Channel change, bankfull and effective discharges on a vertically accreting, meandering, gravel-bed river

Along the lower reaches of the Waipaoa River, New Zealand, cross-section survey data indicate there was a 23 per cent decrease in bankfull width and a 22 per cent reduction in channel cross-section area between 1948 and 2000, as the channel responded to increased inputs of fine (suspended) sediment following deforestation of the headwaters in late C19 and early C20. We determined the bankfull discharge within a similar to 39 km long reach by routing known discharges through the one-dimensional MIKE 11 flow model. The model runs suggest that the bankfull discharge varies between similar to 800 and similar to 2300 m(3) s(-1) and that the average recurrence interval is 4 2 years on the annual maximum series; by contrast, the effective flow (360 m(3) s(-1) ) is equaled or exceeded three times a year. The variability in bankfull discharge arises because the banks tend to be lower in places where flood flows are constricted than in reaches where overbank flow is dispersed over a wide area, and because scour has counteracted aggradation in some locations. There is no downstream variation in Shields stress, or in relative shear stress, within the study reach. Bankfull shear stress is, on average, five times greater than the shear stress required to initiate motion. At the effective discharge it is more than twice the threshold value. The effective discharge probably has more relevance than the bankfull discharge to the overall picture of sediment movement in the lower reaches of the Waipaoa River but, because width is constrained by the stability and resistance of the bank material to erosion during high flows that also scour the bed, the overall channel geometry is likely determined by discharges at or near bankfull. Copyright (c) 2006 John Wiley & Sons, Ltd.