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Leaf
<br />using the baseline bedload sediment rating ciirve
<br />(Figure 3). Column 4 summarizes the minimum
<br />transport requirement of 1,000 tons/year. Column 5
<br />summarizes the minimum transport requiremerit in
<br />any given year. For example, in 1957, the niinirnum
<br />amount of sediment to be transported in 5,644 t;ons.
<br />This represents storage of bedload sediment in the
<br />channel as the result of the cumulative eflFec9;s of pre-
<br />vious dry years when transport capacity was mot suffi-
<br />cient to move all of the required 1,000 tons/year. In
<br />1957, a high runoff year, the maximum transport
<br />capacity is 41,851 tons. As seen in column 6, 8 of the
<br />29 days of greater than bankfull flow would be
<br />required to move the minimum transport of 5,644
<br />tons. In 1958, the maximum transport capacity is
<br />1,174 tons, and six of the seven days of greEiter than
<br />bankfull flow would be required to move t:he 1,000
<br />tons minimum during that year.
<br />Out of the total of 411 days that mean daiily flows
<br />equalled or exceeded bankfull flow during the 59-year
<br />record period, 202 days would be utilized for- cha.nnel
<br />maintenance flows at point A(Table 2). The frequency
<br />and duration of these bankfull and greater flows vary
<br />from zero during dry years to 79 days during wet
<br />years. On average, the minimum required flow dura-
<br />tion is 3.4 days. Assuming an average annual flow of
<br />20,569 acre-feet from the data presented for the
<br />record in Table 2, a total of 1,213,579 acre-feet was
<br />generated during the record period. Of thi:a total, a
<br />minimurn of 86,516 acre-feet would be utilized for
<br />channel maintenance flow purposes at point A.
<br />Assuming that a project were constructed at point
<br />B, a monitoring and accounting procedure woul.d be
<br />developed in order to measure sediment buildup and
<br />provide for operational releases and thereby assure a
<br />channel maintenance flow regime at point A similar
<br />to the one presented in Table 2. Leaf and Sundeen
<br />(1988) have proposed similar procedures for determin-
<br />ing operational releases to assure a flow regime which
<br />would mitigate wetlands impacts downstream from
<br />the proposed Homestake II Project in Coloracio.
<br />Stream systems can go through periods of overall
<br />degradation/scour or overall aggradation/deposition
<br />and still not be unstable over the long-term of many
<br />years. ]Eiow much is a question of magnitude and
<br />thresholds which is inherently site specific.
<br />The key data in this example are (1) sediment load-
<br />ing rates from the regional relationship of Figure 1,
<br />(2) required sediment transport for channel mainte-
<br />nance, and (3) the existence of a critical reach or
<br />"weak link." For actual accounting, such data must be
<br />based on extensive site specific monitoring. Stream-
<br />flow data, bedload transport data (sediment quantity,
<br />size, and source), and channel stability evaluations
<br />(Pfankuch, 1975) must be collected pre- and post-
<br />watershed activity.
<br />This example assumes a moderately-sized water
<br />storage project as the watershed activity that could
<br />potentially impact stream channel stability. Knowing
<br />the potential amount of sediment accumulation and
<br />effect on channel stability after a project of this mag-
<br />nitude is built has to be site specific. If there is negli-
<br />gible introduced sediment and the channel below the
<br />diversion project is extremely stable, then channel
<br />maintenance bypass flows may not be an issue.
<br />It is incumbent on hydrologists and others con-
<br />cerned with stream channel stability to objectively
<br />determine if channel maintenance bypass flows are
<br />needed. Weak links must be established and the mini-
<br />mum flow regime necessary to transport post-project
<br />sediment loads must be determined. Most important,
<br />land use practices that will minimize erosion at its
<br />source on the watershed must be employed, thereby
<br />reducing the need for excessive channel maintenance
<br />flows.
<br />Finally, the example had used a rather long exist-
<br />ing streamflow record to illustrate the channel main-
<br />tenance flow evaluation procedure. Equally valid and
<br />convincing results can be obtained using a properly
<br />calibrated and validated watershed simulation model
<br />such as WATBAL (Leaf, 1975) to synthesize a long-
<br />term, site-specific, streamflow record when actual
<br />data are not available (Leaf and Sundeen, 1988).
<br />CONCLUSIONS
<br />This hypothetical example illustrates an account-
<br />ing procedure that allows sediment buildup within
<br />threshold limits. This procedure gets away from chan-
<br />nel maintenance flows always being put in the con-
<br />text of a required yearly bypass flow whether needed
<br />or not. Flows should be bypassed only if they can do
<br />the job of moving the required amounts of sediment.
<br />Flows that do not have the transport energy or
<br />stream power to move the sediment need not be
<br />bypassed for this purpose.
<br />LITERATURE CITED
<br />American Society of Civil Engineers, 1975. Sedimentation Engi-
<br />neering. ASCE Manuals and Reports on Engineering PrACtice.
<br />ASCE, New York, New York, No. 54, p. 483.
<br />Anderson, H. W., M. D. Hoover, and K. G. ReinhArt, 1976. Effects of
<br />Foreat Management on Floods, Sedimentation, And Wnter Sup-
<br />ply. U.S. Dept. Agric. For. Serv. Gen. Tech. Rept. PSW-18, Pacific
<br />Southwest Forest and Range Expt. Station, Berkeley, Californirx,
<br />115 pp.
<br />Andrews, E. D., 1980. Effective und Bankfull DischArges of
<br />Streams in the Yampa River Basin, Colorado And Wyoming.
<br />Jour. of Hydrology 46, Elsevier Scien. Pub. Co., Amsterdam.
<br />JAWRA 874 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
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