WRS began working with mesoscale physical models in 2011 when it was contracted to design a bank stabilization project on the North Platte River near the Nebraska/Wyoming boarder (Figure 2). At issue was the need to stabilize her approach to the weir so that it would yield a consistent discharge rating. The analysis plan was to conduct a 1-D HEC-RAS analysis to establish water levels, and boundary conditions to support more advanced modeling. The advanced modeling was to conduct 2-dimensional numeric analysis and physical modeling concurrently. Countermeasures consisting of bendway weirs were to be designed and tested in the numeric and physical models concurrently believing that the physical and numeric models would inform each other. In practice however, building a finite element mesh, and validating a stable numeric model is time consuming requiring significant skilled labor, and every geometric change to the bendway weir design necessitated a time consuming re-meshing running of the numeric model. In contrast, 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 nearly instant feedback from the physical model. As a consequence, a preliminary design of the optimum bendway weir arrangement was obtained on a matter hours.
The overall size of the model was less than 4m long and 1.5 m wide. With the size limitations the horizontal scaling was 70:1. This model had a distortion of 3.5 which is near the upper limit of which we like to work. Currently, most of our models are distorted by a ratio of 2.5 or less. It became clear early in this project that only optimum bendway weir arrangement needed to be analyzed using the numeric 2-D model, from which the concept of Physical-First was developed. In this methodology the physical model is built, tested and modified to determent the best design to meet the requirements of the project. Optimum project conditions obtained from the physical model are then modeled using a multi-dimensional model. In early projects 2-D models such as RMA-2 or Flow 2-D were commonly used. Currently, almost all of our physical modeling projects are also being numerically modeled in 3-Dimensional. The physical model provides one of the only ways to validate the numeric models because it is seldom that project flow conditions can be observed in the field. Once the numeric model is validated to project conditions, the numeric results of local flow depths and velocities can be determined for final sizing and dimensioning of the final project. For example the velocities derived from the two-dimensional Platte River model were used design the weirs so that they would not move. Approximately 18 months after construction, the Platte River weirs are were performing as anticipated from the models.
This approach to conducting combined multi-dimensional numerical and physical hydrodynamic modeling has been successfully applied to several 4 channel and bridge projects on Brush Creek in Mission Hills, Kansas, and for a spillway redesign and renovation for Kansas Fish and Wildlife. The Physical-First has evolved with each project we have conducted. In many ways the rational for these models is similar to that of larger models, namely that the usage of physical models provide a much greater insight to the complex three-dimensional hydrodynamic processes associated with each project. This allows for the analysis and design to specifically target geometries of structures and countermeasures to minimize adverse hydraulic and erosional conditions. ADVANTAGES The primary advantage of using mesoscale models and a physical first approach is primarily economic which will be discussed in the next presentation of this series. However, it is our experience that, although the design, construction and testing of the physical models add a modest amount to the cost to engineer and design. However, these costs are significantly offset by cost savings elsewhere. In every physical model these authors have conducted (including all of the authors earlier models), we have always observed unexpected hydraulic phenomenon. In the Platte River model, the 2-D analysis of existing conditions showed a strong transvers velocity vector immediately upstream of the weir and aligned parallel to the weir plate pointing from the right side of the weir to the left. Without the physical model, this vector was considered to be an anomaly of the numeric model. Only when the physical model was tested, did we learn that the vector represented the core of a transverse vortex that scoured nearly all of the material upstream of the weir (Figure 4). Although the 2-dimensional model identified an issue at this location, The tactile nature of the physical models is a distinct advantage especially when working with clients, city councils, planning commissions, and funding agencies. The physical models allow for clear explanation of complex concepts that are not easily explained using words charts and graphs. What-If scenarios can often be quickly demonstrated to an often interested yet skeptical constituency. Projects are streamlined because client and regulator buy-in is more easily accomplished.