Measured total sediment loads (suspended loads and bedloads) for 93 United States streams
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Geology topics
Garnett P. Williams
Publications and source records attributed to Garnett P. Williams.
The case of the shrinking channels; the North Platte and Platte rivers in Nebraska
Since the latter part of the 19th century, channels of North Platte and Platte Rivers in western and central Nebraska have changed considerably. In the 365-km reach from Minatare to Overton, the channel by 2969 ws only about 0.1-0.2 as wide in 1865. The 1969 channel for this reach was less braided and slightly more sinuous than the 1938 channel. (No data are available for braiding and sinuosity prior to 1938.) From Overton to lGrand Island the 1969 channel was about o.6-0.7 as wide as in 1865, and various changes in braiding and sinuosity took place between 1938 and 1969. The decreases in channel width are related to decreases in water discharge. Such flow reductions have resulted primarily from regulating effects of major upstream dams and greater use of the river water. Much of the former river channel is now overgrown with vegetation. (Woodard-USGS)
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Hydraulic geometry of river cross sections; theory of minimum variance
This study deals with the rates at which mean velocity, mean depth, and water-surface width increase with water discharge at a cross section on an alluvial stream. Such relations often follow power laws, the exponents in which are called hydraulic exponents. The Langbein (1964) minimum-variance theory is examined in regard to its validity and its ability to predict observed hydraulic exponents. The variables used with the theory were velocity, depth, width, bed shear stress, friction factor, slope (energy gradient), and stream power. Slope is often constant, in which case only velocity, depth, width, shear and friction factor need be considered. The theory was tested against a wide range of field data from various geographic areas of the United States. The original theory was intended to produce only the average hydraulic exponents for a group of cross sections in a similar type of geologic or hydraulic environment. The theory does predict these average exponents with a reasonable degree of accuracy. An attempt to forecast the exponents at any selected cross section was moderately successful. Empirical equations are more accurate than the minimum variance, Gauckler-Manning, or Chezy methods. Predictions of the exponent of width are most reliable, the exponent of depth fair, and the exponent of mean velocity poor. (Woodard-USGS)
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Erosional and Depositional Aspects of Hurricane Camille in Virginia, 1969
Probably the worst natural disaster in central Virginia's recorded history was the flood resuiting from an 8-hour deluge of about 28 inches (710 mm) of rain on the night of August 19-20, 1969. This study examines some of the intensive sediment erosion and deposition that resulted from the storm and flood. Most of the 150 people whom the flood killed in this mountainous area died from broken bones and other blunt-force injuries, rather than by drowning. The transport of sediment and other debris by the water therefore was very significant in loss of life and in property damage. Erosion resulted mainly from debris avalanches down the mountain-sides and channel scour along streams and head-water tributaries. Total amounts of sediment yield from certain mountainous areas in Nelson County were about 3.2-4.6 million cubic feet per square mile, probably the equivalent of several thousand years of normal denudation. Characteristics of the debris avalanches were that (1) they usually followed pre-existing depressions on hillsides and occurred on slopes greater than 35 percent, (2) the upslope tip of the avalanche scar tended to be located at the steepest part of the hillside, where the convex slope merged with the concave or planar zone immediately below, (3) hillsides facing north, northeast and east were more susceptible to avalanching than slopes facing other directions, and (4) debris-avalanches caused rapid and devastating surges of water and sediment in the mountain-stream channels. Such surges in some instances temporarily blocked the channel flow upstream. Slightly more than half of the total sediment contributed to the stream system was from erosion of stream channels. Channel erosion was very irregularly distributed; some ravines 10-20 feet wide and 5-10 feet deep were scoured in places which formerly had only a very small channel, whereas other channels only a few hundred yards away experienced little or no channel erosion. By the use of figures for the total amount of sediment removed from a drainage basin and the duration of the storm, estimates were made of the storm-average sediment-transport rate at the mouth of various basins. For drainage basins ranging up to about 1.5 square miles, the estimated storm-average sediment-transport rates varied from practically nothing to as much as 172,000 pounds per second (7.4 million tons per day). The types of sediment deposits were (1) debris-avalanche deposits, rather rare, at the base of hillslopes, (2) mountain-stream channel deposits, usually in scattered sediment patches but locally occurring as large wedge-shaped deposits behind debris dams, (3) alluvial fans, (4) delta-like deposits at the junction of a stream and major highway, where water backed up during the flood due to plugging of a culvert, and (5) accretion deposits on flood plains. The highway deltas and some downstream flood-plain sediments consisted mostly of sand-sized grains, but the other types of deposits usually contained particles ranging from silt or clay to boulders 5-10 feet in diameter. Changes in grain size and in volume of deposition with distance downstream were measured, and sedimentary features of the various types of deposits are described.
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