Hi all,
I received a request to post an unedited version of the questions from FEMA to the USACE regarding accreditation of the Indianapolis North Flood Damage Reduction Project (see previous post).
Here you go!!
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This responds to your submittal dated November 7, 2022, that the Department of Homeland Security’s Federal Emergency Management Agency (FEMA) issue a revision to the Flood Insurance Rate Map (FIRM) for the City of Indianapolis, Marion County, IN. We have reviewed your request and determined that the following data items are required to complete our review.
Duplicate Effective Model
1. The duplicate effective hydraulic model is a copy of the hydraulic analysis used for the currently effective published FEMA flood hazard data, referred to as the current effective model. For situations where the effective hydraulic model is a HEC-2 model, as with the White River, the duplicate effective model is created by directly converting the HEC-2 model to HEC-RAS and making minor adjustments which are necessary to get the model to run on the new platform. Our review of the submitted models for the White River revealed discrepancies between the submitted duplicate effective HEC-RAS model layout and the effective study. Because of this, the submitted model cannot be considered a true duplicate effective model. To get an accurate estimate of the difference in base flood elevations (BFEs) that result from updating the modeling software, the layout of the duplicate effective model should match that of the HEC2 study as much as possible within the revision area. It is recommended that the duplicate effective model is truncated to the intended limits of revision to reduce the effort needed to create the duplicate effective model.
a. The submitted duplicate effective model does not include cross sections from previously approved LOMR Case No. 17-05-3161P, effective 10/26/2017. Review of the model from this case revealed only two additional cross sections would need to be added to the submitted models. These sections are incorporated in the NFHL database and are at reach stations 243.22 and 243.24 (between lettered sections BG and BH). Although the accreditation of the levee system will supersede LOMR 17-05-3161P, the duplicate effective model should be revised to incorporate these cross sections so that it accurately represents the effective hydraulic model.
b. There are numerous cross sections within the intended revision area where the incorporation of blocked obstructions is inconsistent between the effective analysis and the submitted duplicate effective model. For example, blocked obstructions are present within an overbank of a cross section in the effective model, but not in the duplicate effective model. Additionally, the stationing of the blocked obstructions for several cross-sections does not match between the models. Table 1 in the attachments shows the locations where discrepancies occur.
c. For cross sections not directly bounding structures, there are differences between the effective and duplicate effective models for the overbank and main channel lengths. Table 2 in the attachments shows the locations where this discrepancy occurs.
d. Cross section 244.75 in the submitted duplicate effective model appears to be a newly added cross section as it is not defined in the effective model and is not near a structure. This section appears to be upstream of the area intended for revision, however this will need to be addressed if the limit of revision is extended further upstream.
Corrected Effective Model
2. The corrected effective model is a model that corrects any modeling errors in the duplicate effective model, makes changes necessary to ensure the model meets current FEMA standards, and/or incorporates more detailed topographic information than that used in the effective model. A corrected effective model is required to ensure the final preproject and post-project models meet FEMA standards.
a. The channel distances between cross sections defined in the model used to make mapping revisions should match the channel distances shown on the topographic work map within 5-percent of the effective FIRM scale, or 5-percent of each other, whichever is larger. Based on the cross section alignment and river centerline defined in the submitted existing (post-project) conditions georeferenced model, most of the distances between cross sections in the revision area do not meet this requirement. Table 3 in the attachments provides the calculations for the existing (post-project) conditions model.
Pre-Project and Post-Project Models
3. FEMA Guidance Document 95 (Levees) specifies that both “with levee” and “natural valley” analyses should be performed for a hydraulic analysis in an area with a levee/floodwall. This is done to provide an estimate of the changes in water surface elevations that result from the construction of the levee in addition to help determine the area to be mapped as protected by the levee system. The submitted draft Levee System Evaluation Report (Section 8.2.2B) indicates that minor changes were made to the effective HEC-2 model to reflect present day conditions. Specifically, “effective flow areas were modified based upon existing bridges, buildings, flood depths, and most importantly, existing and proposed levees.” As there is a difference in the placement of effective flow areas due to the as-built levee project compared to the effective model, please provide a natural valley (without levee) analysis.
