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<br />3.3 CORP Design storms
<br />
<br />For drainage basins less than five square miles, a two-hour storm
<br />distribution without area adjustments of the point rainfall values
<br />shall be used for CUHP. For drainage basins between five and ten
<br />square miles, a two-hour storm distribution is used but the
<br />incremental rainfall values are adjusted for the large basin area
<br />in accordance with suggested procedures in the NOAA Atlas for
<br />Colorado. The adjustment is an attempt to relate the average of
<br />all point values for a given duration and frequency within a basin
<br />to the average depth over the basin for the same duration and
<br />frequency.
<br />
<br />For drainage basins between ten and twenty square miles, a three-
<br />hour storm distribution with adjustment for area shall be used.
<br />The distribution for the last hour was obtained by uniformly
<br />distributing the difference between the two- and three-hour point
<br />rainfall values. The adjustment for area was obtained from the
<br />NOAA Atlas for Colorado. The incremental rainfall distribution for
<br />all basin areas up to 20 square miles is presented in Table 3-1.
<br />
<br />3.4 Intensity-Duration-Frequency (IDF) Curves
<br />
<br />Intensity-Duration-Frequency (IDF) curves are necessary to utilize
<br />the Rational Method for runoff analysis. The one-hour design point
<br />rainfall values obtained from the NOAA Atlas for Colorado are
<br />required for the development of the IDF curves. The one-hour point
<br />rainfall values applicable for the City of Greeley are presented
<br />below.
<br />
<br />
<br />..........ii...9N~.;~9~.......pb!~.......~i~~f.T;..(~~g~~~)......................}...........i....
<br />
<br />1.04
<br />
<br />':;:::,::::::;:;::::::-:.;::.;::::-:::::> >';<:::,.::::: ,-: ';':'::-,::':";';'
<br />.i!99mt.~.~#'.II
<br />2.78
<br />
<br />1.49
<br />
<br />1. 76
<br />
<br />2.51
<br />
<br />The IDF curves were developed by distributing the one-hour point
<br />rainfall values using the factors obtained from the NOAA Atlas as
<br />presented below.
<br />
<br />'-""-,-,,-,,:- - .,--,,-.-,-,-.--.'-:-'-..,'-'-...-,.-,-.--. "., ,,__ __,n. .".
<br /> ",,, ,,,.'.".--.'.'.'......._-,....-........,,-,.,-., . ..." -- --.. .,'",.,-, ,,-...',-.,,-.-.-.-----....' ,,',".-,"...-".-...-, -"',"""" -,.
<br /> ...,_'....d______.........".",.._._. c.,-"','",,,,__ ........ ....... .,.,.......,--..,-........"."....
<br />.".......__,_, ,n..
<br /> .. ... . y....... ,- ................."..,.-......- . ,
<br />. ..ii...FACTORSFOR .....,....,...,.....,............. --,---,-'. -,. --..
<br />t>o@.'.[i~()its OF X;l!lSs :C~g!l~jtQW.......
<br /> ... ..........................
<br /> ., -------,--,-,..".
<br />Duration (minutes ) I 5 I 1 0 I 1 5 I 3 0 I
<br />Rat io t 0 1 -hour Depth 0 . 2 9 0 . 4 5 0 . 57 0 . 7 9
<br />
<br />The point values were then converted to intensities and plotted on
<br />Figure 3-1. The data are also presented in Tables 3-1, 3-2, and
<br />3.3.
<br />
<br />3 - 3
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