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Ms. Erica Crosby <br />RE: Weld County. Hokestra Pit <br />April 3, 2008 <br />Page 4 of 4 <br />returns to the River. This means that reservoir IV will not likely be significantly lower than the <br />surrounding reservoirs. Therefore, a scenario where floodwaters would fill reservoir II prior to <br />reservoir IV and cause significant erosion to the berm are unlikely. <br />c. The stability model assumed the following assumptions: <br />•. The pit depth is 17 feet below the crest of the embankment. Design depth will vary between <br />12 and 15 feet. 17 feet is therefore a conservative approach. <br />• Embankment side slopes for reservoir II are at a 4:1. <br />• Embankment slopes for the St. Vrain are slightly steeper than 3:1. <br />• The river invert is at 12 feet below the bank. The river invert elevation was taken from <br />existing information. <br />• NWS and HWS of the St. Vrain were taken from the HEC-RAS model. <br />• .Soil information was derived from on-site soil tests. A copy of the tests is included. <br />;Type of '.Unit Moisture ;Saturated Cohesion 'Friction Angle <br />'Material '(pcf) Weight (pcf) ,Intercept (psf)(deg) _ <br />sand & gravel ~ 111.4 114.9'. 0' 35 <br />shale bedrock 128.9 132.4 2000' 35~ <br />__ _ <br />_ _ _ _ . _ <br />engineered fill`. 122.7 127.1 100' 20, <br />* Processed shale <br />• The existing riprap was not considered in the model. The model is therefore more <br />conservative than existing conditions. The existing riprap will add weight and stability to the <br />sand and gravel slopes that were assumed in the model; hence the stability of the slopes <br />would increase along with the factor of safety. <br />Results of the model are included as an appendix to this letter. <br />Erica, I trust this letter addresses your comments. You can trust that Weld County, as a public entity will <br />act. in accordance with protecting the public safety and welfare. Again, I appreciate your time and effort <br />on this project. Let me know if you need anything further.. <br />Sincerely, <br />Apple ate Group Inc. <br />Je ' er J Lee <br />Pr ect Engi eer <br />JJL/ta <br />Attachment <br />cc: Greg Nelson, Weld County Public Works <br />AG File #06-125 <br />\\Hercules\projects\06125 Hokestra lining\DMG permit\Submittal\Adequacy review\ILeply2.docx <br />