Analytical Framework for Evaluating Channel Maintenance Flows in Colorado

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An Analytical Framework for Evaluating Channel Maintenance Flows in Colorado 100 year flood events (Whittaker and Jaeggi, 1982; Grant et al., 1990). Ouerview of Channel Processes Channels throughout the Front Range of Colorado vary between the two extremes of self-formed sand/gravel streams and step/pool streams discussed above. In general, these channels fall into one of the following categories which will be discussed below: (1) Sand/Gravel-Bed Alluvial Channels, (2) Mobile Gravel Bed Streams, and (3) Step/Pool Channels. Sand/Gravel-Bed Alluvial Channels. This type of self-formed channel includes low-gradient sand/gravel bed streams at the lowest elevations of the Montane zone. In sand channels, the sand typical- ly moves in wave forms (ripples, dunes, plane bed) over a wide range of flows (Simons and Richardson, 1961). Sand/gravel channels are sinuous, have large width-to-depth ratios, and are characterized by rapid shifting of bed material, with continuous lateral migration. They are the "D" type channels classified by Rosgen (1985). This continual and frequent scour and deposition in sand/gravel-bed streams is characteristic of large bedload transport. Annual rates in such systems can exceed several hundred tons per square mile of drainage area. Sediment transport in such channels is derived from both flow induced and introduced sources. Mobile Gravel-Bed Streams. The processes in mobile gravel bed streams have been intensively stud- ied in recent years by Andrews (19$4), Hey, 1988), Hey and Thorne (1986), Bathhurst (1985), and Parker et al. (1982). The mobile gravel beds in this category are moderately steep self-formed streams in the Mon- tane and Alpine/Subalpine zones. They are the "B" and "C" type channels as classified by Rosgen (1985). The bedload transport can be moderate where sedi- ment is derived from introduced sources. On stable forested watersheds with little introduced sediment, the bedload transport is extremely low, averaging no more than 0.1 to perhaps 7 tons per square mile (Leaf, 1970). The entire channel perimeter is comprised of sedi- ment that has been transported by the stream. Such streams typically flow through intermontane valleys and not in steep narrow canyons (Andrews, 1984). They are typically at elevations from 6,000 to approxi- mately 9,500 feet above mean sea level. Another char- acteristic of this channel category is a prominent flood plain. In undisturbed watersheds, the primary erosion source is from the adjacent banks. However, cohesive bank material and the density of vegetation bordering subalpine gravel-bed streams exert an important con- trol on channel stability (Heede, 1972; Pfankuch, 1975; Rosgen, 1985) and channe] geometry (Andrews, 1984; Hey and Thorne, 1986). The bed-material transport rate controls the quan- tity of course sediment available to the stream. This flow-induced feature of a self-regulated sediment sup- ply explains the small loads and long-term stability of such channels in the Alpine/Subalpine zone. Step-Pool Channels. Step-pool channels are straight, steep (5 to 25 percent bed slope) streams. These streams generally are tributary to the mobile gravel-bed streams previously discussed. In the Alpine/Subalpine zone, step-pool channels transport a negligible amount of introduced sediment. Virtually all of the transported sediment results from bed and bank erosion (Leaf, 1966; Anderson et al., 1976; Sted- nick, 1987), and banks are usually resistant due to cohesive soils, large boulders, and dense vegetation. Thus, the bedload transport rate is extremely low. Published average annual sediment yields from watersheds drained by step-pool streams vary from 1.0 to perhaps 20 tons per square mile (Leaf, 1970; Caine, 1974), Free adjustment of channel width and slope typically is limited by geomorphic and/or other constraints. Step-pool streams do not have a well- defined flood plain. They are small "A" and "B" type streams as classified by Rosgen (1985). In contrast to mobile gravel beds, the larger frac- tions of the bed material in step-pool channels are not available for transport even during moderate flood events (Whittaker and Jaeggi, 1982; Grant et al., 1990), and vertical sorting of the bed material in step- pool channels is poor. The larger fractions of the bed material can be boulder-size and may extend well into the subsurface. Vegetation exerts an important influence on the hydraulics and resultant stability of step-pool streams. Logs which fall across these channels create steps which accumulate gravel upstream. Step length decreases with increasing channel gradient and increasing median bed material size (Heede, 1972). Log steps dissipate considerable energy so that veloci- ties are not high (Heede, 1972; Steppuhn, 1970). Within each reach, the cumulative heights (above the bed) of gravel and log steps are sufficient to reduce natural gradients from 50 to almost 100 percent (Heede, 1972). Due to the very low yields of sediment, it takes many years for these steps to fill in with bed- load material. Many reaches (steps) have geometric and hydraulic characteristics similar to those of mobile gravel-bed .IOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 869 JAWRA