Li Bway Baek, ness eee ey q U.S. Army Coastal Engineering Research Center ie eee INTERACT HONS OF THE BEACH - OCEAN -ATMOSPHERE | SYSTEM AT VIRGINIA BEACH, VIRGINIA TECHNICAL MEMORANDUM NO. 7 DEPARTMENT OF THE ARMY CORPS OF ENGINEERS h Thbbeoo TOEO O WNC CN IOHM/181N INTERACTIONS OF THE BEACH -OCEAN-ATMOSPHERE SYSTEM AT VIRGINIA BEACH, VIRGINIA by W. Harrison and W.C. Krumbein TECHNICAL MEMORANDUM NO.7 December 1964 Material contained herein is public property and not subject to copyright. Reprint or re-publication of any of this material shall give appropriate credit to U.S.Army Coastal Engineering Research Center LIMITED FREE DISTRIBUTION OF THIS PUBLICATION WITHIN THE UNITED STATES IS MADE BY THE U.S.ARMY COASTAL ENGINEERING RESEARCH CENTER 5201 LITTLE FALLS ROAD, N. W., WASHINGTON D.C. 20016 FOREWORD An understanding of the changes taking place in a beach environment requires the sorting out and ranking in importance of the various processes acting upon each of the environmental factors being analyzed. Also required is a knowledge of the time lag between the inception of a group of "processes" and the moment of their maximum effect on the "response" being analyzed. The approach to these two problems followed in this study involves the use of some 27 variables of the beach-ocean-atmosphere system at Virginia Beach, Virginia. The sorting out and ranking of the variables in a given analysis is approached through linear and quadratic multiregression analysis, as programmed for high-speed computers. One of the variables in the system is selected as the dependent variable and studied in relation to several controlling independent variables, by taking the latter one at a time, two at a time, and so on until all of the independent variables are included simultaneously. Thus the technique of sequential multiregression analysis is the tool used in the investigation. In addition, the dependent variable at time to is studied in its relation to the independent variables as measured at successive lag perdodsmti. stowed backwards) dingtimer This report was prepared by Wyman Harrison, formerly Associate Professor of Marine Science, Virginia Institute of Marine Science, in pursuance of Contract DA-49-055-CIV-ENG-64-5 with the Coastal Engineering Research Center, in collabora- tion with W. C. Krumbein, Professor of Geology, Northwestern University, and consultant to the Coastal Engineering Research Center. This study was supported by the Coastal Engineering Research Center (formerly the Beach Erosion Board of the Corps of Engineers), under the general supervision of J. V. Hall, Jr., G. M. Watts, and N. E. Taney, of the Engineering Development Division. The Computing Centers at Northwestern University and the College of William and Mary extended every cooperation. Programs used in the analysis were largely developed with supporting funds from the Geography Branch of the Office of Naval Research, under general supervision of Evelyn Pruitt and R. A. Alexander. Betty Benson of the North- western Computing Center was most helpful in developing the IBM 709 computer programs used in the study, and R. Libutti and J. Curran of IBM aided in rewriting her programs for use on the 1620 computer. The Virginia Beach City Engineer, C. Kiley, and his assistant, A. Gregg, aided by laying groundwork for the field studies. The following graduate students of the Virginia Institute of Marine Science aided in sampling: D. R. Tuck, Jr., M. P. Lynch, R. Morales-Alamo, R. B. Stone, and W. S. Wilson; C. Kyte of Antioch College, N. Peterson of Hofstra College, and G. Williamson of Old Dominion College also aided in field work and data reduction. Wave data were furnished by the Research Division of the Coastal Engineering Research Center. The U. S. Army Transportation Corps, Fort Story, Virginia, provided amphibious support for overwater sampling. Weather data were supplied by the U. S. Weather Bureau Station at Fort Story and tide data were furnished by the U. S. Coast and Geodetic Survey. D. R. Tuck, Jr. and R. Barnes, Virginia Institute of Marine Science, were in charge of data reduction. The authors are indebted to the following colleagues for critical review of the manuscript: Douglas Inman of Scripps Institution of Oceanography: Per Bruun of the University of Florida; and Arthur Brebner and Ian White of Queen's University (Ontario). The freedom and encouragement given to Wyman Harrison in this study by William J. Hargis, Jr., Director of the Virginia Institute of Marine Science, is most gratefully acknowledged. An addendum to this report represents a cooperative effort between the U. S. Coast and Geodetic Survey, the U. S. Weather Bureau, and the U. S. Army Coastal Engineering Research Center, in the general area of research on coastal vulnerability. Time for the research was granted by the U. S. Coast and Geodetic Survey to Wyman Harrison in his capacity as Chief of the Marine Geology Section and by the U. S. Weather Bureau to Arthur Pore in his capacity as a member of the Storm Surge Unit of the Office of Meteorological Research. This report is published under authority of Public Law 166, 79th Congress, approved July 31, 1945, as supplemented by Public Law 172, 88th Congress, approved 7 November 1963. TABLE OF CONTENTS Page No. ABSTRACT - - - = - = = = = ie & a es a Mi 4 INTRODUCTION - - - - - = = = = = 2 es eS S 2 General Considerations - - - - - - = - = = 2 Area of Investigation - - - - = = = = = = 3 METHODS) - - - - = - = = = = = a = e ui 6 Measured Variables - - - - - = = - = © Z 6 Approach - - - mec - =e - = So 2 8 Method of Analysis used in this Report - - - - - - 9 The sum of Squares Criterion - - - - - - - - alal Implications of Linear Regression Analysis - - - - - 16 LONGSHORE CURRENT VELOCITY - - - - - = = = = = 18 Results oF Amalyeais S| = SS te) 9s co a & = & 18 Discussion - See Mate hs ee a til eines sea 20 Summary - - - - - - - = = = = aR A = 27 THE PROBLEM OF NON-LINEARITY - - - ~ - = = = & Da DEPOSITION AND EROSION ON THE LOWER FORESHORE - - - - - 30 Time Lag in Peak Interaction - - - - - - - - 30 Measurement of the Dependent Variable - - - - - - 32 Measurement of the Independent Variables - - - - - 36 Net Deposition (J¢) - - - - - - = = = = = 36 Net Erosion (K¢) - - - - = = = = = = = 39 BOTTOM SLOPE AND PARTICLE SIZE IN THE SHOALING WAVE ZONE - - - 44 Mean Slope in the Shoaling Wave Zone (Sa) - - - - - 44 Average Mean Grain Size in the Shoaling Zone (Mz) 5 - - - 48 UU RE SEU. eet anne Ue tien MERE RANI AMP eR AE it aS ee 51 REFERENCES) tate pene) cre Ee RS ina 1 clark IC DMO TMS Mar eNGE at 53 LIST OF FIGURES - - - - = - = = = = = = 2 56 APPENDIX A - (Description of variables and method of measurement )- SY APPENDIX B - (Summary tables of the computer output) - - - - 65 ADDENDUM - "Alternative Multiregression Technique for Obtaining Predictor Equations" by W. Harrison and N. A. Pore (8 pp.) - = - - = - - = = = = = A-1 ry An r ult ta eu; yar! tee INTERACTIONS OF THE BEACH-OCEAN-ATMOSPHERE SYSTEM AT VIRGINIA BEACH, VIRGINIA by W. Harrison (1) 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. 