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Clarification of Recommendations for Lake Embankment Slope Protection Terracon <br />Bayshore Development -Weld County, Colorado <br />Terracon Project No. 25045300 <br />Station 51+50 to 55+00, and Station 63+00 to 65+50 (spillway location). These <br />areas are the only locations where the lake embankment will be subjected fo <br />water flows by the St. Vrain River during normal flow conditions. As previously <br />outlined, these areas have previously been heavily armored with recycled <br />concrete materials which extend to the bottom of the existing river channel. In <br />these areas, the existing slope configuration is generally about 1-1/2 :1 to 2:1 <br />and the Waters of the US delineation is generally located within or directly <br />adjacent to the existing armoring. It is the goal of the proposed construction to <br />not disturb the existing armoring within the river channel or encroach the Waters <br />of the US boundary. The proposed rip rap improvement would be keyed behind <br />the existing armoring which extends to the river channel at the toe, and extend <br />from that point at a 2-1/2:1 slope to the crest of the lake embankment. <br />Based on our observations of the existing concrete materials in these locations, it <br />is our opinion that these materials will be sufficient to resist undercutting of the <br />embankment edge. Along the remainder of the southern lake embankment <br />which is not located directly adjacent to the St. Vrain River, the proposed rip rap <br />shall be embedded a minimum of about 2 to 3 feet at the toe of slope to provide <br />for undercutting protection and extend to the crest of the slope at an inclination of <br />2-1/2:1 as outlined in our previous letter. Attached to this letter are cross <br />sections of the proposed typical details/sections of the slope protection measures <br />and a plan view showing the extents of the proposed rip rap protection along the <br />southern lake embankment. <br />iii. As previously outlined, during normal flow conditions, the newly placed rip rap <br />materials will not be in contact with waters flowing within the St Vrain River. As <br />such, no impacts to the St. Vrain River and associated downstream parties are <br />anticipated. In fact, rip rap should reduce scour and surtace erosion from runoff <br />which could cause transport of sedimentation downstream. <br />iv. Generally, with all other factors being equivalent, the stability of an embankment <br />decreases with increasing height or increasing water head differential across the <br />embankment. As such, the width of the embankment used for the stability <br />analyses simulating the highest (worst case) embankment heights and head <br />differentials were used. Based on the proposed grading plans, a minimum <br />embankment width of ten (10) times the height of the spillway crest in relation to <br />the river channel base will be maintained (i.e., a ten foot elevation differential will <br />be a minimum of 100 feet in width). As noted, in the previous letter, the Factor of <br />Safety (FS) for the worst case conditions meets or exceeds the required FS of <br />1.2 for rapid drawdown and 1.5 for static conditions, respectively. Supplemental <br />stability analysis runs with a narrow configuration are attached for reference. <br />3 <br />