Section 5: Hydrologic and Hydraulic Studies

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Before starting the design

The roadway designer should visit the site to observe existing conditions and limitations. The designer must avoid designs that may cause increased flooding or increased damage to adjacent, downstream, or upstream properties. TxDOT can be held liable for damages even if a design only increases the frequency of flooding to properties that are already regularly inundated. Overlays in areas where the water overtops the roadway may require milling prior to resurfacing in order to maintain the same elevation and not increase flooding. Problems caused by culverts and bridges can usually be avoided by alternative designs that will not significantly increase the cost. Ultimately, TxDOT must not be perceived as causing flooding to adjacent, downstream, or upstream property owners.

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If the project is within a participating community

The designer should contact the local FPA. The FPA can be a source of valuable data to aid in the hydraulic design, such as existing flooding issues that the project may exacerbate if unaddressed or may alleviate with minor modification to the project. The FPA may have information or a localized study establishing an approximate BFE for Zone A, or may have the complete study data used to establish BFEs for the entire zone. The FPA may also have knowledge of a CLOMR submitted to FEMA by others, or other changes to the area.

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If the project is within or crossing an SFHA

The designer should determine the SFHA zone designation with the current, correct FIRM. The FIRMs are available at Locate the project site on the FIRM to determine whether it crosses or is in any SFHAs Zoned A, AE (A1-30), AO, AH, V, or VE (V1-30). Actions for the various SFHAs are as follows:

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  • Zone A - No BFE or only an approximate BFE is available. A full hydrologic and hydraulic analysis and coordination with the local FPA is required regardless of results (rise, no rise or lowering of the BFE). Any rise in the BFE which extends beyond the project ROW requires discussion with the FPA to make sure the rise is acceptable. The rise must not cause damage to properties in order to be acceptable. The FPA may also be able to use the TxDOT hydrologic and hydraulic analysis to establish informal BFEs in that area.

    The designer should compare the flood footprint from the design hydraulic analysis with the FIRM Zone A footprint, and explore any serious discrepancies.

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  • Zone AE - BFE's have been established by formal hydrologic and hydraulic study. The steps in analyzing a Zone AE are as follows:


    1. Existing Model: The designer is required to obtain from the FPA or FEMA the effective hydraulic model or study data to use for the analysis. Consult DES-HYD for instructions on ordering the model data. If no model is available, go to B below. The entire length of the model usually does not need to be used; the designer should select the appropriate reach for the analysis. However, the selected reach shall fit seamlessly into the entire model; that is, water surface elevations and velocities must match exactly at both the downstream and upstream ends of the selected reach.

    2. Updated Model (if necessary): Older studies that were modeled in HEC-2, WSPRO, or some other program should be converted to HEC-RAS. Corrections to the model must be made because of differences in modeling practices, such as the tendency of piers to be modeled as ground points in HEC-2. Differences in programmed algorithms within the software will cause differences in the water surface elevations. The reasons for the differences are explained in detail in a memorandum from FEMA dated April 30, 2001, titled " Policy for use of HEC-RAS in the NFIP." FEMA requires that the revised and unrevised BFEs match within 0.5 foot at the bounding cross sections between the output of the older model and HEC-RAS model ( 44 CFR 65.6(a)(2)).

    3. Corrected Existing Model: The designer should examine the effective (or updated) hydraulic model for errors such as unrealistic or incorrect flows (Q), inaccurate survey data, missing bridges, and bridges where hydraulically inefficient rails were excluded in the model. The model should be labeled “corrected effective” after the corrections have been made. The water surface elevations in the corrected effective model may be higher or lower than in the existing effective model, but the designer has no requirement to file a CLOMR/LOMR other than supplying the FPA with a copy of the corrected model.

    4. Proposed Model: The designer should then utilize the corrected effective hydraulic model to include the new structure. The model should be labeled “proposed”.


    Sometimes the effective hydraulic model or its data are not available, or the data are unreadable and therefore unusable. In such situations, a new complete HEC-RAS model to approximate the current model must be developed as follows:

    1. The designer must obtain any available information from the FIS and FIRM, such as flowrate(s), cross section topography, floodplain footprint, and BFE's at pertinent cross-sections.

