Laserfiche WebLink
1 <br />1 <br /> <br /> <br /> <br /> <br /> <br />1 <br /> <br />TABLE 5 <br />WHITE RIVER CROSS-SECTION IDENTIFICATION <br />REACH <br />NUMBER FLOW VALUES (CFS) <br />50-YR, '100-YR <br />(NEAREST <br />ITYDROLOGIC POINT) CROSS- <br />SECTION CROSS-SECTION TYPE <br />NUMBER DERIVATION <br />OF FLOOD <br />DEPTHS <br />1 6934, 8133 (W1) Z-Z' 3 CALCULATED <br />2 9565, 12501 (W2) A-A' 1 CALCULATED <br />3 10137, 13508 (W3) NONE CANYON REACH N/A <br />4 11007, 15079 (W4) B-B' 1 CALCULATED <br />5 12051, 17021 (W7) D-D'* 1 CALCULATED <br />6 12051, 17021 (W7) E-E' 2 (FROM B-B') CALCULATED <br />7 13500, 20000 (W9) NONE DETAILED STUDY <br />ALREADY PERFORMED FEMA STUDY <br />8 13500, 20000 (W9) FA-FA' 3 CALCULATED <br />9 13500, 20000 (W9) NONE CANYON REACH N/A <br />10 14539, 21873 (W10) F-F' 4 FEMA STUDY <br />* Map-derived cross-section C-C' was prepared, but it was not used. It was omitted because the field-surveyed <br />cross-section, D-D', located in the same reach, better represents that reach. <br />1 4.4 Computing Flood Depths <br />Flood depths were calculated at each cross-section through a computer program (QANY) which <br />uses Manning's equation. The input parameters for the calculation of flow included: x and y <br />coordinates for points on the cross-section, Manning's 'n' values (ranging from 0.03 to 0.032 for <br />the channel and 0.045 to 0.065 for the overbank) (Reference 7), the channel slope (interpreted <br />from USGS quadrangle maps), and depth values. The program calculated flow and velocity <br />values for each depth value entered. The target flow values were those values established in <br />Section 3.0 and shown on Table 3. Rating curves were established from the calculated depths. <br />From the rating curves 50-year and 100-year flood elevations were established at each location. <br />In performing the depth calculations the flow on the river the day of the survey was obtained <br />from the USGS and subtracted from the established 50-year and 100-year flows for each <br />particular hydrologic point. The depth of water on the day of the survey was estimated and <br />added to the flood depth calculations to obtain the true flood depths shown in Table 6. The <br />methodology used for computing flood depths at the cross-sections is similar to accepted <br />methodologies used by state and federal agencies in performing detailed analyses, except that <br />there is no balancing of energy from one cross-section to the next. <br />1 <br />18 <br />