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4.3 Field Surveys and Cross - Section Preparation <br />The project budget and schedule only allowed for field surveying of three cross - sections on the <br />Gunnison River. The selection of the field- surveyed cross - sections was based on the availability <br />of a nearby benchmark, the accessibility of the location, and the ability to survey the opposite <br />bank. Field estimates were made of the average water depth, but the channel bottoms were not <br />surveyed. Estimations of the Mannings 'n' value were made at each surveyed cross - section <br />' location during the field survey. <br />For some reaches, field- surveyed cross - sections from other stream reaches were transferred to the <br />reach in question. Appropriate adjustments were made to ground elevations. When flow depths <br />were computed, the appropriate slope values, Manning's 'n' values, and flow values were used. <br />' Map- derived cross - sections were utilized for the remaining reaches. Some map- derived cross - <br />sections were modified to more accurately represent actual channel shapes. Canyon reaches on <br />' the Gunnison were not generally represented by cross - sections since the level of detail obtained <br />could not be shown on the base maps. There was one exception where a cross - section was used <br />for a canyon reach, and it is discussed below. <br />The cross - sections used for the computation of flood depths can be grouped into three different <br />' types: <br />Type 1 - Field surveyed cross - sections used only in the representative reach; <br />' Type 2 - Field surveyed cross - sections transferred from a reach with similar <br />characteristics using input parameters specific to the reach in question; <br />Type 3 - Hybrid cross - sections using map - derived data and field observations; <br />By using these categories to identify cross - section sources, 8 of the 12 reaches on the Gunnison <br />River were divided into the following groups: <br />' Type 1 Cross - sections - 3 reaches <br />Type 2 Cross - sections - 2 reaches <br />IType 3 Cross - sections - 3 reaches <br />There were 4 additional reaches which were classified as follows: 3 canyon reaches for which <br />' no cross - sections were developed; 1 reaach where a detailed study had already been performed. <br />Table 5 identifies the cross - sections, the group type, and flow values used to calculate the flood <br />' depths. The cross - sections, with 50 -year and 100 -year flood lines illustrated, are presented after <br />Section 4.4 in Figures 7A -7H. <br />15 <br />