A velocimetric survey of the Lower Missouri River from river mile 492.38 to 290.20, July-October 2011 and July 2012

Abstract

Velocimetric surveys were made by the U.S. Geological Survey in 2011 and 2012 to provide data for the U.S. Army Corps of Engineers’ ongoing study of bed degradation in the Lower Missouri River. Using Acoustic Doppler Current Profile technology, velocity data were collected at 87 river miles along the Lower Missouri River from Rulo, Nebraska to Waverly, Missouri, from July to October 2011 and in July 2012, for a total of 118 velocimetric surveys. Multiple-repeat velocimetric surveys were done eight times at three river miles from July to October 2011. Synoptic velocimetric surveys spanning 2–4 days were done twice at ten river miles, once in July 2011 and once in October 2011. Additional synoptic velocimetric surveys were done at proximal river miles in October 2011 and July 2012. Main-channel, near-bed, near-bank, and whole-river velocities were extracted from the Acoustic Doppler Current Profile data using AdMap and compiled as an average of reciprocal pairs for each survey. In addition, the mean velocity computed by the Winriver II software for each survey was integrated with the extracted data. Multiple-repeat velocimetric surveys in the vicinity of Kansas City and Waverly, Missouri, in 2011 indicated that main-channel, near-bed, near-bank, and whole-river velocities generally declined with respect to declining daily mean discharges at the St. Joseph and Waverly, Missouri streamgaging stations. Statistical analysis of the four extracted velocity types indicated that multiple-repeat velocimetric surveys were strongly correlated with daily mean discharges at nearby streamgaging stations (coefficient of determination greater than 0.75). Main-channel velocity exceeded whole-river velocity by an average of 25 percent at river mile 357.70, 22 percent at river mile 357.09, and 6.8 percent at river mile 290.20 for all velocimetric surveys at each location, respectively. Mainchannel, near-bed, near-bank, and whole-river velocities declined about 48 percent from July to October 2011 at the Kansas City sites and about 38 percent at the Waverly site. Winriver II mean velocity from multiple-repeat velocimetric surveys indicated that the relation between all velocities was inconsistent from July to October 2011. Percent changes in channel width from July to October 2011 were typically less than concurrent changes in channel area and instantaneous discharge. The combined synoptic and multiple-repeat survey data for July 2011 did not indicate a clear longitudinal trend of velocity as instantaneous discharge increased downstream. Main-channel velocity exceeded whole-river velocity by an average of 22 percent during July 2011 and in some cases by more than 40 percent (river miles 452.50 and 308.80). Evaluation of 10 pairs of synoptic and synoptic-repeat velocimetric surveys with multiple-repeat velocimetric surveys from July to October 2011 indicated that all velocity types and channel width decreased by about one-third. Channel area and instantaneous discharge decreased by more than 50 percent from July to October 2011 and the percent by which main-channel velocity exceeded whole-river velocity decreased slightly from 22 percent in July to 19 percent. Comparing high (July 2011) to low (October 2011) discharge, where the channel width and area expanded by a factor of nearly 3.0 or more at high discharge (river miles 492.38 to 452.50), main-channel, near-bed, near-bank, and whole-river velocities increased by factors in the range of 1.1 to 1.7 and Winriver II mean velocity decreased. At the Kansas City river miles, all velocity types and channel areas nearly doubled at high discharge and channel widths remained similar to those at low discharge. Multiple-repeat and synoptic velocimetric surveys evaluated in October 2011 indicated that main-channel, near-bed, nearbank, and whole-river velocities generally increased downstream from river miles 424.20 to 404.70 and then decreased, until river mile 290.20, where they increased slightly. Of the July 2012 synoptic velocimetric surveys, velocities near St. Joseph, Missouri, indicated no longitudinal trends in the main-channel, near-bed, near-bank, and whole-river velocities. The Kansas City and Waverly synoptic velocimetric surveys indicated a general decrease in these velocities proceeding downstream. For all 2012 surveys, near-bed velocity was closest in magnitude to Winriver II mean velocity and near-bed and whole-river velocities decreased with increasing channel area. For the entire study, variations in near-bank velocity may have been due to the influence of channel structures and their diversion of higher velocities away from the channel edges.