4. Comparing the georeferenced existing (post-project) conditions model geometry to the provided terrain data, several cross sections do not appear to accurately reflect the existing topography and the channel bank stations do not align with the actual location of the main channel. For some cross sections, the placement of ineffective flow areas makes the difference between cross section geometry and existing topography unimportant with regards to overall conveyance calculations. However, there are some cross sections where the discrepancies may be important for estimating BFEs. For an example, see Figure 1 in the attachments. Typically, the most current topographic information is used to update cross section geometry in a model as it is considered the best data available for use in the analysis. It is recommended that this is done to ensure the model can provide the most realistic estimate of flooding based on current conditions.
5. The locations of the levee embankments and floodwalls are currently represented in the submitted existing (postproject) conditions HEC-RAS model in a variety of ways, as listed below.
a. The levee/floodwall is represented as high ground with an ineffective flow area placed at the high point. Additional topography landward of the levee exists.
b. The cross section geometry is cut-off at high ground at what is assumed to be the location of the levee/floodwall. Additional topography landward of the levee does not exist.
c. An ineffective flow area is placed at the approximate location of the floodwall without the indication of high ground at the location. It appears this configuration is used only for cross sections where the floodwall exists.
To ensure the model clearly presents the location of the levee/floodwall at all locations, the levee/floodwall should be documented/modeled in the post-project conditions model in a consistent manner. As separate pre-project and postproject conditions models are necessary to estimate changes in flood levels resulting from the levee construction, option B is not recommended.
6. In the submitted existing (post-project) conditions model, the water surface elevations of the 500-year flood event are not contained by the ground geometry at cross sections 243.05, 237.07, 243.075, and 243.08. Based on FEMA Guidance Document 80 (Hydraulics: One-Dimensional Analysis) Section 2.2.2, cross sections in the model must “extend beyond the most extreme event modeled.” Please ensure that all cross sections in the pre-project and postproject conditions models contain the 500-year flood event.
7. In determining the known water surface downstream boundary condition for the truncated version of the submitted existing (post-project) conditions model, water surface elevations for cross section 237.87 of the submitted corrected effective model, which incorporated updated discharge values, were employed. For LOMRs utilizing truncated models, known water surface boundary conditions are set to the values of the effective models. For all submitted models that are truncated from the effective HEC-2 analysis, please incorporate elevation values that correlate with the HEC-2 results in the known water surface downstream boundary condition option.
8. The submitted existing (post-project) conditions floodway analysis uses a combination of methods 1 and 4 to calculate the floodway encroachments. For FEMA floodway analyses, method 4 is used to determine the initial encroachment stations needed to meet surcharge requirements. The values of the encroachment stations are then defined in method 1 in the finalized model that will be used to map the floodway. Please ensure that the post-project floodway analysis incorporates method 1 to define the floodway encroachment stations at all cross sections.
9. The submitted existing (post-project) conditions floodway analysis shows a surcharge value of 0.15’ at cross section 241.9. Based on state requirements, surcharges must be below 0.15’. Please ensure all surcharges in the post-project floodway analysis are below 0.15’.
10. The top of road for the bridge at Reach Station 242.36 in the submitted duplicate effective, corrected effective, and existing (post-project) conditions hydraulic models indicates that the bridge deck is approximately 6 inches thick. Because the bridge has low flow during the base flood, the top of road does not affect the computed BFEs. However, the top of road is needed to show on the revised flood profile for the White River. Please confirm the top of road elevation information for this structure in all submitted hydraulic models.
Mapping Items
11. Please provide a topographic work map that shows all applicable items listed below. Please also provide digital CAD or GIS data for the topographic work map and ensure the digital data are spatially referenced so that the data may be used for accurate mapping.
a. The location and alignment of the levee/floodwall for the Indianapolis North Levee System.
b. Boundary delineations of the post-project conditions base floodplain, 0.2-percent-annual-chance floodplain, regulatory floodway, and area to be protected by the levee system.
c. Boundary delineations of the currently effective base floodplain, 0.2-percent-annual-chance floodplain, and regulatory floodway.
d. Logical tie-ins between the revised and effective flood hazard boundary delineations.
e. Topographic contour information used for the boundary delineations of the base floodplain and 0.2-percentannual-chance floodplain.
f. Locations and alignments of all cross-sections used in the hydraulic model.
g. The flow line used in the hydraulic model.
h. Reference to a datum, such as North American Vertical Datum of 1988.
12. Please provide an annotated FIRM at the scale of the effective FIRM, that shows the revised boundary delineations of the base floodplain, 0.2-percent-annual-chance floodplain, regulatory floodway, and area protected by the levee system shown on the submitted work map and how they tie into the boundary delineations shown on the effective FIRM at the downstream and upstream ends of the revised reach.
