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<br /> <br />12/20/79 Memo, p. 4, #8 <br /> <br />"8. Curve numbers used to calculate flow for the <br />permanent diversions appear low. What assumptions went <br />into the selection of curve numbers. Show calculation <br />of average curve numbers. If the runoff from reclaimed <br />surfaces is assumed to be less than that from natural <br />surfaces, ditches should be redesigned using higher <br />curve numbers." <br />LRCWE has recalculated the curve numbers for EFC, <br />using CN 90 (AMC-II) for disturbed areas, CN 68 <br />for undisturbed areas. Average CN calculations <br />were performed using standard area-weighting <br />techniques, as shown on p. 816-194. Figure 32, <br />p. 780-153, has been revised to show the new <br />average CN, runoff volume and peak discharge (100 <br />year - 24 hour). Backup calculations have been <br />included in Appendix C which has been added to Exhibit <br />13. <br />See also p. 780-151(a) and revised Figure 32 <br />12/20/79 Memo, p. 4, #9 <br />"9. The "n" values used to determine flow velocities <br />in the permanent ditches appear high. Demonstrate the <br />applicability of these values. Provide example calcu- <br />lations of velocity." <br />LRCWE has re-sized the permanent diversion ditches <br />for EFC, using Charts 15, 17, 19 and 21 from <br />"Design Charts For Open-Channel Flow" previously <br />cited. For grass channels an "n" of 0.03 was <br />used; riprap lined channels were assigned an "n" <br />of 0.045. The revised Figure 32 reflects these <br />lower "n" values. Backup calculations have been <br />included in Appendix C which has been added to <br />Exhibit 13. <br />See also, p. 780-151(a) <br />