For a detailed discussion on this topic, go to the HEC-RAS Documentation Webpage and download the document called "Modeler Application Guidance for Steady vs. Unsteady, and 1D vs 2D vs 3D Hydraulic Modeling". Here is a link to that document:

The following is a short summary of what is in that document. First the user should think about "When do I need unsteady flow modeling over steady flow modeling? Where should I use 2D Flow Areas over 1D river reaches and/or storage areas ?"

To answer the questions of Steady vs Unsteady Flow, and 1D vs 2D modeling approaches, requires additional information regarding the purpose of the model and the data available. Each river system will have site specific information that must be considered in order to answer the questions of Steady versus Unsteady flow and 1D versus 2D modeling. The following is a partial list of some of the things that the modeler should typically consider when trying to make a modeling approach decision:

  1. Physical description of the river channels, floodplain areas, bridges/culverts, other hydraulic structures, levees, roads, etc. that the model will be applied to.
  2. What is the typical size, length, and complexity of the systems that these models will be applied to? Is it a 1 mile, 10, 50, 100, 500, or 1000 mile river system
  3. What is the source and level of accuracy of the terrain data, cross section data, and hydraulic structure data?
  4. What is the general level of accuracy of the hydrology used to drive the models?
  5. Will this model be used for Planning type studies, or will it be used for real time modeling and mapping?
  6. .What type of events (hydrology and boundary conditions) will the models be used to analyze (Dambreak, flash floods, normal rainfall runoff events, steady flows)?
  7. What is the typical duration of a flood event on this river system? (1-3 days, 1- week, 1-month, 6-months, years)
  8. Is the flow path of the water generally known for the full range of events?
  9. Are there unique aspects of the system that will significantly affect the computed results? Such as: is the river tidally influenced; do wind speed and directions affect the water surface elevations; is the river affected by floating ice or ice jams; does there tend to be debris issues during flood, and does the debris tend to pile up at hydraulic structures; will levee overtopping, breaching, and interior flow routing need to be addressed, Are there any significant bridges and culverts that will cause water to backup behind them during significant flood events, etc…?
  10. What are the required outputs from the model (water surface elevations, water depths, arrival times, average velocities, detailed velocities in two dimensions at specific point locations, etc…)?
  11. What is the model purpose and expected level of accuracy required?
  12. How much time and money do you have to get it done?
  13. What experience does the modeler have with 1D and/or 2D modeling?

With that said, I will try to offer both a theoretical opinion and a practical application opinion to the question posed above.

Note: a much more detailed and extensive discussion on this subject is contained in HEC Training Document 41 (TD-41) "Modeler Application Guidance for Steady vs Unsteady, and 1D vs 2D vs 3D Hydraulic Modeling". This document can be obtained from the Publications area of HEC's website.