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May 05 2011

1-D, 2-D, 3-D, 4

As a I said before, when you need to use a hydraulic model to analyze a problem there are four options, a scaled physical model, three dimensional (3-D) model, two dimensional (2-D) model, or a one dimensional (1-D) model. 

Scaled physical models and 3-D models definitely have there place for large complex hydraulic analysis.  Though I have observed both I have very little experience with either, and at best can point to other resources.  For physical hydraulic modeling studies the best facilities I know of are the CSU, USU, and IU hydraulics labs.

In any river, canal, culvert, stream, creek, etc. all flow is 3-D, but for most hydraulic modeling studies a 1-D or a 2-D model will provide all the information required for an analysis and/or design.  Understanding the underlying assumptions of each model is very important when deciding on which type of model to use.  The Colorado Floodplain and Stormwater Criteria Manual (Chapter 12) provides a helpful table comparing the differences between 1-D and 2-D modeling.

    1-D_2-D modeling table

Because of the differences in how each model computes all of the hydraulic parameters, 2-D models have definite advantages over 1-D models in several situations.  These include:

  • Complex Floodplain Geometry
    • Flow Splits
    • Wide Floodplains
    • Variations in Channel and Floodplain Flow Paths
  • Complex Bridge Crossings
    • Multiple Openings
    • Roadway Overtopping
    • Crossings Near Channel Bends
    • Skewed Embankments
    • Skewed or Complex Pier Configurations
    • Extreme Constriction at Crossing
  • Alluvial Fans
  • Braided or Anabranched Streams
  • Asymmetric Floodplains
  • Highly Meandering

In addition to these site characteristics some hydraulic studies depend on additional, more accurate, data provided by 2-D models.  These include:

  • Bank Protection Design
  • Levee Protection Design
  • Scour Analysis
  • Habitat Analysis

The next time you need to use a hydraulic model make sure you think through your specific site and project characteristics before choosing which tool to use.

For additional reading on this topic please see the resources linked above, and the NCHRP report Criteria for Selecting Hydraulic Models.

2 comments

1 ping

  1. Anthony henry

    I’d like to add that there is are quasi-2D, and coupled 1D – 2D approaches you can also use.

    Qusai-2D is 1D modeling with prescribed flow directions splits. Essentially you define the main channel with the conventional cross sections but you can model the flood plains with storage units and flood plain sections, irregular weirs or other routing units to account for the flow across the flood plain. The flood plain and main channel are typically linked with lateral weirs allowing flow back and forth between them. Using this method appropriately you can set up a 1D model that can correctly account and model many of the situations you describe 2D being an advantage for. Different modeling platforms are better at setting up these simulations that others.

    A lot of modeling packages now also offer a coupled 1D – 2D engine for running a simulation where you use 1D in the main channel and 2D for the flood plain. Some care needs to be taken with this as difference modeling packages couple the 1D to 2D in different ways.

  2. Lisa researching hydraulic tools

    Thanks for sharing this chart. What a help it really is and the break down of models is fantastic.

  1. » The Big Assumption Hydraulically Inclined

    […] I also had some exposure to HEC-RAS, but didn’t even know that 2-D modeling existed nor how it was different than 1-D modeling.  Although I new how to setup a basic HEC-RAS model I did not fully understand how everything […]

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