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

Drainage Design Manual for Maricopa County
Hydrology: Application
Compute the Time of Concentration (T c ) and Intensity (i) for concentration point C1 using
Equation (3.2) and the data from Table 9.5. Use a log interpolation to compute i.
Concentration Point C1:
T c
11.4L 0.5 0.52 – 0.31
= K b S
Start with an initial estimate for T c of 15 minutes.
- From Table 9.5, i = 5.26 inches/hour for a 15-minute duration.
Recompute i for T c of 13.3 minutes.
i – 0.38
c
= 11.4( 0.729) 0.5 ( 0.091) 0.52 ( 473.0) 0.31
T c
= 0.415 – 0.38
– i – 0.38
T c
= 0.415( 5.26) – 0.38 = 0.221 hours = 13.3 min
- From Table 9.5, i = 5.26 inches/hour for 15 minutes, and i = 3.55 inches/hour for 30-
minutes
( ( ( 13.3 – 15) ⁄ ( 30 – 15 ))
( log 103.55 – log 10 5.26) + log 10 5.26)
i = 10 =
5.50 inches/hour
T c
= 0.415( 5.50) – 0.38 = 0.217 hours = 13.0 min
Difference is less than 1%. Use T c = 13.0 min, i = 5.50 inches/hour
Compute the peak discharge at C1:
Concentration Point C1:
Q = ( 0.63) ( 5.50) ( 78.59) = 272 cfs
7. The peak discharge at concentration point C2 can be computed in a similar manner. Keep in
mind that the Combined Watershed Method is not implemented in DDMSW.
8. Compute the peak discharge for concentration points C1 and C2 using the Triangular Hydrograph
Method.
9-22 August 15, 2013