Levee Assessment Workshop

Objective

(105 mins) – In this hands-on workshop participants use their previously generated HEC-FDA study files to evaluate the economic feasibility and project performance of various levee alternatives. Participants recommend the “best" alternative.

Downloadable Workshop Materials

Software Version

HEC-FDA Version 2.1 Beta will be used during this course.  Download the portable version of the software by following the instructions provided, here: Download the HEC-FDA Software. Detailed instructions are included on this page.

Workshop Datafiles

Download Zipped Workshop Datafiles:  

  • Review the Example Study Data to familiarize with some of the data in this workshop. Note: This workshop is the only workshop that deviates from the Example Study Data, because we need more than one levee in our FDA study to meet the workshop objectives.
  • The starting point for this Workshop 5 is the resulting study files from Workshop 4. You may EITHER:
    • Use your own study files from Workshop 4, OR 
    • Download the solution files at the bottom of the Workshop 4 page 

Workshop Overview

The purpose of this workshop is for students to be able to:

  • Evaluate three levee plans (aka. scenarios) for an impact area during analysis of the without-project condition.
  • Formulate and evaluate alternative levee sizes for the impact area.
  • Compute and review results for the base year and most likely future year
  • Specify a recommended levee plan based on the economic and hydraulic performance formulation/evaluation results.
  • Determine if the recommended plan (scenario) meets present FEMA floodplain levee certification criteria.

At the conclusion of the workshop the student will be able to perform a conventional levee analysis as required by present USACE guidance and ascertain if the recommended plan, if any, meets Federal Emergency Management Agency (FEMA) criteria for levee certification.

Introduction

Having completed Workshops 1-4, you should have the ability to model an alternative. Provided the inputs, you can produce an estimate of equivalent annual damages for multiple alternatives to compare to the without-project existing conditions. In this workshop, you'll model two different sized levees. You'll be asked to think critically as you compare the results of the two levees against the without-project conditions and consider which levee you might choose if you were so tasked.

A levee alternative is modeled similarly to the way you modeled the without project conditions. To model a levee, you need to:

  • Create Lateral Structure - a lateral structure indicates the levee's system response function, or the relationship between stage and levee failure
  • Create new Stage-Discharge Function - with a levee in place, stage for a given discharge increases in the channel
  • Create new Scenarios - two new scenarios per levee are required, base and future year
  • Create new Alternative - with new base and future year Scenarios, equivalent annual damages can be estimated

The proposed levees have heights that reach elevations of 946 ft and 948 ft. The levees are being proposed to benefit the Left-Bank Impact Area and have the exact same footprint, with only the height being different. Only one of the two levee options can be chosen.

Instructions

Task 1 – Open the Starting Workshop 5 Project

  1. Open the HEC-FDA Version 2 software. 
  2. Save and unzip the provided Workshop 4 solution file.
  3. From the File menu, click Open. The Open Study dialog opens as a tab.
  4. Click the ellipse button in the Study Path box to open the Select File browser window.
  5. Navigate to the Workshop 5 starting project (e.g., C:\Workshops\FDA\startingWS\), select the *.sqlite HEC-FDA file and click Open.
  6. From the Open Study tab, click OK.

Alternatively, just reopen your Workshop 4 solution. From the File menu select the top recently opened study (number 1 in the list).

Task 2 – Enter Stage-Discharge Functions for With-project Conditions

Steps

  1. From the Study Tree, right-click on Stage-Discharge Functions and select Create New Stage-Discharge Relationship. The Create New Stage-Discharge Relationship opens as a tab.

  2. Enter a name for the relationship in the Name box (e.g., Rating Curve With Project), and enter a description for the relationship (e.g., The stage-discharge function below represents the with-project condition. Observe that some stages are higher for a given discharge.).

  3. For Distribution Type, select Triangular and use the information in Table 1 to create the new stage-discharge relationships for the with-project condition.

    Table 1

    Loading

  4. Click Save.

Task 3 – Enter the Three Lateral Structures

For each levee, follow the instructions below to create two new lateral structures.

