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<br />CHAPTER 5
<br />HYDROLOGY
<br />
<br />SECTION 1.0
<br />RAINFALL
<br />
<br />COLORADO STATEWIDE
<br />DRAINAGE AND FLOODPLAIN MANAGEMENT CRITERIA MANUAL
<br />
<br />is necessary to determine the region and apply appropriate equations for the
<br />drainage basin. If the drainage basin is located within few miles of a regional
<br />boundary, computations shall be made using equations for both regions and the
<br />average rainfall values shall be used for the rainfall-runoff analysis.
<br />
<br />The 1-hour frequency values for 2- and 100-year stonn events can be estimated
<br />utilizing the appropriate regional equations from Table CH5-T102. Once computed,
<br />the 2-year and 100-year, 1-hour values can be plotted on Figure CH5-F113, and a
<br />straight line between the two values can be drawn. Then, the 1-hour values for retum
<br />periods between 2- and 100-year events can be obtained from the line.
<br />
<br />Rainfall depths for the 2-hour and 3-hour events can be estimated using the following
<br />fonnulas (NOAA Atlas 2, 1973).
<br />
<br />Region 1
<br />Region 2
<br />Region 3 &4
<br />
<br />Dx,2= 0.342:Dx.e + 0.658:Dx,1
<br />Dx2= 0.341 Dxe + 0.659 DX1
<br />Dx:2= 0.250'Dx:e + 0.750'Dx:,
<br />
<br />(Eq. CH5-1 00)
<br />(Eq. CH5-101)
<br />(Eq. CH5-102)
<br />
<br />Where Dx,2= "X"-year, 2-hour rainfall depth (Inches)
<br />Dx,,= "X"-year, 1-hour rainfall depth (Inches)
<br />Ox,e= "X"-year, 6-hour rainfall depth (Inches)
<br />
<br />Region 1
<br />Region 2
<br />Region 3&4
<br />
<br />Dx,3= 0.597:Dx.e + 0.403:Dx,1
<br />Dx,3= 0.569px,e + 0.431px,1
<br />Dx,3= 0.467 Dx,e + 0.533 DX.l
<br />
<br />(Eq. CH5-103)
<br />(Eq. CH5-1 04)
<br />(Eq. CH5-105)
<br />
<br />Where Dx,3= "X"-year, 3-hour rainfall depth (Inches)
<br />DX,l= "X"-year, 1-hour rainfall depth (Inches)
<br />Ox,e= "X"-year, 6-hour rainfall depth (Inches)
<br />
<br />Based on Figure 17 in the NOAA Atlas 2, the 12-hour duration rainfall depth for the
<br />desired recurrence frequency is essentially the average of the 6-hour and 24-hour
<br />stonn events (NOAA, 1973).
<br />
<br />Dx,'2= (DX,6 + Dx,24)/2
<br />
<br />Where DX,12= "X"-year, 12-hour rainfall depth (Inches)
<br />Dx.e= "X"-year, 6-hour rainfall depth (Inches)
<br />Dx,24= "X"_year, 24-hour rainfall depth (Inches)
<br />
<br />Rainfall depths for durations less than 1-hour can be estimated using the adjustment
<br />ratios supplied in Table CH5-T101 and the estimated "X"-year, 1-hour rainfall depth
<br />(NOAA, 1973). These adjustment ratios were originally published in the US Weather
<br />Bureau Technical Paper No. 40 in 1961, and later evaluated and adopted by NOAA.
<br />
<br />(Eq. CH5-1 06)
<br />
<br />Dx,V= Dx.,'RATIOx,y
<br />
<br />(Eq. CH5-107)
<br />
<br />Where Dx,y= "X"-year, Y-minute rainfall depth (Inches)
<br />DX,l = "X"-year, 1-hour rainfall depth (Inches)
<br />RA TIOx,y = Ratio to convert "X"-year, 1-hour rainfall depth to the "X"-year, Y-
<br />minute depth
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<br />AUGUST 2002
<br />
<br />RAINFALL
<br />
<br />CH5-1 05
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