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<br />0;)1593 <br /> <br />32.8--S <br /> <br />WATER INFORMATION MANAGEMENT SYSTEM HANDBOOK <br /> <br />_. <br /> <br />c. A recession segment composed of a series of single day decrements <br />in flow will generally prevent adverse streambank effects from rapid <br />drawdown. <br /> <br />If local data and professional judgement indicate the need, the hydrologist <br />should modify this step to make allowance for situations which do not fit <br />the above assumptions. <br /> <br />Considering that segment of the flow-duration curve which lies between the <br />bankfull discharge (Q6/QB) ratio point and the QA/QB ratio point <br />'(ex. 2), locate graphlcally the rising-recession flow steps as follows: <br /> <br />(1) Beginning at the QB/QB ratio point on the flow-duration curve, <br />extend a vertical line down to intersect the 0.90 ratio level. From this <br />intersection, extend a horizontal line to intersect the flow-duration <br />curve. This becomes the first step of the rise-recession flow component. <br />The duration of this component is determined by converting to days and <br />subtracting the percent time increment limits, in days, of the step (that <br />is, the difference between the bankfull discharge time increment and the <br />0.90 flow-duration curve intersection point time increment). <br /> <br />(2) Succeeding rise-recession steps and their durations are determined <br />in a similar fashion. For example, extend a vertical line from the 0.90 <br />curve intersection point down to intersect the 0.80 ratio level, then <br />extend a line horizontally to intersect the flow-duration curve. This <br />becomes the second rise-recession flow step. <br /> <br />(3) Proceed in the above manner, using 0.1 decrements in the y-axis <br />ratio levels, until a decrement value falls below the QA/Qa ratio. At <br />the QA/QB ratio extend a horizontal line to the flow duratlon curve. <br />Next extend a vertical line from this last flow-duration intersection point <br />down to intersect the x-axis (A time scale). Note: Ten percent decrements <br />are used to reasonably approximate the rise-recession limbs of the natural <br />hydrograph. <br /> <br />(4) Multiply the calculated durations by 0.5. The product represents <br />the rise-recession durations which will be required. Finally, 1 day, <br />representing the recession limb duration, is subtracted from each <br />rise-recession duration having a magnitude equal to or greater than 2 <br />days. The remainder represents the duration for the individual rise step. <br />For example, calculating the rise portion of the 0.8 to 0.7 ratio step: <br />Qrise(0.8_0.7) = (0.5 QR(0.8-0.7)) - 1 day. Note: See appendix 8 for an <br /> <br />example of these calculations. <br /> <br />Common Step 3 - Calculate required baseflow ratio and duration. <br /> <br />Consider the location of the QLl/QB ratio point on the flow-duration <br />curve; extend a line from this point vertically to intersect the x-axis <br />(6 time scale). The duration of the QLl baseflow component is the <br /> <br />ID No. 3 <br />8/28/87 <br />