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|Title:||Shoaling in harbor entrances : hydraulic model investigation|
|Authors:||United States. Army. Corps of Engineers. Lower Mississippi Valley Division.|
Melton, Bertrand K.
Franco, John J.
|Publisher:||Cold Regions Research and Engineering Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
|Series/Report no.:||Technical report (U.S. Army Engineer Waterways Experiment Station) ; HL-79-5.|
Abstract: This report covers the results of a general investigation to determine and demonstrate some of the principles involved in shoaling in harbor entrances and some of the factors to be considered in the development of solutions to the problems. The model used was not a reproduction of a reach of any stream but was designed to fit in an existing facility and provide for two bends and a straight reach between the bends. The model was of the movable-bed type with scales established arbitrarily as 1:400 horizontally and 1:100 vertically. Tests were conducted with the harbor entrance located along the concave bank of the upper bend, in the straight reach, and along the convex bank of the lower bend simultaneously. Results of this investigation provided the following general indications: (A.) There is a natural tendency for shoaling in entrances to harbors involving openings in bank lines of alluvial streams because of the sudden expansion in channel width and the lowering of the water level causing sediment-laden bottom currents to move toward the opening. (B.) The tendency for shoaling in harbor entrances varies with the size of opening in the bank line, the location of the entrance with respect to the channel alignment, and flow conditions. (C.) Harbor entrances located along the concave bank of a bend would tend to have less shoaling problems than if located in a straight reach or along the convex side of a bend. Entrances located in straight reaches would tend to have less shoaling problems than those located on the convex side of a bend. (D.) Shoaling in harbor entrances depends on the principle of lateral differential in water level. An opening in the bank line required for a harbor entrance tends to cause a lowering of the water level at the entrance. When there is a lowering of the water level on one side of the channel, there is a greater tendency for the slower moving bottom currents with sediment to move toward the lower elevation. Therefore, plans for the elimination or reduction of shoaling in entrances should be designed to reduce the amount of lowering of the water level near the entrance or to prevent the movement of bottom currents toward the lower elevation. (E.) Shoaling in harbor entrances located on the concave or convex side of a bend will depend to a considerable extent on the superelevation of the water surface near the entrance which is affected by the curvature of the bend, width of channel , and current velocities during the range of stage and discharge. In general, shoaling will tend to be less with the entrance located downstream of the point bar or axis of the bend. (F.) Shoaling in harbor entrances located in straight reaches will depend on the harbor entrance location with respect to the channel over the crossing and the tendency for the channel to be unstable. Channels in long straight reaches will tend to meander between banks and are affected by changes in stages and discharge and the alignment of the channel with respect to the bend upstream. (G.) Some reduction in shoaling in harbor entrances can be accomplished by modification of the bank line or with structures near the entrance designed to increase the water level near the entrance, or by structures designed to prevent bottom currents from moving toward the entrance. (H.) L-head dikes and in some cases wing dikes, can be used to prevent bottom currents from moving toward the entrance by permitting surface flow over the top of the dikes toward the entrance. The effectiveness of these dikes would depend on their location, length, elevation, and alignment with respect to the entrance and stream channel.
|Rights:||Approved for public release; distribution is unlimited.|
|Appears in Collections:||Technical Report|
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|TR-HL-79-5.pdf||6.42 MB||Adobe PDF|