Table 2-2. Ditch flow travel times
STEP 11. Find the rainfall intensity (I) for a 2-year, 60-minute storm from the world isohyetal map figure 1-4 on
page 1-7.
STEP 12. Adjust the rainfall intensity to match the TOC using the Standard Intensity-Duration Curves (figure 1-
5, page 1-9).
STEP 13. Calculate Q (discharge) using the "C" value from the step 9, times the "Iadj" from Step 12, and the "A"
from Step 3 as follows:
Q = CIA
You now have enough information to put the Rational Method to use in an example. First, let's assume that our
example area meets the criteria of the Rational Method.
Its total area is less than 1,000 acres.
The watershed has no lakes or flowing streams in it.
The basin has a regular shape.
EN5465
2-8
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