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 de- pend 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 manifest- ing at times of falling tide, is the combination made up of lower fore- shore 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. INTRODUCTION General Considerations Study of the beach-ocean-atmosphere system under natural field conditions progresses from initial descriptive studies, consisting mainly of masses of seemingly unrelated observations, to the analysis of "“cause- and-effect" (process-and-response) relationships between oceanic and atmospheric forces and their resulting products, the beaches. While in the simplest sense it is possible to choose some intuitively rational group of environmental processes that act as causal factors, and a like group of environmental responses that act as effects, this simple picture is complicated by numerous interrelationships among the various causal factors as well as among the response elements. This study is mainly a "search procedure" for identifying inter- actions among subsets of the numerous variables that operate in the natural beach environment. A number of "causes" and "effects" common to the general environment are measured and evaluated, consistent with avail- able time and resources. Least-squares techniques are employed in this search for relationships between the various individual "effects" and their attendant "causes." When the beach-ocean-atmosphere complex is considered as a whole, it is evident that no single functional equation can at present fully express the complex interrelations that occur in nature. The tendency has been to "fragment" the system into portions, either in controlled wave-tank experiments or studies of a limited number of variables in the field, that may help explain some facets of the many phenomena composing the whole. It is one purpose of this paper to refrain from expressing the data to be presented, either within the framework of process-response models (Krumbein and Sloss, 1963, Chapter 7; Whitten, 1964), or as more formal deterministic models. Rather, the intent is to "sort out" sets of variables to see whether they may provide a basis on which more formal models can be erected. The method used here is a form of sequential multiple linear re- gression to be described later. It is recognized that this is only one of several search procedures that may be used, and it is anticipated that opportunity may arise for extending the study in later publications by using alternative methods, such as factor analysis and discriminant func- tions, to see whether some optimum model for search can be identified. Area of Investigation This study was conducted at Virginia Beach, Virginia, an oceanic beach situated near the entrance to Chesapeake Bay, at the center of the mid-Atlantic Bight (fig. 1A). The study was concentrated along a strip of shoreline between 6 1/4 and 8 miles south of Cape Henry (fig. 1B). A relatively small, controlled inlet occurs near the center of the study strip. (The strip is bounded by the northern and southern transects of figure 1C). A study by Harrison, Krumbein, and Wilson (1964) indicates that the influence of the inlet on the adjacent beach is confined to a zone 500-900 feet to either side of its mouth and that it exerts no measureable influence at the northern or southern transects where all of the measurements for this study were made. Surveys by the U. S. Army Corps of Engineers show that beach ‘slopes in the foreshore area range from about 1:17 to 1:28, while beach slopes between roughly the 6 and 20-foot contours range between 1:50 and 1:60 (We S. Congress, 1953, p. 13). Details of beach profile modifications in the study area are presented by Harrison and Wagner (1964), along with several maps of the nearshore bottom that show the presence of NSXVL SJYSM SLNANSYNSVAW HOIHM LV SLOASNVYL GNV NOILVOILSSANI 3O VSYV ONIMOHS SdVW ‘| AYNOIS NOILVOSILSSANI 40 VayV S3LON3G EES 44 0008 () 0009 OO0O0€E (0) = [ = SS | ABE 0091 008 L SuaLaW aay dIHSLH9I1 130N! 330qnu XY LOSSNVYL 4 SJNGCINW y1vmMayuvog \\ Se See 4 ON] Hinos—s-\ BS nod JNVAdDVSIHD @ HOvad MANTONEA LOASNVYL NYSHLYON 39V9 SAAVM~- of 292 3AM = ae NOILVLS YSHLVAM AYNSH 3dVO bar-trough rhythms. Because an artificial beach-nourishment project has been in operation within the study area since 1956, the measurements for this study were taken during times of little or no pumping of sand. Physical and dynamic properties of 219 sand samples from the area have been studied by Harrison and Morales-Alamo (1964), and the averages of several properties for various dynamic zones of the beach may be sum- marized as follows: Zone Mean Size in mm Sorting Reynolds Number Nominal |Sieve Coefficient (Under Average Sea- Dia. Dia.* Water Conditions) Shoaling-wave ©,25 0.22 O70 4.8 Breaking-wave 0.30 0.27 0.70 Soak Swash 0.28 0.25 On55 6.2 Swash-berm 0.37 0.33 0.54 IDO *Converted from nominal diameter (Int.-Agcy Comm, Wat. Res., 1957, fig. 5) Sand samples with mean grain sizes approximating the above average values exhibit about 10% (by number) of heavy minerals and 2% or less of rock or shell fragments. Thus, the beach is composed largely of medium to fine quartz sand. Small samples of the average sand of the beach exhibit rel- atively low Reynolds numbers under average temperature and salinity con- ditions of the seawater. Tides at Virginia Beach are of the semi-diunal (equal) type and have a mean range of 3.0 feet, as against a spring range of 36) FRAKES Tidal currents in the study area are related to the ebb and flood through the entrance to Chesapeake Bay. They are generally reversing in nature, and are usually parallel to the shore. Harrison, Brehmer, and Stone (1964, fig. 4) present evidence from which it may be calculated that peak tidal-current velocities some 3,000 feet from shore and a few feet above the bottom do not exceed roughly 0.5 knot (0.85 ft /sec) in the study area. Peak velocities measured one meter above the bottom at the end of the 15th Street fishing pier (fig. 1C, "northern transect") approximated 0.68 ft/ sec. This pier marks the northern terminus of the study strip, and it is here that the tidal currents are at a maximum for the study area. The wave climate for the study area may be estimated from an analysis by Harrison of four years of wave records from a stepped-resis- tance wave gage (fig. 1B) maintained by the U. S. Navy in 20 feet of water off Cape Henry and from five years of wave observations at Chesapeake Lightship (fig. 1A), in 60 