Chapter 5 - Publications, US Army Corps of Engineers

EM 1110-2-1701
31 Dec 1985
5-7. llow-Duration Metti
(1) The basis of this method is a flow-duration curve, usually
constructed from historical records, which describes the percent of
time different levels of streamflow are equaled or exceeded (Figure
5-15). This curve can be readily converted to a power-duration curve
through application of the water power equation, and from the latter
curve an estimate can be made of the sitets energy potential. The
primary advantages and disadvantages of the flow-duration method are
summarized in Section 5-4b, together with a discussion of the types of
studies for which this method is appropriate.
(2) Traditionally, duration-curve energy analyses have been
based on flows for the entire year, and this is often satisfactory for
preliminary energy potential studies. However, when a project
advances to the point where marketing of the Pwer is being studied,
it is usually necessary to prepare duration curves describing the
plantfs energy output by month or by season. The dependable capacity
for most small projects is based on the average capacity available
during the peak demand months (Section 6-7g), and to do this analYsis)
it is necessary to have a power-duration curve based on flows for the
peak demand months.
(3) The following sections describe the basic steps for
computing average annual energy and dependable capacity using the
flow-duration method. The discussion includes a sample calculation
for a typical low-head run-of-river project with no wndage.
b. }ata Rea~ements. Table 5-2 provides a summary of the basic
assumptions and input data requirements for this method. Further
information on specific items is provided in the corres~nding
paragraphs of Section 5-6.
c. Deve~ Curve. The first step is to compile a
flow-duration curve using the available streamflow record, adjusted if
necessary to reflect depletions and current streamflow regulation.
For preliminary studies, flow would be aggregated in classes (flow
ranges) which would produce 20 to 30 well-distributed points on the
duration curve. For more detailed studies, a larger number of classes
should be used. The actual compilation of the duration curve is
usually done with a computer model. Figure 5-15 illustrates a flowduration
curve for the example project. From the area under the
curve, the average annual flow is computed to be 390 cfs.
d. ~ Flow-Duration wve. If less than thirty years of
flow data is available, nearby stations with longer periods of record
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