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The question remains as to whether flow becomes supercritical (Froude number exceeds 1) at higher flood discharges. At higher flows, channel-bank roughness (vegetation or bank irregularities) can markedly increase the total channel roughness as shown in figure 3. During larger floods, channel erosion is common. Usually when large floods occur in small steep basins, large amounts of channel erosion occur and sediment is subsequently transported. About 60,600 tons (55,000 metric tons) of sediment were eroded from the valley floor of Loveland Heights Tributary to the Big Thompson River, Colo. during the 1976 flood (Andrews and Costa, 1979). Additional energy is consumed in transporting the bed material. Flume studies by Bathurst and others (1979) indicate there is a sharp increase in resistance when bed material moves. Rubey (1933) showed that energy is required to move sediment. Transport of fine- g ra i ned sed i men t tends to reduce tu.rbu I ence and hence, flow res i stance. t/{P However, this decrease in resistance is normally offset by a much greater increase in flow resistance caused by the formation of dunes (Vanoni and Nomicos, 1960). The movement of the bed material probably makes the channel react I ike an alluvial channel in which bed-forms produce stand~ ing waves and additional energy losses as a result of-an increase in I friction. Critical and supercritical flow can occur locally in these channels, in smooth bedrock channels)and in fine-grained alluvial chan- nels. Dobbie and Wolf (1953), Thompson and Campbell (1979), and the author bel ieve that dlJring large floods, n values are much higher than those normally selected and that flows in steep natural channels con- taining cobbles and boulders approach, but do not exceed, critical flow. For these condi tions of steep slopes and extreme flows, a 1 imi ting assumption of critical depth in subsequent hydraulic analyses appears reasonable.