BANKFULL DISCHARGE

THE BASIC IDEA

Dynamically stable stream channels formed in fully alluvial materials (sediments that can be eroded and deposited by the stream) tend to have widths, depths, and slopes that reflect a balance among the geological and hydrologic variables that interact to create the fluvial system. Many scientists and engineers have found the concept of channel-forming discharge to be a useful tool in understanding and managing streams. This concept is based on the idea that even though channel width, depth, and slope vary along a stream and through time, average values of width, depth and slope tend to be constant for a reach with a given drainage area if:

1. The stream bed and banks are alluvial,

2. There have not been any extreme floods, droughts, earthquakes, forest fires, or other catastrophic events in the recent past,

3. The watershed is largely free of human-caused disturbances, such as land use changes, grazing, mining, road building, dams, or channelization, and

4. Furthermore, the channel geometry is such that the greatest discharge the channel will carry without overflowing is not a rare flood (which moves tremendous amounts of sediment, but occurs only rarely) or a low flow (which occurs frequently, but has relatively little erosive power), but is an intermediate magnitude, such as the one- or two-year flood.

This characteristic discharge is referred to as the “channel-forming” or “dominant” discharge, Q_cf. This discharge therefore dominates channel form and process, at least for streams in humid regions and for perennial streams in semi-arid environments (Soar and Thorne, 2001 and Biedenharn et al., 2001).

Clearly, very few stream reaches meet the four criteria outlined above, and few can be described as "dynamically stable" or "fully alluvial" without qualification. Therefore, channel geometries often vary considerably from sizes needed to convey Q_cf. Many workers ignore departures from ideal conditions, and determine bankfull stage based on field indicators like permanent vegetation or terraces, but this approach can lead to errors.

SIGNIFICANCE OF Q_cf TO PROPOSED CHANNEL MODIFICATION OR STREAMBANK STABILIZATION

One of the principal reasons for estimating "channel forming discharge" (or dominant discharge) is to insure that any planned modification to channel cross section (as a result of proposed bank or channel protection measures along a reach of stream) will be compatible with this discharge. There are several approaches that can be employed to estimate this channel forming discharge. Under the right conditions, other types of flows, including "bankfull discharge," can be used as surrogates. The characteristics of these flows, their method of measurement, and their applicability as a surrogate for Q_cf is discussed in the next section.

METHODS FOR DETERMINING Q_cf

At least three approaches are available for determining Q_cf ; effective discharge (Q_eff), bankfull discharge (Q_bf), or the discharge that corresponds to a given return interval, Q_ri (Table 1).

BANKFULL DISCHARGE

The expression, "bankfull discharge," Q_bf, should be used to refer to the maximum discharge that the channel can convey without overflow onto the floodplain. Although this definition, proposed by Copeland et al. (2001), differs from that used by others (e.g., Rosgen, 1996), it eliminates confusion. As noted above, theoretically Q_bf and Q_eff are generally equivalent in channels that have remained stable for a period of time, thus allowing the channel morphology to adjust to the current hydrologic and sediment regime of the watershed (e.g., Pickup, 1976, Andrews, 1980, Soar, 2000, but see Emmitt & Wolman, 2001). In such a channel, the bankfull discharge corresponds to a sharp change in the slope of the rating curve (Figure 2).

Figure 2. Bankfull stage and discharge reflected in rating curve. From FISRWG (1998).

It must be noted, however, that in an unstable channel that is adjusting its morphology to changes in the hydrologic or sediment regime, Q_bf can vary markedly from Q_eff. Therefore, the expression "bankfull discharge" should never be used to refer to Q_ri or Q_eff. The relationship of Q_bf to Q_ri and Q_eff is useful as an indicator of channel stability and evolution (Schumm et al., 1984; Simon, 1989; Thorne et al., 1996). The Q_bf from ‘template’ or ‘reference’ reaches (stable reaches from similar watersheds) has been used as a guideline for relevant dimensions of the restored channel (Rosgen, 1996). Field indicators of Q_bf are often unreliable (Williams, 1978). Problems associated with basing design on Q_bf are discussed by FISRWG (1998) and Biedenharn et al. (2000).

