Analytical Framework for Evaluating Channel Maintenance Flows in Colorado

Home Browse Search

An Analytical Framework for Evaluating Channel Maintenance Flows in Colorado in the Montane zone. Sedirnent introduced from gully erosion can account for 60 percent of the sediment load in streams (Gary, 1975). Figure 1 shows the regional relationship between average annual precipitation and sediment yield within the Front Range of Colorado. The data used to develop Figure 1 were obtained from a number of pub- lished sources and represent primarily bedload sedi- ment yields from the semi-arid plains, the Montane, and Alpine/Subalpine zones. The correlation in Figure 1 of more sediment yield with less annual precipita- tion results from the lower precipitation values being from rainfall dominated zones (i.e., Montane and Semi-arid) where surface runoff causes soil erosion and introduced sediment. The higher precipitation values are from the snowmelt dominated Alpine/Sub- alpine zone where subsurface runoff results in negli- gible introduced sediment. Of significant importance in FSgure 1 is the wide range of yields as illustrated by the following summary (Table 1). The regional relationship of Figure 1 will be used in this paper's example of an analytical accounting procedure for quantifying instream channel mainte- nance flows. While Figure 1 is used to illustrate the procedure, it cannot be emphasized enough that site specific studies on sediment sources/yields, hydrology and channel processes before and after disturbances within a watershed are essential for accurate quan- tification. CHANNEL PROCESSES An important concept having to do with channel maintenance flows is that of "dynamic equilibrium" (Leopold et al., 1964). Dynamic equilibrium implies a balance between discharge, slope, bed sediment dis- charge, and particle diameter of bed material. Dynamic equilibrium means the sediment material supplied to andlor stored in the stream channel is bal- anced with the energy available to transport the material. Dynamic equilibrium is not a steady state from year to year, and annual variations in scour or deposition may occur. Such net channel adjustments can occur and channel stability remain as long as these forcing functions stay below the threshold. The discharge required to supply the transport energy or stream power when dynamic equilibrium exists is called the effective or formative discharge. The con- cept of dynamic equilibrium implies a"]ive" yet stable bed in the sense that bed sediment is introduced at a rate sufficient to offset scour resulting from bed mate- rial movement. In self-formed channels or "channels flowing within an unlimited envelope of deposited sediment identical in character with that transport- ed" (Lacey, 1963), the formative discharge is equated to "bankfull discharge." A flow of this magnitude just fills the channel to the level of the adjacent flood plain. Hydrologically, bankfull discharge is a frequent flow which has a recurrence interval of 1.2 to perhaps three years and transports the largest fraction of annual sediment load over a period of years (Andrews, 19$0). It is widely held (although debated) that the channel-forming flow in natural alluvial streams and rivers is bankfull discharge provided this discharge can transport virtually all sizes of the bed and bank material. But not all streams have this characteristic. Recognizing this, Leopold et al. (1964) stated that ". .. Obviously mountain streams which contain large boulders in their beds will not be adjusted to flows that are incapable of moving the sizes of material found within the channel. The thresh- old and channel forming discharge will be one competent to move the size available. The larger the material, the larger will be the discharge necessary to provide the stress required for mov- ing the material. . ." In steep, step/pool Alpine/Subalpine zone channels, flows in excess of the approximate 1.5 year recurrence interval magnitude are necessary to move the large sizes available. Indeed, many of the morphological features of such streams are formed during the 50 to TABLE 1. Average Annual Precipitation and Sediment Yield from Montaine, Subalpine, and Alpine Zones in Colorado. Average Annual Hydrologic Annual Annual Sediment Yield Location Setting Ppt. (in.) (tons{mi2) Niwot Ridge (Caine, 1974) Alpine 50 1.15 Fraser Experimental Forest (Leaf, 1970) Subalpine 30 9.0 Manitou Experimental Foreat (Gary, 1975) Montane 15 300+ .IOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 867 JAWRA