Written by Aaron A. Lee | WEST Consultants
I’ve gotten a chance to play around with the alpha version of HEC-RAS 4.2 and check out the program’s new two-dimensional (2D) modeling capabilities. From what I’ve seen this will be a really useful feature! Being the alpha version of RAS2D there are still features in development, and is bound to be a bit “buggy”, but it is definitely worth looking forward to.
What is it?
The new build of RAS will allow users to connect 2D flow elements to a 1D river system. You will now be able to model overland areas as dynamic, 2 dimensional grids, rather than level pool storage areas. The figure below shows the 1D and 2D features together in the geometry window.
- The 2D area can be drawn in as a polygon (much like a storage area)
- The grid cells are automatically generated at a user-specified size within the 2D area
- Cells can be added, removed, or edited manually
How does it work?
The 2D mesh and the 1D system are tightly coupled during an unsteady simulation. This means that water surface elevation is calculated at each XS and each grid cell for every timestep, allowing direct feedback at the connections. 2D flow areas can be linked to the 1D system the same way storage areas are. The Figure shown below is a schematic of how the 2D mesh is built, and how RAS routes flow from cell to cell.
Upon creating the 2D mesh, you need to load a digital terrain file (.flt format). Both the pre- and post-processing steps for the 2D flow area are done through RAS Mapper.
- RAS pre-processes the 2D mesh separately from the 1D system. During this process RAS creates an elevation-storage curve for each cell, and calculates hydraulic properties for each cell face. These hydraulic properties are similar to the cross section hydraulic properties (HTAB curves).
- The Cell Center is where water surface elevation is computed for the whole cell.
- Cell Faces control flow between cells by acting as a detailed XS. Station/elevation data is captured directly from the underlying terrain file.
- Cell Face Points are used for stationing to connect to a lateral structure. They also represent the ends of cell faces.
- Manning’s n-values for each cell will be assigned by a spatially varied polygon or manually entered.
Computationally, RAS will allow the user to choose between using 2D Diffusion Wave equations (default), or the full 2D Dynamic Wave equations. Most flood applications should be adequately modeled using the Diffusion Wave equations.
What are the advantages?
Besides computing in 2-dimensions, the main advantage is the program’s ability to maintain computational robustness while preserving the details of the underlying terrain. Smaller features (i.e. drainage ditches) that run through large cells will be captured in the hydraulic properties of the cell faces. Therefore, these features will be preserved and accounted for both computationally and visually, even though they are smaller than the grid cell size, as demonstrated in the RAS2D output displayed in the figure below. Traditionally, many 2D models require cell size to be consistent with the size of the features to be included. Not the case with RAS2D.
Additional advantages include:
- Cells can be any size and shape. This allows the user to model odd-shaped features within the 2D flow area as well as provide more computational detail around areas of interest.
- Faster computation times. RAS uses an implicit scheme to calculate water surface and flow at each XS and cell simultaneously. The implicit scheme is also more stable, which allows for larger cell sizes.
- Detailed mapping. RAS Mapper will be able to post-process results to map depth and velocity grids on detailed terrain.
- RAS2D will be able to utilize multiple processors (if available).