DISCHARGE FOR A SPECIFIC RETURN INTERVAL,Q_ri

If gage data are available, the discharge with a given return interval is often assumed to be the channel-forming discharge, e.g., Q_cf = Q₂, where Q₂ is the two-year return interval discharge (Hey 1994, Ministry of Natural Resources 1994, Riley 1998). In general, Q_bf in stable channels corresponds to a flood recurrence interval of approximately 1 to 2.5 years in the partial duration series (Leopold et al., 1964, Andrews, 1980), although intervals outside this range are not uncommon. Recurrence interval relations are intrinsically different for channels with flashy hydrology than for those with less variable flows. Because of such discrepancies, many studies have concluded that recurrence interval approaches tend to generate poor estimates of Q_bf (Williams, 1978, Kondolf et al., 2001) and of Q_eff (Pickup, 1976, Doyle et al., 1999). Hence, assuming a priori that Q_ri is related to either Q_bf or Q_eff should be avoided, although it may be useful at times to serve as a first estimate of Q_eff and/or Q_bf in stable channels, particularly those with snowmelt hydrology (Doyle et al., 1999). The Q_ri approach is based on the assumption of stationary hydrologic conditions and thus is weak when applied to situations such as urbanizing watersheds where land use changes are forcing changes in hydrology and geomorphology.

UNGAGED SITES

Most watersheds are ungaged, and sediment transport data are collected at only a relatively few gages. When gage records are not available, estimates of Q_ri can be made based on similar gaged watersheds or from regression formulas (Jennings et. al., 1994, Wharton et al., 1989) developed using appropriate regional data sets. Calculation of Q_eff will require synthesis of a flow duration curve. Two methods are described by Biedenharn et al. (2001): the drainage area-flow duration curve method (Hey, 1975) and the regionalized duration curve method (Watson et al., 1997). It should be noted that both methods simply provide an approximation to the true flow duration curve for the site because perfect hydrologic similarity never occurs. Accordingly, caution is advised. Some workers have used sediment-discharge rating curves coupled with detailed geomorphic analysis to find Q_eff when historical hydrologic data were unavailable (Boyd et al., 2000).

A RANGE OF DISCHARGES

The quantities Q_eff, Q_bf, and Q_ri are estimates of Q_cf, and thus more than one of these should be considered (Biedenharn et al., 2001). Computed effective and bankfull discharges outside the range between the 1 and 3 year recurrence intervals should be questioned. The computed effective and recurrence interval discharges should be compared with field evidence to ascertain if these discharges have geomorphic significance. Channel performance should be examined for a range of discharges that represent key levels for aquatic habitat, riparian vegetation, channel stability, or flow conveyance (Copeland et al., 2001).

For the purposes of this manual, Annual High Water (AHW) serves as a guide to identify where vegetative techniques should be positioned on a given streambank. Note that in some stable streams systems, AHW may be equivalent to bankfull discharge. The AHW may or may not represent the average annual peak discharge or the elevation for the one-year event, but it does represent the lower limit suitable for establishing permanent woody vegetation. This is important because vegetation usually should not be planted below the AHW elevation. Generally the channel boundary below the AHW elevation is subjected to higher velocities and boundary shear stresses than regions above, therefore AHW roughly delineates the upper boundary of the zone in which structural support should be applied to the toe of the bank. The Annual Low Water (ALW) indicates the general elevation the roots must be able to penetrate down to in order to have access to water during the dry season. ALW approximates the depth of the vadose zone in a streambank soil profile. The elevation of the vadose zone is increasingly dictated by soil type as distance from the stream lengthens.

Design High Water (DHW), as calculated and specified by the designer, defines the upper elevation extent of a structure or technique, not including the required level of freeboard. The DHW elevation simply depicts the extent to which a technique may be inundated during a rare (design) hydrologic event. The designer must select techniques and materials suitable for hydraulic (i.e., waves, currents, seepage forces) or gravitational loading anticipated under design conditions. Design hydraulic loading may or may not coincide with the highest water levels. The relationship between river stage and hydraulic loading is site-specific due to differences in energy slope, channel roughness, and channel geometry. See Figure 3 for visual representation of DHW, AHW, and ALW and examples of techniques suitable for particular elevations.

