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Detailled Studies There are two types of detailed floodplain studies, floodplain IIlfOrmatlon studies aAd master drainageway planning studies. Floodplain information studies delineate floodplains; master drainageway planning studies delineatE! floodplains, consider alternative approaches 10 problems. and recommE!Od preferred alternatives. Another category of study incorporates features of both approximatE! and detailed studies; mapping of dam failure flood zones is discussed after tile steps in floodplain information studies have bee'n explained. Floodplain Information StudiElS Delineation of floodplains through detailed methods includes flood history research, review of watershed development patterns, hydrologic evaluation of the we,tershed to develop streamflow rates (discharges for the study streams), hydraulic computations to obtain the floodwater surface elevations. and a portrayal of flood outlines on detailed topographic mapping, There are four basic steps that should be followed when Obtaining flood- plain information and data through detailed engineerinq procedures: 1. bas'~ mapping and field surveys Sllould be performed; 2. the proper hydrologic method for a particular drainage basin should be selE,cted and fOllowed; 3. the water surface elevations (flood levels) which are representative of a projected event should be computed; 4. the water surface elevation data should be transferred to a flood outline map assuring that datum consistency between thE' floodplain delineations and on-1tw-ground conditions is maintained. Base Mapping The 1 OO.year floodplain information and data must ultimately be delineated on a map of suitable scale and detail. The map may be an existing map (USGS quadrangle map, town, county, etc.) or a iarge scale topographic map, FlOOdplain outlines shown on large scale topographiC ('" = 1 00' or 1" ~ 200' with 2 foot contours) maps with cultural features are desirable because the floodplain outlines can be defined more accurately, and physi. cal featLres can be shown more clearly. (Accuracy is important for effective floodpla n zoning and regulation. Administrative prOblems and the need for costly and time-consuming field surveys increase as map accuracy de. creases. Maps which are "nlarged from small scale maps have only the accurac,/ equivalent to that of the original small scale map.) 'RectifiE'd photographic contour maps with flood outlines must be used with great caution when establishing regUlatory elevations or floodplain boun- daries. These maps may be distorted. Contours drawn to scale may not match Itle photo background because of this distortion, On th" other hand, line maps showing pfanimetric features and ortho-photograhic maps are true to scale within their stated accuracy. The CWCS encourages local governments to obtain large-scale planimetric or ortho.photo contour maps for floodplain administration purposes. Large- scale maps can also be used for other community purposes such as planning and design of highways and st/'l~ets, water and sewer lines, and land use planninlJ, and for planninll and design of private developm"nts. United States National Map Accuracy Standard,. IA] Horizontal accuracynFor maps on publication scales larfjer than 1/20,000, not more than 20 percent of the points tested shall be in error by more than 1/30 inch, measured on the publication scale; for maps on publication scales of 1 :20,000 or smaller, 1/50 inch. These limits at accuracy shall apply in all cases to positions of well-defined points only. Well.defined points are those that are easily visible or recoverable on the ground. such as the following: monuments or markers, such as bench marks, property boundary monuments; intersections of roads, railroads, etc.; corners of large buildings or structures (or center points of small buildings); etc. In general what is well-defined will also be determined by what is plottable on the scale of the map within 1/100 inch_ Thus while the inter section of two road or property lines meeting at right angles would come within a sensiblB interpretation, identification of the intersection of such lines mel~ting at an acute angle would obviously not be practicable with 1/100 inch. Similarly, features not identifiable upon the ground within close limits are not to be considered as best points witl"1in the limits Quoted, even though their positions may be scaled closely upon the map. In this class would come timber lines, soil boundaries, etc. 17