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I I I I I I I I I I I I I I I I I I I 7 V HYDROLOGIC ANALYSIS in Lakewood and McIntyre Gulches that can be directly used to estimate the frequency of either major floods or major rainfall events. Therefore, it was necessary to prepare estimated rain- fall events and flood hydrographs indirectly using records and data available from the surrounding area. Major floods in the area result primarily from heavy rainfall. Major floods do not result from snow melt, nor is snow melt considered to increase rainfall floods significantly, because of the relatively low elevation of the contributing watershed. Introduction The Urban Drainage and Flood Control District directed that the hydrology and the description of the analysis as prepared by McCall-Ellingson & Morrill, Inc. be accepted for this study. The remainder of this chapter is largely a reproduction of the hydrology prepared by McCall-Ellingson & Morrill in a report titled "Phase A, Preliminary Report on Major Drainageway Plan- ning on Lakewood and McIntyre Gulches, Volume 1 Report," dated December, 1975, revised January, 1976. The report included an analysis of McIntyre Gulch, which is not a part of this study. Certain minor revisions of peak flow rates have been made after careful evaluation, but the changes are of only academic sig- nificance. Portions of the original text which referred to hydrology for existing basin conditions, rather than future, have been deleted because they are not applicable to this report. The table numbers used in the original report have been revised to conform to the format and sequence of this report. Procedure The Lakewood and McIntyre Gulch hydrographs were prepared using primarily the data and methods presented in the Urban Storm Drain- age Criteria Manual, Volume I, Denver Regional Council of Gov- ernments, 1969, which is referred to herein as the Drainage Criteria Manual. The general sequence followed in preparing the hydrographs is as follows: Floods There are neither stream flow records nor precipitation records 1. Estimate precipitation duration and depth for each fre- quency from rainfall records in the areas. 2. Arrange the total rainfall amount into a typical rainfall pattern for the area. 3. Estimate the effective rainfall by deducting watershed retention and losses from total precipitation. 4. Determine hydrograph shape using watershed characteristics. A computer program was developed to calculate the large number of hydrographs that were required. General Flood hydrographs were prepared for 24 hydrologic points along Lakewood Gulch amd McIntyre Gulch as shown on Sheet 2 of 23. Hydrographs were computed at each point for 2, 5, 10, and 100- year frequency floods for future conditions of basin development.