Analytical Estimation of Geomorphic Discharge Indices for Small Intermittent Streams

Effective, f-load, and functional-equivalent discharges are metrics that can be used as design flows for stream channel restoration and for eco-hydrologic and geomorphic studies. Previously published analytical estimators of these discharge indices are reviewed and new estimators are derived for gamma, lognormal, mixed gamma, and mixed lognormal flow frequency distributions of daily flows combined with a power-function approximation of sediment rating curves. For 19 small streams in Ontario, Canada, goodness-of-fit test statistics do not clearly show that mixed gamma distributions improve the fit with observed daily flow frequency distributions; however, the discharge indices determined analytically using mixed gamma distributions better match empirical estimates of these indices than those computed based upon gamma, lognormal, and mixed lognormal distributions. Comparative analysis of the three discharge indices also suggests that f-load discharge with a reasonable f value provides a more consistent design flow for the restoration of small streams.