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<br />18
<br />
<br />LAWN LAKE DAM AND CASCADE LAKE DAM FAILURES, COLORADO
<br />
<br />pocket at the back of the report. Storage-capacity data
<br />and other pertinent information for the two lakes are
<br />shown in table 3.
<br />The vertical datum for the Lawn Lake topographic
<br />maps was approximated from an existing U.S. Geolog-
<br />ical Survey 7Y,-min quadrangle map. as the site was so
<br />remote from any available accurate vertical control.
<br />These surveys indicated that the failure of Lawn Lake
<br />dam released 674 acre-ft of water on July 15, 1982. As
<br />indicated in figure 12, a substantial amount of water re-
<br />mained as dead capacity, and almost corresponding to
<br />the original moraine-dammed mountain lake. Although
<br />the lake level at the time of failure was 1 ft deep in the
<br />overflow spillway, the water released was considerably
<br />less than previously believed to have been stored behind
<br />the dam (Office of the State Engineer, 1983). The main
<br />reason for this difference appeared to be that the lake
<br />bed immediately upstream from the dam embankment
<br />was 6.4 ft higher than the inlet elevation of the outlet
<br />pipe which was located on bedrock. This outflow con-
<br />trol probably had been present since construction of the
<br />dam, and was not reflected in earlier lake surveys.
<br />Determination of the capacity of Cascade Lake dam
<br />was more difficult. as severe bed erosion (fig. 9E) oc-
<br />curred after the dam failure. Postflood surveys indicated
<br />that approximately 1.53 acre-ft (table 3) of sediment
<br />
<br />was removed from the lakebed during the flood.
<br />Therefore, field inspections and surveys were made to
<br />reconstruct the preflood lakebed. All the evidence in-
<br />dicated that the preflood lakebed was at an elevation
<br />of 8,560 ft, plus or minus 1 ft. Contours in plate 2 reflect
<br />postflood contours; extensive scour occurred with a
<br />maximum of approximately 12 ft. Storage-capacity data
<br />and other pertinent data for Cascade Lake, assuming
<br />a lakebed elevation of 8,560 ft, are shown in table 3.
<br />Cascade Lake contained approximately 12.1 acre-ft of
<br />water prior to the flood, and 25.1 acre-ft of water at the
<br />time of the Cascade Lake dam failure. The total volume
<br />of water released from the failures of the Lawn Lake dam
<br />and the Cascade Lake dam was approximately 686
<br />acre-ft.
<br />For the period 0845 MDT on July 15, to 1200 MDT
<br />on July 16, C. W. Huntley (U.S. Bureau of Reclamation,
<br />written commun., 1982) estimated the inflow to Lake
<br />Estes, resulting from the failures of Lawn Lake and
<br />Cascade Lake dams, was 663 acre-ft. The difference be-
<br />tween the volume released from the two lakes and the
<br />volume entering Lake Estes probably was the result of
<br />water remaining in valley storage, which had not reached
<br />Lake Estes by July 16 at 1200 MDT, and of
<br />evapotranspiration and infiltration (particularly in
<br />Horseshoe Park). Errors in the methods of determining
<br />
<br />TABLE 3.-Storage capacity and other pertinent information for Lawn Lake and Cascade Lake
<br />
<br /> Lawn Lake
<br /> Surfaceb Storageb Surfaceb Storageb
<br />Contour- capacity, Contour. capacity,
<br />elevation, area, in "' elevation, area, in "'
<br />in feet acres acr&feet "'''''' ""'" acre-feet
<br />10,975.69 0.0 10,990 42.29 362.19
<br />10,978 8.93 10,992 45.09 449.55
<br />10,980 23.14 30.96 10,994 47.29 541.92
<br />10,982 27.82 81.85 10,996 49.33 638.53
<br />10,984 31.01 140,66 10,998 50.73 738.59
<br />10,986 35.24 206.86 11,000 52.50 841.81
<br />10,988 38.93 281.00 11,002 54.59 948.89
<br /> 11,004 56.57 1,006,40
<br /> CSlleade Lake
<br /> St,..... Storage
<br />Contoure 8uriaoe capacity, Contourc Burl,,, capacity,
<br />elevation, area, in "' elevation. area, f.n in
<br />in feet ""'" acre-feet "'met ''''' acre-feet
<br />8,448 0.0 ~.O 8,464 0.70 '2.10
<br />8,450 .02 .01 8,466 .89 '3.69
<br />8,452 .07 d.09 8,468 1.24 85.81
<br />8,454 .11 d.27 8,470 1.86 '8.89
<br />8,456 .18 d,56 8,472 2.25 '12.99
<br />8,458 .23 d,97 8,474 2,80 '19.56
<br />8,460 .33 d1.53/0' 8,476 3,41 '25,76
<br />8,462 ,54 8.86 8,478 3.84 '33.01
<br />llApproxiInate mean sea level; water-surface elevation at failurll was 10,996.42 feet.
<br />b Above dead etorage.
<br />cMean sea level; water-surface elevation at. failure WILS 8,476.8(1 feet.
<br />dEstimat.ed amount below preflood lakebed elevatioD of 8,460 feet.
<br />6Storage capacit.y above ~timated preflood lakebed elevation of 8,460 feet..
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