feet of water. The brief summary that follows indicates that roughly two-thirds of the waves that could be expected to strike Virginia Beach come from only three directions and that mean heights and periods are relatively low. Significant Wave Analysis Visual Swell Observation Wave Height Mean ist Mode (sec) (@Eo) (£b2 ) Wave Period Direction in Range 05° malolieg (True N) From Which Swells Come Direction (sec) Percent of Total Swells A breakdown of the 15 most-frequent associations of wave period, height, and angle of approach at the Chesapeake Lightship is given in Harrison and Wilson (1965), together with details of the refraction of these wave fronts as they move into the study area. Surf statistics at the Virginia Beach Life Boat Station, 1.6 miles north of Rudee Inlet (fig. 1C), have been complied by Helle (1958). ‘Three years' records, for observations every four hours, reVeal that surf is 4 feet or higher 10% of the year, 3 feet or higher 50% of the year, and 2 feet or higher 95% of the year. Surf tends to be highest in early fall when the angle of wave approach tends to be from the east (or ENE). Surf is also high in January, when it is largely out of the northeast. The average period of the surf tends to be greatest April through July (around 6.0 seconds). Wind data from the U. S. Weather Bureau's Cape Henry station (fig. 1B) fora 16-year period is summarized in the Virginia Beach, Va., Erosion Control Study (U. S. Congress, 1953, plate 5). Analysis of these data shows that whereas prevailing winds are from the southern quadrants, vel- ocities and total wind movements are greater from the northern quadrants. Northeast winds tend to be most common in September. METHODS Measured Variables A list of the variables measured in the beach-ocean-atmosphere system at Virginia Beach is presented in table 1, and descriptions of the techniques used in their measurement are given in Appendix A. Of all of the measurements used in this study, most were taken in the months of February, June, and July. Some were taken in March and April. Thus, a winter and a summer set of data were available for combination and study. (Only summer data were used in the evaluation of the dependent variables Jp and Kp). Table 1.--Variables Used in Analysis of Interactions of Beach-Ocean-Atmosphere System. (See Appendix A for Descriptions and Units Used. ) Symbol Dimensions Variable Description Ba L Depth of water at wave breaking C Tite Velocity of tidal current Cy ur-t Velocity of tidal current flowing opposite to longshore current C. tr-t Velocity of tidal current flowing in same direction as longshore current D L Depth of water table at top of uprush h L Still-water depth Hy L Height of breaking wave Ho L Height of wave (expressed as deep-water height) H)/Lo 0) Wave steepness of wave (expressed as deep-water wave) Jp L Net deposition at lower foreshore stations in 24. 5-hour period Kp L Net erosion at lower foreshore stations in 24.5-hour period Lo Th Wave length (expressed as length in deep water) (M,) 5 L Average mean nominal grain diameter over bottom in shoaling-wave zone R L Range of tide over one tidal cycle 8, O Mean slope of lower foreshore of beach 8, O Mean slope of beach over inner portion of shoaling- wave zone a8 ay Wave period UW, ur? Mean wind velocity directed against the longshore current Vor un Mean wind velocity in an offshore direction ue tr Mean wind velocity in an onshore direction Up Lt Mean wind velocity parallel to shore Us ure Mean wind velocity in same direction as longshore current Vv prt Mean velocity of longshore current a O Angle of wave approach Te po-t Rate of rise of still-water level Ne Lr Rate of fall of still-water level fe) Ma Water density Wind, tide, and most wave measurements were made every two hours, both day and night. Some of the other variables listed in table 1 (Ba, D, Hp, @, and 0) were measured four times a day at 0700, 1100, 1500, and 1900 hours. (Values at 2300 and 0300 hours were arrived at by linear interpolation.) The variables Jp, Kp, (M,),, Sp, and 5. were usually measured oncé each day at low tide, while (>), and 5. were measured Zz every 4 hours during the February field period. Approach Designation of "cause" and "effect", or "stimulus" and "response", is rather arbitrary in a system of complexly interlocked variables (like those of table 1) 5 all of which may vary simultaneously. In a general way it is possible to say that relationships between the variables in a specific beach environment are conditioned by the natural ranges in magnitude of the variables and by their natural frequencies of occurrence. Ranges in the magnitude of wave height, tidal-current velocity, wind velocity in various diections, and grain size distribution in the various littoral zones are all characteristic for a given beach. Also character- istic are the frequency of occurrence of waves of a given height, currents of a given velocity, and so on. Ideally then, to understand the interaction of a variable singled out as an “effect" with a number of other variables designated as "causes", one must measure both "causes" and "effects" over the range of values that they assume in the study area. Understanding of erosion of the foreshore of a beach as it is "caused" by winds, and currents, for example, can come only when foreshore slope, wind velocity, wave height, longshore- current velocity, etc., are measured through their expectable range of values and over a long enough time period so that adequate representations of natural frequencies are obtained. It is to be noted that consideration must be given to the fact that the moment of maximum interaction of an effect with its several causes may be influenced by a time delay. The well-documented delay in the response of laboratory beach slopes to unvarying wave trains in wave-tank experi- mentation is a good example of the delay factor in this cause-and-effect relationship. It is also to be noted that a given variable designated as an "effect" in the beach-ocean-atmosphere system may in fact be little in- fluenced by some of the variables that are considered+as "causes". Wind velocity parallel to the shore taken as an effect, for example, is in- tuitively independent of water density, angle of wave approach, and mean grain size on the foreshore taken as causes. For this reason it becomes intuitively "unprofitable" to investigate wind velocity parallel to the shore as a function of such improbable causes. Where it appears, however, that one of the measured variables is significantly dependent upon most of the other measured variables, it becomes desirable to investigate that inferred dependency. Method of Analysis Used in This Report Among several methods available for analyzing observational data that involve interrelationships among independent variables, the