Insert 90 mm for I, one gets 41.3 min. Since 41.3 > the assumed 30 min, try the
intensity for 41 minutes from Figure 3-1 which is 72 mm/hr.
Using 72 mm, one gets 45.2 min. Repeat the process with 70 mm/hr for 45 min and
a time of 45.7 min was found. This value is close to the 45.2 min.
Use 46 minutes for segment 1