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<br />Channel: Camp Creek dredged channel near Seymour, Illinois) <br />see USDA Technical Bulletin No. 129, Plate 29-C for photo- <br />graph and Table 9, page 86, for data. <br /> <br />Description: Course straight) 661 feet long. Cross <br />section, very little variation in shape) variation in <br />size moderate, but changes. not abrupt, Side slopes <br />fairly regular, bottom uneven and irregular, Soil, <br />lower part yellowish gray "lay) upper part, light <br />gray silty clay loam, Condition, side slopes cover- <br />ed with heavy growth of poplar trees 2 to 3 inches in <br />diameter, large willows and climbing vines) thick <br />growth of water weed on bottom) summer condition with <br />vegetation in full foliage, <br /> <br /> <br />~ <br /> <br />B,7. <br /> <br />factors requiring most careful consideration are obstructions and vege- <br />tation, Many agricultural flood plains have fairly dense networks of <br />fences to be evaluated as obstructions in Step 4. Vegetation probably <br />would be judged on the basis of growing season conditions, <br /> <br />Field and office work, <br /> <br />It is suggested that field parties record adequate notes on field <br />conditions pertinent to the five factors affecting n at the'time cross <br />section surveys are being made, The actual estimates of n may then be <br />made in the office. This will require training of both field and <br />office personnel. The conditions to be covered by field notes and <br />considered in the estimate of n apply to a reach of channel and flood <br />plain, It is not adequate to consider only those conditions in the <br />immediate vicinity of a cross section. Note the sketch on Figure B,l. <br />With cross sections located as shown, field.notes should describe the <br />channel and flood plain conditions through the reach indicated as a <br />basis for estimating the n values (assuming subdivided sections) to <br />be incorporated in the hydraulic computations at Section 2. <br /> <br />Figure B,2 shows a sample set of notes that illustrate the type <br />of field information to be recorded as a basis for estimating n, <br />Field men should be trained to recognize and record in brief state- <br />ments those conditions that are necessary for realistic evaluation <br />of the five factors discussed under procedural Steps 1 to 6, <br /> <br />Example 1. Estimation of n for a reach. <br /> <br />This example is based on a case where n has been determined so <br />that comparison between the estimated and actual n can be shown. <br /> <br />, <br /> <br />Average cross section approximates a trapezoid with <br />side slopes about 1,5 to 1 and bottom width.about 10 <br />feet, At bankfull stage, average depth and surface <br />width are about 8.5 and 40 feet respectively, <br />