Determining which flood events should be evaluated isn’t always straightforward—especially for smaller, less obvious events. For major floods, the decision is usually clear. But for moderate or borderline events, identifying what qualifies as “damaging” requires thoughtful analysis.

Event Selection vs. Full-Year Screening

While you could run the full suite of CWMS models across the entire water year to detect damaging events under both regulated and unregulated conditions, this approach is data-heavy and time-consuming. It's not practical for annual reporting.

Instead, save the full modeling workflow for:

  • Analyzing specific flood events already flagged for potential inclusion in the AFDR report to Congress.
  • Generating detailed results once an event has already been identified as damaging.

Key Questions to Consider When Selecting Events

  1. Where are your critical control points and major economic centers?
    1. Identify flows/stages at which these locations begin to incur damages (e.g., infrastructure impacts, floodplain encroachment, agricultural loss).
  2. What are your district’s rebuild and replant timelines?
    1. This matters especially for back-to-back events. For example:
      1. Midwest: 60-day rebuild, 14-day replant.
      2. Gulf/Atlantic coasts: potentially annual due to hurricane season.
  3. How are you calculating unregulated flows?
    1. These help estimate what would have happened without flood control infrastructure in place, critical for assessing the value of the project.

Leveraging Daily CWMS Operations Runs

If you’ve configured HEC-ResSim to compute unregulated flows as part of your daily CWMS operations, you're already set up for an efficient screening process.

Here's how to use it:

  • Review regulated vs. unregulated flow comparisons at key control points.
  • If unregulated flows exceed damage thresholds (e.g., bankfull, flood stage, crop saturation), flag the event for further analysis.
  • Use these comparisons to quickly scan for potential FDR events without rerunning the full model suite.

Complex Scenarios

In large watersheds or those with distributed impacts, things get trickier.

For example:

  • One event might cause damage upstream.
  • A different event might impact downstream areas weeks later.

How do you classify them—as one event or multiple? This is where rebuild and replant times become critical in determining whether the region had time to recover between events.

Collaboration is Key

Water managers and economists should work together when identifying events. Input from both groups ensures that selected events are:

  • Hydrologically significant
  • Economically impactful
  • Within the practical scope of analysis

Example Scenarios

Here are a few example scenarios to review.

Example 1 - A Quiet Year with Isolated Impacts

The year has been relatively calm. Reservoirs handled moderate floods well, and no significant downstream damage occurred (thanks to regulation).

At your quarterly AFDR meeting, the team reviews the last four months using your daily CWMS operations model. Here's what you find:

  • Two similar-sized events caused potential damage at upstream control points, 45 days apart.
  • One larger event caused potential damage basin-wide, including the downstream control point.
  • Your district uses a 60-day rebuild and 14-day replant window.

What decision should the team make?

You decided to evaluate the larger basin-wide event for inclusion in the FDR report. The upstream events are too close together to count as separate recovery periods—but one of them might still be considered for agricultural damages, if it overlapped with planting season and was severe enough.

Example 2 - Back-to-Back Major Events

Multiple back-to-back storms fill the reservoirs deep into their flood pools. Minor flooding occurs from unregulated local flows downstream. 30 days after the event, the H&H team meets. Flows are now below flood stage, and reservoir levels have drawn down. You’ve been running daily CWMS models and have good data coverage.

Although several distinct rain events occurred, the team determines:

  • The main 4-week window of high-impact flooding should be evaluated as a single FDR event.
  • Due to the duration and lack of sufficient recovery time, no separate agriculture events can be run.