    2. The designer must use the available information to develop a hydraulic model as if a Zone AE was not present.

    3. In a Zone AE without a floodway, the designer must compare the output from the new HEC-RAS model with the published BFE's from the FIS and the floodplain footprint from the FIRM. The model must match the published BFEs within 0.10 foot. If only the FIRM is available or if the FIS has no BFEs listed, the model must match the BFEs on the FIRM to 0.5 foot or less.4. In a Zone AE with a floodway, flow should be confined to the floodway limits and the elevations match the published floodway elevations to within 0.10 foot (23 CFR 650A, Attach 2).5. Once the new HEC-RAS model meets the allowable differences, the model should be labeled “replacement effective” model and steps A1 through A4 above should be used to progress toward a “proposed” model.

If the FEMA data or model is not used, the situation and process must be fully explained in the hydrologic and hydraulic report and noted on the plan sheet. The argument that the hydraulic data is only available in hard copy and must be manually entered is not an acceptable explanation for not using the FEMA model.On occasion, a model is obtained which contains numerous errors throughout the entire reach. TxDOT is not responsible for quality control and comprehensive updating of the NFIP models. In these cases, either limit the reach used for the TxDOT study to the shortest length practical, limit the corrections to the cross-sections closest to the bridge, or both. Consult the DES-HYD if a corrected or updated hydraulic model appears to be warranted.If the flowrates used in the existing model appear to be in error, the designer is encouraged to develop a new hydrologic model and compare the results. If the existing flowrates are not used, the justification must be explained in the hydrologic and hydraulic report and noted on the plan sheet. The hydrologic process must also be included in the report. Designers shall not, in any circumstance, develop a new model without documented justification.

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  • Zone AE with Regulatory Floodway - Once a regulatory floodway, or floodway, has been determined and mapped on a FIRM, FEMA requires a study to prove no rise of the water surface for any work in a floodway. The designer must obtain the floodway model to determine the limits of the floodway and the effect of the proposed structure on the floodway. The floodway model is not to be confused with the current effective floodplain model. The floodway model is almost identical to the floodplain model with the exception that it contains the floodway boundaries. This model should be obtained along with the floodplain model.

    If the entire structure of a bridge, including abutments, bridge superstructure and piers, can be documented to be well outside the limits of a regulatory floodway, and is above the BFE in the floodway, then the current effective hydraulic model may not need to be obtained. The NFIP allows that any work, including fill, in the flood fringe or above the floodway does not require a study because the study establishing the regulatory floodway assumed that the flood fringe was already filled. If the bridge design meets these criteria, the normally required analysis can be replaced by a prominently placed note in the plans. However, if there are insurable structures in the floodplain, TxDOT requires that the roadway designer acquire and modify the floodplain model as outlined in Zone AE above.

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  • Zone AH - Places of no outflow, such as playas, in which the BFE has been determined are labeled Zone AH. Structures in playas are equalizers and as such, the bridges and culverts typically need no hydraulic modeling. However, the designer is required to calculate how much the BFE will be raised because the roadway and structure will reduce storage in the playa if any roadway work will be in the Zone AH and below the BFE.
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  • Zone AO - Average flood depths are given instead of a flood elevation. The designer should examine the source of the flooding to make sure the project will not trap flood waters or block drainage. The design may require relief structures in any elevated roadway or extended bridge approaches.
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  • Coastal Zones V, VE (V1-30), A, and AE - Coastal zones are flooded by the Gulf of Mexico storm waters instead of riverine flows. The FEMA modeling for coastal zones is not the same as for riverine modeling. The designer does not need to acquire the model but must consider tidal flows, wave actions, and storm precipitation. The designer should make sure the project will not trap flood waters or block drainage.

High Bridges

For some bridges, the geometry is such that the bridge either spans the entire floodplain or the low chord is well above the BFE. In these situations, or where the proposed work can be documented to be well above the BFE and outside the limits of the floodplain, a hydraulic analysis may not be necessary. The normally required analysis can be replaced by a prominently placed note in the plans.

Fred Hartman Bridge facing Baytown from
the ship channel, mostly above the BFE. (click in image to see full-size image) Anchor: #FMLKIELMgrtop

Figure 5-1. Fred Hartman Bridge facing Baytown from the ship channel, mostly above the BFE.

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