Now that you are satisfied with your time-series data, you are ready to create a CWMS forecast. A CWMS Forecast is a simulation of watershed processes and consequences of potential flooding from various applications configured in CWMS. The hydrologic application, HEC-HMS, computes flow from forecasted and observed meteorological time-series. The reservoir operation application, HEC-ResSim, computes flow and release schedules. The hydraulic application, HEC-RAS, computes stage and inundation maps. The impact analysis application, HEC-FIA, computes flood consequences and action reports.

Prior to creating a CWMS forecast, you will have already configured CWMS. Specifically, you will have created Model Alternatives, assigned Model Alternative Keys (Model Linking Editor), defined the Program Order, defined Forecast Runs, and configured the Model Linking (Modeling Module).

For your routine CWMS forecasting, you might create a CWMS Forecast using a typical forecast run that includes something similar to the following model alternatives: normal conditions (for HEC-HMS), normal reservoir operations (for HEC-ResSim), normal river conditions (for HEC-RAS), and no flooding consequences (for HEC-FIA).

If a high rainfall event is forecasted, you might create a CWMS Forecast using a forecast run with model alternatives similar to the following: wet basin conditions (for HEC-HMS), restricted downstream channel capacity (for HEC-ResSim), high channel flow (for HEC-RAS), and evacuate with warning (for HEC-FIA).

In the Modeling module, you specify a forecast time window, create and manage forecast extract and post lists, extract time-series data for the forecast, create and manage forecast alternatives, edit selected model parameters, and run the models. Figure 1 shows the data flow concepts through the six standard CWMS analysis applications (HEC-MetVue, MFP, HEC-HMS, HEC-ResSim, HEC-RAS, and HEC-FIA).

Hydrologic Modeling (MFP, HEC-HMS) 

You use the meteorological preprocessors HEC-MetVue and MFP to combine observed and forecasted hydrometeorological data into a single dataset that can be used by the watershed runoff model, HEC-HMS.

Using the observed and future precipitation data, HEC-HMS computes runoff, including uncontrolled local flows that are typically used by HEC-ResSim.

The types of data you might typically change for a forecast are loss rates and baseflow.

Reservoir Simulation (HEC-ResSim)

After the upstream boundary flows and uncontrolled local flows have been computed by HEC-HMS, you will run the reservoir simulation application HEC-ResSim to use those flows during the operation of the reservoirs to compute regulated flows. Reservoir operations might include zones and rules for day-to-day operations, along with zones and rules for extreme events.

The types of data you might typically change for your forecast are the starting conditions for reservoirs. As long as the reservoir starting conditions are linked to real-time data (starting elevations or storages, reservoir or outlet releases, and any other boundary conditions needed by your model), then you shouldn’t have to make any revisions to your HEC-ResSim model in order to run your routine CWMS forecasts.

River Analysis (HEC-RAS)

Regulated flows computed by HEC-ResSim are typically used as boundary conditions to the hydraulic routing model, HEC-RAS. Computed flows from HEC-HMS or supplemental models can also be used as input to HEC-RAS. In turn, HEC-RAS computes stage, which is input to HEC-FIA.

The types of data you might typically change for your forecast are n values at selected cross sections to match profile information better throughout the lookback period.

Impact Analysis (HEC-FIA)

HEC-FIA analyzes impacts within inundated areas based on computed hydrographs from HEC-ResSim and stages from HEC-RAS. HEC-FIA calculates flood damage determined by rising stage in a stream, river, lake, or reservoir. HEC-FIA also computes action reports that tell responders what they need to do based on the computed stage values.

The types of data you might typically change for your forecast are adjustments to the levee failure stages to see the associated damage and impacts during an event.