Bed-sediment grain-size change, and implications for sediment management

David M. Rubin1, David J. Topping2, Henry Chezar3, Brian Lockwood4, James P. Bennett5, and Stevan Gyetvai5

1USGS Santa Cruz, CA2USGS Flagstaff, AZ3USGS Menlo Park, CA4University of California at Santa Cruz, CA5USGS Denver, CO

Subtitle: Bed-sediment grain size: a neglected control of the Colorado River ecosystem.

Since completion of the dam, grain size of bed sediment has been a more important control of concentration, grain size, and transport of suspended sand than flow.

Concentration of suspended sediment is a major control of:

• Amount of deposition and erosion of bars

• Rate at which sand is exported from the canyon

• Grain size of sand deposited on bars

• Turbidity, and subsequent biological effects (Mike Yard, Joe Shannon)

• Background• Monitoring technology• Recent results• Implications for sediment management

• Background• Monitoring technology

• Pipe dredge • Underwater video microscope • calculations

• Recent results• Implications for sediment management

• Background• Monitoring technology

• Pipe dredge • Underwater video microscope • calculations

• Recent results• Implications for sediment management

• Background• Monitoring technology

• Pipe dredge • Underwater video microscope • calculations

• Recent results• Implications for sediment management

From Rubin and Topping, 2001, Water Resources Research.

• Background• Monitoring technology• Recent results

• Winnowing of bed through time• Changes with distance• Relation to other studies

• Implications for sediment management

Mile 2-3

Mile 7-8

• Background• Monitoring technology• Recent results

• Winnowing of bed through time• Changes with distance • Relation to other studies

• Implications for sediment management

• Background• Monitoring technology• Recent results

• Winnowing of bed through time• Changes with distance• Relation to other studies

• Implications for sediment management

Measurements of fine-sediment abundance

• Bed-sediment grain-size studies (USGS )• Bed-sediment spatial distribution (USGS )• Bar topography (NAU, USU)• Sediment budget (USGS )

Bed-sediment grain-size

Point measurements of grain size do not measure system-wide sand storage.

measures abundance of fine sediment, but only that part accessible to flow.

Spatial distribution of bed-sediment

Measures changes in distribution, of sediment in large reaches of the river, but does not detect changes in grain size of sand or detect changes in net volume in a reach.

Bar-topography studies

Measures changes in volume, but study areas are small and not fully representative of the river. Longer study reaches (which are proposed for future monitoring) are expected to be more representative.

Sediment budget (inputs and outputs)

Difficult to determine accurately over long time intervals, because the net budget is a small difference between 2 large numbers (input and output). Accurate over short times, when differences are greater.

Does not identify sediment sources or depositional sites.

Although the 4 techniques have different strengths and limitations, they all have one feature in common: they have all failed to detect the multi-year accumulation of sand that was previously hypothesized to occur.

Instead, they demonstrate: depletion of fine sediment on the bedrapid export of tributary sanddecreasing size of sand bars.

• Background• Monitoring technology• Recent results• Implications for sediment management

(Results of NAU bar-topography studies.)

CONCLUSIONS

Tributary inputs enrich the river bed in fine sediment. This fine sediment increases the transport rate, resulting in a more rapid export of sand than would otherwise be expected. As a result, current operations are failing to meet AMWG’s sediment objective of maintaining the bar size that existed in the early 1990’s.

To meet the AMWG sediment objectives, therefore, requires new management strategies that can exploit recent tributary inputs before that sediment is lost downstream.

Management options that are likely to be more successful than present operations include:

• Implementing bar-building flows during or shortly after large tributary inputs

• Maintaining low flows (≤8,000-10,000 cfs) after tributary inputs, until bar-building can be implemented

Because of the limited quantities of tributary sediment, even these options may not be successful in restoring sediment resources. Restoration of bars to the size that existed in the early 1990’s may not be possible without adding sediment.

References

Rubin, D.M., Nelson, J.M., and Topping, D.J., 1998, Relation of inversely graded deposits to suspended-sediment grain-size evolution during the 1996 flood experiment in Grand Canyon: Geology, v. 26, p. 99-102.

Rubin, D.M., Topping, D.J., Schmidt, J.C., and Hazel, Joe, Summary and discussion of recent research findings related to dam operations and sand bar resources of the Colorado River ecosystem: unpublished memo to Barry Gold, August 29. 2000.

Rubin, D.M., and Topping, D.J., 2001, Quantifying the relative importance of flow regulation and grain-size regulation of suspended-sediment transport (), and tracking changes in bed-sediment grain size (): Water Resources Research, v 37, p, 133-146.

Topping, D.J., Rubin, D.M., and Vierra, L.E., Jr., 2000, Colorado River sediment transport: Part 1: Natural sediment supply limitation and the influence of Glen Canyon Dam: Water Resources Research, v. 36, p. 55-542.

Topping, D.J., Rubin, D.M., Nelson, J.M., Kinzel, III, P.J., and Corson, I.C., 2000, Colorado River sediment transport 2. Systematic bed-elevation and grain-size effects of sand supply limitation: Water Resources Research, v. 36, p. 543-570.

Topping, D.J., Rubin, D.M., Nelson, J.M., Kinzel, P.J., III, and Bennett, J.P., 1999, Linkage between grain-size evolution and sediment depletion during Colorado River floods, in Webb, R.H., Schmidt, J.C., Marzolf, G.R., and Valdez, R.A., eds., The 1996 controlled flood in Grand Canyon: Washington, D.C., American Geophysical Union, Geophysical Monograph 110, p. 71-98.