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Title: Weir jetty performance : hydraulic and sedimentary considerations, hydraulic model investigation
Authors: Coastal Engineering Research Center (U.S.)
Seabergh, William C.
Keywords: Jetties
Hydraulic structures
Coastal structures
Hydraulic models
Sediment transport
Publisher: Hydraulics Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
Series/Report no.: Technical report (U.S. Army Engineer Waterways Experiment Station) ; HL-83-5.
Description: Technical Report
Abstract: The weir jetty concept offers one alternative for jetty design at tidal inlets. Instead of impounding sand in a fillet upcoast of the jetty system where it is difficult to handle for sand bypassing, a low weir section of the jetty permits movement of the sand by wave-and tide-generated currents over the low weir into a deposition basin where it can be handled by a dredge in more protected waters. The hydraulic model investigation was performed to study important design parameters for a weir jetty system including weir length, elevation, orientation with respect to the shoreline and the conventional portion of the jetty structure, tidal currents over the weir section, flow patterns in the vicinity of the weir section, sediment movement over the weir and effects of the weir jetty on accretion, and erosion upcoast of the jetty system. The study was accomplished with a fixed-bed undistorted, 1:100 scale model. Large ocean and bay areas were reproduced in a 150-ft by 305-ft facility. Tides were reproduced in the model and two types of inlet-bay systems were simulated--one system in which the bay nearly fills (high Keulegan K value) and the other in which the bay only partially fills (low Keulegan K value). The conditions provided extremes of velocity-tidal elevation flow relationships over the weir. Sediment tracers and a movable-bed beach section provided the means to examine deposition basin filling, fillet accretion patterns for upcoast waves, and fillet removal by waves from the downcoast direction for several weir jetty orientations, including weir angles with the shoreline of 30, 45, 60, and 90 deg. Results indicate the mean tide level weir elevation is the most practical elevation for providing wave protection for a dredge, good sediment transport across the weir, and good flood-ebb tidal flow relationships, i.e., moderate flood flow currents and little or no ebb flow. Strong ebb flow currents over the weir are not desirable as they might aid in migration of the navigation channel through the deposition basin. Jetty systems with the outer, more oceanward portions parallel to each other and at minimum spacing provide the best flow characteristics when tidal current migration through the deposition basin region is considered. Wave-generated currents upcoast of the weir jetty are not entirely captured by the weir but some current, and thus sediment, moves oceanward along the outer portion of the jetty. Also reflected waves off the jetty and weir structure combine with incident waves to form a shortcrested wave field which aids in removal of sediment from the upcoast beach to various degrees, depending on the structure's angle with respect to the shoreline and the incident wave angle. The effects of groins adjacent to the weir section were examined with regard to providing additional fillet storage and reducing the sediment movement in an oceanward direction along the jetty. Positive results were found for each variation tested. NOTE: This file is large. Allow your browser several minutes to download the file.
Rights: Approved for public release; distribution is unlimited.
Appears in Collections:Technical Report

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