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<br />I- <br />II <br />fl <br />I <br />ill <br />'I <br />I <br />IIJ <br />II <br />I) <br />i.1 <br /> <br />II <br /> <br />J <br />I <br />I <br />.. <br />. <br />. <br /> <br />.1 <br /> <br />~ - <br /> <br />PART II <br /> <br />RAINFALL <br /> <br />Selection of a rainfall event for design of drainage facil- <br />ities is done on the basis of economic considerations. Costs <br />associated with the magnitude and frequency of damage caused by <br />storm water quantities in excess of system capacity must be <br />balanced with the capital and maintenance costs of the drainage <br />system. The minimum design criteria for sizing facilities in <br />the Pikes Peak Region shall be a 100 year storm event for all <br />facilities which would convey in excess of 500 cubic feet per <br />second (cfs) as a result of the 100 year event and a five year <br />storm event for all other facilities. <br /> <br />Storage structures with embankment heights in excess of 10 <br />feet, storage volume greater than 1,000 acre-feet, or a surface <br />area in excess of 20 acres fall under the regulations of the <br />State of Colorado Division of Water Resources. These structures <br />will be designed on a more stringent standard which, dependent <br />on an assigned hazard rating, may be a probable maximum precipi- <br />-tation or approximately four times the 100 year precipitation. <br /> <br />The terminology "5 year storm" or "100 year storm" refers <br />to the reciprocal of the annual probability of occurrence. The <br />probability of annual occurrence of the 5 year and 100 year <br />storms would be 20 percent and one percent respectively. A <br />second parameter for selecting a design rainfall event is the <br />storm duration. Typically a shorter duration storm will produce <br />higher flood runoff in smaller basins and the longer duration <br />storms will produce more runoff in the larger basins. The~~.iiK' <br />lt6ur'sb5i'm. wHItis ua lly ~~.res1.i.lt 'in ..atIi gherpeak-run6f f "'t'hari"}:Ile <br />~4 hour storm ..wllen:applied to- urp~i'l.--:o.;r..?J!lE!9'~=~l.l;ua:tions~ecaus~ <br />~:Of relat-;vely.short. timeso-f:.:c6ilcen'fration, The designer should <br />review both the 24 and 6 hour storm for a given facility and base <br />hydraulic design criteria on the precipitation event which gives <br />the greatest peak runoff for the appropriate storm frequency. <br />The 24 hour storm should be used when considering storage or <br />det~ntion facilities. <br /> <br />Figures 11-1 through 11-3 are depth-duration curves for three <br />locations in this region. The depth-duration curves represent <br />a statistical analysis of a limited amount of data which extrapo- <br />lates the probability of occurrence..()f a given precipitation depth <br />from a single rainfall event. \Existingdatais used to estimate <br />th~se curves and a separate analysis is accomplished for durations <br />"f 6 and ,24 hour storms. An' ana-lysis' of~this sort is used to~- "0. <br />develop isopl~vial maps for El Paso and Teller County. The iso- <br />pluvial maps shown in Figures 11-4 through 11-7 represent 6 hour <br />and 24 hour storm events for five year and 100 year return <br />frequencies. Figures 11-1 thro"'1h 11-3 are included to allow <br />rainfall for frequencies other than the five and 100 year events <br />toe.~~evaluated. It should be remembered that if the depth- <br /> <br />3 <br />