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) J ) The SPF is important because it constitutes the outer limits for any detailed study, Unless there is future development which will create run- off, and change the flood or flooding boundaries, and change the floodway boundaries, there is little reason to have detailed inventory of property outside the SPF boundaries, Procedure. Elevation-frequency relationships at each cross section are utilized to outline the areas flooded at various frequencies of events, These areas should be delineated on topographic maps and aerial photographs, For most pre-authorization feasibility studies of urban areas, except where steep elevations and financial constraints prohibit, two-foot contour maps should be used to mark off the area flooded, Coastal floodplains may require one-foot contours, In other cases, such as Continuing Authorities and pre-authorization reconnaissance reports, it may be difficult to get more detailed than five-foot_,contours, A field investigator should check the location of the overflow area tentatively outlined on the maps, This information should be substantiated by consultation with local engineers and other public works officials and from interviews with residents who have knowledge of the limits of previous floods. Delineation of overflow areas may also be based on hydraulic backwater computations and correlations on topographic maps with known high water marks, The field investigator also needs to know as much as possible of the specific action and course of floods in each area, since the area subject to flooding and damages can vary appreciably with local circumstances and hydraulic conditions, Coordination with the hydrologic and hydraulic engineers studying the problem, and with local interests to V-19