Levee System Components
13. The submitted documentation includes survey points and notes compiled for use in the National Levee Database (NLD). The notes included with the survey indicate that the data associated with Phase 3B-2, was collected while the project was incomplete and thus will not represent as-built elevations. Additionally, the top-of-levee elevations were not provided on the submitted as-built documents. Please provide verification of the top-of-levee elevations in this area to confirm freeboard requirements are met.
14. Section 8.2.3 (Risk & Uncertainty Analysis) of the Levee System Evaluation Report (LSER) submitted November 7, 2022, references Section III “Stage-Discharge Uncertainty” of EC 1105-2-205. We have been unable to locate this document on the USACE website. Please clarify if this engineering circular methodology was incorporated into another document or that the methodology still represents current practices and conditions for this project.
15. The LSER submitted November 7, 2022, specifies that the I-Wall completed in 2019 as part of Phase 3B may currently have settlement that is still occurring. However, it was specified that this project has relatively little fill and was constructed in in-situ soils, so settlement is expected to be minimal. As a settlement analysis was not conducted for this project area, please confirm USACE still recommends a positive evaluation for accreditation without this analysis.
16. The LSER submitted November 7, 2022, specifies that there is an automatic floodwall closure structure along Westfield Boulevard that was installed in 2019. The provided as-builts and other documentation indicate that ground elevation along this section is below the proposed Base Flood Elevation (BFE), however it is unclear if this data represents the elevations after completion of the project for the invert of the structure. Please provide verification of the invert elevations of this automatic closure structure. In addition, FEMA Levee Guidance requires all automatic closures to be documented in the Operation Plan and must include traffic control to prevent damage to the closure and also include provisions for monitoring and manual backup for the activation of the automatic closure.
Regulatory Items (Responsibility of Community)
17. The submitted application included an Operations, Maintenance, Repair, Replacement and Rehabilitation Manual (O&M) dated September 2020 and a May 2019 Flood Response Plan (FRP). To meet the requirements specified in 44 CFR 65.10(c) and (d), please provide evidence that the most recent operation and maintenance plans have been formally adopted by a Federal or State agency, an agency created by Federal or State law, or an agency of a community participating in the NFIP.
18. Section 3-8 of the submitted O&M (Duties of the Superintendent) specifies that the Superintendent shall oversee all operations and maintenance of the levee system and ensure that key workers have read those portions of the manual that pertain to their duties. Please provide an addendum or appendix to the O&M to identify the City of Indianapolis personnel (name or title) who will be responsible for the actions and assignments of operations and maintenance of the levee system.
19. Before this LOMR can be processed, please provide community acknowledgment in the form of a letter stating that the community has reviewed the revision request and understands the effects of the revision on flooding conditions in the community, and that any existing or proposed structures to be removed from the Special Flood Hazard Area
(SFHA), the area that would be inundated by the base flood, are or will be reasonably safe from flooding. Alternatively, please submit Application/Certification Form 1, entitled “Overview & Concurrence Form,” signed by a community official.
20. The State of Indiana requires approval before FEMA issues a LOMR affecting the Base Flood Elevations and/or the regulatory floodway. Before this LOMR can be processed, please submit documentation of Indiana Department of Natural Resources approval.
21. Before this LOMR can be processed, please submit a finalized copy of the public notice distributed by City of Indianapolis officials stating their intent to revise the regulatory floodway or provide a statement that city officials have notified all affected property owners, in compliance with Paragraph 65.7 (b)(1) of the National Flood Insurance Program regulations. These notices must include the extent of revision, the changes to the regulatory floodway, and contact information for any interested parties. An example of public notice can be found in Figure 3 of the MT-2 instructions, which may be accessed at https://www.fema.gov/flood-maps/change-your-flood-zone/paper-applicationforms/mt-2.
22. Before this LOMR can be processed, please submit documentation of the individual legal notices that were sent to property owners who are affected by any increases in width and/or shifting of the base floodplain and/or increase in Base Flood Elevations. Legal notice may take the form of certified mailing receipts or certification that all property owners have been notified, with an accompanying mailing list and a copy of letter sent. Examples of individual notices can be found on pages 12 through 17 of the instructions for the MT-2 application/certification forms, which may be accessed at https://www.fema.gov/flood-maps/change-your-flood-zone/paper-application-forms/mt-2.
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