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Page 6a of 9 <br />Minor Revision No. 131 <br />Reclaimed Ditch 7 Channel lining <br />The peak flows used to size riprap for Ditch 7 were calculated on pages 5 and 6. Since <br />flow from point B flows into point C as shown on Map 6A -5D, the peak flow used for <br />riprap sizing in Ditch 7 is the summation of flow from points B and point C, resulting in a <br />peak flow of 15.3 cfs. A Manning's n of 0.035 will be used for consistency. <br />Ditch 7 varies in grade from approximately 12 -45 %, as shown on page 6e (Figure 1). The <br />12% section parallels the road and the design will remain valid; no changes will be made <br />to the specified riprap size. <br />After the ditch segment leaves the road, it becomes very steep, approximately 40% for <br />200'. In this segment, D50 = 18 ", with grouted riprap will be used. The grout will be a <br />low strength concrete, and the purpose is to fill the voids between the rock in order to <br />anchor the riprap on the steep slope. To the extent possible, the grout will be placed <br />between the rocks, so the roughness ( Manning's N) will not change. <br />In the main (middle) segment of Ditch 7, the grade varies from approximately 25 -35 %. <br />Therefore, 35% will be used to size the riprap. <br />The lower segment of Ditch 7 varies in grade from approximately 38 -45 %. Therefore, <br />45% will be used to size the riprap. Using a Manning's n of 0.035 would specify a <br />D50 =19 ". Using a Manning's n of 0.030 would specify a D50 =27 ". Therefore, D50 = 19" <br />to 27" is acceptable. D50 = 26 -30" is currently in place and performing well. The voids <br />between the rocks have begun to fill in with sediment which is key to long -term channel <br />stability. Using a larger D50 riprap will provide a more conservative design. <br />y p,E S <br />�0,FSSION NO*\O 41 <br />Vol. 6A MR-13109/13 <br />