Analytical Framework for Evaluating Channel Maintenance Flows in Colorado

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Leaf channels; however, the "self-formang" processe-s in each reach are limited in large part by the irifluence of rock outcrops, boulders, and streamside foressts which form steps and maintain channel stability. GEOMORPHIC THRESHOLDS An understanding of the concept of geornorphic threshold is important in evaluating channel mainte- nance flow requirements. Simply put, this conr.ept recognizes that most streams will remain stable with- in a range of hydrologic conditions and sediment inputs. Channel stability, i.e, the magnitude of this range can be implicitly evaluated using the pi•ocedure developed by Pfankuch (1975). Also, as previously dis- cussed, a comprehensive quantification procedure for determining the magnitude of change in hydro]ogy and/or sediment sources and supply is available in the chapters of WR,ENSS previously mentionedl. Ot;her engineering procedures such as Meyer-Peter-Muller (1948) can be used where applicable for deterrnining a stream's ability to transport bedload sedimer.it under a particular flow regime. OPERATIONAL CONSIDERATIONS In this discussion a channel maintenancE: flow is that flow regime necessary to move sedime:nt "over the long haul" in a given stream reach. Quantification of this flow regime requires an evaluation of the local hydrology, sediment sources and yields, channel pro- cesses, and the inherent stability of the stream chan- nel. Then, the problem becomes one of a site specific evaluation of how much change in hydrology, sedi- ment sources and yield, and baseline flow regrime the channel can tolerate without excessive or accelerated changes in channel geometry and rates of lateral migration. In self-formed channels, at least bankfull flows must occur often enough to transport the quantity and maximum size of the bedload sediment (Leopold et al., 1964). In channels where morphology is not entirely determined by flow (as in step-pool c;hannels for example) the required maximum discharge is not well known. For such channels, a channel mainte- nance flow regime appears to be necessary only where introduced sediment sources contribute significantly to yield. The proposed criteria for determining an opera- tional channel maintenance flow regime consist;s of two important components: (1) Only flows equal to or greater than "bankfull discharge" are utilized to move the required amount and sizes of bedload sediment. These flows should be allowed to occur on average at least once each 1.5 to 3 years. Actual amounts and timing of flow occurrences will depend on site specific monitoring of sediment buildup and potential channel instability. However, in all stream systems, the required discharge will be that needed to move introduced sediment if any. (2) The flows above must transport the average annual yield of sediment from all sources over the long term (say 25 to 50+ years). In dry years, for example, when the duration of flows greater than or equal to bankfull are short or non-existent, the mean annual bedload sediment yield may not be transported that year. In this case, sedi- ment derived from introduced sources will be stored in the channel. That sediment which is stored in the channel must be transported during subsequent high runoff years when the duration of bankfull or greater flows is adequate to move the stored sediment plus the average annual sediment yield for that year. In summary, the proposed quantification of a chan- nel maintenance flow regime involves an accounting system based on site-specific evaluations of the hydrology, sediment yield characteristics, and channel processes. Example calculations which illustrate application of these simple criteria follow. EXAMPLE A small storage reservoir and diversion project is proposed at point B on the hypothetical Front Range watershed shown in Figure 2. A preliminary channel maintenance flow evaluation is required as part of the feasibility study of the proposed project. The drainage area upstream of the reservoir is 40.43 square miles of Alpine/Subalpine zone. The lower basin downstream drains 40.44 square miles of the Montane zone. The critical reach of channel selected for channel maintenance flow evaluation is some distance tlownstream from the reservoir at point A. The channel at this location is a C3 mobile gravel bed. Bankfull flow at point A is approximately 165 c.f. s. Average annual precipitation in the ]ower basin is 22 inches. Referring to Figure 1, this value corre- sponds to an average annual bedload sediment yield of approximately 41 tons/sq. mi. This amounts to 1,658 tons per year of bedload sediment transport in the reach of channel between A and B. Of this total baseline yield, it was conservatively estimated that the stream must be capable of transporting at least JAWRA 870 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION