7 – 1
Chapter 7
7.2.1.
T = 25 years, T
d
= 30 min.
Using equation (7.2.1b),
P
30min
= 0.51P
15min
+ 0.49P
60min
Also, P
25,60
= aP
2,60
+ bP
100,60
. For T = 25 yr., a = 0.293 and b = 0.669 (From
Table 7.2.1). Using Figures 7.2.14 (e) and (f), P
2,60

1.45 in and P
100,60

3.125 in
for Chicago, Illinois). Hence,
P
25,60
= 0.293(1.45) + 0.669(3.125) = 2.515 in.
Similarly, P
25,15
= aP
2,15
+ bP
100,15
. Using Figures 7.2.14 (c) and (d), P
2,15

0.86 in
and P
100,15

1.65 in for Chicago, Illinois). Hence,
P
25,15
= 0.293(0.86) + 0.669(1.65) = 1.356 in.
Thus, P
25,30
= 0.51(1.356) + 0.49(2.515) = 1.924 in
7.2.2.
T = 2, 10, 25, 100 year, T
d
= 15 min
From Figures 7.2.14 (c) and (d) (HYDRO35), P
2,15

1.0 in and P
100,15

1.78 in
(for Memphis, Tennessee). Hence,
P
2,15
= 1.0 in, P
100,15
= 1.78 in may be taken. For the remaining return periods,
P
T,15
= aP
2,15
+ bP
100,15
where the values of a and b are given in Table 7.2.1.
P
10,15
= 0.496(1.0) + 0.449(1.78) = 1.295 in
P
25,15
= 0.293(1.0) + 0.669(1.78) = 1.484 in
7.2.3.
T = 2 and 25years. The intensitydurationfrequency relationship is
The intensitydurationfrequency (IDF) curves for Memphis, Tennessee, are
determined by using the HYDRO35 maps.
P
2,5

0.488 in, P
2,15

1.0 in, P
2,60

1.76 in. Precipitation values for 10 and 30
minute durations are computed by the interpolation method.
P
2,10
= 0.41P
2,5
+ 0.59P
2,15
= 0.41(0.488) + 0.59(1.0) = 0.790 in.
P
2,30
= 0.41P
2,15
+ 0.59P
2,60
= 0.41(1.0) + 0.59(1.76) = 1.448 in.
Also, the 25year return period precipitation depths are computed for the selected
durations.
P
25yr
= aP
2yr
+ bP
100yr
where a = 0.293 and b = 0.669 for T = 25 yr. (Table 7.2.1).
P
100,5

0.83 in, P
100,15

1.775 in, P
100,60

3.625 in (from HYDRO35 maps)
P
100,10
= 0.41P
100,5
+ 0.59P
100,15
= 0.41(0.83) + 0.59(1.775) = 1.388 in.
P
100,30
= 0.41P
100,15
+ 0.59P
100,60
= 0.41(1.775) + 0.59(3.625) = 2.867 in.
Hence, P
25,5
= 0.293P
2,5
+ 0.669P
100,5
= 0.293(0.488) + 0.669(0.83) = 0.698 in
P
25,10
= 0.293P
2,10
+ 0.669P
100,10
= 0.293(0.790) + 0.669(1.388) = 1.160 in
P
25,15
= 0.293P
2,15
+ 0.669P
100,15
= 0.293(1.0) + 0.669(1.775) = 1.480 in
P
25,30
= 0.293P
2,30
+ 0.669P
100,30
= 0.293(1.448) + 0.669(2.867) = 2.342 in
P
25,60
= 0.293P
2,60
+ 0.669P
100,60
= 0.293(1.76) + 0.669(3.625) = 2.941 in
The result is given in tabular form and in Figure as shown below. The rainfall
intensities are derived by dividing the rainfall depths by their corresponding
duration.