one chosen here is that of sequential multiple linear regression. It in- volves measures of the relationships of a given dependent variable ("effect") in terms of several controlling environmental "causes" (in- dependent variables), by taking the latter one at a time, two at a time, and so on, until all of the environmental processes are included simul- taneously. This sort of analysis is commonly called stepwise regression, and it may be conducted with regression techniques or multiple and partial correlation techniques. These methods also permit study of interrelations among the independent variables themselves, and they are useful for eval- uation of data redundancy. Redundant variables, that is, variables that in large part restate what some other variable has already measured, are common in early stages of quantification in the observational sciences, especially when physical models are not clearly discernible in the complex of observations. In these cases a method for "sorting out" a set of independent variables in terms of their importance or meaningfulness in controlling the response of some dependent variable, Y, helps to reduce the number of variables in the set to more manageable proportions. We shall illustrate the method with a subset of data from a larger example to be treated more fully in a later section of this paper. The problem here is to examine the nearshore bottom slope just seaward of the zone of breaking waves, as it responds to several shore process elements. The full example includes a dozen independent variables, designated as) Xi, K2, ..., X12. We sellect five of these, retaining the same number designations that they have in the larger example, for ease of later com- parsion. As set up, our introductory example includes the following var- iables: Bottom Slope In Shoaling-wave Zone M Mean grain size xa Wave period X2 Wave height x4 Angle of wave approach X9 Still-water depth at time of slope measurement X10 In conventional step-wise regression the "strongest" single X- variable is first obtained, and this is then "held constant," statisti- cally, to identify the second strongest variable. Multiple and partial correlation is commonly used, and some step-wise computer programs have built-in provisions that fix the relative importance of a given X for all subsequent stages of analysis. This sometimes leads to spurious results, in that a variable that may be weak in combinations of two or three Xs may rise in relative importance as combinations of four or five Xs are considered. Hence, our analysis is based on a sequential regression analysis of all possible combinations of Xs, so that every X has a chance to enter into every possible combination. The reader is referred to Kemeny, and others (1958, chap. 5), and to Rao (1952, chap. 1) for reviews of the matrix algebra that follows. The procedure adopted uses the computational form for the Y and five Xs of our illustration, in terms of the general linear model: Yi = 6, + BX1 + Bok2 + By X4 + BoX9 + By oX10 + e; which is expressed computationally in matrix form as: (1) where g aS 2) (6) se cl vector-Y, S is a 6 x 6 matrix of squares and cross-pro- ducts of the Xs, and 8 is a 6 x 1 vector of the estimated’ Bs “inedevanne the matrix equation is: {D> I loa & N Ba we bo) SG ff Bg |L | ase 2X1 mal mee ey, | abel | ey. || | mane ; (2) 2X5 Dod wie so wa || | sya) aon The computer first inyerts the entire matrix and post-multiplies the inverse by g, to obtain B. The proportion of the total sum of squares of Y attributable to all five Xs is then computed and expressed as a per- centage. This was found to be 78.7% in our example, which suggests that the set of five Xs taken together yields a fairly satisfactory predictor equation for nearshore bottom slope. In extracting all possible combinations of Xs from the matrix in equation (2), the computer program is so arranged that it always starts with N in the upper left corner of 8, and always has ZY as the first element of ¢ in equation (1). Thus, in the first computer loop the Xs are taken one at a time, yielding the following as the first two sub-matri- ees N 2X1 Bo ZY N 2X2 Bo LY and @ = 2X1 =x1° By 2ZX1LY 2X2 EXO= Bo 2ZXCY When all such combinations are computed, the next computer loop takes the 3 xX 3 matrices having pairs of Xs, again starting with N as the upper left element of S, and ZY as the first element of g. The first part of this loop has the submatrix for Xl and X2, the second has X1 and x4, and so on for all combinations of our example. The first submatrix in this stage aS: N IX1 IX2 Bo ZY mal | eae ebe|O/E, = TX1Y mS we wes | |. SX2Y Of course, the individual Bs change in value with each stage, but by continuing the looping process until all five Xs are used, it is pos- sible to scan the computer output to identify the strongest individual X, the strongest pair of Xs, and so on. The data for this example are given in table A and the complete output is given in table B. Before discussing the results, some remarks on the method of computation are appropriate. The total number combina- tions obtained by this procedure is 2*-l, where k is the total number of Xs used with a given Y. In our example k = 5, and we obtain 31 elements of output. This rises rapidly as k increases: for 12 Xs the output has 4095 elements and for 13 Xs it is 8191. Thus there are practical limi- tations on the size of problem that can be economically handled, to say nothing of the sheer bulk of output for large sets of Xs. For many problems with k of the order of 10 or more, the sequence may be carried as far as say six Xs at a time, which commonly yields most of the infor- mation in the set of data. The computational procedure may be simplified by using deviation matrices, but the present procedure is given here, to agree with the computer program as described in Krumbein, Benson, and Hempkins (1964). The Sum of Squares Criterion Once the Bs are estimated for each combination of Xs, the raw sum of squares of the computed Y values is obtained by multiplying the transpose of the B-vector by the vector g: i Seo 8 The sum of squares of the observed values of Y is computed as: es Neue 2a, SSYopg = 2 (Yq - Y)" = 2Y,° - Y2Y, Table A.- Subset of Data From Virginia Beach Study Beach Mean Wave Wave Angle of Water Sample Slope Grain Size Period Height Wave Approach Depth Number (Sg) (M,) s (T) (Ho) (a) (h) O1 0.68 0.79 7.80 1.82 30.00 12.0 02 0.85 0.65 8.00 8.84 25.00 11.40 03 0.66 0.81 9.03 512 35.00 10.70 ou 0.50 0.74 6.06 5.43 hO.00 aL 560) 05 1.86 0.22 5.90 AL 2) 30.00 Tal 30) 06 2.33 0.23 8.40 1.09 30.00 10.70 O7 2, Af 0.25 12.00 N15) 25.00 aa. NO) 08 1.83 0.26 4.80 8.53 25.00 12.80 09 1.68 0.41 10.80 So dal 10.00 13}, 30) 10 BeOS Os55 10.40 6 (EO 30.00 13}5:30 ala TGs} 0.47 10.80 1.04 30.00 14.10 1D 1.84 0.59 7.90 5 O2 35.00 13.40 13 ILS OH 0.47 4.30 Aga 30.00 13550 14 1.82 0.50 10.80 0.62 35.00 1334 3}0 15) 185 On52 3.80 1.69 30.00 14.40 16 LS 0.47 4.10 1622 20.00 14.10 ale( 15 Sul 0.42 4.50 QS} 30.00 15), 30 18 1.38 0.37 6.10 Ll 20.00 14.00 Table B.