9658 - Laserfiche WebLink

Home Browse Search

recession, with peak discharges arriving in early to late spring (Fig. 22). The bed near the gage responds to the seasonal flooding by scouring, or increasing the thalweg depth as each years flood rises (Fig. 23). Although significant scour or fill occurs during a wide range of discharges, all discharges greater than 50 m3/s (1770 fe/s) scour the bed. Bed elevation data were recorded on average twice each month between 1968 and 1979, and between 1985' and 1997. These measurements occurred relatively infrequently in relationship to changes in discharge. Accordingly., the precise discharges at which scour occurs could not be directly extracted from the discharge records. Therefore, we compared the largest discharges during periods when the bed changed less than 0.25 m with the largest discharges during periods when the be~ scoured and filled more than 0.25 m. Results of a two-tailed t-test show that, with 95% certainty, the mean maximum discharge during periods of scour was larger than the mean maximum discharge during periods of bed stability (Table 5). The highest discharge during a period of bed stability was 41.3 m3/s (1460 fe/s), but in some cases the bed scoured at lower discharges. The lowest peak discharge during a scour event was 37 m3/s (1290 fe/s). All discharges above 41 m3/s caused scour. Figu~e 24 shows the distribution from which these means were calculated. Discharge values during periods of bed stability are normally distributed. Discharges during periods of scour were skewed and bi-modal. Because we did not know which exact date scour began, some discharges which were not sufficient to move the bed were unavoidably included in the period of scour. The first mode probably represents these discharges. The second mode more accurately represents the discharges responsible for scouring the bed. The most common discharges in the second mode occur between 45 and 75 m3/s (1590 and 2650 fe/s). Discharges during periods where 24