  • From the Study Tree, right-click on Lateral Structures and select, Create New Lateral Structure. The Create New Lateral Structure dialog opens as a tab. Enter the appropriate levee system name, description, top of levee elevation, and system response curve, and Save each levee.

  • Use the Create New Lateral Structure dialog to add the following two levees:
    1. Name: Levee 1; Description: With-project scenario for proposed levee 1 for the left-bank with a top elevation of 945 feet.
      1. Top of Levee Elevation: 945
      2. System Response Curve: Default
      3. Click Save and Close to close the dialog.
    2. Name: Levee 2; Description: With-project scenario for proposed levee 2 for the left-bank with a top elevation of 946 feet.
      1. Top of Levee Elevation: 946
      2. System Response Curve: Default
      3. Click Save and Close to close the dialog.
    3. Name: Levee 3; Description: With-project scenario for proposed levee 3 for the left-bank with a top elevation of 947 feet.
      1. Top of Levee Elevation: 947
      2. System Response Curve: Default
      3. Click Save and Close to close the dialog.

Task 3 (A) – Create Base Year Scenarios for the Two Proposed Levees

The Without Base Year and Without Future Year scenarios have been created for you, but now you need to create two new scenarios for the base year, one for each levee. Table 2 provides an overview table of the two new base year levee scenarios.

Table 2: Base Year Levee Scenarios Overview
ScenarioYearImpact AreaStage-DamageFrequency RelationshipStage-DischargeLateral StructureThreshold Stage
Base Year Levee 12031Left-BankBase Year Without ProjectFlow-Frequency LPIIIRating Curve With ProjectLevee 1 (or 945 ft Levee)(set by selected lateral structure)
Right-BankBase Year Without ProjectFlow-Frequency LPIIIRating Curve With Project
User-Defined*
Base Year Levee 22031Left-BankBase Year Without ProjectFlow-Frequency LPIIIRating Curve With ProjectLevee 2 (or 946 ft Levee)(set by selected lateral structure)
Right-BankBase Year Without ProjectFlow-Frequency LPIIIRating Curve With Project
User-Defined*
Base Year Levee 32031Left-BankBase Year Without ProjectFlow-Frequency LPIIIRating Curve With ProjectLevee 3 (or 947 ft Levee)(set by selected lateral structure)
Right-BankBase Year Without ProjectFlow-Frequency LPIIIRating Curve With Project
User-Defined*

* The "User-Defined" instructions are provided below.

Create With-Project Levee 1 Base Year Scenario

  1. From the Study Tree, right-click on Scenarios and select Create New Scenario. The Create New Scenario dialog opens as a tab.
  2. Enter a name for the new scenario in the Name box (e.g., Base Year Levee 1), and enter a description (e.g., Base year with-project scenario for proposed levee 1 for the left-bank with a top elevation of 946 feet.).
  3. Enter the Year: 2031
  4. From the Stage-Damage list select: Base Year Without Project. 
  5. From the Left-Bank Impact Area tab: 
    1. Recall that the left-bank impact area contains the proposed levee, so you need to select the appropriate levee created in Task 3. (e.g., Levee 1, depending on the name you gave the stage-damage function). Also, this is a with-project condition, so select the appropriate stage-discharge function that you created in Task 2.  

    2. Select the following for the Left-Bank impact area:

  6. From the Right-Bank Impact Area tab:
  7. The right-bank impact area does not contain the levee; therefore, do not add a lateral structure. However, the right-bank still requires the with-project condition stage-discharge function imported in Task 2.

    1. Select the following for the Right-Bank impact area:

    2. Now you must enter the Threshold Stage manually. Uncheck the Calculate Default Threshold checkbox and enter the project performance threshold in the Threshold Stage box and in HEC-FDA Version 2.0, click Set.

      Threshold Stage (Right-Bank)

      Review the Target Threshold section of the Scenarios page in HEC-FDA User Manual. As explained in this section, for impact areas without levees, and for scenarios that reflects with-project conditions, then the user must enter the threshold stage as the default threshold stage from the without-project condition (for the specific analysis year). With-project system performance then reflects the stage at which significant damage begins in the without-project condition.

      Warning!

