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A DEFINITION OF DOMINANTDISCHARGEM. A. BENSON a & D. M. THOMAS aa U. S Geological Survey , Washington, D. C.Published online: 04 Jan 2010.

To cite this article: M. A. BENSON & D. M. THOMAS (1966) A DEFINITION OFDOMINANT DISCHARGE, International Association of Scientific Hydrology. Bulletin,11:2, 76-80, DOI: 10.1080/02626666609493460

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A DEFINITION OF DOMINANT DISCHARGE

M. A. BENSON and D. M. THOMAS

U. S Geological Survey, Washington, D. C.

ABSTRACT

The concept of dominant (stream) discharge in geomorphology has not been firmly defined. Bydefining it, on rational grounds, as that discharge at which, over a long period of time, most sedimenthas been transported, a definite value can be computed for a stream for which there are available duration­curve values of daily discharge and a sediment-rating curve. This has been done for 5 eastern and 4western rivers. The dominant discharge, when expressed in terms of percentage duration, is remarkablyconsistent, and is found to be a lower discharge than has generally been considered as dominant.

INTRODUCTION

The concept of dominant discharge in geomorphic processes is somewhat tenuous. Inglis(1941) said that "Dominant discharge is the discharge which controls the meander length andbreadth. It appears to be slightly in excess of bank-full stage."

Wolman and Miller (1960) summarized a study by Wolman that included observations onthe discharges that were most responsible for shaping a channel. "In an analysis of factorscontrolling bank erosion, Wolman (1959) has shown that lateral cutting of the cohesive channelbank of a small stream in Maryland occurs mostly during the winter months, when flows of asize which occurs eight to ten times per year attack previously wetted banks." Although theposition of such discharges on the daily-duration curve is uncertain, the effective magnitudeapparently is considerably lower than that of bankful stage.

Leopold, Wolman, and Miller (1964) say, "The most meaningful discharge for any discussionof channel morphology is that which forms or maintains the channel. --the effective dischargecan often be approximated by bankfull discharge. In many rivers the bankfull discharge is onethat has a recurrence interval of about 1.5 years."

Carlston (1965) has concluded that the dominant discharge which controls meander wavelength is a range of flows, possibily falling-stage flows, between the mean of the month ofmaximumdischarge and mean annual discharge.

There appears to be some uncertainty as to whether the discharge that controls or can bestbe related to meander length can be equated with the discharge most meaningful in shaping ormaintaining a channel. Both have been associated with bankfull stage (Inglis, 1941, and Leopold,Wolman, and Miller, 1964), and both have been associated with something much less thanbankfull stage (Wolman and Miller, 1960, and Carlston, 1965).

DEFINITION

The forming and maintenance of channel cross sections and the movement of meandersmust be accompanied by the movement of sediment. It seems, therefore, logical to suggest adefinition of dominant discharge as the discharge that over a long time period transports themost sediment. Such a discharge may be computed by combining information from a dailysediment-rating curve and a discharge-duration curve. Because only information on suspendedsediment load is generally available, only suspended sediment can be considered and it isnecessary therefore to assume that total sediment load is proportional to suspended load.

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ApPLICATION

Sediment-rating and discharge-duration information have been combined for several streamswhere necessary data were available. Figure 1 shows histograms of discharged suspendedsediment for equal intervals of stream discharge at each station. Each histogram has a peakordinate. For each peak ordinate it can be said that there are more tons of sediment transported

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Fig. la - Histograms of suspended sediment discharge (eastern U. S.).

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within that range of stream discharge than within any other equivalent discharge range. Fromduration curves the percent of daily flows that exceed the mid-range discharge of the peak ordi­nate were determined and are summarized in table 1. Flow days only were considered in com­puting the duration value; therefore for ephemeral streams the indicated duration is the per­centage of flow days that exceed the peak ordinate discharge rather than percentage of time thatflow exceeds the peak ordinate discharge.

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Fig. Ib - Histograms of suspended sediment discharge (southwestern U. S.).

CONCLUSIONS

With one exception the percentage of flow days exceeding the mid-range of the maximumhistogram ordinate lies within a fairly narrow range, 7.6 to 18.5 percent, and averages 12.4 per­cent. The exception is the Rio Grande at Bernalillo, New Mexico, for which the dominant di­scharge has a 42.1 percent duration. The reason that this stream diverges from the gerenalpattern is not known. It might be that the transported sediment size is smaller than at other sta­tions, but data are unavailable to investigate this possibility.

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TABLE 1

Summary of discharge intervals transporting greatest amounts of suspended sediment at gaging stations

Contrib. Average Discharge Class Duration Average l.5-yr.Stream drainage Discharge Lower Upper Lower Upper duration Peak

area (cfs) (cfs) (cfs) limit limit % discharge(sq. mi.) % % (cfs)

I

Rio Grande nearIBernalillo, N ..M 14,360 1.152 600 800 50.2 35.2 42.7

Rio Puerco near Bernardo,N.M. 6,220 400 500

(only flow days) 211.9 16.7 13.7 15.2(all days) 56.5 4.6 3.8

Galisteo Creek at Domingo,N.M. 640 70 80

(only flow days) 50.6 10.0 8.9 9.4(all days) 10.1 2.6 2.4

Colorado River nr GrandCanyon, Ariz. 137,800 17,010 45,000 50,000 9.1 7.1 8.1 74,000

E. B. Delaware R. at Fish'sEddy, N. Y. 783 1,670 4,000 4,400 8.3 6.8 7.6

W. B. Delaware R. atHale Eddy, N. Y. 593 1,066 1,600 2,400 19.4 10.1 14.8

Neversink R. at Godeffroy,N.Y. 302 604 800 1,000 22 15 18.5

Neshaminy Cr. at Langhorne,Pa. 210 276 400 500 15 11 13.0 8,000

Potomac R. at Point ofRocks, Md. 9,651 9,248 15,000 20,000 15.8 9.9 12.8 78,000

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In any case, the dominant discharge defined in this manner is for most rivers much less thanbankfull-stage discharge and lies somewhere between the mean annual flow and the point indi­cated by Wolman's investigations.

The shapes of the histograms indicate that, although there are peaks, sediment loads ofalmost the same amount (per equal discharge intervals) are transported over a large range ofdischarge. Therefore the concept of a dominant discharge may not be too meaningful. Onlyfurther study can show whether the dominant discharge determined in this manner has anyvalue in defining channel morphology.

REFERENCES CITED

CARLSTON, C. W., 1965, The relation of free meander geometry to stream discharge and its geomorphicimplications: Am. Jour. Sci., v. 263, pp. 864-885.

INGLIS, C. c., 1941, Digest of answers to the Central Board of Irrigation questionaire on meanderingof rivers with comments on factors controlling meandering and suggestions for future action, inEdgecombe, A.R.B., ed. 1941, annual report (technical) of the Central Board of Irrigation, India,1939-40: Pub. 24, pp. 100-114.

LEOPOLD, L. B., WOLMAN, M. G., and MILLER, J. P., 1964, Fluvial processes in geomorphology: SanFrancisco, W. H. Freeman and Co., 522 p.

WOLMAN, M. G., and MILLER, J. P., 1960, Magnitude and frequency of forces in geomorphic processes:Jour. Geology, V. 68, No.1, pp. 54-74.

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