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SECTION III <br />2NERAL: DESIGN CRITERIA <br />DESIGN STORM <br />The selection of a "design storm' is the most basic consideration of any <br />drainage study. In the semi -arid climate normal to Greeley, the amount of <br />annual rainfall is of little importance in an urban drainage study. In this <br />region the high intensity, short duration storm is more common and should be <br />used as a basis for design. The nearest weather station which maintains <br />long term rainfall records is at Fort Collins. Records from this station can <br />be considered valid for Greeley even though there are recognized geographical <br />and climatic differences. <br />The maximum intensity storm of record is 1.10 inches of rainfall in a <br />15- minute period, or an hourly rate of 4.40 inches per hour. This is con- <br />sidered a 100 -year storm, or in other terms, this is a storm that can be <br />expected once in each 100 -year period. A twelve -hour duration storm, wherein <br />3.00 inches of rainfall falls, occurs once every three to four years. A storm <br />of 1.00 inches intensity per hour for a one -hour duration occurs once every <br />two years, according to the Fort Collins records. <br />In any given year nature may produce a storm of any intensity. The 100 - <br />year storm, like the 10 -year storm, may occur at any time. From an ideologi <br />cal standpoint the engineer prefers to design for the maximum storm of record; <br />then life and property are given complete protection. Economic considera- <br />tions, however, prohibit such an ideological design. It is not proper <br />planning to spend more money on a program to protect property than the value <br />of the property being protected. Normal engineering and economic practices <br />require that the cost of an improvement be compensated for in full by <br />- 11 - <br />