Appendix A: A Practical Guide to Effective Discharge Calculations
Application 2: Channel Stability Assessment Using the SAM Hydraulic Package
Abiaca Creek, Mississippi has been monitored for five years as part of the DEC Monitoring
Project by the U.S. Army Corps of Engineers Vicksburg District and Waterways Experiment Station.
Based on observations of the thalweg profiles and channel morphology for this period, Watson et al.
(1996) concluded that a study site (Site No. 6) on Abiaca Creek was in dynamic equilibrium. Data from
this site were used by Watson et al. (1997) to develop a stable channel design procedure for sand-bed
streams based on the minimum-slope extremal hypothesis (Chang, 1979).
The design procedure was developed and tested using hydrological and effective discharge data
generated using the SAM hydraulic design package (Thomas et al., 1994) and HEC-6T (Thomas, 1996).
SAM was applied to generate a series of 21 combinations of width, depth, and slope that satisfy water and
sediment continuity for a given flow and sediment concentration. The morphology corresponding to the
minimum slope was selected from the 21 combinations as the preferred stable channel design. The flow
used in these computations was the effective discharge, calculated using the procedure presented here, with
logarithmic class intervals.
15-minute discharges for a complete hydrological year were simulated for the preferred design and
9 of the other 20 alternative morphologies. For each condition, HEC-6T, a 1-D sediment routing model,
was used to simulate expected changes to the initial channel geometry. Of the ten morphologies tested, the
preferred design (based on the minimum-slope condition), resulted in the smallest change in channel slope
(0.24%) and generated no change in bed elevation. On this basis, it was selected as the most stable design.
In this application, the effective discharge was found to provide a reasonable representation of the
range of flows actually experienced by the stream, for the purpose of stable channel design.
Determination of effective discharge is an initial step in the Waterways Experiment Station (WES)
best practice hydraulic design procedure for restoring channels. The preferred cross-sectional geometry
is a compound configuration composed of a primary channel, designed to carry the effective or `channel
forming' discharge, and an overbank area designed to carry the additional flow for a specific flood
discharge. The effective discharge is calculated during a geomorphic assessment that is undertaken at a
stable location upstream of the project reach. This ensures that the restored channel design will transport
the sediment load from upstream with minimal net aggradation or degradation in the medium term. The
procedure then equates the target bankfull discharge for the primary channel in the restored reach to the
Hydraulic geometry relationships appropriate to the type of channel are applied to determine a
range of possible stable bankfull widths as a function of effective discharge within user-defined confidence
limits (Soar et al., 1998). A selected width, within the confidence band, is then used, together with the