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Title: Equilibrium characteristics of sand beaches in the offshore zone
Authors: Massachusetts Institute of Technology. Hydrodynamics Laboratory
United States. Army. Office of the Chief of Engineers
Eagleson, Peter S.
Glenne, Bard
Dracup, John A.
Keywords: Equilibrium profile
Sediment sorting
Sand movement
Sedimentation and deposition
Publisher: United States, Beach Erosion Board
Engineer Research and Development Center (U.S.)
Series/Report no.: Technical memorandum (United States. Beach Erosion Board) ; no. 126.
Description: Technical Memorandum
Abstract: This report describes a theoretical and experimental investigation of equilibrium profiles and sediment sorting in the offshore zone of sand beaches. The investigation was designed to test the applicability of existing idealized theories to the prediction of equilibrium characteristics of laboratory sand beaches. Two different sediment motion equilibrium criteria are considered; the first, in which the moments on a stationary particle are in equilibrium, is called incipient motion, and the second, in which the particle is oscillating with no net motion, is called established motion. The experimental equipment consisted of a 100 foot long, glass-walled wave tank containing a bulkhead-type wave machine and an experimental beach. For the profile experiments the beach consisted of a well-sorted, highly rounded silica sand, placed at initially constant slope. The initial slopes used were 1 on 20, 1 on 30 and 1 on 45 and the incident wave steepneses, Ho/Lo, ranged from 0.025 to 0.048. Profiles were measured at at equilibrium. For the sorting experiments the beach was impermeable and rigid but was covered with a fine layer of rounded silica sand having known initial areal weight and local size distributions. Weight and size distributions were measured at equilibrium for incident waves in the above range. Results indicate the existing theories to provide good quantitative predictions of 1) seaward limit of profile modification and 2) whether a given beach will build or erode under the action of a given incident wave. Quantitative prediction of profile shape is good only near the offshore extreme of profile modification. The sorting experiments bear out the qualitative theoretical predictions of an increase in size sorting in the onshore direction and a tendency toward the formation of bi-modal size-frequency distributions. The predicted increase in mean diameter in the onshore direction is found only near the offshore limit of sorting action. Further experiments are needed to define the effect of the breaker, bottom roughness, sand ripples and shoaling depths on the vertical distribution of fluid mass transport velocities, the fundamental quantity on which the theories are based.
Rights: Approved for public release; distribution is unlimited.
Appears in Collections:Technical Memorandum

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