Laserfiche WebLink
<br />4.2.2 Kain stem InflowinK Loads <br /> <br />Measurements of instantaneous sediment transport rates and size <br />distributions are not available at the main stem gages and could not be <br />obtained within the scope of this study. Therefore, it was decided to develop <br />an inflowing load that is in equilibrium with the transport capacity of the <br />upstream end of the reach. This was accomplished by operating HEC-6 for a <br />range of discharges, each with very short duration so that insignificant <br />changes to the bed material size distribution or bed elevations would take <br />place. An inf lowing load curve with zero sediment load was provided. The <br />calculated load passing each section for those conditions is equivalent to the <br />equilibrium transport rate. This information is obtained for each grain size <br />from a "C" level print out. The inflowing load curve used was computed as the <br />average of these calculated equilibrium loads at the three most upstream <br />sections. These calculated inflowing load curves are shown on Figures 4.5 to <br />4.8. <br /> <br />Transport capacity was calculated using the Toffaleti procedure. The <br />Toffaleti method was chosen because this reach of the Arkansas is similar to <br />sand bed streams for which the Toffaleti method has worked satisfactorily in <br />the past. Should a different transport function be selected at a later date, <br />the main stem inflowing load cut'Ves would have to be recalculated. <br /> <br />4.2.3 Tributary InflowinK Loads <br /> <br />Four tributaries occur within the study reach: Timpas Creek, Apishapa <br />River, Huerfano River, and Chico Creek. Although instantaneous sediment <br />transport measurements were made on these tributaries in conjunction wi th <br />early John Martin sedimentation surveys, the original data are no longer <br />available. The data have been averaged into monthly and/or yearly values <br />however, which are available. Therefore, these data were converted to units <br />of tons/day for sediment and cfs for water and plotted to yield an <br />"instantaneous" relationship based on the temporal averages. Konthly averages <br />were used for the Apishapa and Huerfano; yearly for Chico and Timpas. These <br />load curves, while yielding correct average volumes of sediment, do not <br />reflect the true relationship between instantaneous water and sediment <br />discharges. Adjustment of these load curves during calibration of the model <br />was considered but not found to be necessary. <br /> <br />Average grain size distributions for the tributary suspended loads were <br />given in the John Martin Dam Sedimentation Report (6). These distributions <br />were applied directly to the total load curves developed as described above to <br />yield loads for the individual grain sizes. The measured suspended load <br />includes, of course, no information on bed load (actually "unmeasured load") <br />which, although a smaIl portion of the total (lO-20~, perhaps), contains all <br />the coarser size materials. (The suspended gradations include as a maximum <br />size, coarse sand.) The bed load component for the tributaries was estimated <br />by applying the percentage of total bed material load calculated for the main <br />stem for the coarser size fractions to the tributary loads. This may result <br />in too small a load for the coarser fractions. <br /> <br />19 <br />