Floodplain Information Report Volume 2 White River from Rio Blanco Lake to the Utah-Colorado State Line

F1 n 1 r 4.3 Field Surveys and Cross - Section Preparation The project budget and schedule only allowed for field surveying of three cross - sections on the White River. The selection of the field- surveyed cross - sections was based on the availability of a nearby benchmark, the accessibility of the location, and the ability to survey the opposite bank. Field estimates were made of the average water depth, but the channel bottoms were not surveyed. Estimations of the Mannings 'n' value were made at each surveyed cross - section location during the field survey. For some reaches, field- surveyed cross - sections from other stream reaches were transferred to the reach in question. Appropriate adjustments were made to ground elevations. When flow depths were computed, the appropriate slope values, Manning's 'n' values, and flow values were used. Map- derived cross - sections were utilized for the remaining reaches. Some map- derived cross - sections were modified to more accurately represent actual channel shapes. When the USGS maps were felt to lack sufficient detail, the map data were combined with field - surveyed ,data from other locations to prepare hybrid cross - sections. Canyon reaches on the White River were not represented by cross - sections since the level of detail obtained could not be displayed on the base maps. The cross - sections used for the computation of flood depths can be grouped into four different categories: Type 1 - Field - surveyed cross - sections used only in the representative reach; Type 2 - Field- surveyed cross - sections transferred from a reach with similar characteristics using input parameters specific to the reach in question; Type 3 - Hybrid cross - sections using map- derived data and field observations; Type 4 - Map- derived cross- sections using flood depths from published report; By using these categories to identify cross - section sources, 7 of the 10 reaches on the White River can be divided into the following groups: Type 1 Cross - sections - 3 reaches Type 2 Cross - sections - 1 reach Type 3 Cross - sections - 2 reaches Type 4 Cross - sections - 1 reach There were 3 additional reaches which were classified as follows: 2 canyon reaches for which no cross - sections were developed; 1 reach where a detailed study had already been performed. Table S identifies the cross - sections, the group type, and flow values used to calculate the flood depths. The cross - sections, with 50 -year and 100 -year flood lines illustrated, are presented after Section 4.4 in Figures 7A -7F. 17