HEC-RAS Two-Dimensional Sediment Transport User's Manual




















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HEC-RAS Two-Dimensional Sediment Transport Users Manual




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Alejandro Sanchez, CEIWR-HHT
Stanford Gibson, CEIWR-HHT




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U.S .Army Corps of Engineers
Institute for Water Resources
Hydrologic Engineering Center (CEIWR-HEC)
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HEC-RAS; River Analysis System; HEC; Hydrologic Engineering Center; two-dimensional; R&D; research & development; United Kingdom; research project; models; hydraulic; benchmarking; test; grid cell sizes; grid; bathymetry; cell face; terrain; elevation; flow solver; time steps; result; water surface; velocities; computational interval










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HEC-RAS
Two-Dimensional Sediment Transport Technical Reference Manual


December 2020












U.S. Army Corps of Engineers
Institute for Water Resources
Hydrologic Engineering Center
609 Second Street
Davis, CA 95616
(530) 756-1104
(530) 756-8250 FAX
www.hec.usace.army.mil
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Table of Contents


Table of Contents
Table of Contents
List of Figures
List of Tables
Chapter 1
Introduction
Chapter 2
Performing a 2D Sediment Transport Analysis
2.1 Hydraulic Best Practices for a 2D Sediment Model
2.1.1 Hydraulic Warm Up
2.2 Sediment Data
2.2.1 Initial Conditions and Transport Parameters
2.2.2 Associated Bed Gradation Templates with Bed Material Layers
2.2.3 Sediment Boundary Conditions
2.2.4 User-Defined Grain Classes
2.2.5 Cohesive Options
2.2.6 Transport Methods
2.2.7 Transport Function Calibration and Modification
2.2.8 2D Options
2.2.9 Bed Mixing Options
2.2.10 Bed Gradations
2.3 Viewing Sediment Results in RASMapper
2.3.1 Plotting 2D Sediment Time Series
2.3.2 Plotting 2D Sediment Transects/Profile Lines
2.4 Sediment Computation Options and Tolerances
2.4.1 Sediment Computation Options and Tollerances
2.4.2 2D Computational Options
2.5 Sediment Output Options
2.5.1 Output Level
2.5.2 Output Interval Multiples
2.5.3 Customized Sediment Output Variables
2.5.4 Sediment Hotstart
2.6 Viewing Results
2.6.1 Exploring the HDF5 File
2.6.2 Log Output
Chapter 3
References


