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Title: Supplementary tests of Masonboro Inlet fixed-bed model: hydraulic model investigation
Authors: Seabergh, William C.
Sager, Richard A.
Keywords: Fixed-Bed Models
Hydraulic Models
Hydraulic Similitude
Masonboro Inlet
Tidal Inlets
Tidal Models
Publisher: Coastal Engineering Research Center (U.S.) General Investigation of Tidal Inlets Research Program
Engineer Research and Development Center (U.S.)
Description: General Investigation of Tidal Inlets Report
Abstract: This report describes supplemental tests of the Masonboro Inlet fixed-bed model not reported in "Physical Model Simulation of the Hydraulics of Masonboro Inlet, North Carolina," GITI Report 15. The supplemental tests consist of three separate studies performed in the Masonboro Inlet fixed-bed model. The first study examines the effects of closing various bay channels on the inlet's hydraulics. The second study examines the effects of the addition of a south jetty to the existing condition which has a single north jetty and examines the resulting hydraulics for various weir elevations on both jetties. The third study examines the use of a tracer material and closely parallels the hydraulic testing sequence discussed in the previous Masonboro Inlet report. The tracer tests include verification of the 1969-1971 shoaling trends, testing of the prejetty 1964 condition, testing of the single jetty plan with the 1964 bathymetry, and testing of the postjetty construction 1966 condition. These tests were performed to evaluate the effectiveness of using sediment tracer materials in inlet model studies. Results indicate that the closure of any of the three interior channels in Masonboro Inlet produces a significant change in the inlet hydraulics and would likely produce a significant change in morphology as indicated by the prototype case history. The important changes in flow patterns at the inlet entrance occur for the ebb flow, with flood patterns at the entrance staying fairly consistent. Keulegan K values show show decreases for each closure condition, with closure of Banks Channel (the largest interior channel) providing the greatest decrease. These decreases in K are the result of a reduction in flow area and an increase in the effective length of the inlet for each closure condition. This in turn increases the bay superelevation because of the reduction in K. Shinn Creek, the interior channel perpendicular to the coast, aids in directing ebb flow oceanward, perpendicular to the coast as seen in evaluating flow volumes through range 1 for the various conditions. This indicates that the initial scour along the outer portion of the north jetty could have been caused by this ebb flow effect. This result must also be coupled with wave effects which probably contributed to filling the dredged channel location. Weir jetty testing indicated the effect of a south jetty is to centralize the flood flow through the inlet gorge, and the presence of a weir on the south jetty does not change this basic flow pattern. As a result of this flow pattern flow increases through Shinn Creek, the channel directly behind the inlet gorge. Flow velocities over the 0.0 mlw weirs are slightly greater than those over the +2.0 mlw weir jetties (0.2 to 0.4 fps greater). However, the flow volume over the 0.0 mlw weir sections is substantially greater, which slightly reduces flow entering the jetties between their oceanward end. Ebb velocities seem to be better centralized between the jetties for the +2.0 mlw weirs and the non-weir south jetty conditions than for the 0.0 mlw weirs. Flow over the +2.0 mlw weirs cuts off very early during the ebb flows since the tide level has dropped to just below the mtl once ebb flow has begun to reach faster velocities. For the 0.0 mlw weir condition, however, ebb flow over the weirs continues until nearly low water. The continued drawdown of ebb flow over the 0.0 mlw weirs aids in dispersing the flow over the entire region between the jetties and seemingly could aid currents to flow along the inner walls of the jetties rather than concentraing in the central region between the jetties where the navigation channel is located. Plane 3 (dual weirs at 0.0 mlw) produced a much greater ebb velocity predominance between the channels than did Plan 2 (dual weirs at +2 mlw). Results of the sediment traces testing indicates that short-term fill and scour trends can be predicted qualitatively. Also the reduction in model distortion to values less than this study's 5:1 scale distortion would appear to improve fill and scour tests, especially in areas of steep slope such as the inlet's gorge.
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