Analytical Framework for Evaluating Channel Maintenance Flows in Colorado

Home Browse Search

JOUftNAL OF THE AMERICAN WATER R.ESOUR.CES ASSOCJATION VOL. 34, NO. 4 AMERICAN WATER RESOURCES ASSOCIATION AUGUST 1998 AN ANALYTICAL FR.AMEWORK FOR EVALUATING CHANNEL MAINTENANCE FLOWS IN COLORADO1 Charles F. Leaf2 ABSTRACT: An accounting procedure is developed which deter- mines a flow regime that ia capable of tranaporting an amount of bedload sediment necessary to ensure channel atability down- stream. The method allowa for sediment buildup in the channel within geomorphic threshold limits during low flow periods. During periods of high runoff, enough water is bypassed to transport the stored aediment. The procedure utilizes only those tlows of suffi- cient magnitude to maintain channel stability over the long run (25-50+ yeara). An example is presented which determines the vol- ume of water and frequency of release for channel maintenance purposes downatream from a hypothetical water diveraion pmject. Of some 1,200,000 acre feet generated during a 59-year period, 86,500 acre feet was required for channel maintenance flows. Bypass flowa were not required each year, but only during those yeara when average daily flow reached bankfull or greater. Such releases were made on 202 of the 411 days when average flows either equalled or exceeded bankfull diacharge. (KEY 1'ER.MS: sediment transport; geomorphic threshold; channel pmcessea; channel maintenance flowa; sediment yield, fluvial geo- morphology.) INTRODUCTION The purpose of this paper is to propose a prelimi- nary analytical framework by which channel mainte- nance flows can be rationally quantified for the Alpine/Subalpine and Montane climatic zones of the Colorado Front Range. In this paper, "Channel Maintenance Flow" refers to the flow regime that transports a volume of bedload sediment necessary to maintain the stability of a stream channel subsequent to localized disturbances such as road building, logging, ski area developments, and regional disturbances like water diversion pro- jects. Any determination of this flow regime must be based on a site-specific and explicit evaluation of the following elements: (1) the proposed activity, (2) hydrology, (3) sources and yields of sediment, and (4) channel processes. Man's activities can have a wide range of effects on a stream. Stream channels reflect the current water- shed condition. For example, critical watersheds (USFS, 1981) are identified by the U.S. Forest Service where road construction and timber harvest have increased sediment loads to streams beyond the threshold of channel stability. A disturbance within a watershed will impact stream channel stability because it affects the exist- ing interrelationships among the elements of hydrolo- gy, sediment sources/yields, and channel processes. The relationship of channe] stability to these ele- ments is an issue of thresholds. Site specific evalua- tions require analytical procedures that compare hydrology, sediment levels, and channel processes within a watershed for conditions before and after dis- turbances to thresholds where channel stability could be affected. It is believed that our status of knowledge is suffi- cient to do at least a preliminary determination of channel maintenance in AlpinelSubalpine and Mon- tane watersheds. For example the WRENSS (Water Resources Evaluation of Nonpoint Silvicultural Sources) Procedural Handbook (EPA and USFS, 1980) with chapters on Hydrology (Troendle and Leaf, 1980), Surface Erosion (Warrington, 1980), Soil Mass Movement (Swanston and Swanson, 1980), and 1'otal Potential Sediment (Rosgen, 1980) provides the hydrologist with ways for systematically evaluating the hydrologic effects of land use activities. In addi- tion, more than 75 years of watershed management research has been summarized in the literature (Leaf, 1975; Gary, 1975; 7roendle and Kaufmann, 1987). 1Paper No. 93159 of the Journal of the American Water Resources Association. Discussions are open until April 1, 1999. ZPlatte River Hydrologic Research Center, 59365 WC Road R., Merino, Colorado 80741. .JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 865 JAWRA