A geomorphic approach to the analysis of bedload and bed morphology of the Lower Mississippi River near the Old River Control Structure

作者：Knox Richard L; Latrubesse Edgardo M^*

来源：Geomorphology, 2016, 268: 35-47.

DOI：10.1016/j.geomorph.2016.05.034

• Lower Mississippi River
• Old River Control Structure
• Bed forms
• Bedload
• Geomorphic change
• Channel geometry

The Mississippi River is the ultimate single-thread meandering river. Five hundred kilometers upstream from its mouth, about 25% of the river's discharge and sediment load is diverted into the Atchafalaya River. This diversion is controlled by the Old River Control Structure (ORCS), built by the U.S. Army Corps of Engineers (USACE) in stages since 1961, to stop the avulsion of the Mississippi River into the Atchafalaya. The effects of ORCS on sediment and water discharge and geomorphic change to the Lower Mississippi River (LMR) channel are not yet completely understood and require placing the river into a geomorphic context, first classifying the channel into similar categories before evaluating change. The objectives of this study are to estimate the LMR bedload, develop and apply a geomorphic classification of the LMR near the ORCS, and explore geomorphic change within the classified areas.
We studied a 115-km-long stretch between ORCS and Baton Rouge that is highly impacted by engineering. We used six sets of bathymetric multibeam echosounder surveys conducted by the USACE, multitemporal cartographies, and a field survey supported by multibeam echosounder bathymetry, acoustic Doppler current profiler (ADCP) measurements, sediment samples collection, and geomorphic observations. A three-dimensional method for estimating bedload from time-elapsed bathymetric surveys was developed and applied on seven sets of time-elapsed surveys downstream from ORCS from 2010, 2011, and 2012. We estimate that the fraction of bedload as a percentage of total sand load between 2003 and 2011 was 13.2%. A bedform classification scheme, based on bedform height was developed. The bed was almost completely mantled by sandy bedforms above the -24-m isoline.
The studied reach was divided into ten zones according to four geomorphic types based on channel planform, geologic controls, presence of islands, and other morphometric parameters. These zones were shown to be physically distinct in terms of bedform location, depth to bedform size, channel sinuosity, channel width-to-depth ratios, and adjustments in the thalweg and channel width from 1948 to 2012.
The 1948 to 1975 period displayed a relatively large amount of thalweg aggradation, with a spatial average of 1.8 m and an average channel narrowing of 80 m. The 1975 to 2012 period had much lower spatial aggradation of 0.5 m and a lower average channel narrowing of 35 m. Thalweg sinuosity did not adjust very much, with an average reduction of 0.02 from 1948 to 2012 and an average of zero adjustment between 1975 and 2012.
The shape and spatial extent of a very large, anomalous bar near ORCS varied at seasonal and decadal timescales and may be influenced by the ORCS. Bed morphology was specific to geomorphic zones, which were shown to be physically significant, and should be employed in future studies of large rivers.

• 出版日期2016-9-1