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<br />-\ ~~..
<br />
<br />'\
<br />_.
<br />~..,.,.
<br />
<br />L. Ncar l~{)uldcr. Located below tile confluence of Sixlnilc Canyon
<br />
<br />with Lefthand Creek, the gage provided a record from 1929 to 1931, from
<br />
<br />1947 to 1953. and [rem 1956 [0 1957. A generalized skew coefficient of
<br />
<br />+1, obtained tram a Corps of Engineers regional study of mountain streams .~
<br />
<br />in Colorado, was used to help shape the distribution. Frequency data for
<br />
<br />these streams is shown on attachment 2. The resulting curve, with confi-"
<br />
<br />dence limits and adjUStments tor length of record, is shown on attachment 3.
<br />
<br />c. At Mouth at Longmont. Eighteen years of record, 1927 to 1942, 1954
<br />
<br />and 1955, were used in the WRC frequency analysis. The largest annual
<br />
<br />~, ... '
<br />
<br />peak discharge 'of 812 cubic feet per second occurred in September 1938.
<br />
<br />A value of +1 was selected for the generalized skew coefficient. The
<br />
<br />frequency curve, with confidence limits and adjustments for"length of
<br />
<br />record, is shown on attachment 4.
<br />
<br />4. MODELING STUDIES.
<br />
<br />a. General. Th, Lefthand Creek model that was developed for the St.
<br />
<br />Vrain Creek study wa:: used, with some modifications, in this study. Both
<br />
<br />studies utilized the Environmental Protection Agency's Storm Water Manage-
<br />
<br />ment Model (SWMM). ~torms varying in frequency from 10- to 500-yearsand
<br />
<br />.in duration from 1- to 6-hours were modeled by Sl.:HM to obtain the discharges
<br />
<br />presented belo\o1.
<br />
<br />b. Rainfall. R'.in[all values for the 10-, 50-, and 100-year, 1-,
<br />
<br />3-, and o-nour event~ were obtained from the "PreCipitation-Frequency ~tlas
<br />
<br />of the Hestern Unite, States, Atlas 2, Volume Ill, Colorado," published
<br />
<br />by the National OceaIiic and Atmospheric Administrat:ion in 1973. In each
<br />
<br />case, a 500-year value was estimated 'by extrapolating t:he selected Atlas
<br />
<br />2
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