Appendix A: A Practical Guide to Effective Discharge Calculations
indirectly from an estimate of the return period for either the bankfull or effective discharge. In practice,
problems often arise when attempting to use bankfull discharge to determine the dominant discharge.
Problems center on the wide range of definitions of bankfull stage that exist (Williams, 1978). Although
several criteria have been identified to assist in field identification of bankfull stage, ranging from vegetation
boundaries to morphological breaks in bank profiles, considerable experience is required to apply these
in practice, especially on rivers which have in the past undergone aggradation or degradation.
In many studies channel-forming discharge is estimated from the recurrence interval for bankfull
discharge. Leopold and Wolman (1957) suggested that the bankfull flow has a recurrence interval of
between one and two years. Dury (1973) concluded that the bankfull discharge is approximately 97% of
the 1.58 year discharge, or the most probable annual flood. Hey (1975) showed that for three British
gravel-bed rivers, the 1.5 year flow in an annual maximum series passed through the scatter of bankfull
discharges measured along the course of the rivers. Richards (1982) suggested that, in a partial duration
series, bankfull discharge equals the most probable annual flood, which has a one-year recurrence interval.
For expediency, bankfull discharge is often assumed to have a recurrence interval of 1.5 years and Leopold
(1994) stated that most investigations have concluded that the bankfull discharge recurrence intervals range
from 1.0 to 2.5 years. However, there are many instances where the bankfull discharge does not fall within
this range. For example, Pickup and Warner (1976) determined bankfull recurrence intervals may range
from 4 to 10 years in the annual maximum series. Therefore, extreme caution must be used when estimating
the dominant discharge using a flow of specific recurrence interval.
Although the channel forming discharge concept is not universally accepted, most river engineers
and scientists agree that the concept has merit, at least for perennial and ephemeral streams in humid and
semi-arid environments. There are three possible approaches to determining the channel-forming discharge:
1) bankfull discharge; 2) flow of a given recurrence interval, and; 3) effective discharge. Ideally, the
method used should have general applicability, the capability to be applied consistently, and integrate the
physical processes responsible for determining the channel dimensions. Of the three possible approaches
listed above, only the effective discharge has the potential to meet these requirements. Another advantage
of the effective discharge is that it can be calculated for post-project conditions, where the hydrologic
regime may be significantly different from the existing regime due to the construction of dams, diversions,
or there major channel improvement activities. Selection of the appropriate method will be based on data
availability, physical characteristics of the site, level of study and time, and funding constraints. If possible,
it is recommended that all three methods be used and cross-checked against each other to reduce the
uncertainty in the final estimate.
HYDROLOGICAL DATA AND CALCULATIONS
The range of flows experienced by the river during the period of record is divided into a number
of classes and then the total amount of sediment transported by each class is calculated. This is achieved