The Cells/Layers tab (Figure 4.29) allows the user to adjust the default water quality cell size. By default, for reaches the water quality cell length (or segments) is defined by the HEC-RAS cross-section and for reservoirs the layer height is set to 3.0 feet (ft), or 0.91 meters (m) depending on the watershed units of measure and the watershed display units. The water quality computational points are located exactly in the center of the water quality cell/layer.
Figure 4.29 – Geometry Editor – Cells/Layers Tab
From the Cells/Layers tab (Figure 4.29), the water quality cells, and reservoir layer heights can be viewed and adjusted either globally (for the selected geometry), or by individual reach(es) or reservoir(s). Note, the watershed units of measure and the watershed display units determine the display units for length and height in the Cells/Layers tab (Figure 4.29).
In many cases, the water quality model can be run using this default configuration. However, in situations where hydraulic cross-sections have been placed very close together, some of these water quality cells may be very small. Small water quality cells surrounded by larger cells may present a challenging computational problem that may lead to instability in the water quality model. Furthermore, a single small water quality cell will force the model to choose a small computational time step that will lead to longer simulation times. Therefore, if a small water quality cell is not necessary for the purpose of the water quality analysis, then the user may wish to group small water quality cells (created by default from HEC-RAS cross-sections placed close together) into larger water quality cells.
The Cells/Layers tab (Figure 4.29) is divided into three panels. The top, Cell Length panel, contains two radio buttons for modifying the water cell lengths. The middle, Reservoir Layers Heights panel, contains an Edit (Figure 4.29) button for modifying the reservoir layer heights. The bottom of the Cells/Layers tab (Figure 4.29) contains the Show Table of WQ Cells (Figure 4.29) button for viewing the individual water quality cell lengths by HEC-ResSim reach and HEC-RAS cross-section. The following sections provide instructions for viewing and editing the water quality cells, and reservoir layer heights.
