Page 321 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SITE UTILITY SYSTEMS

SITE UTILITY SYSTEMS 6.43

FIGURE 6.25 Factor for adjusting future condition runoff CN based on percent of imperviousness.

FIGURE 6.26 Factor for adjusting future condition runoff CN based on hydraulic length modified.

Assuming that 25 percent will be covered, Fig. 6.25 gives the adjustment factor based
on percent of imperviousness. Entering with the CN of 80 and percentage of new imper-
vious area of 25, 1.16 is read.
4. The length of run along the slope of the new development that is modified, and which
will affect the overland flow time, must also be determined and expressed as a percent
of the total length. Assuming that 50 percent of the original hydraulic length is modified,
Fig. 6.26 gives the adjustment factor based on hydraulic length modified. Entering with
a CN of 80 and 50 percent length, 1.31 is read.
5. The future condition peak discharge can now be found by multiplying the base peak dis-
charge by both the impervious area factor and the hydraulic length modified factors. Thus,

459 1 16 1 31 697 5× . × . = . (roundto 698 cfs)
The effect of this proposed development is to increase the peak discharge from 399 to
698 cfs.

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