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422 BIOLOGICAL REPoRT 19 Kennedy 1982), was adjustment of the initial bed- sediment diameters to account for the spatial het- erogeneity not reflected in the sparse field meas- urements available. Once the model had been verified, it was used to study a series of schematic scenarios in the interest of identifying the primary cause of the degradation and forecasting future degradation. These scenarios were as follows: 81: Nominal2O-year simulation, a continuation of the 1960-80 verification run. 82: Effect of out-of-basin diversion, simulated by reducing the Gavins Point Dam releases by an average of 116 ems (4,100 cfs) over the annual cycle. 84: Effect of channelization, studied by widen- ing the navigation channel from 183 to 244 m. 85: Effect of channelization, studied by widen- ing the navigation channel from 183 to 305 m. 88: Effect of artificial armoring as a means of local degradation control, studied by increasing by about 10% the amount of bed sediment between 2.4 and 19.1 mm. 89: Effect of modulation of Gavins Point Dam releases, studied by constant release of the mean annual discharge of 823 ems (29,000 cfs). Figure 4 shows the computed time-evolution of bed level at 8ioux City, Iowa, under these various scenarios. The initial rapid degradation for all runs is caused by the use of an initial armoring factor of zero, thereby failing to account for any residual protection of the bed surface by interlock~ ing immobile particles in 1980. All the runs tend to reach a kind of equilibrium in 2-6 years. Then all but run 85 show the effects of the arrival of the degradation wave from upstream. The asymptotic approach to a new equilibrium is visible from about 15 years onward. Runs 82 and 89 show that reduction and modu- lation of the mainstem water inflow can reduce the 2.5 ~ ~ 2.0 ~ o ~ 1.5 Cl w o w 1.0 > ~ => 0.5 ::;; => o ~~:--:,.-_.. .--:::::::-::.:;;;;;;~-;;;:::-~~;:;.;-;;.~:'"~~."'~~~.~",.... --------------------- .. _.. -.. SI: NOMINAL 2O-YEAR SIMULATION - S2: UPSTREAM DIVERSION --- - - - $4: 6O-METER WIDENING - --- S5: 120-METER WIDENING ............... S8: LOCALIZED ARTIFICIALAAMORlNG .-.-.- S9:S1CAIlYFLOWAT822CMS '~ Ji ji j ,\ ] ,'11 ,1 00 2 4 6 8 10 12 TIME (YEARS) ultimate degradation by about 10 and 20 em, re- spectively. Runs 84 and S8 show that a 61-m wid- ening and artificial armoring could reduce the ulti- mate degradation by about 30 em. Run 85 shows that a 122-m widening, which represents a return to nearly the pre-channelization natural-channel width, would virtually eliminate further degrada- tion, though possibly at the expense of maintaining adequate navigation depths for part of the year. Analysis of similar results for these and other sce- narios at all computational points of the model can be found in Holly and Karim (1983). Additional analyses of the effect of the Platte River sediment Inflow in arresting degradation at and below Omaha can be found in Holly et al. (1986). Although this Missouri River model has not been in active use in the past few years, it could be used to~~~ersedime~m~~mscenariosm :relevance to wildlife habitat. These could include: recovery of the temporal pattern. of the natural (pre-dam) hydrograph, perhaps with some storage modulation; development of a sediment-bypass sys- tem for Gavins Point Dam; and artificial sediment supply to the tailwater of Gavins Point Dam. Simulation Deficiencies Although the Missouri River application has been presented somewhat optimistically, mobile- bed modelling is still in the dark ages insofar as reliable fidelity to physical processes is concerned, especially in 1-0 applications. A few of the more prominent weaknesses and areas of concern are listed below. Spatial Averaging In their inability to resolve spatial heterogene- ity of bed sediment and transport within a cross ..-..--.-.. Fig. 4. Cumulative computed degrada- tion at Sioux City, 1980-2000. 14 18 20 16