FLOOD02408

• County: Statewide
• Community: Nationwide
• Basin: Statewide
• Title: Design Hydrographs for Very Small Wathersheds from Rainfall
• Date: 7/1/1962
• Prepared By: USDA Hydrograph Laboratory, Civil Engineering Section Colorado State University
• Floodplain - Doc Type: Educational/Technical/Reference Information

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Potter (18) has recently consolidated much of
his earlier work into a very clear method of estimat-
ing noods from areas ranging in size between O. 16
and 25 square miles. Peak rates of runoff with re-
turn periods from ten to fifty years can be obtained
from a topographic index and the 10-year 60-minute
rainfall. Four graphical correlations are presented,
each applicable to a zone of characteristic underlying
rock formation. Based, as it is, on ninety-five gaged
watersheds the method represents considerable uni-
fication over most of the United States east of the 1050
meridian, to which it applies.

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A recent discussion of unit hyclrograph shapes
for watersheds of 27. Z. 50 and 290 acres has been
presented by Minshall (14). He shows that the "unit
hydrograph" is not a constant for one small water-
shed but that its peak rate and the time from begin-
ning of excess rainfall to peak rate of runoff may be
related to rainfall intensity and storm (time) pattern.

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Nash (16) has discussed various methods of
determining the relation between effective rainfall
and storm runoff as particular cases of the general
unit-hydrograph theory. His remarks are valuable
to anyone relating physical characteristics of a catch.
ment to its indical response.

No review of literature would be complete
without mentioning some of the attempts to arrive at
hydrographs purely from mathematical considera-
tions. Although apparently overlooked by many later
writers, the three parts which Zoch (28, 29, 30) pub-
lished of an incomplete series, twenty-five years ago,
showed great promise. Commencing with simplifying
assumptions of rectangular or triangular watersheds
and uniform rainfall rates he progressed through to
the development of complex mathematical expressions
for runoff under various complex conditions. More
recent examples of this approach are papers by
Dooge (5) and Edson (6).

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