2D Sediment Transport (2024)

The class kicks off with six mini-case studies, including laboratory and prototype models that highlight the capabilities of the 2D sediment capabilities in HEC-RAS.

Sediment results are more sensitive to good hydraulic and mesh development than hydraulic water surface or floodplain analysis.  This session introduces some of the 2D mesh best practices - including channel alignment and structure definition - that set up a good sediment, as well as some of the new diagnostic tools to identify mesh issues.

Gibson

A good 2D sediment model starts with an excellent 2D hydraulic model.  This session introduces some of the 2D mesh and hydraulic best practices that precede sediment modeling.  We will also cover turbulence approaches as well as some of the new diagnostic tools to identify problematic cells and model instabilities.

Gibson

The 2D model in HEC-RAS uses a disequilibrium, advection-diffusion approach, with is fundamentally different than the 1D algorithms.  This presentation will introduce the differences between equilibrium and disequilibrium approaches to sediment modeling.  It will also introduce the governing equations, the advection-diffusion approach, how the erosion and deposition rate terms work, and the 2d sediment transport equations (van-Rijn, Wu, and Soulsby-van Rijn).

Sanchez

This presentation will introduce the basic sediment inputs in HEC-RAS required to run a model: mainly bed gradation and sediment boundary conditions.  We will describe how to define bed gradations spatially in RAS Mapper with a Bed Gradation Layer and how to connect it to sediment data in the Sediment Editor.  This session will also introduce the 2D sediment boundary conditions. Then we will introduce the sediment results tools in RAS Mapper, including how to add and access sediment results, and some of the most helpful results to evaluate different models.

Gibson

Most of the runtime in 2D sediment models is associated with solving the sediment and bed mixing equations for each grain size.  However, recent versions of HEC-RAS include a 2D sediment “Capacity Only” approach, that compute fixed-bed, sediment capacities and can route sediment concentrations without erosion or deposition. These tools can provide sediment process insights at runtimes comparable to the hydraulic model.   This session introduces 2D, fixed bed, sediment modeling options and discusses their potential for screening alternatives and reducing the number of mobile bed simulations required.

The 2D sediment model has four “warm-up” phases.  The standard, 2D, hydraulic warm up is even more critical for sediment models.  But in addition to that phase, the sediment model can initialize the concentration, bed gradation, and/or the bed elevation.  Initializing the bed so the bed gradation, hydraulics, sediment flux, and transport function are in quasi-equilibrium at the beginning of a simulation will help the modeler to isolate the impact of management alternatives or morphological events from numerical artifacts of initial conditions.  This session will also discuss how to limit the computational burden of this warmup phase with a hotstart file. 

Sanchez

The presentation will describe model calibration parameters and their sensitivity. Recommendations and guidance will be shown on how to perform a model calibration or sensitivity analysis. The presentation will also describe recommended parameter bounds and typical values. The effect of model parameters on results will also be covered. 

This presentation will describe other 2D options and parameters including outer loop convergence, base bed slope parameters, avalanching, splash erosion, and other options.

This presentation will explore several common problems 2D sediment modelers encounter and best practices to resolve or work-around them.