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<br />3.0 ENGINEERING METHODS <br /> <br />hydrologic and hyaraullc stUdy methOdS were useU1Co ae~eTmIrre-- <br />the flood hazard data required for this. study. Floods having <br />recurrence intervals of 10-, 50-, 100-, and 500-years have <br />been selected as having special significance for flood plain <br />management and for flood insurance premium rates. The anal- <br />yses reported here reflect current conditions in the water- <br />sheds of the streams. <br /> <br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish the <br />peak discharge-frequency relationships for floods of the <br />selected recurrence intervals for each stream studied in <br />detail in the community. <br /> <br />Boulder Creek at Boulder drains 240 square miles, including <br />109 square miles in South Boulder Creek. Most of this <br />area is on the steep mountain slopes. Floods are usually <br />caused by intense rainfall in the spring and summer <br />months. Discharge-probability relationships for Boulder <br />Creek and South Boulder Creek at Boulder are based on <br />statistical analyses of stream flow records in the basin <br />and analyses of rainfall and runoff characteristic of the <br />region. The effect of reservoirs upstream from Boulder <br />was considered. The following table gives values of <br />the discharges for Boulder Creek and South Boulder <br />Creek at Boulder which were used for this study. <br /> <br />Stream <br /> <br />Peak Discharge <br />(Cubic Feet per Second) <br />10-Year 50-Year 100 -Year 500-Year <br /> <br />Boulder Creek <br />South Boulder Creek <br /> <br />3,500 <br />1,200 <br /> <br />8,450 <br />3,100 <br /> <br />11,800 <br />5,000 <br /> <br />21,400 <br />15,500 <br /> <br />The elevation-frequency curves up to the laO-year flood <br />level were derived from data used in the development of <br />the profiles. The probability curve was extended <br />logarithmically to the 500-year value and the elevation <br />determined by use of a rating curve. <br /> <br />Peak discharges for Gregory Creek, Bluebell Canyon Creek, <br />King Gulch, Skunk Canyon Creek, Bear Canyon Creek, Viele <br />Channel, Anderson Channel and Davids Draw were taken from <br />a report prepared by Wright-McLaughlin Engineers (Refer- <br />ences 1 and 2) as part of a plan to manage the city's <br />storm drainage. <br /> <br />4 <br />