Appendix A: A Practical Guide to Effective Discharge Calculations
Step 1 - Flow Frequency Distribution
1)
Evaluate Flow Record
The flow record is a historic record of discharges at a gaging station. The record from a single
gaging station can be used to develop the flow frequency distribution if the gage is in close proximity to the
study site and the discharge record at the gage is representative of the flow regime there. If a gaging record
is either unavailable or unrepresentative, the flow frequency distribution can be derived using either the
basin-area or regionalized duration curve method.
2)
Check the Period of Record
It is recommended that the length of period of record be at least 10 years and that measurements
be continuous to the present day. Discharge data can still be used if there are short gaps in the record, but
caution must be exercised when collecting data from a discontinuous record. The flow frequency curve
will not be representative of the natural sequence of flows over the medium term if the length of record is
less than 10 years or if the record has been influenced by changes in the watershed runoff regime. If this
is the case, a flow duration curve should be developed using either the basin-area or regionalized duration
curve method.
3)
Determine the Discharge-Averaging Time Base
To construct the flow frequency distribution, the time base should be sufficiently short to ensure that
short-duration, high magnitude events are properly represented. If 15-minute data are unavailable, then
either 1-hour or mean daily data can be used, but caution must be exercised when using mean daily data
to develop a flow frequency distribution for a stream which exhibits a flashy regime.
4)
Calculate Discharge Range
The range of discharges is calculated by subtracting the minimum discharge in the flow record from
the maximum discharge.
5)
Calculate Discharge Class Interval
It is recommended that the initial attempt to construct the flow frequency distribution should use 25,
arithmetic class intervals. Therefore, the class interval is the discharge range, calculated in Step (4), divided
by 25. The class interval should not be approximated by rounding. The relative proportions of the bed
material load moving in suspension and as bed load should be estimated during site reconnaissance. For
rivers in which the bed material load moves predominantly as suspended load, the first discharge class goes
from zero to the class interval, the second class is determined by adding the class interval to the upper value
of the previous class, and so on until the upper limit of the discharge range is reached. For gravel-bed
rivers, where bed material load moves predominantly as bed load, the minimum discharge used in
277
Previous Page
