Sediment Transport Analysis in HEC-RAS

Introduction

This blog post will introduce the steps involved in performing a simple sediment transport analysis using the HEC-RAS program.  HEC-RAS is capable of mobile boundary, and sediment transport modeling. A boundary is mobile when flow carries a lot of sediment through suspension and is in contact with the streambed. In these cases, the following aspects of the channel change with space and time: depth, bed with, and longitudinal slope.

An HEC-RAS sediment model requires a geometry file, a flow file (quasi-unsteady or unsteady), a sediment data file, and a sediment analysis plan file. This blog post will focus on creating all these files except for the geometry file.

Sediment Transport Modeling in HEC-RAS

Enter Geometric Data

Firstly, enter one-dimensional (1D) geometric data as you would for any hydraulic model. Before entering sediment data, ensure that your hydraulic model runs without errors.

Flow Data

Next, enter the flow data (unsteady or quasi-unsteady). Although unsteady models more accurately represent real-world conditions, quasi-unsteady models are more stable. Therefore, I will focus on quasi-unsteady models in this blog post. However, you should apply unsteady flow data if you are modeling a feature with significant storage such as a reservoir or lake. In addition, unsteady flow data is appropriate for complex flow boundaries such as groundwater interflow, internal boundary gate controls, and lateral structures.

Quasi-unsteady flow represents a continuous hydrograph with a series of discrete steady flow profiles. The result is a histogram.

For quasi-unsteady flow, HEC-RAS divides each steady flow profile into three time steps. These time steps are entered as a Flow Series boundary condition at the upstream end of the reach.

The coarsest time step is flow duration. Flow, stage, temperature, and sediment are constant over this time step. Secondly, flow duration is further subdivided into computational increments, which are the primary quasi-unsteady flow hydraulic and sediment time step. HEC-RAS updates the bed geometry and hydrodynamics after each computational increment. Finally, computational increments are divided into bed mixing time steps. HEC-RAS updates the composition of the bed mixing layers (e.g., active, cover, inactive) during each bed mixing time step. Note that sediment transport depends on water viscosity. Therefore, you should also enter temperature data in the quasi-steady flow data.

For the downstream boundary condition, users can enter three boundary condition types. These include normal depth, stage series, and rating curve. Models should only include one downstream boundary condition. In addition, users can enter internal boundary condition types. These include lateral flow series, uniform lateral flow, and internal stage boundary condition (BC).

Enter Sediment Data

When entering sediment data, HEC-RAS requires the Initial Conditions and Transport Parameters tab and the Boundary Conditions Tab. In contrast, the USDA-ARS Bank Stability and Toe Erosion Model (BSTEM) tab is optional.

Firstly, I will discuss entering the data on the Initial Conditions and Transport Parameters tab.

First, use the dropdown menu to define the number of mobile bed channels for a reach. For example, the image below illustrates a multi-channel cross-section. In this case, there are 3 mobile bed channels.

Next, select the Transport Function, Sorting Method, and Fall Velocity Method. There are several choices for each of these methods. You can learn more about each equation set/method in the HEC-RAS User’s Manual. Finally, click the Define/Edit Bed Gradation button and create a new Bed Gradation Template by clicking the little paper symbol. Then enter gradation information.

Secondly, enter data in the Boundary Conditions tab. Users can enter a Rating Curve, Sediment Load Series, or Equilibrium Load. Unlike the quasi-unsteady flow boundary conditions, HEC-RAS requires sediment data at the upstream end of each reach. In addition, users can include optional, local, and lateral sediment loads.

Run Sediment Transport Analysis

Finally, run the sediment transport analysis.