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|Management of environmental resources of cutoff bends along the Tennessee-Tombigbee Waterway
|United States. Army. Corps of Engineers. Mobile District.
Shields, F. Douglas
|Environmental Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
|Miscellaneous paper (U.S. Army Engineer Waterways Experiment Station) ; EL-87-12.
Abstract: Construction of the Tennessee-Tombigbee Waterway (TTW) involved construction of a navigation channel across the necks of 35 meander bends of the Tombigbee River. The cutoff bends, or bendways, constitute- a valuable resource that the Corps of Engineers is committed to maintain. This study was conducted to provide a rational basis for refinement of the management strategy for the 30 TTW bendways located downstream of Aberdeen Lock and Dam. The five bendways in Aberdeen Pool were not considered in this study. Bendway sedimentation may be conceptually divided into two phases: blockage and infilling. Sedimentation during the blockage phase is primarily deposition of bars of bed material in the upstream and downstream bend entrances. The infilling phase begins when the upstream bend entrance is blocked to top bank elevation and is characterized by deposition of fine sediment more or less uniformly along the length of the bend. Discharge, suspended sediment concentration, and repetitive hydrographic survey records for 20 man-made cutoff bends and associated cut channels and for one natural chute cutoff were used to study the rate of sedimentation during the blockage phase.. Cutoff bend volume was found to be a log-decay function of cumulative water discharge in the main channel. Regression functions show that the log-decay constant is inversely proportional to the average bed-material concentration in the master stream during the period of interest divided by either the sine of the angle between the cut channel and the bend entrance or by the average difference in mean bed elevations of the cut and bend entrances. The derived regression functions may be used to predict log-decay constants for cutoff bends along river reaches similar to those studied, The variation of bendway channel cross-sectional area with both time and distance along the bend showed that for most bends, deposition patterns tend to be bimodal, with a primary location for deposition just below the upstream entrance and a secondary location just above the downstream entrance. Extrapolation of trends observed in blockage-phase bends showed that the TTW bends will experience large losses of channel volume during the blockage phase if allowed to block naturally. Sequential aerial photographs of seven cutoff bends were used to study infilling rates. The perimeter of the treeline and the area enclosed by the treeline around each lake were measured for each photography coverage date. Area decreased rapidly at the end of the blockage phase and then stabilized and mildly fluctuated about a value 40 to 60 percent of the initial value, Perimeter declined only slightly or remained constant over the same time period, Specific recommendations are provided for management of each of the 30 TTW bendways downstream of Aberdeen Lock and Dam, Construction of blockage embankments at the upstream end is recommended for 27 of the bendways. Three of these 27 blocks were constructed in 1985.
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