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I I I I I I I I I I I I I I I I I I I 18 07/16/99 5C Sheetflow Assumptions: broad, unconfined, shallow flooding Limitations: not for use in areas of undulating terrain Recommended Applications: shallow flooding across uniformly sloping surfaces Please refer to Appendix 2 ofFEMA 37, Flood Insurance Study Guidelines and Specificationsfor Study Contractors, for guidelines on the analysis and mapping of shallow flooding. Although it is stated in Appendix 2 that the procedures are not to be used for alluvial fan flooding, the approach established by this document enables the use of those methods described in Appendix 2, except for highly active conical fans that are studied using the FAN program. 5D Hydraulic Analytical Methods Assumptions: stable flow path, uncertainty is to a degree that may be disregarded Limitations: not for use with active alluvial fan flooding Recommended Applications: entrenched stable channels and channel networks, constructed channels, urbanized areas Inactive, yet floodprone areas should use "riverine" hydraulic analytical methods. Where flow paths are stable and flow is reasonably confined, standard hydraulic engineering methods such as backwater computations may bc used to define the elevation (or depth), velocity and extent ofthe I DO-year flood. Hydraulic mcthods may also bc used for stable channel networks when applicable. For example, relict alluvial fans or inactive fans with stable channels, as determined by a geomorphic analysis, may be subject to flow splits throughout the distributary system that exists. Hydraulic modeling can generally handle split flow analyses through stream junctions of this type. In general, for stable channels on alluvial fans, physically based methods that consider site processes and hydraulics, such as channel geometry, grade and roughness, and channel bank and bed material are preferred. Where precise computations of water surface profiles using energy and momentum based methods may not be feasible based on the scope of the study, the use of normal depth calculations for dcfinition of approximate I DO-year flood boundaries may be warranted. Chapter 5 ofFEMA 37 provides guidelines for hydraulic analytical methods. Within Section D of Chapter 5, several methods applicable to conditions found on alluvial fans are described. These methods include two-dimensional water surface models, modeling techniques of streams with supercritical flow regimes, and split-flow analysis.