The water quality libraries compute the kinetics of specified constituents within a discrete water quality cell. The complete mass balance involves adding a sediment layer to the computational grid. Therefore, a water quality cell consists of an aquatic element, representing the water column, and a bed sediment element, representing a biologically active sediment layer beneath the water column. These elements interact with each other across a sediment-water interface. Kinetics for the water column and active sediment layer are fully coupled in the Nutrient Simulation Modules (NSM), whereas these are not coupled in the General constituent Simulation Module (GSM).
The water quality model geometry (see Section 1.2) sets the Water Quality (WQ) domain, which includes the river-reservoir network connectivity and discretization information. The geometry dataset is based on an HEC-ResSim network, an HEC-RAS geometry shapefile, and a steady flow model output file (see Chapter 4 for instructions on creating and defining the geometry).
The HEC-RAS geometry shapefile is used to establish the water quality cells for the river reach. By default, a single cell is configured between HEC-RAS cross-sections. These default water cell configurations can be adjusted if necessary. For example, if very small water quality cells are surrounded by larger cells, this may present a challenging computational problem that could lead to instability in the water quality model. A single small water quality cell forces the model to choose a correspondingly small time-step in order to satisfy the Courant and Peclet conditions (discussed in more detail in Chapter 13). Smaller computational time-steps lead to longer simulation compute times. Therefore, if a small water quality cell is not necessary for the water quality analysis, 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.