Comparison of Sediment-Transport and Bar-Response Results From the 1996 and 2004 Controlled-Flood Experiments on the Colorado River in Grand Canyon

HR: 11:35h
AN: H52A-06 [Abstracts]
TI: Comparison of Sediment-Transport and Bar-Response Results From the 1996 and 2004 Controlled-Flood Experiments on the Colorado River in Grand Canyon
AU: * Topping, D J
EM: dtopping@usgs.gov
AF: USGS, 2255 N. Gemini Dr., Flagstaff, AZ 86001 United States
AU: Rubin, D M
H52A-06 AF: USGS, 400 Natural Bridges Dr., Santa Cruz, CA 95060 United States
AU: Schmidt, J C
H52A-06 AF: Dept. of Geog. and Earth Res., Utah State Univ., Logan, UT 84322
AU: Hazel, J E
H52A-06 AF: Dept. of Geol., NAU, Flagstaff, AZ 86001 United States
AU: Wright, S A
H52A-06 AF: USGS, 2255 N. Gemini Dr., Flagstaff, AZ 86001 United States
AU: Melis, T S
H52A-06 AF: USGS, 2255 N. Gemini Dr., Flagstaff, AZ 86001 United States
AU: Kaplinski, M
H52A-06 AF: Dept. of Geol., NAU, Flagstaff, AZ 86001 United States
AB: The sediment-transport paradigm for the regulated Colorado River in Grand Canyon (GC) prior to the 7-day 45,000 ft³/s 1996 controlled-flood experiment was that, under normal releases from Glen Canyon Dam, tributary-supplied sand would accumulate in the channel over multiple years and could then be transferred from the channel bed to eddies during controlled floods, increasing both the area and volume of eddy sandbars. Work conducted during and after the 1996 flood indicates that this paradigm was based on assumptions that were either false or only partially true (Rubin et al., EOS, 2002). First, sand did not accumulate in the channel over years. Second, sand deposited at higher elevations in eddy sandbars during the 1996 flood was derived mostly from the lower parts of these bars (not from the channel bed) resulting in bars that were smaller in area and volume (although they did contain more sand at higher elevations). Tributary inputs of sand were low in the year preceding the 1996 flood and dam releases were moderate to high. Thus, the 1996 flood was conducting during a period when the Colorado River in GC was relatively sand-depleted. The design of the 2004 controlled-flood experiment was to: (1) keep dam releases relatively low (<10,000 ft³/s) in Sept.-Nov. 2004 to allow accumulation and retention of new tributary sand in the channel, and (2) after retention of >800,000 metric tons of new sand in Marble Canyon (the first 99 km of GC), release a 60-hr flow of 41,000 ft³/s from the dam to transfer this sand from the channel bed into the eddies. More sand, silt, and clay were present in Marble Canyon (MC) during the 2004 experiment than during the 1996 experiment. At the lower end of MC, suspended-silt & clay concentrations were 3x higher and suspended-sand concentrations were ~30% higher than during the 1996 flood. Furthermore, during the 2004 flood, sand concentrations were higher in the upstream half of MC than in the downstream half of MC. In contrast, during the 1996 flood, sand concentrations likely increased downstream throughout MC. The spatial pattern in concentration during the 2004 flood resulted from effective retention of tributary-supplied sand in the upstream half of MC during the lower releases preceding the flood. The response of the eddy sandbars during the 2004 flood correlates with this observed spatial pattern in sand concentration. About 2/3 of the bars surveyed in the upstream half of MC were larger than they were after the 1996 flood, whereas only 1/3 of the bars surveyed in the downstream half of MC were larger in both area and volume than they were after the 1996 flood. In contrast, less sand was present in GC downstream from MC during the 2004 flood than during the 1996 flood. At 2 gaging stations located 43- and 260-km downstream from MC, sand concentrations were ~30% lower than during the 1996 flood. As in MC, the response of eddy sandbars in this downstream reach also reflects the difference in sand concentration between the two experiments, with fewer surveyed bars being larger after the 2004 flood than after the 1996 flood. Therefore, it appears that the 800,000 metric tons of new sand in retention prior to the 2004 flood was sufficient to result in substantial increases in sandbar area and volume in only the first 50 km (i.e., the upstream half of MC) of the 400-km long reach of the Colorado River in GC.
DE: 1808 Dams
DE: 1821 Floods
DE: 1825 Geomorphology: fluvial (1625)
DE: 1861 Sedimentation (4863)
DE: 1862 Sediment transport (4558)
SC: Hydrology [H]
MN: Fall Meeting 2005