FLOOD07468

• County: Statewide
• Community: State of Colorado
• Stream Name: All
• Basin: Statewide
• Title: The Design of Encroachments on Floodplains Using Risk Analysis
• Date: 10/1/1980
• Prepared For: USDOT
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

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6.2.5 Hydrographs

There are several techniques for developing hydrographs from limited data.
While discharge hydrograph may be adequately approximated by a triangular
shape, a stage hydrograph tends to be more of a trapezoidal shape with a
longer duration near the peak.

Hydrographs may be developed by using a measured outflow hydrograph as a
pattern, or where no hydrograph is available, by assumming the hydrograph
shape.

6.2.5.1. Using a Measured Hydrograph as a Pattern

If a measured hydrograph is available, curves simi1iar to the measured
hydrograph can be drawn to complete the family of hydrographs. First,
mark each flood peak as illustrated in Figure 6.6a, and then draw
simi1iar curves through the peaks as illustrated in Figure 6.6b.

The measured hydrograph should be based on measurement from an existing bridge
or transfer of data from another bridge on the same stream. To be precise, the'
hydrographs should represent upstream stage-time relationships that reflect
ponding responses from each alternate design. In a practical sense, however,
several compromises as presented in the "overtopping times" section, are acceptbb1e.
First, the range of bridge alternates is not likely to significantly change the
shape of the hydrographs so that one set is adequate. Second, relative rather
than absolute stages are important so that even a downstream hydrograph will
suffice. Finally, transferring a hydrograph from another bridge on the same
stream is preferable to relying strictly on a computed hydrograph.

6.2.5.2. Computed Hydrographs

Measured hydrographs are not available on some streams and an analytical
hydrograph will have to be developed. A hydrograph can be developed by a
combination of methods attributed to the U.S. Water and Power Resource Service I
(21) and the Soil Conservation Service (22). Referring to Figure 6.7a, the time
to peak, Tp, is estimated by ,

Tp = Tc = (11.9 L3)O.385
H

where: Tp = time to peak, hr.
L = length of longest watercourse, mi.
H = elevation difference, ft.
Tc = time of concentration, hr.

An average constant can be used to compute the time of recession, Tr, so that:

Tr = 1.67 Tp

For a given peak discharge, Qp, a triangular discharge hydrograph, as i11ustrat~d
in Figure 6.6a, is defined by Tp and Tr.

A stage hydrograph can be developed from a triangular discharge hydrograph
by combining it with a rating curve, as illustrated in Figure 6.7b.

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