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Typical Drainage Basin [81 Vertical accuracy--As a'pplied to 'contour maps on all publication scales. vertical accuracy shall be such that not more than 10 percent of the elevations tested shall be in error more than one-half the contour mterval. In checking elevations taken from the map, the apparent vertical error may be decreased by assuming a horizontal displacement Within the permissible hOrizontal error for a map of that scale. [C] The accuracy of any map may be tested'by comparing the positions of points whose locations or elevations are shown upon it with corresponding positions as determined by surveys of a higher accuracy. Tests shall be made by the producing agency, which shall also determine which of its maps are to be tested, and the extent of such testing. [DJ Published maps meeting these accuracy requirements shall note this fact on their legends, as follows: "This map complies with national map accuracy standards." IE] Published maps whose errors exceed those aforestated shall omit from their legends all mention of standard accuracy. IF] When a published map is a considerable enlargement of a map drawing (manuscript) or of a published map, that fact shall be state.d in the legend. For example "This map is an enlargement of a 1 :2Q,QQO-scale map drawing," or "This map is an enlargement of a 1 :24,OOO-scale published map." IGI To facilitate ready interchange and use of basic information for map con- struction among all federal mapmaking agencies, manuscript maps and published maps, wherever economically reasonable and consistent with the uses to which the map is to be put. shall conform to latitude and longitude boundaries, being 15 minutes of latitude and longitude, or 7.5 minutes, or 3. 3/4 minutes in size. Local governments can use the National Map Accuracy Standards as their basic criteria in determining whether maps for which they will enter into a contract or existing maps they use are sufficiently accurate. If maps meet the National Map Accuracy Standards, that fact must be stated on the maps, Field Surveys Preparation of accurate field surveys is a prerequisite to any topographic mapping project. The field survey ties specifiC points on the map to known pOints on the ground and assures anyone using the maps of their relationship to conditions in the field. The provision of surveying monuments in the fieid ensures that the maps will b easier for local officials and property owners to use. The field surveys for base maps must be performed at a stan- dard of third-order control as established by the National Geodetic Survey, These standards are stated in a document entitied "Classification. Standards of Accuracy, and Generai Specifications of Geodetic Control Surveys," dated June, 1980. In addition, the CWCB has prepared specifications for obtaining both field surveys and large-scale topographic mapping which are available for local governments to use. The specifications are entitled, "Specifications for Photogrammetric Services for Floodplain Studies in Colorado" dated August, 1982@ Hydrologic Methods Hydrology deals with the water balance in a watershed, Hydrologic methods are used to determine flood flows and volumes, Science has now developed reasonably consistent methods for determining representative flow values,- A hydrologic investigation for a floodplain study will first determine the potential cause of flooding and then determine volumes or rates of flow that would be expected for a given set of probabilities, In a detailed hydrologic analysis these calculations will be performed for the 10-,50-,100-, and 500- year frequency flood discharges. The analysis will be based on existing drainage basin conditions except where future development within the basin may be significant. The selected flood discharges may be computed by either an analysis of available streamflow records or by a synthetic rainfa/l- runoff method, During the hydrologic investigations of a flOOdplain study the engineer will calculate the 1 OO-year flow. A 1 OO-year flood is a flood that would be expec' ted to happen once every 1 00 years or that has a one percent chance of occurring during any given year. Sections 24-65-105,24-65.1-103. and 24- 65.1-403(3)(b), CRS, 1973, as revised, established the 1 OO-year flood stan- dard as the basis for land-use planning and regulatory activities. The CWCS recommends the following guide in selecting the appropriate method(s) for a hydrologic investigation in a particular community, For developing urban areas, the hydrologist should select a synthetic rainfall-runoff method and correlate it to avaiiable gauge records, Acceptable models are: 1) U.S. Army Corps of Engineers, Flood Hydrograph Package (HEC-1) (Davis, CA 1969.) , 2) Wright-McLaughlin Engineering, Urban Storm Drainage Criteria Manual, Volumes I and IIl0enver, CO 1969.)