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<br />M'lS8~ <br /> <br />32.8--4 <br /> <br />'. <br /> <br />WATER INFORMATION MANAGEMENT SYSTEM HANDBOOK <br /> <br />line to the flow duration curve. Next extend a vertical line <br />from this last flow-duration intersection point down to <br />intersect the x-axis (:c time scale). Note: Ten percent <br />decrements are used to reasonably approximate the rise-recession <br />limbs of the natural hydrograph. <br /> <br />(4) Multiply the calculated durations by 0.5. The product <br />represents the rise-recession durations which will be required. <br />Finally, 1 day, representing the recession limb duration, is <br />subtracted from each rise-recession duration having a magnitude <br />equal to or greater than 2 days. The remainder represents the <br />duration for the individual rise step. For example, calculating <br />the rise portion of the 0.8 to 0.1 ratio step as follows. <br /> <br />Q = <br />rise <br />(0.8-0.1) <br /> <br />(0.5 Q ) - 1 day. <br />R <br />(0.8-0.1> <br /> <br />Note: See Section 36.2 for an example of these calculations. <br /> <br />C. Common SteD ~. Calculate required baseflow ratio and <br />duration. <br /> <br />Consider the location of the Q /Q ratio point on the <br />flow-duration curvej extend a ~lneBfrom this point vertically to <br />intersect the x-axis (S time scale). The duration of the QLl <br />baseflow component is the difference in time increments, <br />converted to days, between the last rise-recession step and the <br />QL1/QB time scale intersection points. <br /> <br />The QL2/QB ratio point, the lowest flow (highest exceedance <br />value] on the flow-duration curve, is used to estimate the <br />amount of low-flow that may occur during the remaining time <br />increment (that is, from the QI.,/QB intersection point to <br />the end of the time scale). Tne required baseflow is Q or <br />the natural flow whichever is the lesser. The Q flowLtate <br />and duration are computed only for the purpose o~2estimating the <br />total low-flow volume contained in the flow component which is <br />less than baseflow. <br /> <br />D. Common SteD 4. Using local data and professional judgement <br />determine if additional flows are needed to prevent vegetation <br />encroachment. <br /> <br />If in the user's professional judgement the potential for <br />vegetation encroachment is low, proceed to Common Step 8. If <br />potential for vegetation growth between the top of the inner <br />berm and the start of the channel banks is high, quantify the <br />necessary flows using Common Steps 5 through 1. Base the <br />assessment of vegetation growth potential upon the following. <br /> <br />*-FSH 4/89 AMEND 3-* <br />