Mississippi river bedform roughness and streamflow conditions near Vicksburg, Mississippi : data collection summary and analysis

Abstract

Bedforms are a consequence of flow of sufficient magnitude over a mobile sediment bed. They are a primary component of the drag acting upon a moving stream, yet are infrequently explicitly treated in numerical models of fluvial sediment transport. This study aims to document the collection of bathymetric data in the Mississippi River in an area of persistent and dynamic bedforms over a range of flow conditions, statistically examine bedform geometry, and parameterize results for inclusion in numerical models. Bathymetric data were collected several times to measure rates of bedform transport. Linear profiles of the bedforms were extracted from the bathymetry and analyzed for roughness and dune population statistics. These statistics are compared with the flow conditions under which the bedforms were observed. Bedforms increase in size with discharge and decrease in steepness (height:length ratio). At extremely high discharges, bedforms begin to decrease in size. In comparing results with methods for calculating form drag coefficients, it was observed that the dunes at higher river stages, despite their greater size, may present less resistance to flow due to their reduced steepness and reduced relative heights (dune height:flow depth). DISCLAIMER: The contents of this report are not to be used for advertising, publication, or promotional purposes. Citation of trade names does not constitute an official endorsement or approval of the use of such commercial products. All product names and trademarks cited are the property of their respective owners. The findings of this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents.

On the Capabilities of Emerging Nonintrusive Methods to Estimate Bedform Characteristics and Bedload Rates

“…These results are in contrast with the theoretical findings of M. S. and Van Rijn (1984) for dunes in equilibrium whereby it is found that dune wavelength is mainly controlled by the water depth. However, these trends are confirmed by the analysis with an alternative method conducted by Ramirez et al (2018) for the same flows that is, the heights and wavelengths for the same locations were 1.44 and 80.1 m for the low flow and 3.1 and 64.5 m for the high flow, respectively. As expected, the differences between the bed geometries for the two flows result in commensurate differences between the transported sediment (see Figures 4g and 4h).…”

“…This morphing appears to be driven by secondary dunes translating over the primary, larger, dunes according to Ramirez et al. (2018). Along channel bedform profiles at the same location for both flowrates are illustrated in Figures 3c and 3d.…”

“…Comparing the maps at the low and high flows, it can be noticed that overall the dune wavelengths for the lower flow are larger than for the larger flow. This morphing appears to be driven by secondary dunes translating over the primary, larger, dunes according to Ramirez et al (2018). Along channel bedform profiles at the same location for both flowrates are illustrated in Figures 3c and 3d.…”

“…The MBES measurements presented herein are an integral part of a comprehensive study conducted by the US Army Corps of Engineers (USACE) over extended spatial and temporal scales with the intent to provide calibration and validation data for developing numerical models focused on sediment transport processes (Ramirez et al., 2018). As illustrated in the legend of Figure 2, the comprehensive data acquisition campaigns include MBES surveys, measurement of suspended sediment (red dots), acquisition of ADCP data from fixed locations (FV ADCP) and transects (MV ADCP), sampling of bed material at selected locations (yellow squares), and water surface profiling over extended distances (450 km).…”

“…The first AMV workflow step is the creation of the acoustic maps as a continuous depth‐data layer covering the targeted area on the channel bottom. River management agencies regularly acquire such maps to document bedform characteristics and their distribution across river cross‐sections and reaches (Ramirez et al., 2018). There are multiple instruments for non‐intrusively obtaining bathymetric maps (see Figure 1, Step 1a).…”