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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />III HYDROLOGIC ANALYSIS <br /> <br />Floods <br /> <br /> <br />There are neither stream flow records nor precipitation records <br /> <br /> <br />in Lakewood and McIntyre gulches that can be directly used to <br /> <br /> <br />estimate the frequency of either major floods or major rainfall <br /> <br /> <br />events. Therefore, it was necessary to prepare estimated rain- <br /> <br /> <br />fall events and flood hydrographs indirectly using records and <br /> <br /> <br />data available from the surrounding area. Major floods in the <br /> <br /> <br />area result primarily from heavy rainfall. Major floods do not <br /> <br /> <br />result from snow melt, nor is snow melt considered to increase <br /> <br /> <br />rainfall floods significantly, because of the relatively low <br /> <br /> <br />elevation of the contributing watershed. <br /> <br />6 <br /> <br />Introduction <br /> <br /> <br />The Urban Drainage and Flood Control District directed that the <br /> <br /> <br />hydrology and the description of the analysis as prepared by <br /> <br /> <br />McCall-Ellingson & Morrill, Inc. be accepted for this study. <br /> <br /> <br />The remainder of this chapter is a reproduction of the hydrology <br /> <br /> <br />prepared by McCall-Ellingson & Morrill in a report titled <br /> <br /> <br />"Phase A Preliminary Report on Major Drainageway planning on <br /> <br /> <br />Lakewood and Mc:tntyre Gulches, Volume 1 Report," dated December, <br /> <br /> <br />1975. The original report included an analysis of McIntyre <br /> <br /> <br />Gulch, which is not being re-analyzed in this report. Certain <br /> <br /> <br />minor revisions of peak flow rates have been made after careful <br /> <br /> <br />evaluation, but the changes are of only academic significance. <br /> <br /> <br />Figures III-I, :tII-2 and 111-3 have been added at the end of <br /> <br /> <br />this Section to illustrate graphically the hydrologic analysis. <br /> <br /> <br />The figures are developed from data of the original report. <br /> <br />General <br /> <br />Flood hydrographs were prepared for 24 hydrologic points along <br />Lakewood Gulch and McIntyre Gulch as shown on Sheet 2 of 22. <br />Hydrographs wer~ computed at each point for 2, 5, 10, and 100- <br />year frequency floods, for both existing and future conditions <br />of basin development. <br /> <br />Procedure <br /> <br /> <br />The Lakewood and McIntyre gulch hydrographs were prepared using <br /> <br /> <br />primarily the data and methods presented in the urban Storm <br /> <br /> <br />Drainage Criteria Manual, Volume I, Denver Regional Council of <br /> <br /> <br />Governments, 1969, which is referred to herein as the Drainage <br /> <br /> <br />Criteria Manual. The general sequence followed in preparing <br /> <br /> <br />the hydrographs is as follows: <br /> <br /> <br />1. Estimate precipitation duration and depth for each <br /> <br /> <br />frequency from rainfall records in the area. <br /> <br /> <br />2. Arrange the total rainfall amount into a typical rain- <br /> <br /> <br />fall pattern for the area. <br />