Arkansas River Channel Capacity and Riparian Habitat Planning Study

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Type III Analysis on five stream gages as listed above (in accordance with Bulletin 17B, EM 1110 -2 -1415, ER 1110 -2 -1450 and the HEC -FFA User Manual. The flow versus the drainage area for the 2 -, 10 -, 50- and 100 -year flows was plotted using an Excel spreadsheet. These plots showed that the peak flows for the 10, 50, and 100 -year frequency primarily occurred at the La Junta stream gage. The 2 -year peak flow occurred at the Nepesta stream gage. The flow typically attenuates downstream as it approaches -the Las Animas stream gage. After discussing these plots with experts at BBC in Davis, California, we decided that some of the missing years of data (flood of 1921} needed to be interpolated and added so that these plots better represent the actual trend of flows. The Moments (mean, standard deviation and skew) versus drainage area was then plotted in a spreadsheet for each of the group of statistics. The mean and standard deviation behaved very well (gradual change in slope of the curves) for ;the period of record. The skew showed no apparent pattern. The Moments versus time was used as a tool to investigate how purported changes in the watershed affect the discharge frequency. A trend analysis was to develop by taking the 10 -year running moments. First we ran the FFA program using 10 -year data sets. Then we plotted them in Excel using the mid -year to define the points. Thr1 plots showed relatively smooth curves. The mean versus time showed a relatively steady trend versus time. The standard deviation versus time showed an increasing trend versus time, which is primarily due to the floods of 1999. The plots of the skews were less consistent. Table 3.2 lists the frequency discharge peaks :generated by the FFA program for selected stations on the Arkansas River. TABLE 3.2 DISCHARGE FREQUENCY PEAKS 10.0 % - 2.0 % - 1.0 %- 0.2 %v- Drainage Chance Chance Chance Chance LOCATION Area Flood Flood Flood Flood , (sq nu) Discharge Discharge Discharge Discharge (cfs) (cfs) (cfs) (cfs) Arkansas River near Avondale, CO 6327 1 16500 33600 44400 81900 Arkansas River near Nepesta, CO 9345 ` 19700 39000 50600 88500 Arkansas River at Catlin Dam at Fowler, CO 10901 18400 39700 53500 102000 Arkansas River at La Junta, CO 12210 .25100 53300 69400 118000 Arkansas River at Las Animas, CO 13976 ! 17700 40100 53800 98500 28 I L I 1'�- 1 I.