Sediment - Bed IC

Owned by Kester Scandrett
Last updated: 23 Feb, 2024 by Jeffrey Jung
6 min read

The form provided options for the user to set the initial conditions for the sediment bed. Right mouse click on Sediment-Bed in  Model Control form to access the Initialization of Sediment Beds form as shown in Figure 1. Here two main options are provided, Grid Cells Options frame allows the user to control which cells the initialization of the bed is applied to. The Initial of Sediment Beds  frame provides options for setting the bed. These are described further below. 

Grid Cells Options

There are two options for determining which cells apply the sediment bed initialization: All grid cells and Only grid cells inside polygons as presented Figure 1. EFDC+ Explorer will use all of the cells in the model if the polygon file is not available. When selecting Only grid cells inside polygons, it is required to upload a polygon file (in P2D or DAT formats) that contains one or more polygons.


Figure 1.  Initialization of Sediment Beds.

Initialization of Sediment Beds

There are several ways to build sediment beds in EFDC+ Explorer as shown in the Initialization of Sediment Beds frame in Figure 1. The simplest option is to create a uniform bed. Other options include using a polygon digital surface model, using sediment cores with grain size, each of which is described below. 

Create Uniform Bed

To set up a simple horizontally uniform sediment bed for the model the user can use the with Inorganic Sediment Bed Constructor: Uniform Layers tool ( shown in Figure 2 ). This tool is accessed from the Create Uniform Bed button in the main Sediments tab. The Bed Constructor asks the user to input the number of layers, the number, and types of sediments, and allows the user to specify the sediment fractions, thickness, and bulk densities for each layer. The user must still set the cohesive and non-cohesive erosion, and deposition parameters, but once the user finishes this option, the sediment bed configuration is ready for EFDC.

In the Bed Layer Settings frame the user may specify the number of bed layers and subsequently the thickness, porosity (thus void ratio), and mass distribution by layer. Once the sediment bed is generated the user can modify the sediment bed properties in 2DH View as needed. The layer configuration settings can be saved and later retrieved using the Save and the Load buttons in the Bed Properties Definitions frame.

Figure 2.  Uniform Sediment Bed.


It is generally expected that creating a sediment bed is done in the early phases of generating a sediment model. EE will attempt to update parameters in the Morphology & Consolidation tab (under Sediment modules | Settings) such as the deposition porosity and cohesive void ratio to match the data entered in the Bed Constructor. This is not a bug and is behaving as expected.

Also, note that the Inorganic Sediment Bed Constructor: Uniform Layer tool adds two layers of "empty" sediment layers to allow for deposition and layer addition. The Maximum Layer Thickness setting in the Morphology & Consolidation tab controls when EFDC will create a new layer during deposition. If the top layer is already active and the thickness exceeds the maximum layer thickness, EFDC will just keep adding sediment to the top layer. This can result in a very thick layer, which is not a code problem but is a model configuration problem and should be avoided. This is expected behavior and is not a bug.

Use Polygon DSM

A very useful feature of EFDC+ Explorer is its ability to build the EFDC sediment files from a defined digital sediment model (DSM) that was generated by some third-party package (e.g., Spatial Explorer). This option is available by selecting Use Polygon DSM option in Figure 1. The DSM format requires a polygon followed by the layer thickness, bulk density, porosity, and grain size distribution for each depth available (sediment depth intervals are based on data). More information on the data structure is available in Appendix B - Data Formats. EFDC+ Explorer uses the DSM coupled with the number of size classes requested, the maximum size for each class, the number of sediment layers, and the layer options (e.g., minimum layer thickness) to build the EFDC sediment files. The "Max (μm)" sediment diameters, one for each sediment class, are needed to break the sediment grain size curves into ranges. These diameters are not class diameters but represent the grain size breakpoints whose geometric mean of the upper and lower limits is the corresponding sediment class' diameter.

When the user clicks on the Apply button after all the inputs have been provided, EFDC+ Explorer generates the sediment map in memory and then writes out the files. The current sediment map (if any) will be replaced by the new one. It is recommended that the project be saved into a new sub-directory prior to implementing this option in order to save different versions of the sediment bed configurations.

Sediment Cores with Grainsize                       

Another option provided in EE is the ability to build a sediment bed from sediment core data. Figure 3 shows an example of a digital sediment model derived from sediment cores. The data contained grain size distributions by depth interval. The black solid circle symbols show the locations of the cores. The plot shows the resulting depth-averaged d50 grain size.


Figure 3.  A digital sediment model generated from sediment cores with grain size.


Process by which EE Builds a 3D Initial Sediment Bed Condition Using Sediment Core Data

This description is provided to help users better understand how EFDC+ Explorer (EE) processes the sediment core data to configure a three-dimensional sediment bed for a model’s initial condition.

Once a user imports the sediment core data,

1. EE will calculate the total thickness of the sediment bed over the domain using the total sample depths from the sediment core data input. This assumes that the core samples were probed until refusal, meaning they reached bedrock. An inverse distance method is used to interpolate the total bed thickness across the entire domain in a horizontal direction.

2. Then, EE will determine the thickness of each layer of the sediment based on the user-defined parameters: Number of Initial Layers, Minimum Surface Sediment Layer Thickness, and Minimum Subsurface Layer Thickness.

  • The “Number of Initial Layers” specifies the number of layers in which the input sediment core data information will be applied. This parameter differs from “# Sediment Bed Layers” in the major settings (the total number of sediment bed layers to be simulated).*
  • Among the “Initial Layers” identified above, the top layer thickness is determined to be the same (and interpolated horizontally) as the thickness of the top samples in the input core data. But if the data top sample thickness is smaller than the user-defined “Minimum Surface Sediment Layer Thickness,” then EE will use the user-defined parameter value for the top layer thickness.
  • Next, the layers beneath the top layer will have a uniform thickness, calculated as (total thickness – top layer thickness) divided by the number of the below layers. This computation considers the user-defined "Minimum Subsurface Sediment Layer Thickness," which might lead to an adjustment in the total layer thickness determined in Step 1.

3. Then, EE will define the properties for porosity, wet density, and sediment class mass fractions (specified based on the max size table) for each sediment bed layer (vertical variation) of each model cell (horizontal variation).

  • The linear interpolation is used to convert the core data input values over sample depths to the modeled bed layers in a vertical direction.
  • The inverse distance method is used to interpolate the core data input value across the entire domain in a horizontal direction.


Figure 4.  Core Sample Data and Sediment Bed IC.


* We suggest setting the “Number of Initial Layers” to be smaller (2 or more) than “# Sediment Bed Layers” for computational stability purposes. For example, if the “# Sediment Bed Layers” of 10 is used, it is recommended to set the “Number of Initial Layers” as 8 (or less). In that case, the input sediment core data information will be assigned to the bottom 8 layers (Initial Layers) of the 10 layers.  As an initial condition, the remaining top 2 layers will be defined as empty layers (zero thickness with no sediment mass).