      This quick how-to does not show you the value you should enter for your study, instead it just shows you how-to find the value. Please use your own Without Base Year scenario to determine the appropriate Threshold Stage value to set for the Right-Bank impact area.

      Quick How-to find a without-project scenario's threshold stage

    3. The default Damage Category selection is RES and Asset Category selection is Structure. Click Preview Compute.

  8. Click Save, and click Close.

Create With-Project Levee 2 and Levee 3 Base Year Scenarios

Repeat the steps from the previous section for Levee 2 and then again for Levee 3. Be sure to:

  • Enter the correct analysis year (2031),
  • Select the correct lateral structure (your Levee 2, then your Levee 3) on the correct Impact Area (Left-Bank), and
  • Enter the appropriate Threshold Stage for the Right-Bank impact area (use the Without Base Year scenario threshold value).

Task 3 (B) – Create Future Year Scenarios for the Proposed Levees

Repeat the steps from Task 3 for the future year conditions. Be sure to:

  • Enter the correct analysis year (2080),
  • Select the correct lateral structure on the correct impact area (Left-Bank), and
  • Enter the appropriate Threshold Stage for the Right-Bank impact areas (use the Without Future Year default threshold value).

Task 4 – Compute All Scenarios

Compute the new Levee scenarios:

  1. From the Study Tree, right-click on Scenarios and click Compute Scenarios.
  2. From the Compute Scenarios tab, click the checkbox to select the new scenarios, and then click Compute. A message box opens asking if you'd like to view the results. Click Yes.
  3. The Summary Results tab opens. Close the Compute Log tab.

Task 5 – Create and Compute Two Alternatives for the Two Levees

Create and Compute Levee 1 Alternative

  1. From the Study Tree, right-click on Alternative and select Create New Alternative. The Create New Alternative dialog opens as a tab.

  2. Enter a name for the new alternative in the Name box (e.g., Levee_1), and enter a description (e.g., Levee 1 with-project conditions for the Base Year and Most Likely Future Year).

  3. From the Base Year Scenario box, select the Base Year Levee 1 scenario you created in Task 3. From the Year box, enter: 2031.
  4. From the Future Year Scenario box, select the Future Year Levee 1 scenario you created in Task 3. From the Year box, enter 2080.
  5. Click Save. Click Close.

  6. From the Study Tree, right-click on the created alternative, and click View Results. Close the Compute Log.

Create and Compute Levee 2 Alternative

  1. From the Study Tree, right-click on Alternative and select Create New Alternative. The Create New Alternative dialog opens as a tab.
  2. Enter a name for the new alternative in the Name box (e.g., Levee_2), and enter a description (e.g., Levee 2 with-project conditions for the Base Year and Most Likely Future Year).
  3. From the Base Year Scenario box, select the Base Year Levee 2 scenario you created in Task 3. From the Year box, enter: 2031.
  4. From the Future Year Scenario box, select the Future Year Levee 2 scenario you created in Task 3. From the Year box, enter 2080.
  5. Click Save. Click Close.
  6. From the Study Tree, right-click on the created alternative, and click View Results. Close the Compute Log.

Create and Compute Levee 3 Alternative

Repeat the above steps and select the appropriate base and future year scenarios and enter the correct analysis years.

Task 6 – Create and Compute Alternative Comparison Report

  1. From the Study Tree, right-click on Alternative Comparison Report and select Create New Alternative Comparison Report. The Create New Alternative Comparison Report dialog opens as a tab.
  2. Enter a name for the new alternative comparison report in the Name box (e.g., Comparison), and enter a description (e.g., alternative comparison report to provide damage reduced between the without-project condition and with-project conditions).
  3. From the Without Project list select: Without Project.
  4. From the With Project list select: Levee_1.
  5. Click Add Comparison. From the newly added With Project list select: Levee_2.
  6. Click Add Comparison. From the newly added With Project list select: Levee_3.

  7. Click Save. Click Close. The new comparison report is added to the Study Tree.
  8. From the Study Tree, right-click on the created comparison report, and click View Results. Close the Compute Log.

Workshop Discussion Questions

Project Performance

Review the Project Performance reports to answer the following questions.

