USE OF A HIGH-SHEAR STRESS MOBILE FLUME TO DETERMINE CRICITAL STESSES FOR RIO GRANDE CHANNEL SEDIMENTS

A mobile high-shear stress flume was used to model the susceptibility to erosion of sediment at three sites in the channel of the Rio Grande along the U.S. Mexico border between El Paso and Ft. Quitman, Texas. A combination of erodability measurements, high-precision topographic mapping using GPS, and surface geophysical measurements demonstrates the utility of this method for improving our understanding of the stream and for solving environmental problems along this highly stressed environment. At the two arroyos, Alamo and Balluco, the addition of coarse material makes the channel-filling sediments more difficult to erode downstream from the arroyos. At one site, thin beds of interstratified sand and mud make the channel much more difficult to erode. Because the EM-31 conductivity device was able to map even thin muddy layers in the shallow subsurface this site illustrates the ability of these combined techniques to map areas resistant, or particularly susceptible to erosion. The critical shear stresses required to eroded channel sediments were two-times greater downstream of the arroyos. This highlights a natural process that probably effects the Rio Grande along much of its length, accretion of more poorly erodable bars within the channel during low-flow years. These changes were associated with widening of the channel and the formation of mid-channel and side-channel bars. Arroyo mouths may form avulsion nodes because of this characteristic. The rapid deposition of Rio Grande sand at the mouth of Balluco arroyo following a flood may be an example of why the Rio Grande repeatedly avulsed prior to the construction of Elephant Butte Dam.