HR: 0800h
AN: H51I-0902    [Abstracts]
TI: Dominance of Changes in Bed-Sand Grain Size Over Bed-Sand Area in Regulating Suspended-Sand Concentration: Examples From the Colorado River
AU: * Topping, D J
EM: dtopping@usgs.gov
AF: USGS, 2255 N. Gemini Dr., Flagstaff, AZ 86001, United States
AU: Rubin, D M
EM: drubin@usgs.gov
AF: USGS, 400 Natural Bridges Dr., Santa Cruz, CA 95060, United States
AU: Wright, S A
EM: sawright@usgs.gov
AF: USGS, Placer Hall 6000 J St., Sacramento, CA 95819, United States
AU: Melis, T S
EM: tmelis@usgs.gov
AF: USGS, 2255 N. Gemini Dr., Flagstaff, AZ 86001, United States
AB: Sand transport in the Colorado River in Marble and Grand Canyons was naturally limited by the upstream supply of sand. Prior to the 1963 closure of Glen Canyon Dam, the river exhibited the following four effects of sand supply limitation: (1) hysteresis in suspended-sand concentration, (2) hysteresis in suspended-sand grain size coupled to the hysteresis in suspended-sand concentration, (3) production of inversely graded flood deposits, and (4) development or modification of a lag between the time of a flood peak and the time of either maximum or minimum (depending on reach geometry) bed elevation. Construction and operation of the dam have enhanced the degree to which the first two of these four effects are evident, but has not affected the degree to which the last two effects of sand supply limitation are evident in the Colorado River in Marble and Grand Canyons. The first three of these effects involve coupled changes in suspended-sand concentration and grain size that are controlled by changes in the upstream supply of sand. Tributaries downstream from Glen Canyon Dam are now the sole suppliers of sand to the Colorado River in Marble and Grand Canyons. During floods on these tributaries, sand on the bed of the Colorado River fines; this causes the suspended sand to fine and the suspended-sand concentration to increase, even when the discharge of water remains constant. Subsequently, the bed is winnowed of finer sand, the suspended sand coarsens, and the suspended-sand concentration decreases independently of discharge. Also associated with these changes in the upstream sand supply are likely changes in the fraction of the bed that is covered by sand. Thus, suspended-sand concentration in the Colorado River is probably regulated by both changes in bed-sand grain size and changes in bed-sand area. A physically based flow and suspended-sediment transport model has been recently developed, tested, and applied to data from the Colorado River to evaluate the relative importance of changes in bed-sand grain size and changes in bed-sand area in regulating suspended-sand concentration. Although the model was developed using approximations for steady, uniform flow and other simplifications that are not met in the Colorado River, the results nevertheless support the hypothesis that changes in bed-sand grain size are much more important than changes in bed-sand area in regulating the concentration of suspended sand. Because changes in bed-sand volume and bed-sand area are not uniquely related nor necessarily positively correlated, decreases in the upstream supply of sand may actually be associated with increases in the area of the sand on the bed (e.g., as observed during artificial flood tests released from the dam in 1996, 2000, and 2004). Thus, changes in bed- sand grain size may override the effects of changes in bed-sand area in regulating suspended-sand concentration.
DE: 1862 Sediment transport (4558)
DE: 1894 Instruments and techniques: modeling
DE: 1895 Instruments and techniques: monitoring
SC: Hydrology [H]
MN: 2007 Fall Meeting