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Title: Interactions of the beach-ocean-atmosphere system at Virginia Beach, Virginia
Authors: Harrison, Wyman, 1931-
Krumbein, William Christian, 1902-1979
Keywords: Shorelines
Virginia Beach (Va.)
Publisher: Coastal Engineering Research Center (U.S.)
Series/Report no.: Technical memorandum;no. 7
Abstract: Abstract:A number of interactions among beach variables are investigated by sequential linear multiregression analysis, as programmed for high-speed computers. The study includes the influence of beach geometry, wave characteristics, tidal effects, and local wind condition's on the velocity of longshore currents, deposition and erosion on the lower foreshore, and the response of grain size and beach slope to shore processes. Results show that if about six variables are segregated out of any group of about a dozen, these six account for essentially all of the variability that is explained by all twelve. This, the regression method serves to condense relatively large data matrices to more compact form. The most-influential combinations of variables arbitrarily designated as "process" variables are in general agreement with significant variables of wave-tank experimentation, and substantiate intuitive judgments regarding the relative importance of these variables on natural beaches. The results suggest that certain additional variables, seldom examined under controlled conditions, be studied in combination with variables normally examined in wave tanks. The combination of six variables found to be most influential in the determination of longshore-current velocity in the study area is made up of wave period, wave height, lower-foreshore slope, wind velocity onshore, wind velocity offshore, and angle of wave approach, in order of decreasing importance the significance of wind velocity on and offshore is believed to lie mainly in the ability of the wind to alter the form of incoming swells. A special regression analysis for quadratic effects reveals that water density is highly non-linear in its effect on longshore-current velocity. Bottom slope in the shoaling-wave zone, some 250 feet seaward of the breakers, is found to be controlled primarily by average mean grain size of the bottom materials, wave period, wave length, wave steepness, water depth, and tidal-current velocity. This combination exerts its maximum influence on the slope through a lag in time of between 4 and 8 hours, and apparently to a lesser extent between 16 and 20 hours. Mean grain size of the beach slope in the shoaling-wave zone is found to depend upon the combination: mean bottom slope, wave period, wave steepness, wind velocity parallel to shore, angle of wave approach, and tidal-current velocity, and this combination is most influential after an 8-12 hour lag in time. When mean slope, the most-dominant independent variable, is removed from the analysis, water density and tidal-current velocity appear as the most influential variables on mean grain size. Wind velocity parallel to shore is believed important because it will influence the velocities of the tidal currents that flow parallel to the shore in the study area. Angle of wave-front approach may at times significantly augment or decrease tidal-current velocities near the bed and thereby the sizes of particles moved. Wind velocity onshore and offshore at times interlocks with water density, as density varies when stratified shelf waters undergo turnover. Fluid drag velocities vary as water density varies, and differing sizes of particles will be moved. Net deposition on the lower foreshore during June and July is most influenced by slope of the foreshore. Wave period, wave height, wind velocity on-shore, angle of wave approach, and water density are variables that form the most-influential combination when in conjunction with lower foreshore slope. This combination expresses itself 8 to 12 and 20 to 24 hours prior to the low-tide time of measurement of net deposition; that is, during times of rising tide. The regression analysis suggests that net erosion on the foreshore is not nearly as much influenced by beach slope angle as is net deposition, but that lower-foreshore slope is still the most consistently important variable to net erosion through time. The combination of five variables suggested as most influential to net erosion during June and July includes lower-foreshore slope, wave period, wind velocity offshore, angle of wave approach, and depth to the water table at the top of the uprush. Of secondary importance, and manifesting at times of falling tide, is the combination made up of lower foreshore slope, wave period, wave height, and angle of wave approach. Problem areas reviewed in the study are related to redundant and noisy data and to the linear model used. Descriptions of the regression techniques for the linear and higher-order models are also given.
Description: Technical Memorandum
Rights: Approved for public release; distribution is unlimited.
Appears in Collections:Technical Memorandum

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