List of Figures
Page
No.
Figure 21. HEC-RAS Unsteady Computation Options and Tolerances editor.
Figure 22. Sediment Data editor.
Figure 23. Bed Gradation Template editor.
Figure 24. Crating a Bed Material Layer in RASMapper.
Figure 25. Sediment Bed Material Layer underlying a 2D mesh and in the RASMapper tree under Map Layers.
Figure 26. Give the layer classifications names that will show up in the sediment editor under Layer Properties.
Figure 27. RAS Mapper Manage Layer Associations editor.
Figure 28. Bed Gradation Template editor.
Figure 29. Example of a Bed Layer Group definition in the Define Gradation Layers editor.
Figure 210. Example of a Bed Layer Group assigned to a computational cell.
Figure 211. Simple, single gradation, 2D sediment specification with one Bed Gradation associated with one Bed Material Type.
Figure 212. More detailed bed material specification, where the Bed Material Layer was imported from a shape file.
Figure 213. Boundary Conditions tab in the Sediment Data editor.
Figure 214. Sediment Rating Curve editor.
Figure 215. Sediment Load Series editor.
Figure 216. Opening the Unsteady Temperature editor from the Unsteady Flow Data editor.
Figure 217. Specifying a temperature time series.
Figure 218. Opening the User Defined Grain Classes editor from the Sediment Data editor.
Figure 219. User Defined Grain Classes editor.
Figure 220. Selecting the Density Method in the Define Grain Classes and Sediment Properties editor.
Figure 221. Opening the Cohesive Options editor from the Sediment Data editor.
Figure 222. Cohesive Options editor.
Figure 223. Selecting the Hwang (1989) floc settling velocity formula.
Figure 224. Specifying a floc settling velocity as a function of suspended sediment concentration.
Figure 225. Example consolidation curve of dry bulk density as a function of time.
Figure 226. Opening the Transport Methods editor from the Sediment Data editor.
Figure 227. Schematic of sediment and current velocity profiles.
Figure 228. Setting the Total-load correction factor options in the Transport Model editor.
Figure 229. Setting the bed-load correction factor options in the Transport Model editor.
Figure 230. Setting the suspended-load correction factor method in the Transport Model editor.
Figure 231. Specifying a constant total-load diffusion coefficient.
Figure 232. Specifying a total-load diffusion coefficient based on weighted bed and suspended load coefficients.
Figure 233. Specifying a total-load adaptation length.
Figure 234. Specifying a weighted total-load adaptation length.
Figure 235. Specifying a suspended-load adaptation coefficient.
Figure 236. Opening the Transport Function Calibration and Modification editor from the Sediment Data editor.
Figure 237. Transport Function Calibration and Modification editor.
Figure 238. Opening the 2D Options editor from the Sediment Data editor.
Figure 239. 2D Options editor.
Figure 240. Setting the hindered settling method to the Richardson and Zaki (1954) method.
Figure 241. Accessing the Bed Mixing Options editor from the Sediment Data editor.
Figure 242. Bed Mixing Options editor.
Figure 243. Bed Gradation editor with an example grain size distribution.
Figure 244. Setting sample specific cohesive parameters in the Bed Gradation editor.
Figure 245. Right click on the Plan Description under Results and Select Create a New Results Map Layer to add Sediment results (or non-default hydraulic results).
Figure 246. Two types of sediment results (Bed and Transport) in the Results Map Parameter editor.
Figure 247. Example use of the animation bar to view sediment concentrations in time.
Figure 248. To edit the color ramp and display properties of the seidment maps, right click on the map and select Layer Properties. Then Click Edit under the Surface menu.
Figure 249. Setting the Max and Min to equal values (whichever abolute value is larger) with opposite signs, centers the Bed Change plot, making all depsoitin and erosion the same colors and the range of no change white.
Figure 250. Save customized sediment color ramps that persist throughtou the project with the Save Color Ramp button.
Figure 251. View a time series of a sediment (or hydraulic) result by selecting (chekcing) it in the RASMapper result tree and then right clicking on the cell. Select Plot Time Series and then the result map to launch the time series for that cell or cell face.
Figure 252. Right Click on the Feature menu to create a Polyline Layer. Give it a name.
Figure 253. Draw the polyline layer along the 2D sediment results transect.
Figure 254. Plot a bed change transect with a polyline profile plot.
Figure 255. Bed Gradation editor with an example grain size distribution.
Figure 256. General tab of the Sediment Computation Options and Tolerances editor.
Figure 257. Specifying the warmup periods for 2D simulations.
Figure 258. Specifying the warmup method for 2D simulations.
Figure 259. Outer loop Convergence Parameters.
Figure 260. Calculation of computational bed layer thickness from a single user-specified initial bed layer or bed gradation.
Figure 261. Calculation of computational bed layer thickness and composition from user-specified initial bed layers.
Figure 262. Computational Sediment Layer Parameters section in the HEC-RAS Sediment Computation Options and Tolerances editor.
Figure 263. Example of a power-law parameterization of the critical shear for erosion as a function of the dry bulk density.
Figure 264. Opening the Sediment Output Options editor from the Unsteady Flow Analysis editor.
Figure 265. Sediment Output Options editor.
Figure 266. Opening the User Defined Grain Classes editor from the Sediment Data editor.





List of Tables

TablePage
NumberNo.







Introduction

The Corp's Hydrologic Engineering Center River Analysis System (HEC-RAS) is designed to simulate one-dimensional (1D) steady, unsteady flow. The latest release of HEC-RAS V6.0 also simulates unsteady two-dimensional horizontal (2D) sediment transport, and bed change, sorting, and layering. Sediment transport is computed with a non-equilibrium total-load formulation. The total-load transport equation is solved with implicit Finite-Volume methods on the same unstructured polygonal mesh as the flow solver. Sediment transport is coupled to the flow model at the time step level. One powerful feature of the 2D flow solvers is that they use the subgrid topographic variations directly into the model thus improving the accuracy of the solution and permitting the use of relatively coarse meshes resulting in reduced computational times. The sediment transport model is designed to work within the subgrid framework of the flow model, and computes subgrid erosion and deposition rates, bed elevations, gradations, and bed layering.
This document discusses how to utilize the 2D sediment user-interface, the model input and output, and how-to setup and run a 2D sediment transport model in HEC-RAS. The document is intended as supplemental to the 1D Sediment Users Manual as many concepts and features are covered in detail in that document. Most of the 1D sediment capabilities are supported in 2D sediment and many new features have been added to 2D sediment which are not available yet in 1D sediment. Some of the new sediment features include variable density bed sorting and layering model, flocculation, consolidation, hiding and exposure effects, multiple new transport potential formula. However, as a beta release, there are still several computational and user-interface limitations and known issues including the inability to hot-start sediment, the inability to couple 1D and 2D sediment, the inability to modify terrains based on computed bed change, inability to visualize subgrid output directly in HEC-RAS, the inability to specify avalanching parameters in the user-interface, and the inability to specify subsidense in 2D areas. Lastly, the 2D sediment transport feature in HEC-RAS V6.0 is a beta feature and should not be used for design purposes.




Performing a 2D SedimentTransport Analysis