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instances where the bankfull discharge does not fall within thisrange. For example, Williams (1978) showed that for thirty-fivefloodplains in the United States (US) the recurrence interval ofbankfull discharge varied between 1.01 to 32 years, and found thatonly about a third of those streams had a bankfull discharge with arecurrence interval between 1 to 5 years. In a similar study, Pickupand Warner (1976) determined that bankfull recurrence intervalsranged from 4 to 10 years on the annual series.Table 3.2 – RecommendedFrequencies for BankfullDischarge (after Soar 2000)Discharge frequencyRecommended by1 to 5 years Wolman and Leopold (1957)1.5 year Leopold et al. (1964); Leopold (1994); Hey (1975)1.58 year Dury (1973, 1976); Riley (1976)1.02 to 2.69 years Woodyer (1968)1.01 to 32 years Williams (1978)1.18 to 3.26 years Andrews (1980)1 to 10 years, 2 year USACE (1994)2 years Bray (1973, 1982)If a specified recurrence interval flow is used to estimate thechannel-forming discharge, a range of 1 to 3 years should be used.However, because of the uncertainties discussed above, it isrecommended that discharges in this range be compared tobankfull stage in the field to verify that the discharges do havemorphological significance.3.5.3 Effective DischargeThe effective discharge is defined as the increment ofdischarge that transports the largest fraction of the annualsediment load over a period of years (Andrews 1980). Theeffective discharge incorporates the principle prescribed byWolman and Miller (1960) that the channel-forming or dominantdischarge is a function of both the magnitude of sedimenttransportingevents and the frequency of occurrence. Anadvantage of using the effective discharge is that it is a calculatedvalue integrating discharge and sediment transport regimes of thestream.Equivalence between bankfull and effective discharges fornatural alluvial streams that are in regime has been demonstratedfor a range of river types (sand, gravel, cobble, and boulder-bed),and in different hydrological environments if the flow regime isadequately defined and the appropriate component of the36 Fundamentals of Fluvial Geomorphology and Stream Processes
sediment load is correctly identified (Andrews 1980; Carling 1988;Hey 1997). However, Benson and Thomas (1966), Pickup andWarner (1976), Webb and Walling (1982), Nolan et al. (1987), andLyons et al. (1992) report that the effective and bankfulldischarges are not always equivalent. This suggests that theeffective discharge may not always be a direct surrogate for thechannel-forming flow or the bankfull discharge.Although the effective discharge is straightforwardconceptually, and has been used for many years, many engineershave expressed concerns that the effective discharge calculationsdo not yield reasonable results in some instances. These problemsmay be attributable to data limitations, insufficient understanding ofthe morphology of the stream or to improper calculation procedure.To minimize these uncertainties a standardized procedure for thedetermination of the effective discharge has been developed and isoutlined in the following paragraphs. This procedure is intended tohelp investigators avoid many of the potential problems that theauthors have experienced in the calculation of the effectivedischarge. Interested readers are referred to Biedenharn et al.(2000a) for a more detailed discussion of effective dischargecalculation.The method most commonly adopted for determining theeffective discharge is to calculate the total load (tons) transportedby the range of flows over a period of time by multiplying thefrequency of occurrence of selected discharge classes (number ofdays) by the median magnitude of the sediment load (tons/day)transported by that class of flows. While this approach has themerit of simplicity, the accuracy of the estimate of the effectivedischarge is clearly dependent on the calculation procedureadopted. The basic inputs required for calculation of effectivedischarge are: 1) flow-duration data, and 2) sediment transport asa function of stream discharge.The first step in an effective-discharge calculation is togroup the discharge data into classes and determine the number ofevents occurring in each class during the period of record. This isusually accomplished from a flow-duration curve, which is acumulative distribution function of measured discharges. A flowdurationcurve shows the percent of time a specific discharge isequaled or exceeded during the period of record, for which thecurve was developed. From the flow-duration curve, the number ofdays that discharges within the specified class interval occurredcan be calculated. The three critical components that must beconsidered when developing a flow-duration curve are the timebase, the number of class interval, and the period of record.Conventionally, values of mean daily discharge are used tocompute the flow-duration curve. Although this is convenient andFundamentals of Fluvial Geomorphology and Stream Processes 37
Page 6 and 7: materials. Also, the threshold mobi
Page 8 and 9: ank of a large river. From an engin
Page 10 and 11: eddies in a stream do not scale on
Page 12 and 13: aiding. Braid bars are sediment fea
Page 14 and 15: Sinuosity (P) is a commonly used pa
Page 16 and 17: outcrops, other non-erodible materi
Page 18 and 19: continent (Inglis 1941, 1947, 1949)
Page 22 and 23: uses readily available mean daily f
Page 24 and 25: of flows. The results are plotted a
Page 26 and 27: Similarly, in regime theory the con
Page 28 and 29: 3.7 STREAM STABILITY ANDINSTABILITY
Page 30 and 31: It should be noted that the map coo
Page 32 and 33: and slope of the reach are not sign
Page 34 and 35: (a) Bed and Bank InstabilityFigure
Page 36 and 37: the downstream limit of the system
Page 38 and 39: dam, where degradation was expected
Page 40 and 41: Figure 3.13 - Channel PatternClassi
Page 42 and 43: definition of a graded stream in wh
Page 44 and 45: midwestern experience, while Rosgen
Page 46 and 47: Schumm et al. (1981, 1984) and thei
Page 48 and 49: ut at a much reduced rate. The sedi
Page 50 and 51: • integration of results into eng
Page 52: 68 Fundamentals of Fluvial Geomorph

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