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<br />I The solution of the equations listed in the previous section assumes that the flow is <br />steady state and is not being gradually decelerated by a backwater condition during the <br />occurance of the peak discharge. <br /> <br />3.1.3 IT MUST BE RELATIVELY STRAIGHT. - While some bending of the <br />reach is allowable and often unavoidable, problems can arise when the reach has too <br />many twists and turns. Energy grade lines can vary from bank to bank as the flow <br />"superelevates" on the outside of bends. This can make it harder to define the reach <br />"fall", or drop in the energy grade line and sharp bends make it harder to accurately set <br />cross sections perpendicular to the flow. Additionally, sharp bends increase the <br />Iikelyhood of zero or reverse flow segments being incorporated in a cross section. <br /> <br />3.1.4 IT MUST BE LONG ENOUGH TO PROVIDE AT LEAST 3 CROSS <br />SECTIONS WITH APPROXIMATELY 0.5 FEET OF FALL OR MORE IN THE ENERGY <br />GRADE LINE BETWEEN THEM. - Ideally, a slope-area should have 3 to 6 cross sections <br />in order to improve the estimate of peak discharge. The distance between the cross <br />sections must be large enough to allow for appreciable fall between them or the equations <br />may not "converge" on a single discharge when solved together. As a general rule, the <br />length of the reach should be more than 75 times the mean channel depth in that reach. <br /> <br />3.2 LAYING OUT THE REACH. The slope-area reach can be laid out using <br />surveying pin flags and tape. The following lists the steps encountered: <br /> <br />3.2.1 STAKE OUT THE HIGH WATER MARKS. - High water marks must <br />be carefully selected on both banks. They can be marked with a pin flag when located <br />on the ground, or marked with tape tied around trees and fence posts. The base of the <br />pin should be right on the high water mark located on the ground. The bottom of the tape <br />should mark the location of the mark on vertical objects. Ideally, there should be 6 to 10 <br />good high water marks on each bank between cross sections. Some typical high water <br />marks are discussed below: <br /> <br />3.2.1.1 SEED LINES - These are found along the bank and mark the <br />furthest extent of the flood, They are made up of weed seeds and small plant debris and <br />may be found by carefully looking through the grass. The seed line is often no wider than <br />a pencil, but forms an excellent high water mark. <br /> <br />3.2.1.2 TRASH CAUGHT IN TREES, ON POLES AND ON FENCE POSTS <br />- This is a less accurate indicator of peak stage, but still very useful. Trash usually <br />consists of grass and weeds which snagged on the object. Care must be taken not to <br />select trash on bushes or saplings that could have been bent down by the flood waters, <br />which when returned to their natural shape, would leave the trash higher than the water <br />surface had actually been. <br /> <br />1-3 <br />