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Section 6: Hydrologic and Hydraulic Results

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Changes to the BFE

Repair, extension, or replacement of any particular bridge or culvert may cause the BFE to be raised, lowered, or not changed at all. Lowering the water surface elevation usually doesn't cause any adverse impacts, but the site should be visited to confirm that there will be no resulting problems. Many bridge replacements result in a lowered water surface elevation because of reduced pier sets, enlarged openings, raised low chords, or improved channels.Raising the water surface elevation requires examination of the adjacent properties to assure that the change will not cause any adverse impacts. A rise usually can be considered as having no impact if the rise is contained within the TxDOT ROW. A rise which extends outside of the ROW may be considered either insignificant or significant. A severe rise in an uninhabited area, not excessively flooding adjacent properties, not damaging the stream banks, and not blocking access to properties might be considered insignificant. A rise in an urban area that is contained within the banks without damage to the stream banks or back flooding the sewers may also be considered insignificant. However, in each situation, the effects have to be examined at the site to confirm that the rises are insignificant.

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Range of Frequencies

The NFIP only considers the 1% AEP. However, the designer should analyze the effect of a proposed structure on the full range of AEP water surface elevations (the 50%, 20%, 10%, 4%, and 2% as well as the 1% AEP). The proposed structure may not cause any problem at the design AEP flow, but may cause a problem at one of the other frequencies. Analyzing only for the 1% and design AEP flows may fail to reveal these problems. See Liability above.Some FEMA models contain only the 1% AEP. The designer will need to develop the full range of flows (50%, 20%, 10%, 4%, and 2% AEP) using a suitable method.

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Conditional Letter Of Map Revision (CLOMR)/Letter Of Map Revision (LOMR)

Changes to the water surface elevation in studied areas are usually reported to FEMA by submission of a CLOMR and LOMR. A CLOMR, if required, is submitted to FEMA prior to initiating work to receive approval for the project design and the impacts on the floodplain; a LOMR is submitted after the project has been completed. The approval of a CLOMR application by FEMA requires significant time (six to 12 months), which needs to be factored into the required design time to prevent delay of the project letting. A community may attempt to require TxDOT to submit a CLOMR and a LOMR, based on a 44CFR 60.3 requirement for participating communities to notify FEMA of all changes to the BFE. However, the requirement does not apply to TxDOT because the State of Texas is not a participating community in the NFIP, as discussed above. Additionally, Non-regulatory Supplement 23 CFR 650A, Attachment 2 states in the first paragraph, "The community, by necessity, is the one who must submit proposals to FEMA for amendments to NFIP ordinances and maps in that community should it be necessary." See also Permit versus FPA Notification. TxDOT provides the technical data to the FPA, as required in 44CFR 60.3, through FPA Notification to enable the FPA to submit it to FEMA.TxDOT will prepare and submit a CLOMR/LOMR for very few situations, as described below. This is more likely to happen on large projects which involve major changes to the floodplain, such as channel realignment or channel restoration. However, the designer must consult the DES-HYD before proceeding with the CLOMR/LOMR process.

ALL CLOMRS MUST BE REVIEWED BY DES-HYD BEFORE SUBMITTAL TO THE FPA.

A CLOMR may be prepared and submitted in the following limited circumstances:TxDOT will not file a CLOMR to better define the floodplain for projects in a Zone A. TxDOT will not file a CLOMR to redefine a Zone AE where TxDOT improvements drop the water surface elevation of the BFE, or otherwise change the floodplain footprint, so as to encourage additional development. Improvements in the floodplain that may result from TxDOT projects are considered incidental. However, as for any project, TxDOT will still provide its plans and studies in these cases for the community records.

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  • An encroachment on a floodway of a SFHA results in a rise (not contained within the TxDOT ROW) of the base flood elevation in a Zone AE with Floodway. Alternatively, a larger bridge or culvert may be preferable.
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  • Increases in water surface elevations (not contained within the TxDOT ROW) exceed the usually available (or remainder of) a cumulative 1-foot rise in a Zone AE. Alternatively, a larger bridge or culvert may be preferable.
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  • An increase in water surface elevation (not contained within the TxDOT ROW) results in a significant increase of the horizontal extent of the floodplain footprint in unusually flat areas and in a Zone AE. Alternatively, a larger bridge or culvert may be preferable.
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  • A major channel relocation in a Zone AE that is outside the TxDOT ROW.
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  • Where a relief structure is outside the SFHA containing the main structure and a risk exists of development immediately downstream of the relief structure that might interfere with the operation of the relief structure. Alternatively, a larger main structure may be preferable.

TxDOT will not file a CLOMR to better define the floodplain for projects in a Zone A. TxDOT will not file a CLOMR to redefine a Zone AE where TxDOT improvements drop the water surface elevation of the BFE, or otherwise change the floodplain footprint, so as to encourage additional development. Improvements in the floodplain that may result from TxDOT projects are considered incidental. However, as for any project, TxDOT will still provide its plans and studies in these cases for the community records.

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FPA Notification Details

The TxDOT office where the plans are being developed shall forward a copy of the plan sheets along with all hydrologic and hydraulic analyses, reports, and electronic copies of the models to the local FPA, or FPAs if more than one community is involved. FPA Notification is required no later than when the project is submitted for letting but should be accomplished as soon as the hydraulic design is complete. The purpose of the FPA Notification is to document to the FPA any changes or non-changes to the BFE. The submitted FPA Notification must be complete enough for the community to apply for a Letter of Map Revision (LOMR) if the local FPA deems it necessary.

The transmittal letter to the FPA should state that the attached information is being sent for the FPA's floodplain records. It must not ask for the FPA's concurrence, approval, or consent. The FPA has no authority to approve, disapprove, or change the modeling or design but may offer valuable guidance to the designer.

The designer should be aware that, while the designer's office is coordinating with a community, it does not mean that the office is automatically coordinating with the FPA. The FPA may be in a completely different department such as Health, and not Engineering. This is a source of confusion, and may result in a project having problems if the FPA is not involved until late in the design. The designer should verify that the FPA is involved and recommend to the community engineer that the local FPA be involved early in the design process.

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Communities Without an FPA

A community which is not participating in the NFIP does not have an FPA. The FPA Notification documentation should be sent to the county engineer or county judge, if in an unincorporated area, or the municipal engineer if in an incorporated non-participating city, town, village, tribe, or municipal utility district (MUD). By coordinating with the community, TxDOT may prevent public concerns from growing into problems, and may raise the awareness of flood issues within the community.

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