Erosion Rates Option 3
Users may use the erosion rates option 3: Equation (E = A*Tau^n) and Bed Properties Using SEDB, BEDLAY, BEDBDN, and BEDDDN, if sediment core datasets are available in addition to the SEDFlume datasets (Figure 1). The sediment core samples present physical characteristics of sediment bed such as porosity, specific gravity, and grain size fractions with respect to sampling depths. Such sediment core samples are usually collected with much higher spatial density and much wider spatial coverage compared to SEDFlume samples. With the erosion rates option 3, the user can use sediment core datasets when configuring initial conditions for physical properties of sediment bed and, at the same time, use SEDFlume datasets when specifying erosion rate properties of sediment bed in the model.
For physical properties of sediment bed, EFDC+ with the erosion rates option 3 uses below input files to configure the initial model conditions for spatially varying physical properties of sediment bed layers, and these files can be created using sediment core datasets by a sediment bed construction tool in EFDC+ Explorer.
sedb.inp: fraction of each sediment class
bedlay.inp: layer thickness
bedbdn.inp: wet density
bedddn.inp: porosity
Note: The erosion rates options 1 and 2 specify the physical properties of sediment bed using SEDFlume datasets (written in erate.sdf file). For example, there are only three valid SEDFlume cores (riverine, floodplain, and marine) in this SEDZLJ example model, so the bed properties in the entire model domain are represented by using only those three data sets.
For erosion properties of sediment bed, the erosion rates option 3 employs the same approach as the erosion rates option 2. Specifically, the SEDFlume datasets provide the measurements of erosion rates as a function of applied shear stress, and the relationship between erosion rates and shear stress can be defined using Jones and Lick (2001)’s approach as:
where E is erosion rate (cm/s), τb is bed shear stress (N/m2), τcr is critical shear stress (N/m2), and A and n are the fitted parameters based on the SEDFlume core data. With the erosion rates option 3, then, the SEDZLJ algorithm in EFDC+ computes the erosion rates using Equation (1) based on user-defined three erosion rate parameters (A, n, and τcr) and a maximum erosion rate (Emax), which are obtained from the SEDFlume study. The maximum erosion rate is employed for each bed layer in order to prevent the prediction of unrealistically high erosion rates, which might occur due to the exponential formulation of Equation (1) when a high shear stress is applied.
Figure 1 Selection of Erosion Rates Option 3: Equation (E = A*Tau^n) and Bed Properties Using SEDB, BEDLAY, BEDBDN, and BEDDDN - General Tab.
The files sedb.inp, bedlay.inp, bedbdn.inp, and bedddn.inp specify the initial conditions for sediment bed in the model, and the user can create those files using a sediment bed construction tool in EFDC+ Explorer. To assign the sediment bed initial conditions, the user may click Assign Bed IC button in the Erosion Rate Options frame in the General tap (Figure 1), then the Initialization of Sediment Beds window will appear as shown in Figure 2.
Two options are available in the Grid Cell Options frame to specify the region for sediment bed initialization:
All Grid Cells: EFDC+ Explorer will initialize all grid cells in the model domain using a single sediment core file
Only Grid Cells Inside Polygons: EFDC+ Explorer will initialize the only grid cells inside the applied polygons using a sediment core file.
In the Initialization of Sediment Beds frame, among three available options (Use Polygon DSM, Use Sediment Core with Grainsize, and Create Uniform Bed), the user needs to select the Use Sediment Core with Grainsize and browse a sediment core file (*.dat) that contains sediment bed sample properties (e.g., sample location, sample depth, layer thickness, porosity, specific gravity, and grain size fractions). The ways to produce the sediment core file (*.dat) and initialize the sediment bed are described in detail at Sediment Bed Construction.
Figure 2 Erosion Rates Option 3 - Initialization of Sediment Beds.
The Erosion Rate Matrix (cm/s) for Active & Deposited Sediment Bed in the Erosion Rates tab (Figure 3) is used as a lookup table for calculating erosion rates in active & deposited bed layers as a function of their D50 size during the model simulation. The user may see that the first row of this matrix presents the Erosion Multiplier (A), Erosion Exponent (n), and Maximum Erosion Rate (cm/s). To fill this lookup table, the user may utilize the SEDFlume core test results or can adjust them manually for model calibration.
Note: The lookup table for critical shear stresses (τcr) in active & deposited bed layers is specified in the Sediment Bed tap.
Figure 3 Erosion Rates Option 3 - Erosion Rates Tab.
After completing the Erosion Rates tab, the users may enter the SEDFlume core datasets to the Core Definitions tab (Figure 4), which would be used for defining the physical and erosion properties of parent-bed layers in a model.
The user may enter the Number of SEDFlume Cores and then assign the measurements for physical and erosion properties associated with each core. For example, if the SEDFlume data consists of the measurements for three sediment cores, the user can set the Number of SEDFlume Cores as three, then the drop-down will be populated with three cores in the Sediment Bed Characteristics frame. The name of each core in the drop-down box can be modified by the user manually.
Water Density can be specified as 1 g/cm3, and Sediment Density can usually be determined as 2.65 g/cm3.
In Bed Layer Properties and Grain Size Distribution frame, the user may enter the physical properties of each SEDFlume core such as critical shear stress (dynes/cm2), dry bulk density (g/cm3), layer thickness (cm), and grain size fractions (%) for each layer. As discussed above, with the erosion rate option 3, EFDC+ will not use the dry bulk density, layer thickness, and grain size fraction data from the SEDFlume cores entered here (instead, EFDC+ uses the sediment bed properties written in sedb.inp, bedlay.inp, bedbdn.inp, and bedddn.inp files for model simulation).
In SEDFlume Measured Erosion Rates matrix, the user may enter the erosion properties measured from each SEDFlume core such as Erosion Multiplier (A), Erosion Exponent (n), and Maximum Erosion Rate (cm/s) for each vertical layer.
The sampling location of each SEDFlume core also needs to be entered in Easting (m) and Northing (m) Coordinates for utilizing the Core Assignment features of EFDC+ Explorer.
Figure 4 Erosion Rates Option 3 - Core Definitions Tab.