- All Combinations of Xs for Beach Data of Table A. i Independent Variable Combinations Percentage of Sum of Squares of Y Accounted for: X1 (Mean Grain Size) 63.1 X2 (Wave Period) Ao Al x4 (Wave Height) OB. of X9 (Angle of Wave Approach) 5.6 X10 (Water Depth) Dae AS ae 65.5 i 4 bees 1 9 64,2 ale 10 66.4 2 ih. a4, 4 2 9 6.5 2 10 ©q a. 4 9 36.1 4 10 ah. y one) 8.9 1 2 & 15-9 a ee 9 66.8 ee 2 10 HLS 1 4k 9 (leg Al aL y 10 74.8 al S 10 68.6. De Ih © 36.5 BD Mh 10 25.9 2 Q I 12.0 h © 1 36.2 Th ee ee) (do) i. ak 10 Wok Tg Ae iS) al 1503 iL « Meo, io 74.9 2 he ©) dO 36.5 1 2 hh 9 10 1801 where the last term on the right is the so-called "correction term", which is then used to obtain the sum of squares of the computed Ys: SsY = SSY 19 comp = Eas aw Thus, the proportion of the total sum of squares attributable to linear regression, expressed as a percentage, can now be computed directly as 100 SSY comp i; SSYops- For example, if SSY obs = 250, and SSY ay = W510). then the linear relation "accounted for" 60% of the total sum of squares of Yobs: The quantity 100 SY om / SsYy is informally referred to as the i t reduction in the stim of 88", h as in the ab i percent reduction in the s squares , inasmuch as in the above example, if the total sum of squares of Y is 250, the 150 units attributable to Yeomp leave, in effect, only 40% of the original variability "unaccounted" for. Hence, the reference to a percent reduction in SSY,p, means that the regression relation has in effect "reduced" the initial variability by 60%. The sum of squares criterion is generally most useful when the number of samples (or times of observation of process elements) is several times as great as the number of process elements measured. Thus, for six Xs the minimum number of times of observation should be 15 to 18. As the number of Xs measured approaches the number of observations, the sum of squares reduction tends to be forced closer to 100%, and may give an impression of explaining a greater part of the variability in Y than is correct. (The unusually high percent reductions in the lower part ‘of table BuO, described later, may be attributable to such effects). The reduction of the sum of squares is a measure of the mathematical association between variables, and is not necessarily the measure of a physical relationship. Where the independent variable has physical mean- ingfulness in the problem, however, it is not extreme to infer that the strength of the mathematical relation is also a measure of the strength of the physical relation. We return to the output for this example, as given in table B. It is interesting to note the wide range in the percentage of the sum of squares of Y accounted for by the various combinations of Xs. In pre- computer days, when only a few variables could be handled feasibly, it is not surprising that different researchers, using different sets of variables, might well have made different inferences, depending upon the particular two or three variables that might have been used, The material in table B can be conveniently arranged as in table C, which lists only the three strongest combinations for each computer loop. There seems to be no doubt in the present subset of data that the effect of mean particle size is by far the strongest single variable in slope response, even though mean grain size is itself influenced by the process elements. For Xs taken two at a time, it is noteworthy that mean grain size (X1) is consistently an element of each pair; and that the com- bination of mean grain size and wave height (X4) is the strongest pair. Table C. - The Three Strongest Combinations of Xs Taken 1, 2, 3, and MWaiti tal Time From Table A i Independent Variable Combinations Percentage of Sum of Squares of Y accounted for: Xa, XB Wh KG) YALO AL 63 h 2h One at a time 9 6 aL, 4 74 ll 10 66 Two at a time ee 65 ae 4 16 iL 4 10 75 Three at a time iL 4 9 74 cles 2 4 10 78 i 2 4 9 76 Four at a time dL, } 9 10 ' ie deere 4 9 10 79 Five at a time NET CONTRIBUTIONS OF RANKED VARIABLES xa 63.1 % x4 11-0) % x2 1.8 % X10 2.2 % x9 0.6% 78.7 % This is borne out by the next stage, in which variables X1 and X4 are consistently present, but now the added contributions by the other three variables does not cover a very wide range. Thus, the three strongest combinations differ by only about 1%. Similarly, combinations of four Xs at a time show a limited range of contributions, with perhaps a sug- gestion that variables X2 (wave period) and X10 (water depth) are slightly stronger than angle of wave approach, X9. In considering the implications of the analysis, we may estimate the relative importance of the several variables in the following way: select Xl first, with 63.1% of the total sum of squares of Y attributed to it. Then, because the gap between the pair (X1, X4) and the next competitor (Xl, X10) is about 10%, choose X4 as the second strongest variable. Its contribution, in the presence of Xl, is (74.1 - 63.1) = 11.0%. From here on the choice is less clear, but if we tentatively accept wave period (X2) as the third strongest, we obtain a contribution of (75.9 - 74.1) = 1.8% from it. Similarly, if we tentatively accept water depth (X10) as the fourth strongest, we obtain (78.1 - 75.9) = 2.2% from it. Lastly, the contribution of angle of wave approach (X9) is found by the relation (Se % - [Sodl,) = 0.6%. The relatively small contributions attributable to X4, X9, and X10 suggest that, except for the two strongest variables, there is little to choose from among the other three, which add on the average about 1.5% each, in contrast to 63% by mean grain size and 11% by wave height. Examination of the weakest variable in the set is also illuminating. As table C shows, wave period "accounts for" only 1.1% of the sum of squares of Y. Yet on the strictly least-squares basis of choice used here, this becomes of about equal rank with X9 and X10, which individually contribute something more than 5% when taken alone. The IBM 1620 and 709 programs used in this study compute the linear coefficients and the sums of squares reduction for all combinations of Xs, as stated, and the output lists the Xs involved and the corresponding per- cent SS reduction. If intermediate output (such as the coefficients) is desired, the program produces this by way of a control card. Details are given in Krumbein, Benson, and Hempkins (1961). Implications of Linear Regression Analysis An important aspect of the present method of analysis is that the general linear model, as used here, examines only the linear re- lations