WSP05649 - Laserfiche WebLink

Home Browse Search

TOPPING ET AL: COLORADO RIVER SEDIMENT TRANSPORT, 2 boundary shear stress field in a reach caused by a change in flow patterns (herein referred to reach-geometric effects) and (2) changes in the upstream supply of sediment. O1anges in bed topography in Marble and Grand Canyons have been used extensively to deduce changes in the amount of sand storage in the river [Gra! et al., 1995, 1997; Konieczkj e/ aI., 1997; Hazel e/ aI., 1999]. However, because substantial changes in bed topog- raphy can be caused by reach-geometric effects, relating bed- topographic changes to changes in sediment storage can be problematic. During the 1996 Grand Canyon flood experiment (described by Schmidt el al. [1999)), a data set was collected that would allow separation of tbe two processes that control bed. topographic change. Daily topographic surveys of the reach at the Grand Canyon gage (at river mile 87.4) were conducted in conjunction with daily measurements of suspended-sediment concentration and grain size and bed-sediment grain size. To monitor changes in suspended-sediment concentration and grain size at other locations during Ihe flood, data were also collected at the Lower Marble Canyon gage (at river mile 61, above the mouth of the little Colorado River), in the eddy at the mouth of Hundred Twenty-Two Mile Creek (herein re- ferred to as 122-mile eddy), and at the National Canyon gage (at river mile 166.1). At the Grand Canyon gage the 1996 flood experiment con- sisted of 3 days of steady 238 m'fs (8400 cfs) discharge, fol- lowed by 5.75 hours of increasing flow, followed by 7 days of steady 1290 m'fs (45,400 cis) discharge, followed by 3.2 days of decreasing flow back to a steady 238 m'fs discharge. Through- out this paper, the day prior to the arrival of the flood is referred to as "day -I," the 7 days of 1290 m3/s discharge 3re referred to as "days 1-7," and the MI day of the receding limb of the flood is referred to as "day + \." 2.1. Methods During the 1996 flood experiment, bed topography in Ihe reach at the Grand Canyon gage was measured daily by (1) sounding under the measurement cableway (herein referred to as the Grand Canyon cab1eway) and (2) surveying five cross sections located at 0, 46, 86, 118, and 158 m above the cableway (Figure 2). These cross sections were surveyed using the meth- odology developed by Gm! et al. [1995]. U.S. Geological Survey (USGS) Ariznna District personnel also surveyed these cross sections 3 weeks prior to and after the flood [Konieczki et aI., 1997J. To allow better topographic interpolation between the cross sections, daily longitudinal sections were also surveyed at locations 113 and '2J3 of the channel width. Daily samples of bed sediment were collected at three to five locations under the Grand Canyon cableway (stations 140, 190, 240,290, and 340) on days -1, 1-3,5-7, and + 1 (Figure 2). To monitor the suspended sediment during the 1996 flood exper- iment, we collected daily samples at the Grand Canyon cable- way and in the I 22-mile eddy. During the experiment, suspend- ed-sediment samples were also collected by other investigators at the Lower Marble Canyon and National Canyon gages. The methods of collection and analyses of these samples are de- scribed by Konieczki el al. [1997] and Topping el al. (1999]. 2.2, Results At the Grand Canyon cableway, hed elevation, sand grain size (both on the bed and in suspension), and suspended-sand concentration all evolved during the 1996 flood experiment in a manner similar to that during the predam annual snowmelt S45 o UPPERGAGE . lOWERGAGE --+- X.SECTlONoa MEASUREMENTCABlEWA'r' -.. . X.seCTlON 1 __- - X.SECTION 2 __ . - X.SECTlON 3 .. .... X-SECTK>NA o BED-SEDIMENT SAMPlING STATION III BED-SEDIMENT & P-61 SUSPENDEo.SEDtMENT SAMPUNG STATION - APPAOX. EDGE OF WATER DURING 1996 flOOD 3150 3100 3050 / I ~ 3000 2950 N/ Ft.OW DIRECTION 2900 3000 3050 3100 3150 K(m) 3200 3250 Figure 2, Map of the study area atlhe Grand Canyon gage showing the locations of the upper and lower gages, the mea. surement cableway, the locations ofthe bed-sediment and P-61 suspended-sediment sampling stations on the cableway, and the cross sections surveyed during the 1996 flood experiment. The cross-hatched area indicates Ihe approximate location of a large lateral recirculation eddy on river left. flood (Figures 3 and 4). In the average predam year the bed at the Grand Canyon cableway aggraded as the water-surface stage increased during the snowmelt flood and then would begin to scour about 4 weeks prior to Ihe peak of the flood (Figure 3a). This scour prior to the peak ofthe snowmelt flood was associated with coarsening of the bed and depletion of the upstream supply of sand [Topping et al., this issue]. During the 1996 flood experiment, as during a predam snowmelt flood, the bed aggraded with the increase in water-surface stage, with maximum hed elevation being attained on days 4-5 of the 7-day flood. Then, also as during a predam snowmelt flood, the bed began to scour prior to the receding limb of the flood (Figure 3b). During the 1996 flood the bed initially fined as it aggraded (with the median size decreasing from 0.4 to about OJ mm between days -1 and I). Then, after day 1 of the flood the bed began to coarsen (Figure 4a). This coarsening contin- ued through at least the first day after the flood began to recede, and, importantly, this coarsening began while the bed was still aggrading. The upstream supply of sand was progressively depleted along at least 170 kIn of the river in Marble and Grand Can- yons during the 1996 flood experiment. As the bed coarsened at the Grand Canyon cableway, the suspended sand coarsened (Figure 4a) and suspended.sand concentrations decreased (Figure 4b). This style of decrease in suspended-sand concen- tration coupled to coarsening was also observed at the three