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B. Aooroximate Studv Methods Areas of expected shallow flood hazard that have no significant development pressure for the near future should be studied by approximate methods. Normally, only the designation Zone A should be used in these areas, with two possible exceptions, In many areas of IOO-year shallow flooding, average flood depths can often be readily determined to be below 1 foot by simple and inexpensive methods. In this situation, with a very limited study, shallow flooding areas may be designated as Zone X. Zone X should also be used whenever the contributing drainage area causing shallow flooding is less than 1 square J1l i 1 c. C. Detailed Studv Methods (1) Ponding, Areas of ponding can be identified through his toric data on pas t flooding, local inquiries, examination of topographic maps, and field reconnais- sance, The SC should determine inflow to, and outflow from, the ponding area and calculate the storage volume and elevations using a simple reservoir routing analysis. Hydrographs, empirical formulas, and design equations for culverts and other manmade structures should be considered. Determination of stage- storage relationships requires some topographic information. Wherever adequate contour interval mapping is available, the SC should determine storage volumes directly from those maps. Otherwise, a limited number of cross sections should be surveyed to determine storage volumes. The number of cross sections needed will depend on the size of the ponding area, but usually one along the maj or axis and two perpendicular to that axis will be sufficient, Where volumes of inflow to ponding areas are sufficient to fill the available storage volume behind low dikes or other large, uniform obstructions, their crest elevation will determine the elevation of flooding in the ponding area. Such areas can usually be delineated based on field reconnaissance, in conjunction with an examination of topographic maps, without detailed calculations or field J; surveys. One BFE should be placed under the Zone AH designation for each ponding area, Whenever BFEs are required, the SC shall establish or confirm ERMs as described in Chapter 3, Section 1. (2) Sheet Runoff, Areas of sheet runoff can be identified from historic data and local inquiries, supplemented by field reconnaissance and examination of topographic maps and aerial photographs. However, the lack of adequate data (e,g" small contour interval mapping) and costly analytic methods pose problems for detailed study of these areas, Sheet runoff typically takes place across broad areas of low relief, This situation makes it likely that sheet runoff depths will be less than I foot, For flood insurance purposes, once a determination has bee'n made that flooding depths are less than 1 foot, the area should be designated as Zone X and more detailed analysis is not required. In certain situations, however, sheet runoff depths may average more than 1 foot. Such may be the case, for instance, when the channel capacity of a perched stream is exceeded, as on a delta formation. A2-4