among the variables, although the model itself can be extended to some non-linear cases, providing that the Bs always remain linear. It is sometimes instructive to examine the matrix of linear correlation coefficients along with the multiple regression output, to see whether additional light is shed on the regression by the linear relations (interlock) among the various Xs used in the study. Table D is the upper right half of the Pearson product-moment- correlation-coefficient matrix for all pairs of variables in table A. Inasmuch as rjj is the same as ry; the matrix is symmetrical and only half of it need be examined. The diagonal in table D carries the value 1.0 for all entries, and merely means that any variable correlated with itself always has r = 1. The top row of the table has the correlation coefficients of Y with each X in turn, and the relation between the values in table B and the corresponding r's in table D is that the 63.1% listed opposite mean grain size in table B is the same as 100 ryx 1 = 100 (-.795)© = 63.2%, which agrees within rounding error. Table D.-Correlation Matrix For All Pairs of Variables In Table A. ij Slope Mean Wave Wave Angle Still- Grain Period Height of Wave Water Size Approach Depth 1.00 -.795 . 104 aa NCH =5236 . 228 Slope 1.000 .062 -205 419 -.060 Mean Grain Size 1.000 - 049 =.035 26356 Wave Period 1.000 -.cl7 =-.305 Wave Height 1.000 =.222 Angle Wave App. Note: Attention is called to negative correlation between (Mz), 1.000 Still-Water Depth and Ss which Is explained on p.45 The most interesting parts of table D are the linear relations among the Xs in this subset of data. For example, ryjx9, between grain size and angle of wave approach, is + 0.419, the largest r in any row other than the first. This correlation with at least one of the process elements is perhaps to be expected, considering that mean grain size is itself in general dependent in part on shore process elements. For a set of data having only 18 observations, any r less than about 0.40 is of doubtful significance. The lack of strong correlations among the variables (except for mean grain size and slope) merely means that the linear relations among the variables are weak, and neither the correlation table nor the re- gression results gives any direct information on non-linear relations that may occur. The problem of non-linearity is always present in linear analysis, and it will be examined in the next section of this paper, where an extension of the linear model is described that permits examination 17 of the second degree (quadratic) effects that may be present. LONGSHORE-CURRENT VELOCITY Longshore-current velocity typifies a variable measured in this study that depends rather obviously upon most of the other measured variables. We may expect from theoretical and laboratory studies (cf. Brebner and Kamphuis, 1963, for example) that the following variables will be significant ones in their influence on longshore-current velocity: beach slope, wave period, wave height, and angle of wave approach. Other measured variables that are not customarily considered in laboratory studies may be expected to exert lesser, but possibly significant, effects upon longshore-current velocity. Among these might be local wind veloci- ties in onshore or offshore directions, which would affect the form of the incoming swells, and water density, which would affect fluid drag and the quantity of sediment transported. Tidal currents opposed to or in the same direction as the longshore currents might also be of signifi- cance. Of the greatest importance in the evaluation of the influence of the several variables considered as causes, on the one considered an effect, is the assessment of the simultaneous influence of each of the various possible combinations of the causal variables. The method used in investigation of this simultaneous influence was described in the previous section. Here we take longshore-current velocity as a dependent variable. Results of Analysis The results of the first stage of the regression analysis are shown in table 2. This table shows the extent to which 13 environmental factors (independent variables, or Xs) reduce the sum of squares of longshore-current velocity when the factors are taken one at a time. The total q, SS reduction by all 13 Xs taken simultaneously is 69.97. (The number of longshore-current measurements used was 53.) An initial in- ference is that some 70% of the interaction of longshore-current velocity with the environment is "explained" by these 13 measured environmental variables. While it is possible that certain important variables may have been omitted from the analysis, it is also likely that a relatively high "noise-level" may be present, due in part to local fluctuations in the phenomena studied, as well as to errors of measurement. Mean long- shore-current velocity, V, for example, shows a great variation about the true mean velocity because the point of actual observation relative to the upstream and downstream rip-current boundaries was hever know. Thus the value used for V actually ranged from zero velocity, up through the true mean velocity, and beyond to the maximum expectable one. Again, noise may have been introduced into the analysis owing to the way that the independent variables were measured through time, relative to the dependent variable. Wave height, for example, could have been measured at the instant that the longshore-current measurement was taken or up to 4 hours before its measurement. In view of these difficulties Table 2.-Per Cent Reduction in Longshore Current Velocity Sum of Squares Attributable to Thirteen Independent Variables, Taken Individually Variable Symbol Position %, Reduction in SS Mean Lognshore Current Velocity Vv Me = Mean Slope Angle, oH Lower Foreshore Sr X1 D)o dll Wave Period ay X2 21.98 Wave Length (deep water value) Lo X38 26.46 Wave Height (deep water value) ial xh 17.89 Wave Steepness (deep water value) Ho/Lo X5 47. 