Figure 3. Elevation diagram of DHW, AHW, and ALW

It is important to remember that there is no simple rule for selecting the design storm event. For any revetment technique, the design storm is selected by a combination of experience, local criteria, and policy. For example, in California, bridges on federal and state highways should pass the 50-year storm with sufficient freeboard for debris. Requirements vary by state, region, and municipality. Considerations for selecting the design storm interval for designing revetments are: risk to personal safety, potential loss of roadway fill that could increase travel times and cause accidents, the cost of rebuilding fill, and the availability of suitable rock. (Racin, 2000).

Design storms should be recalculated for a project site periodically based on recent high-water events, damages, and new gage data. Volume and duration of runoff are responses of global and regional weather patterns, which vary and are not easily forecasted. Velocities and flow rates depend directly on land characteristics and usage, which may change frequently in certain regions (Racin, 2000). Because the high flows produced by a design event vary based on the setting of a project site, it is important to note that DHW is not specified in the technique descriptions. Project designers must use local information and standard engineering procedures to determine the extent to which a revetment or similar structure should be built on a given bank.

The same concept of identifying important levels of flow in stream systems is also termed Stream Zonation and is addressed in this manual. In general, ALW refers to the toe zone and AHW is comprised of the toe and splash zones. The bank and top of bank zone is generally included in DHW.

REFERENCES

Andrews, E. D. (1980). Effective and bankfull discharge of streams in the Yampa Basin, western Wyoming, Journal of Hydrology, 46, 311-330.

Biedenharn, D. S., Copeland, R. R., Thorne, C. R., Soar, P. J., Hey, R. D., & Watson, C. C. (2000). Effective Discharge Calculation: A Practical Guide. Report No. ERDC/CHL TR-00-15, U. S. Army Corps of Engineers, Engineer Research and Development Center, Vicksburg, MS.(pdf)

Biedenharn, D. S., Thorne, C. R., Soar, P. J., Hey, R. D., & Watson, C. C. (2001). Effective discharge calculation guide. International Journal of Sediment Research, 16(4), 445-459.

Boyd, K. F., Doyle, M., & Rotar, M. (2000). Estimation of dominant discharge in an unstable channel environment. Proc., 1999 International Water Resources Engineering Conference (CD-Rom), Environmental and Water Resources Institute, Reston, VA.

Copeland, R. R., McComas, D. N., Thorne, C. R., Soar, P. J., Jonas, M. M., and Fripp, J. B. (2001). Hydraulic Design of Stream Restoration Projects. Technical Report ERDC/CHL TR-01-28, U.S. Army Corps of Engineers: Engineer Research and Development Center; Vicksburg, MS. (pdf)

Doyle, M. W., Boyd, K. F., and Skidmore, P. B. (1999). River restoration channel design: back to the basics of dominant discharge. Second International Conference on National Channel Systems, Ministry of Natural Resources, Ontario, Petersborough, Ontario. (pdf)

Emmett, W. W. and Wolman, M. G. (2001). Effective discharge and gravel-bed rivers. Earth Surface Processes and Landforms, 26, 1369-1380.

Federal Interagency Stream Restoration Working Group (FISRWG). (1998). Stream corridor restoration: principles, processes and practices. National Technical Information Service, U.S. Department of Commerce, Springfield, VA. (pdf)

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Hey, R. D. (1994). Restoration of gravel-bed rivers: Principles and practice.‘‘Natural’’ channel design: Perspectives and practice, Canadian Water Resources Association, Cambridge, Ont., 157–173.

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Riley, A. L. (1998). Restoring streams in cities: A guide for planners, policymakers, and citizens, Island Press, Washington, D.C.

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Schumm, S. A., Harvey, M. D., and Watson, C. C. (1984). Incised channels: morphology, dynamics and control. Water Resources Publications, Littleton, CO.

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