Work by Julien and Wargadalam (1995) has updated the theory and applications of hydraulic
geometry relationships for alluvial channels. In their study, downstream hydraulic geometry of
alluvial channels, in terms of bankfull width, average flow depth, mean flow velocity, and friction
slope, is examined from a 3-dimensional stability analysis of noncohesive particles under 2-
dimensional flows. Four exponent diagrams illustrate good agreement with several empirical
regime equations found in the literature. The analytical formulations were tested with a
comprehensive data set consisting of 835 field channels and 45 laboratory channels. The data
set covers a wide range of flow conditions from meandering to braided sand-bed, and gravel-
bed rivers.
5.4.7 Dominant Discharge in Alluvial Rivers
The hydraulic geometry relations discussed in Section 5.4.6 indicate how the channel
morphology and other characteristics vary with discharge at-a-station or in the downstream
the relations need to be defined to determine the downstream hydraulic geometry of the
channel at a site between two gaged sites. The question then arises about the frequency of
discharge to be used in the hydraulic geometry relations. The downstream hydraulic geometry
relations expressed in Section 5.4.6 relate to the bank-full stage, which for many humid region
U.S. rivers has a frequency of occurrence of one in 1.5 years. For arid region streams, the
bankfull return period may be on the order of five to ten years.
Analysis of bed sediment load estimations indicates that on most rivers, up to 90 percent of the
total transport is caused by flows that are equaled or exceeded about ten percent of the time
only. Thus, the average bed sediment load in a river may be described in terms of a formative
discharge much larger than the mean annual flow. Also, the average channel width, depth and
meander geometry may be defined in terms of different formative discharges rather than an
arbitrarily chosen dominant discharge.
Research by the U.S. Army Corps of Engineers (Watson et al. 1999, Copeland and Hall 1998)
suggests that the effective discharge is the best representation of the channel forming
discharge. The effective discharge is the increment of discharge that transports the most
sediment on an annual basis. This discharge may be determined by integrating a sediment
transport rating curve with the annual flow-duration curve. Where possible, it is important to
attempt to verify this channel-forming discharge with field indicators of bankfull discharge.
Appendix A to the 1999 USACE reference cited above provides a practical guide to effective
discharge calculations.
The concept of the frequency of occurrence of flows is important in the hydraulic design of
highway crossings and encroachments. Both the at-a-station and downstream hydraulic
geometry relations are especially useful when the hydraulic design is based on the frequency
of occurrence of flows. The concept of bank-full condition corresponding to a discharge with a
period of return of 1.5 years for perennial streams and five to ten years for ephemeral streams
is recommended for practical use when detailed analysis of formative discharge is not possible,
or feasible.
5.27
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