4e Mean Wind Velocity Onshore esa X6 6.92 Mean Wind Velocity Offshore Use X/ 5.63 Mean Wind Velocity Directed Ml with Longshore Current We x8 0.68 Mean Velocity Directed Us X9 0.00 against Longshore Current Angle of Wave Approach a X10 1.19 Water Density fe XeLAL 0.65 Tidal Current Velocity with Longshore Current C. x12 P25) Tidal Current Velocity against Current Cy X13 0523} with the noise content of the data, it becomes more understandable that the total % SS reduction by all 13 variables simultaneously was only 69.97%. In actual fact it would be slightly lower than the 69.97 figure if variables X3 and X5, which exhibit considerable "data redundancy" with variables X2 and X4, had been left out of the analysis. Data re- dundaney is brought up again later. Discussion Table 2 shows that the variable X5, wave steepness, has the greatest influence upon longshore-current velocity, followed by the wave factors of Lo, T, and Ho. As noted earlier, the fundamental variables of T (X2) and H(X4) could be expected from theoretical (Putman, Munk, and Traylor, 199) and laboratory (Brebner and Kamphuis, 1963) studies to be among the most- influential in determining longshore-current velocity. Also of expected importance would be beach slope and angle of wave approach, but both of these variables are outranked by wind velocity onshore and offshore when the variables are taken one at a time. As stated, when the independent variables are taken one at a time, their relative rank cannot, in general, be determined directly from the degree to which they reduce the sum of squares of the dependent variable, Some of the variables may be redundant. That such redundancies are pre- sent in the longshore-current example can be seen by adding the individual reductions in the sum of squares of Xp in table 2. This sum is 135. 41%, indicating that some variables, when taken one at a time, show stronger relations than they show when taken in combination with other independent variables. Thus, one may say that there must be, as a minimum, at least 35% of data redundancy and (or) noise in these observations. Before going further it is well to note that variables X3 and X5 (table 1) will exhibit a certain amount of artifically introduced data redundancy because they repeat information found in variables Xe and xh. Thus, wave length L, being directly proportional to the square of the wave period T, might be expected to exhibit a closely similar % SS re- duction to T. Because X3 gives a 4.48% SS reduction (table 2) over that for X2, however, the relation between V and T may be non-linear. More will be said of non-linearity later. Variable X5, then, repeats informa- tion found in the "fundamental" variables X2 and X4. It is stronger than either, however, and by combining these two attributes wave steep- ness be comes a useful variable to measure in studies involving prediction of V. Its non-dimensionality may also enter, inasmuch as H,/Lo is free of any scale factor. The next step is to examine the computer output for the strongest combinations of variables when they are taken in combinations of two, three, four, and so on. Table 3 summarizes certain of the strongest combinations for Xs two through nine at a time; it reveals that wave steepness retains its dominant role in all of the strongest combinations. The strongest doublet is not X5 and X3 (Ho/Lo and Lo), as might be ex- 20 Table 3.-The Strongest Per Cent Reductions in Longshore Current Velocity Sum of Squares Attributable to Each of Several Combinations of Thirteen Independent Variables. (Total Percent Reduction, All Xs = 69.97). Percent Independent Variable Combinations Reduction in SS Two Xs 5 1 5h 2 ie @ wake al 5 53.91 5 10 50.77 3 5 50.09 5 6 49.86 Tiree Xs. 1 5 i 60.69 ateagtame: 1a: 3 5 Si ola il 5 10 57.03 5 ie ila, 56.02 1 2 5 56.02 Four Xs al, 3 5 rie 62.23 aia came: 5 7 10 61.96 iL 5 a WL 61.85 1 2 5 if 61.50 il 5 ri 12 61.37 il 3 5 10 61.19 al Me 5 T 60.90 al 5 i 13} 60.72 Five Xs IN 2y/ 63 5 10 64.53 Bie @) waa IL 3 5 T 10 64.07 4 3 5 i TAL 63.52 12.3 5 ii 68.035 A 5 ih 10) SL 63.02 Six Xs ANG DY 3 5 i 10 66.39 io @, teams dL 3 5 i 16) 65.25 il eyes 5 10 13 65.02 ale Dames) 5 8 10 64.95 yy Be ely 10 64.92 Sevaa ke 1 2 3B 5 6 F 10 67.56 ae a wame lt 2 3 5 il 10 12 66.72 le yeas 5 "/ IO 1 66. 67 a By ln Gl 10 66. 60 AL Ge) aes 5 a 10 13 66.53 2 pected from table 1, but rather is X5 and X7 (H,/L, and Uj). The third strongest doublet is X5 and X10 (a). Although these differ by only a small percentage, they are being taken here as ranked by the least-squares procedure. The practice of inferring the relative rank of combinations of variables from their rank when taken individually is generally not a sound procedure. Slope angle of the lower foreshore (X1), for example, does not appear (table 2) to contribute as much as fully six other variables (Xs 2-7). The method of sequential multiregression analysis shows, however, that Sp is present in the strongest triplet and remains prominent in several of the stronger triplets. From an initial SS reduction of 5.11%, when considered individually, it is seen that in the presence of variables X5 and X7 it contributes fully 6.48% (table 4), for an increase in its origi- nal value of only Me SiGe Thus, the strongest individual variable may be influenced by other variables. However, when the grounds for accepting the strongest variable on a physical basis are sound, it is conventionally taken at face value, and the effects of other independent variables grouped with it are expressed in terms of the added reduction contributed by the combined variable. Continuing in table 3, we noted that the strongest triplet (Xl, X5, X7) now includes beach slope (X1), another fundamental variable, which ranked fully seventh in order of importance when the variable yere considered individually (table 2). Table 4 indicates the change in the original % SS reduction value of Xl when in the presence of X5 and X/. The value of X1 has increased some 1.37%. The strongest combinations of independent variables taken four at a time is composed of Xl, X3, X5, and X/7. Interestingly, the original SS-reduction value of X3 (26.46%, table 2) has been reduced by 24.92% {table 4), in the presence of Xl, X5, and X7. This is largely because it already occurs in variable X5. At this point it is instructive to rewrite the empirical laboratory expressions found by Brebner and Kamphuis (1963, p. 22) to represent the mean longshore-current velocity as they computed it: a) using Airy wave theory, b) Snell's law for wave refraction (assuming a gently sloping plane beach), and c) by expressing the wave parameters in terms of deep-water values. V 1.9 (g Be)4/3 ne (Ho/Ly)'/© (sin 1.650, +0.1 sin 3.300,) Energy (1) Approach V ll 4.0 (g co ae (fe Mesa 1.65, + 0.1 sin 3.300) Momentum (2) pproac Assuming for the moment that there is a reasonable relationship between the major factors that determine longshore-current velocity in the laboratory and those that influence it in nature, we may look for the position of the combination of Xl, X3, X5, and X10 in the ranking of % SS ae ‘ouTy @ Ye SX G JO suCTzeUTQUOD ayy UT (€ SeTqQe4) ySesucTys puodas ST UOTYeUTAWOO STUL yxy 9x ‘eX ‘OLX Y9L°G fo aseeroeg MALT 95°19 Se CiTexGey Kar Gine c6°9 9X AO %99°6T JO eseeroeg Acar 6£°99 Sxacae Kae axa Ging 96°T? ox OTX %59°O gO eseetout 419 °T L0°t9 SKE KeIG Gineis OTXxx %26*he FO eseeroaq 4S°T €3°c9 Cx ‘Tx 1x “Gx 91°92 eX ZLE*T FO eseertouy ora) 69°09 ibaa Ka GxG TESS TC ZOT*T gO eseerouT 6L°9 Te +S IEG €9°S 1x ot’ lit GX on’ Lh GX uoTANGTI7U0D X MON go uoTZeUTqUO) uOTAZeUTQUO)D (2 eTAeL 329) oTAeT Ie A TeuUTSTIO 0 SATISTSY