Drainage Design Manual, Hydrology - Flood Control District of ...

Drainage Design Manual for Maricopa County
Hydrology: Application
4. Compute the Time of Concentration (T c ) and Intensity (i) for each subbasin using Equation
(3.2) and the data from Table 9.5 based on the 100 year event. Use a log interpolation to compute
i.
Subbasin S1:
T c
11.4L 0.5 0.52 – 0.31
= K b S
i – 0.38
c
= 11.4( 0.729) 0.5 ( 0.096) 0.52 ( 473.0) 0.31
T c
= 0.426i – 0.38
Start with an initial estimate for T c of 15 minutes.
– i – 0.38
- From Table 9.5, i = 5.26 inches/hour for a 15-minute duration.
T c
= 0.426( 5.26) – 0.38 = 0.227 hours = 13.6 min
Recompute i for T c of 13.6 minutes.
- From Table 9.5, i = 5.26 inches/hour for 15 minutes, and i = 6.37 inches/hour for 10-minutes
( ( ( 13.6 – 10) ⁄ ( 15 – 10 ))
( log 105.26 – log 10 6.37) + log 10 6.37)
i = 10 =
5.55 inches/hour
T c
= 0.426( 5.55) – 0.38 = 0.222 hours = 13.3 min
Recompute i for T c of 13.3 minutes.
( ( ( 13.3 – 10) ⁄ ( 15 – 10 ))
( log 105.26 – log 10 6.37) + log 10 6.37)
i = 10 =
5.61 inches/hour
T c
= 0.426( 5.61) – 0.38 = 0.221 hours = 13.3 min
Recompute i for T c of 13.3 minutes.
( ( ( 13.3 – 10) ⁄ ( 15 – 10 ))
( log 105.26 – log 10 6.37) + log 10 6.37)
i = 10 =
5.61 inches/hour
T c
= 0.426( 5.61) – 0.38 = 0.221 hours = 13.3 min
Difference is less than 2%. Round T c to nearest minute and recomput i. Use T c = 13 min, and
i = 5.68 inches/hour
9-20 August 15, 2013