Time Step Considerations in Bentley Storm and Sanitary Sewer Models
The Bentley storm and sanitary sewer models analyze time varying (dynamic) conditions (with the exception of the steady solver in the Gradually Varied Flow (GVF) rational solver (StormCAD solver)). Each of the hydraulic solvers:
has its own way of dealing with time step size and different implications of time step size. In addition, if catchment rainfall hydrology is being modeled there are additional different time steps for the hydrology calculations and rainfall data.
There are numerous time steps that can be set in the models. They are generally set in the calculation options of the scenario, although some are properties of the individual catchment or storm. In addition, there are some internal time steps which are based on other parameters such as the time of concentration or the T in the RTK method. The time step choices are described in the sections below.
Hydraulic solver time steps
The explicit (SWMM) solver needs the smallest time step because of potential instability in the explicit method. The default of 0.00833 hr (30 sec) is set as its "Routing time step".
The implicit solver can use a somewhat larger time step and its "Calculation time step" has a default value of 0.025 hr (1.5 min).
The GVF convex solver solves for flow and other hydraulic properties separately and therefore has two different time steps sizes:
"Hydrologic routing time step" which has a default value of 0.1 hr, used for routing gravity flow, "Hydraulic time step" with a default of 1 hr, used as the time step for pressure calculations and the time at which flow is converted into hydraulic properties such as depth and velocity in gravity flow elements.
Because the hydraulic time steps in the GVF convex solver are large and pumps can change on/off status any time within a time step, the pressure portion of the solver will insert additional time steps at times corresponding to pump status changes. Results are only calculated for the pressure subnetwork in which the pump which changed is located. No other subnetworks have results at these times and profiles may not look complete at these times.
Results time steps
Calculations are performed at small time steps which may give results at a finer time resolution than needed. To reduce results file size and improve display performance, the solvers provide ways to only output results at larger time steps.
In the implicit solver, the user would specify the "Output increment" with a default of 0.05 hr (2 default calculation time steps). In the explicit solver, the user would specify the "SWMM output increment" with a default of 0.25 hr (30 default routing time steps).
The GVF-convex provides considerably more flexibility in specifying the "Reporting time step". It can be "All" which corresponds to all hydraulic time steps plus pump switch time steps (default) or either a "Constant" time step which is some multiple of the hydraulic time steps or "Variable" which enables the user to turn off results for periods of time or switch the "constant" time step part way through a run.
Rainfall-runoff hydrology time steps
Rainfall runoff hydrology calculations at catchments and manholes occur at different time steps than the hydraulic solvers. These results are generally interpolated to match hydraulic time steps. (If rainfall runoff is not being calculated, these time steps are ignored.)
There are two general types of hydrology calculations: those using SWMM for calculations and those using the Bentley hydrology engine developed under PondPack for calculations.
SWMM hydrology is used when the user specifies the Runoff method in the catchment properties as EPA-SWMM Runoff or when a manhole has the property "Apply SWMM RTK Unit Hydrograph" set to True. It uses the calculation option of "SWMM Hydrologic Increment" set in the calculation options with a default value of 0.25 hr. The SWMM hydrology calculations have an additional time step, call "Dry time step" which is used for runoff calculations during dry periods.
When any other runoff method is used (e.g. modified rational, unit hydrograph), then the results hydrograph flows from the catchment hydrology are produced at the output time increment for the hydrology solver.
However the actual increment used internally for calculating the hydrograph differs from the hydrograph output time steps and depends on the Runoff method selected. The SCS hydrograph uses 0.1333 tc , where tc is time of concentration for that catchment. The RTK method uses 0.2 T where T is the smallest T value for that catchment in the RTK tables. The Generic unit hydrograph uses a user defined "Convolution time step" which has 0 for a default value. A fatal error is issued if either zero or a large value are given. It is best to set that value equal to the rainfall increment or some small multiple of that value. The Modified rational method uses only 4 points corresponding to the trapezoidal shape of the modified rational hydrograph. See the individual help topics for details on these methods.
Storm event increment
The storm event increment is usually determined by the raw precipitation data when the data are imported from a rain file or are set by the user in the Increment field when manually creating a precipitation vs. time table (either incremental, cumulative or intensity).