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A Simplified Method of Implementing No Rise Analysis in Unnumbered A-Zones

Second National Flood Workshop, Houston, TX Session C4 - Public Awareness/Mitigation. A Simplified Method of Implementing No Rise Analysis in Unnumbered A-Zones Based Upon No Loss of Conveyance. Dwayne E. Culp, Ph. D., P.E., CFM. CFR 60 Requirements. 44 CFR 60.3(a)(3)

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A Simplified Method of Implementing No Rise Analysis in Unnumbered A-Zones

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  1. Second National Flood Workshop, Houston, TX Session C4 - Public Awareness/Mitigation A Simplified Method of Implementing No Rise Analysis in Unnumbered A-Zones Based Upon No Loss of Conveyance Dwayne E. Culp, Ph. D., P.E., CFM

  2. CFR 60 Requirements 44 CFR 60.3(a)(3) Review all permit applications to determine whether proposed building sites will be reasonably safe from flooding. If a proposed building site is in a flood-prone area, all new construction and substantial improvements shall (i) be designed (or modified) and adequately anchored to prevent floatation, collapse, or lateral movement of the structure resulting from hydrodynamic and hydrostatic loads, including the effects of buoyancy, (ii) be constructed with materials resistant to flood damage, (iii) be constructed by methods and practices that minimize flood damages, and (iv) be constructed with electrical, heating, ventilation, plumbing, and air conditioning equipment and other service facilities that are designed and/or located so as to prevent water from entering or accumulating within the components during conditions of flooding.

  3. CFR 60 Requirements 44 CFR 60.3(a)(4) Review subdivision proposals and other proposed new development, including manufactured home parks or subdivisions, to determine whether such proposals will be reasonably safe from flooding. If a subdivision proposal or other proposed new development is in a flood-prone area, any such proposals shall be reviewed to assure that (i) all such proposals are consistent with the need to minimize flood damage within the flood-prone area, (ii) all public utilities and facilities, such as sewer, gas, electrical, and water systems are located and constructed to minimize or eliminate flood damage, and (iii) adequate drainage is provided to reduce exposure to flood hazards.

  4. CFR 60 Requirements 44 CFR 60.3(b)(7) Assure that the flood carrying capacity within the altered or relocated portion of any watercourse is maintained; 44 CFR 60.3(c)(10) Require until a regulatory floodway is designated, that no new construction, substantial improvements, or other development (including fill) shall be permitted within Zones A1-30 and AE on the communities FIRM, unless it is demonstrated that the cumulative effect of the proposed development, when combined with all other existing and anticipated development, will not increase the water surface elevation of the base flood more than one foot at any point within the community.

  5. Basis of Method Determine Base Flood Elevation • Use Maps and Profiles with BFEs if available. • Use the survey methods with local agreement if BFEs are not available. • Demonstrate No Net Fill (no loss of storage) • Demonstrate No Loss of Conveyance Capacity (no loss in carrying capacity of the system)

  6. Determine Base Flood Elevation • When there are elevations • From Profiles (preferred) • From FIRM Panel • When there are no elevations • From FIRM Panel overlaid over a topographical map • From a FIRM Panel with staked elevations of the lines of the FIRM panel

  7. Determine Base Flood Elevation BFE=57.0 feet Property Bridge upstream Bridge downstream

  8. Determine Base Flood Elevation BFE=57.0 feet from profile Bridge upstream Bridge downstream

  9. Determine Base Flood ElevationIf there are No Elevations Property

  10. Determine Base Flood Elevation Property

  11. Determine Base Flood Elevation 54.35 55.15 55.10 53.55 55.00 54.15 54.15 54.35 55.05 54.75 Property

  12. Conveyance Manning’s Equation Conveyance Where, Q is flow (cfs / cms), A is area of flow perpendicular to flow direction (ft2 / m2), R is hydraulic radius (ft / m), n is Manning’s n, K is a conversion factor (1.49 in English units, 1.00 in metric), and X is the conveyance.

  13. Conveyance Note: If the Manning’s n increases, conveyance decreases. If depth of flow increases, both A and R will also increase, and therefore conveyance increases. If depth of flow increases but A decreases, it may be possible to have conveyance increase or decrease.

  14. Goal of the Conveyance Analysis • Demonstrate for multiple cross sections that there is no decrease in conveyance. • Note: City of Houston requires that there is no significant increase in Conveyance too. The City of Houston does allow the most restrictive cross section to demonstrate no significant increase. • For jurisdictions with unnumbered A zones with unmodelled streams, it should be adequate to demonstrate that there is no loss of conveyance.

  15. How to Select Cross Sections • Representative • Existing conditions • Proposed conditions • Perpendicular to direction of channel flow • Include both onsite and offsite areas if necessary • More than one. There will be a need for multiple cross sections. It is suggested that a minimum of two are needed per 500 feet of property width perpendicular to channel flow direction

  16. Example

  17. Example

  18. Conveyance • Calculate Conveyance for Each Section • Wetted Perimeter • In CADD do not use a distorted cross section • Calculate edge of buildings if the buildings act as restrictions to flow • When flow is under buildings without restriction – be aware that piers increase wetted perimeter • Area • Mannings n • Use values for floodplain areas where possible. TxDOT tables or the tables from Chou work good.

  19. Example

  20. No Storage Loss • Demonstrate no storage loss • CADD • Cross Sections and area volume calculations • 3D surfaces

  21. No Storage Loss

  22. No Storage Loss

  23. Why should Agencies Use a Method Like This? • The method is simple to implement • It does not require that the reviewer have significant mathematic ability, or knowledge of specific programs. • It is reproducible • It is more accurate on a site basis than hydrologic or hydraulic models that average conveyance and storage over very large areas • It complies with 44 CFR 60.3(b)(7) as it demonstrates no loss of carrying capacity • It complies with 44 CFR 60.3(c)(10) as it demonstrates that the cumulative effect of the proposed development, will not increase the water surface elevation of the base flood

  24. References Managing Floodplain Development in Approximate Zone A Areas, a guide for obtaining and developing Base (100-year) Flood Elevations, FEMA April 1995. Rules and Regulations for Chapter 19, City of Houston, February 2009. Hydraulic Design Manual, Texas Department of Transportation, October 2011. Chou, Ven Te, Open-Channel Hydraulics, McGraw Hill, 1988.

  25. Questions Dwayne E. Culp, Ph.D., P.E., CFM Jacobs Engineering Group Inc. 5995 Rogerdale Road, Suite B3014B Houston, TX 77072 713-898-1977

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