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APPENDIX 2, SHALLOW FLOODING A2-1 INTRODUCTION Shallow flooding of different types commonly occurs throughout the United States, Areas of shallow flooding include unconfined flows over broad, relatively low relief areas, such as alluvial plains; intermittent flows in arid regions that have not developed a system of well-defined channels; overbank flows that remain unconfined, such as on delta formations; overland flow in urban areas; and flows collecting in depressions to form ponding areas. These have been loosely and inconsistently referred to as "sheet flow" or "ponding." Alluvial fan flooding is to be analyzed using procedures outlined in Appendix Sand not the procedures outlined in this Appendix, For purposes of the NFIP, shallow flooding conditions are defined as flooding that is limited to 3,0 feet or less in depth where no defined channel exists, A2-2 STUDY SCOPE The state of the art for determining shallow flooding hazards, and the cost effectiveness of these determinations, are quite limited. As a result, certain study parameters should be used by the SC to limit the_ detail of studv for shallow flooding determinations. Drainage area size should be considered in determining shallow flooding hazards. Flooding conditions resulting from drainage areas of less than 1 square mile are not generally studied in detail, Calling the community's attention to these hazards by use of approximate study and delineation (described in more detail later in this Appendix) is sufficient, Flooding from sources wi th drainage areas less than 1 square mile is considered to be a local drainage problem. Depths of flooding determined from detailed study of shallow flooding hazards need be computed only to the nearest whole foot, Detailed study should be limited onlv to those areas that have a history of destructive flooding or that have a significant potential for the damage of future development, and where expected 100-year flood depths are 1,0 foot or greater. A2-3 DEFINITION OF FLOOD HAZARD ZONES Flood hazard zones that are relevant to areas susceptible to shallow flooding are listed and described below. Zone X Zone X is the flood insurance rate zone that corresponds to areas outside the SOO-year floodplain, areas within the SOO-year floodplain, areas of lOO-year flooding where average depths are less than I foot, areas of IOO-year flooding where the contributing drainage area is less than 1 square mile, and areas protected from A2-1