Test Your Knowledge - Question 1

Pursuant to ECB 2019-11 (hold Ctrl while clicking to open in another tab), do any of the levee plans meet FEMA’s criteria for NFIP levee accreditation?

You will have to identify the computed assurance of the 1% AEP event using the Annual Exceedance Probability report under Performance outcomes for both levees.  The following ranges of assurance determine the accreditation recommendation:

Assurance for the 1% AEP

Recommendation

Assurance < 65%

Do not accredit

Assurance > 85%

Accredit

65% ≤ Assurance ≤ 85%

More evidence is needed to accredit

Test Your Knowledge - Question 2

Nonfederal sponsors and members of the vertical team are often interested in knowing what a levee's level of risk reduction is in terms of return interval years (e.g., A team with a levee providing 1% annual chance flood risk reduction could say "This is a 100-year levee").  If the criteria for this statement is having 90% assurance that the event would not exceed the levee height, what statement could be said about Levee 2?

Test Your Knowledge - Question 3

What is the probability that Levee 2 will experience overtopping in the next 30 years?

Hint: Look for the Long-Term Exceedance Probability in the Levee2_Base Scenario results.

Test Your Knowledge - Question 4

What is the fifty percent exceedance probability average annual equivalent (AAEQ) damage reduced value for Levee_1 for the Left-Bank?

Hint: Take a look at the Alternative Comparison Report results tables.

Test Your Knowledge - Question 5

Is it always correct to assume that the without-project conditions and with-project conditions stage-discharge (rating) function is the same for a levee project?  Why?  How would you account for a difference using HEC-FDA.

Hint: For a flood-level volume of water, if the water cannot spread out into the floodplain, what does it do? 

Conclusion

Download the solution project below to compare to your project:

Review the best answers to test your FDA knowledge questions and compare with yours to see how you did!

Return to top of page.

Best Answers to Test Your FDA Knowledge Questions

Note: Your results may not exactly match the results shown here. That's a product of variation in the software since this workshop was created and does not indicate that you did something wrong. Your results should follow the same trends and the values should be somewhat close to the ones shown below.

Test Your Knowledge - Question 1

Pursuant to ECB 2019-11 (hold Ctrl while clicking to open in another tab), do any of the levee plans meet FEMA’s criteria for NFIP levee accreditation?

You will have to identify the computed assurance of the 1% AEP event using the Annual Exceedance Probability report under Performance outcomes for all levees.  The following ranges of assurance determine the accreditation recommendation:

Assurance for the 1% AEP

Recommendation

Assurance < 65%

Do not accredit

Assurance > 85%

Accredit

65% ≤ Assurance ≤ 85%

More evidence is needed to accredit

Answer: No. Levee 3 comes closest but falls short with 57.75%.

Test Your Knowledge - Question 2

Nonfederal sponsors and members of the vertical team are often interested in knowing what a levee's level of risk reduction is in terms of return interval years.  If the criteria for this statement is having 90% assurance that the event would not exceed the levee height, what statement could be said about Levee 3?

Answer: The annual exceedance probability (AEP) with 90% assurance has been computed as 0.0279. 1/.0279=35.84

The policy-compliant simplified communication (per paragraph 8j(5) of ER 1105-2-101) would be : “Given irreducible uncertainties inherent in flood frequency analysis, the Levee 3 Plan will pass the 2.8% event with 90% assurance.”


Test Your Knowledge - Question 3

What is the probability that Levee 2 will experience overtopping in the next 30 years?

Hint: Look for the Long-Term Exceedance Probability in the Scenario results.

Answer: .7094, or ~71%

Test Your Knowledge - Question 4

What is the fifty percent exceedance probability equivalent annual damages reduced value for Levee 1 for the Left-Bank? 

Hint: Take a look at the Alternative Comparison Report results tables.

Answer: $296,350.47

Test Your Knowledge - Question 5

Why is there a with-project condition rating curve?

Answer: A levee restricts water to the channel that would have otherwise spread out in the area now kept dry by the levee, so the stages increase for a given level of flow.

Related Topics

For the list of resources below, either select a specific page to view the related information, or click on a page label to see all the pages for a specific topic (e.g., fda_help_resources).