stable numeric model is time consuming, requiring significant skilled labor. Every geometric change to the bendway weir design required a time-consuming re-meshing of the numeric model. Through the use of a three-dimensional physical hydraulic model, WRS was able to optimize the streambank revetments design.

In contrast to pure numerical modeling, once the physical model was constructed and validated to the HEC-RAS boundary conditions, the design team could — in a matter of hours — evaluate and receive near-instant feedback from the physical model. The result: A preliminary design was done in a matter hours, not weeks. Better still, the physical model demonstrated that bendway weir design per FHWA HEC-23 guidelines exacerbated the flow conditions at the weir, creating a large transverse vector vortex and scour hole upstream — a vector the numerical modeling would have treated as simply an anomaly and ignored.

WRS’s modeled solution reduced the total project cost to $175,500, including design, construction and construction oversight — a 61 percent savings. A decade after construction, the weirs remain sediment-free and performing as predicted.

Table 1: North Platte River at State Line Weir Project Cost Analysis

Original Concept (USD)

$ 450,000

Physical First Concept Estimate (USD)

$ 175,500

Project Cost Savings

61% of Original Concept Cost

Project Analysis Cost

29% of Construction Cost