INTERACTIONS OF THE BEACH-OCEAN-ATMOSPHERE SYSTEM 

 AT VIRGINIA BEACH, VIRGINIA 



by 



W. Harrison ,, . 



U. S. Coast and Geodetic Survey, Washington, D. C. and 

 Virginia Institute of Marine Science, Gloucester Point, Virginia 



and 



W. C. Krumbein 

 Northwestern University, Evanston, Illinois 



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 conditions 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. Thus, the regression 

 method serves to condense relatively large data matrices to more com- 

 pact form. 



The most-influential combinations of variables arbitrarily 

 designated as "process" variables are in general agreement with sig- 

 nificant variables of wave-tank experimentation, and substantiate 

 intuitive judgments regarding the relative importance of these vari- 

 ables 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 off- 

 shore 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 



(1) Present address. Study completed while at the Virginia Institute 

 of Marine Science. 



