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tJ u d ~I-e 110 I J Y M1 d t -It ~ m a FORREST M. HOll-Y. JR. AND RoBERT ETI'EMA 41 Sediment Imbalance in Rivers: Simulation Possibilities and Problems by Forrest M. Holly, Jr. and Robert Ettema Iowa Institute of Hydraulic Research Department of Civil and Environmental Engineering University of Iowa Iowa City, Iowa 52242 Abstract. River engineering, having begun the 20th century as a well-intentioned but naive determination to tame and capture nature's water resources, is approaching the 21st century with a sober realization of the complexity and consequences of disturbances to a rivers natural equilibrium and its ecosystem. TIris coming-of-age, born of extensive and sometimes painful experience in rivers'long-term response to man's intervention, has been accompanied, indeed nurtured, by new capabilities in computer-based simulation of nonequilibrium fluvial processes. If it is clear that today's simulation techniques may have led to more careful and successful river engineering in the past, it is not so clear that simulation provides, in itself, a tool of sufficient reliability to guide future river engineering and development. TIris is a result of our continuing feeble understanding of some of the most complex physical processes known to man, and also of our tendency to place too much confidence in these first-generation simulation models and in their practitioners. We describe the common bases of most nonequilibrium simulation models, assess the strengths and future possibilities of such models in providing technical support for river-development decisions, and present an example of application to the Missouri River. The value of models to provide insight into river response, if not finite answers, is illustrated through application of a numerical model to the Missouri River. Alluvial rivers-those that convey not only water and living organisms, but also the sediments com- posing their bed and banks-are among the most complex natural systems dealt with by engineers. Alluvial rivers are architects of their own geometry, shaping and molding their channels and their plan- form meanders, in transporting their imposed water and sediment loads from the watershed to the sea while providing habitat for living organ- isms. The notion of an equilibrium between the chan- nel and planform geometry on one hand, and the imposed water and sediment loads on the other, depends very much on the time scale of interest. At any given moment, or indeed in any given year or perhaps decade, the channel and planform ge- ometry-and thus the riverine habitat-are con- tinually changing in response to short-term floods ,- ~. .''> f~? ~. " !io- ""'""- or droughts, watershed and land-use changes, and perhaps even seismic topographic adjustments. At a much larger time scale, of the order of decades or centuries, a river's channel and planform geome- try, and the size of transported sediments, reflect an equilibrium between the water and sediment loads entering the river from the watershed and the river's capacity to convey these loads. This dynamic equilibrium implies a certain equilibrium habitat, at least insofar as bed and bank geometry, bed and bank vegetation, and suspended sediment are concerned. Now enter man, also known as the river engi- neer. In straightening channels through artificial cutoffs, blocking sediment transport with dams, stabilizing banks, building river-training struc- tures for navigation, paving the watershed, divert- ing tributaries, and so forth, man imposes drastic q25& ~ ~ il ~i ~1 ;t'