Basic Concepts

The 2D Diffusion Wave Transform method explicitly routes excess precipitation throughout a subbasin element using a combination of the continuity and momentum equations.  Unlike unit hydrograph transform methods, this transform method can be used to simulate the non-linear movement of water throughout a subbasin when exposed to large amounts of excess precipitation (Minshall, 1960).  This Transform Method can be combined with all Canopy, Surface, and Loss methods that are currently within HEC-HMS. 

However, only the None, Linear Reservoir, and Constant Monthly Baseflow Methods can be used with this transform method. 

2D Mesh

The 2D Diffusion Wave Method represents the subbasin using a 2D mesh which is comprised of both grid cells and cell faces.  Grid cells do not have to have a flat bottom and cell faces do not have to be straight lines with a single elevation.  Instead, each grid cell and cell face is comprised of hydraulic property tables that are developed using the details of the underlying terrain.  This type of model is often referred to as a “high resolution subgrid model” (Casulli, 2008).  The term “subgrid” implies the use of a detailed underlying terrain (subgrid) to develop the geometric and hydraulic property tables that represent the grid cells and the cell faces.  Currently, users must create a 2D mesh (and any associated connections) within HEC-RAS (version 5.0.7 or newer) and then import to HEC-HMS.  In the future, users will be able to create and modify both 2D meshes and boundary conditions entirely within HEC-HMS.  The 2D mesh preprocessor within HEC-RAS creates: 1) an elevation-volume relationship for each grid cell and 2) cross sectional information (e.g. elevation-wetted perimeter, area, roughness, etc) for each cell face.  The net effects of using a subgrid model such as this are fewer computations, faster run times, greater stability, and improved accuracy.  For more information related to the development of a 2D mesh, users are referred to the HEC-RAS 2D Modeling User's Manual.

The 2D Diffusion Wave Transform can only be used with Unstructured or File-Specified Discretizations.  An Unstructured Discretization can be created by importing a 2D mesh from an HEC-RAS Unsteady Plan HDF file using the File | Import | HEC-RAS HDF File option.  Unsteady Plan HDF files have extensions of ".p##.hdf" where "p##" corresponds to the specific plan of interest.  When importing a 2D mesh from an HEC-RAS Unsteady Plan HDF file, any accompanying boundary conditions for the selected 2D mesh (except for precipitation time series) will be imported and used to create new 2D Connections with the same parameterization.  If a File-Specified Discretization is used, the backing file must be in an HDF 5 format and created using either HEC-RAS or HEC-HMS.

2D Engine

HEC’s 2D engine solves the St. Venant Equations using physically measurable characteristics to route water on the overland surface (U.S. Army Corps of Engineers, 2022).  This engine makes use of an implicit finite volume algorithm which allows for advantages such as:

  • Larger computational time steps than explicit methods,
  • Improved stability and robustness over traditional finite difference and finite element techniques,
  • Efficient wetting and drying of 2D cells, and
  • Subcritical, supercritical, and mixed flow regimes.

Unstructured or structured computational meshes can be utilized within this engine that include triangular, square, rectangular, or even eight-sided elements.  Computational cells and cell faces are pre-processed to contain detailed hydraulic property tables including elevation-volume and elevation-conveyance relationships, amongst others.  This type of model is often referred to as a "high resolution subgrid model" (Casulli, 2008).

The 2D engine can be used to better recreate anticipated non-linear runoff responses when subjected to large amounts of precipitation when compared to unit hydrograph transform methods (Bartles, 2017).  However, 2D overland transform methods require additional data and are more computationally intensive than unit hydrograph transform methods.

For additional details regarding the fundamental equations and solution schemes employed within this transform method please see the HEC-RAS Documentation Page.

Required Parameters

Parameters that are required to utilize the 2D Diffusion Wave method within HEC-HMS include implicit weighting factor, water surface tolerance [ft or m], volume tolerance [ft or m], maximum iterations, time step method, use warm up period, and number of cores.  If the Adaptive Time Step method is selected, additional parameters are required including the maximum Courant number and maximum time step [seconds].  If the Fixed Time Step method is selected, the maximum time step [sec] must is also required.  If the warm up period option is enabled, additional parameters are required including the warm up period [hours] and warm up period fraction.

A tutorial describing a simple example application of this transform method, including parameter estimation and calibration, can be found here: Creating a Simple 2D Flow Model within HEC-HMS.

A tutorial describing a complex example application of this transform method, including parameter estimation and calibration, can be found here: Creating a Complex 2D Flow model within HEC-HMS.