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<br />e <br /> <br />. <br /> <br />, <br /> <br />e <br /> <br />6.1.3.3 Flow Resistance <br /> <br />Mannings roughness ("n") values must be determined at each cross section for <br />the flood plain and main channel. USGS Water Supply Paper 1849 has good <br />guidelines for estimating "n" values for main channels. The USGS is currently <br />conducting a study for FHWA in Louisiana to develop procedures for estimating <br />"n" values for heavily vegetated flood plains. Several hydraulic texts have <br />estimated "n" values for various crops. Some typical "n" values are tabulated <br />below. <br /> <br />Table 6.1 Typical "n" values for Flood Plains <br /> <br />Flood Plain Cover Values of n* <br /> <br />Corn <br />Small Grain <br />Pasture <br />Brush & Waste <br /> <br />.06 - .07** <br />.06 - .10 <br />.04 - .05 <br />.08 - .12 <br /> <br />* Values tabulated from Chow page 104 (23) <br />** Higher values of "n" generally relate to lower depths of flow <br /> <br />6.1.4 Geologic and Soils Data <br />6.1.4.1 Channel Morphology <br /> <br />Sequential aerial photographs should be examined to determine susceptibility <br />of the channel to lateral migration and degradation. <br /> <br />6.1.4.2 Soils Information <br /> <br />Soil samples should be obtained and analyzed for grain size distribution and <br />cohesive properties. Resistance to penetration and soil profiles should also <br />be obtained. <br /> <br />6.1.4.3 Scour History <br /> <br />Past observations of bridge scour should be used to enhance engineering <br />. judgement in making scour predictions for various bridge alternatives. <br /> <br />e <br /> <br />6.1.5 Construction Costs <br /> <br />Construction cost estimates for each alternative are necessary to the LTEC <br />design process. Construction costs include initial embankment, pavement and <br />structural costs. Any maintenance costs which vary with alternatives should <br />also be included. Estimates should include allowance for varying foundation <br />depths to account for scour. Estimates should also allow for other scour <br />protection measures such as spur dikes and riprap. <br /> <br />21 <br />