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<br />..............'
<br />..,...
<br />
<br />~~
<br />
<br />A I-hour distribution developed from NOAA criteria is given in Table 2
<br />
<br />below.
<br />
<br />TABLE 2
<br />I-HOUR RAINFALL DISTRIBUTION
<br />
<br />End of P"riod
<br />(Minut"s)
<br />
<br />Incremental Rainfall
<br />(Percent)
<br />
<br />..
<br />
<br />5
<br />'10
<br />15
<br />20
<br />25
<br />30
<br />35
<br />40
<br />45
<br />50
<br />55
<br />60
<br />
<br />..../
<br />
<br />)
<br />
<br />3
<br />4
<br />4
<br />5
<br />12
<br />16
<br />29
<br />12
<br />5
<br />4
<br />4
<br />2
<br />
<br />,
<br />
<br />c. Losses. A unl"form infiltration rate of 1 inch per hour was used
<br />
<br />for the Lefthand Creek basin. Detention storage was 0.3 inch for the
<br />
<br />6-hour rainfall, 0.4 inch for the J-hour rainfall, and 0.5 inch for the
<br />
<br />I-hour rainfall. Detention storage values ~ere set lower for the longer
<br />
<br />. duration storms to adjust the change in runoff associated with the transi~
<br />
<br />tion of critical'durations as the basin size increased along the discharge
<br />
<br />profile.
<br />
<br />d. Calibration. The WRC frequency analysis of Lefthand Creek near
<br />
<br />Boulder was used as a guide in the calibration of the SWMM model. Cali-
<br />
<br />bration was accomplished by varying detention storage values in the 6-hour
<br />
<br />rainfall model. Att~chment 3 indicates that the modeled curve crosses
<br />
<br />the WRC curve at the lO-year event. Both curves have nearly the same
<br />
<br />slope and fit reasonably well. The'modeled curVe at the mouth (attach-
<br />
<br />ment 4) was drawn to :he 10-year discharge on the WRC curve because the
<br />
<br />SWMM model did not re'lect the numerous ditch diversions that would
<br />
<br />normally influence a lO-year event.
<br />
<br />4
<br />
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