Wave of the Future: 2D Hydraulic Models
Prediction and analysis of surface water flows and flooding is becoming more important than ever. Landowners, government agencies, engineers, and anyone involved in water resources projects have an elevated responsibility to provide the best available data and solutions. Two-dimensional (2D) surface water models are often the best tool to accomplish project goals – luckily that shouldn’t come with significant, if any, increased cost.
With the release of HEC-RAS 5.0, 2D surface water models are now a common tool in our hydraulic modeling toolbox. They can produce more accurate and detailed results in many hydraulic modeling applications. For owners and their projects to benefit most from this accuracy, it helps to understand more about 2D modeling software’s data input needs and its fundamentals.
Many project owners and professionals in our industry have shied away from 2D models thinking they are too expensive and time consuming, maybe even overkill. I am pleased to report this is not the case. They are often a more efficient and better tool for hydraulic analysis and can provide critical information uniquely from a 2D viewpoint. This savings to owners and clients, coupled with HEC-RAS’s ability to provide more detailed results, can tip the scales toward using a 2D model for many projects and studies.
Understanding the Difference Between 1D and 2D
One-dimensional (1D) surface water hydraulic models have a long track record. Historically, they have been used to calculate water discharge, elevation, velocity, and many other parameters for engineers to use in many applications. With 1D, single water surface elevations are computed at each cross-section where the flow is only shown perpendicular to the cross-section and needs to be drawn by the model builder. In order to gain a visual understanding, using 1D models is like driving down a one-way street with a speed-limit sign; you can only drive in one direction and at the signed speed limit.
But using 2D modeling is a game-changer. The assumptions of a single water surface elevation and flow direction perpendicular to the cross-section may be the starkest contrasts between 1D vs 2D models. If we return to our driving example, using a 2D model is like driving the same car around an empty airfield; now you can drive in any direction and at any speed you like!
It’s important to note that some complex 1D models can be constructed to replicate 2D solutions but not without hours of effort, calibration, and engineering expertise.
How 2D Can Save Time and Project Costs
In a 2D model, we use a digital elevation model (DEM) as the basis for computations to determine surface water depths, velocities, and directions. While 1D models are usually based purely on data at cross-sections, a 2D model requires a complete DEM over the entire targeted area (the addition of a DEM is the primary reason there’s an increase in data requirements over a 1D model). While this can be achieved with 2D modeling, most projects already have accompanying topographic surveys and/or design surfaces.
Another advantage for owners and clients is that publicly available DEMs are becoming readily available for little or no cost to support larger-scale modeling efforts. Oftentimes, this results in no additional survey being required beyond what was already planned for on the project.
With 2D modeling, a computational mesh or grid is placed over the DEM to show the movements of water across the DEM in any direction. Instead of the model builder setting assumptions and modeling the direction of water flow, the surveyed DEM and computational grid are used as the basis for the computed direction of surface water flow. The orientation and refinement of the computational grid may be the most influential input by the model builder and should, again, be largely based on the DEM being modeled. Since each grid element computes a discrete value, the resolution of results can be greatly increased giving engineering designers a much clearer picture of the project site.
Efficiency is also a benefit to using 2D models which can often be completed and run in about the same amount of time it takes to set up a 1D model. And, in cases such as multiple conveyance areas or split flow segments, 2D models are more efficient and accurate because the modeler doesn’t need to make nearly as many assumptions. For example, if we need to determine the flow volume in a side channel of a river for irrigation diversion improvements, the 1D modeling effort can require lots of effort and still fall short of a simpler 2D model.
Compatibility with Other Platforms
Another benefit of 2D models is their compatibility with other spatial tools to complete any number of design calculations. These tools can provide:
- scour analysis
- riprap sizing
- depth/shear mapping for aquatic habitat evaluations
- irrigation diversion
- stream restoration design
- and many more design applications
In addition, when coupled with a unit annual hydrograph, a 2D model can even help make decisions around wetlands restoration or groundwater recharge based on the average duration of inundation.
With 2D hydraulic models now commonly available, it’s a win-win modeling approach to help improve water resource projects and studies. And, those with the right knowledge base can use 2D hydraulic modeling efficiently and effectively which won’t result in increased project costs.
So, when thinking about how to accomplish your next project involving surface water, keep in mind that a 2D hydraulic model may likely be to your best advantage.
How You Can Learn More
There is plenty of information on the U.S. Army Corps of Engineers website – along with the most recent HEC-RAS 2D software downloads. We would be happy to share ideas on how this can help your next project and can offer some tips and tricks to getting the data you need.
Matt Barnes, PE., CFM is a professional engineer and certified floodplain manager who works in Morrison-Maierle’s Helena office. A native Montanan, he enjoys being in the outdoors with his wife and son when not working in the outdoors as a specialist in natural resources engineering.
Technical review provided by Mark Franchi, PE