An analysis period defines the simulation time window for a model alternative in an HEC-WAT simulation. Analysis periods are required to create simulations, and an analysis period can only be used once per HEC-WAT alternative. An analysis period can span single or multiple historical hydrologic events or can be used to define the lifecycle length within a Flood Risk Analysis (FRA) simulation.
An analysis period for a historical event, such as the flood of record on a particular river, should be defined so that the start and end times span the full historical event; specifically, the start time should capture the precipitation that drove the flood event and the end time should capture the resulting flood wave and corresponding damage. An analysis period for multiple events, also called a continuous simulation, can be defined by creating one analysis period that captures all the flood events within a continuous historical record (e.g. Oct 1,1950 to Sep 30, 1995)
An analysis period for an FRA simulation is different than when modeling historical events. The analysis period for an FRA simulation is used to set the length of each lifecycle within the FRA compute. Typically, lifecycles are defined to support the USACE requirements for lifecycle analysis and are fifty years in length. If the FRA compute does not analyze damages and a lifecycle analysis is not required, the analysis period can be an arbitrary length; however, 100- or 500-year lifecycles are optimal for results management.
The user can modify the analysis period (time window) through plugins like the Time Window Modifier Plugin or the Time Window Interval Generator Plugin. The Time Window Modifier Plugin provides the user the ability to modify a model's simulation time window based on flow, stage, or elevation during an FRA simulation, while the Time Window Interval Generator creates the ability to evaluate results separately for each year of the analysis period within a continuous simulation (non-FRA). Alternatively, the user can make time window adjustments in the simulation editor. Once the user has defined an alternative (such as the Without Project Conditions alternative), the next step is to create analysis periods (review Creating and Managing Analysis Periods for more information).