Assigning Initial Conditions Based on Data
Setting initial conditions is an essential part of the modeling process because these settings determine the stability of the model at the beginning of a simulation and the accuracy of the output results. The choice of initial conditions depends on the available information about the system. EEMS provides valuable and flexible options for establishing initial conditions. This blog will outline how to configure the initial conditions for bathymetry and water depth/elevation in EEMS.
Bathymetry
Modelers are often required to assign the initial bathymetric conditions across the entire model domain. The form used to assign the initial bathymetry conditions is shown in Figure 1. This option is set by clicking the Bathymetry item in the menu in the Initial Conditions form. Click on the Assign button, and the Apply Cell Properties: Bottom Elevation will then be displayed. There are two approaches for setting the Grid Options: All grid cells or Only grid cells inside polygons. The All grid cell option will typically be selected for the bathymetric setup. In the next step, the Values to Set need to be identified as either a constant value or Scatter (XYZ) data. Constant values should only be used in testing models. In real models, scatter data (XYZ) should be selected when available, as this will provide a realistic representation of the natural variability of the bed.
In the context of interpolation options, four distinct methods are available. If the XYZ data is insufficiently dense to provide at least one value for every relevant model cell, then the Centroid Interpolation option, based on nearest neighbor interpolation, is the preferred choice. Conversely, when the XYZ data is significantly denser than the model cell size, employing cell averaging, specifically the Average Value Interpolation option, is the best approach. The remaining options, Maximum and Minimum Value Interpolation scan the data to select the highest and lowest values within the model cell. The Max Value option scans the data and applies the maximum value found to that cell, while the Min Value option scans the data and applies the minimum value found to the cell.
The bathymetry file with the correct format for the entire model domain must be prepared. This format can be created using Excel or a text editor, with three columns that include the coordinates X and Y and the elevation Z for each grid cell. After inputting the file, click the Apply Defined Conditions button to apply (Figure 2).
Download Online Data
In EE 12.0, you can download data from GEBCO (General Bathymetric Chart of the Oceans) 2021, 2014, and 2008 open sources. The GEBCO 2021 is a global dataset providing elevation data on a 15 arc-second grid. It contains 3,732,480,000 data points and a Type Identifier (TID) Grid for source data (GEBCO_2021_Grid.pdf). The dataset covers land and ice elevations and offers under-ice topography for Greenland and Antarctica. GEBCO 2014 is a comprehensive ocean depth chart developed by the International Hydrographic Organization (IHO) and UNESCO, providing detailed bathymetric data for marine research and navigation (Gebco_2014.pdf). GEBCO 2008, an earlier version, serves as a valuable reference for seafloor topography and bathymetry data published in 2008 (The GEBCO_08 Grid). However, you should use these data sets only if you don’t have access to more robust bathymetry since, these resources may have data of low quality or no data in some areas.
To set this option, click on the Bathymetry item in the menu within the Initial Conditions form (Figure 3). Next, click the Download button, which will prompt the display of the Download Online Data interface. Within the Data Set section, you can choose from three sources, including GEBCO 2021, 2014, and 2008.
The Spatial Information section and Data Extraction Limits are initially configured to cover the entire model domain, but you can manually select a specific area by enabling the Using Polygon File option. After making the selection, click the Download button. You can also save the extracted data to a file by checking the Save Log File checkbox before initiating the download.
Water Depth/Elevation
Along with bathymetry, water depth or elevation is crucial for determining the initial stability of the model at the beginning of a simulation. The depth must be identified at a specific point in time during the simulation, and it can be challenging to determine the optimal value. Therefore, it is recommended that you reevaluate the initial conditions after the first or second run, depending on the output results.
The form used to assign the initial water depth or elevation conditions is depicted in Figure 4. To access this form, click the Water depth/elevation item and then the Assign Elevation button. This opens the Apply Cell Properties: Water Surface Elevations form. From this point onward, you will have the same options and should follow the same steps as those outlined in the setup for bathymetry described above.
Users may encounter several common problems in this process. Data accuracy is paramount, as errors can lead to unreliable simulation outcomes. Balancing data resolution and ensuring it matches the application’s needs without being overly detailed, is a constant concern. Maintaining data consistency, units, and coordinate systems, particularly when integrating datasets, is crucial. Users must also consider data quality, credibility of sources, and the potential for temporal variations. Addressing these issues requires careful data handling and user awareness of limitations and uncertainties.
Do you want to try these options for yourself? You can start by downloading EEMS and activating in the free demo mode and then running our demonstration model. To see these features in action, head over to our YouTube page.
Please contact us if you have comments or questions. For more information on EFDC+ capabilities, contact the DSI team today.