UoTIngT14U09 Akg ow ur SX ATTENPTATPUL 10,7 aseotoeq Jo sseatour pequnoooy gg % peyzunoosoy gs % ATWuTor usyel, SoTqetze, Vuepuedepurl 4seBsuor4g9 UsAeg 94 SuoUYy SeTqetze, Teuotatppy Aq suotangqtaquog - ‘th eTqeL 23 reductions for variables taken four at a time (table ane This combination ranks sixth in importance, but is separated from the most-important combin- ation by only 1.02%. The suggestion, then, is that the method of data analysis reasonably duplicates the expectable influence of combinations of important variables as they have been suggested by theoretical and laboratory approaches, when studied by a straight forward least-squares procedure. Then, an important corollary to this inference is that the data set itself is a reasonably faithful representation of the environmental interactions under study. Exmination of the results (table 3) for five Xs at a time again shows that the variables found to be of significance in the laboratory exert the greatest % SS reduction on V. it is at this point that the variable of angle of wave approach (X10 ) enters into the strongest com- bination. X10 exhibits an increase in its original SS-reduction value of 0.65%, when in the presence (table 4) of X1, X3, X5, and X7. What effect expressing @ and M (using Snell's law and Airy wave theory) would have had in bringing the relative ranking of Xl, X3, and X5 in table 2 more in line with the powers in expression (1) and (2) is unkown. It may be unim- portant in this set of data, owing to complex wave-refraction patterns that may be present. The highest-ranking combinations of six variables at a time include X/7, wind velocity offshore, which probably has a significant effect upon wave steepness, X5. Thus the interlocking nature of the variables again enters the picture. On several occasions during the measurement periods wind shifts from onshore to offshore were noticed in connection with passage of cold fronts. Waves were observed being "knocked down," as spray blew from their crests in a seaward direction. Because of the apparent "corroboration" of the method of analysis for the fundamental variables mentioned above, it seems fair to attach significance to the additional atmospheric variables that turn out to be significant in the least-squares analysis when they combine with these fundamental variables. That is, the indicated importance of variables X/ and possibly X11 (table 3) to long- shore-current velocity may be believed to have true physical significance, when in combination with the variables just mentioned. Once physical significance is attached to these variables it is realistic to enquire into the precise physical relationships involved. As mentioned, for example, strong offshore winds will often rather effectively alter the wave form, and water density enters into fluid drag in the longshore-current trough, the amount of sediment entrained, and thereby the velocity of the longshore current. Bruun (1963) and Inman and Bagnold (1963) give excellent reviews of the problems in understanding the generation of longshore currents. Any inferences advanced here are not capable of immediate verification based upon field studies but may serve as a basis for the design of field measurements. Because of the strictly artificial introduction of data redundancy in variables X3 and X5 (table 2), a new summary (table 5) was prepared from the computer output. This new table lists the strongest. combinations when variables X3 and X5 are removed from the analysis. As expected, 24 variables X2 and X4 are dominant throughout all of the combinations, and variable Xl through all but one combination. Variable X6, wind velocity onshore, takes the place occupied by X10 in the five Xs at a time combina- tion of table 3. Angle of wave approach, X10, then appears in the com- bination of six Xs at a time. Thus, wind velocity again appears important in influencing lonshore-current velocity. Table 5.-The Strongest Percent Reductions in Longshore Current Velocity Sum of Squares Attributable to Combinations of Eleven Independent Variables (x3 and X5 Removed From The Original Thirteen Xs). Percent Independent Variable Combinations Redceution in Ss Two Xs 2 y h1.10 at a time Three Xs 1 2 y U7 £23 at a time owe Xs ah y 7 51.98 at a time Page ots ik 2 6. 7 Sf oO at a time SaexaeXs lene? y © Ff 10 59.71 at a time Seven Xs 1 2 4 6 7% 8. 10 62.04 at a time Table 6 summarizes the combinations of independent variables that least influence V. The table reveals certain variables that contribute to both the weakest and to the strongest combinations. It is instructive to refer to the weakest (table 6) and the strongest (table 2) combinations of independent variables taken six at a time. Intuitively, we might expect that none of the six variables that appear in the strongest combination (Xs 1, 2, 3, 5, 7, 10) would appear in the weakest combination (Xs 8, 9, TO, 1a, 12, 13). It is found, however, that X10 does appear in both com- binations. When the number of variables is fairly large, as ih this study, it is not unusual to find that as the number of Xs per combination is increased, a variable whose net contribution is of the order of lor 2 L, may "rise to the top" when combined with a very strong combination of Xs, whereas when combined with very weak Xs, its own slight contribution is not suffi- cient to raise the total contribution by any significant amount. 25 Table 6.-The Five Weakest Per Cent Reductions in Longshore Current Velocity Sum of Squares Attributable to Each of Several Combinations of Thirteen Independent. Variables. Percent Independent Variable Combinations Reduction in SS Two Xs 9 13 O28} at a time 9 Lal 0.65 6 © 0.68 8 13 Ost 14 13 0.87 Three Xs 8 9 13 Oni at a time 9 LAL 13 0.87 9 12 13 1.28 S} © TAL. ale 8 ileal 3} 1.50 Four Xs 8, 9 abit 13 50 at a time 9 dL 12 13 1.68 © iO) 13 1.88 8 9 Ae) gals} 2.00 ©) 10) Tah 13 2.20 Five Xs 8 9 ii 12 18 245 at a time Sj ©) Sil) Tal 13 2.61 10 Ag 12 as Bo i3 3 © 10 1A 12 3.23 3 ©) NO Ti Je Bul Six Xs eh ©) TO: al, ae 8 3.66 at a time (GOD mae Ove eniale 13 6.02 G8 © aa 2 8 6.16 Ge teh) lO) Ala. = a 6.44 8. 9) IO ey 3} 6.44 Seven Xs Ht}. S) MO) dad 13} 6.51 at a time 1 8 9, 10) th 1p. 13 9.23 1 7% & © I@ as ip 1 AS IL Te (GP ao ala 13 11.88 aL "7 8 © IO 2) 113} i, C2 Eight Xs 1 “8 © IO da. 12 13 I 2A5O at a time 1 6 (Gye). oss KO) ot abal, aay vals) 19.33 4 6 SS) A@- abl 12 18 25.76 i 4 6 8 9 i eats 26.95 aL 4 6 8) 6) © 1a ¢ alg 27-74 26 For example, the combination 1, 5, /, 10 in table 3 accounts for 61.96 %, an increase of 1.27 % over combination 1, 5, 7, In table 6, the combination 8, 9, 13 accounts for 0.77 %, and the combination Sis Op. tO), 13 accounts for 1.88 %, an increase of 1.10%. Thus the position of X10 depends very largely on the strength of the combination in which it occurs, and X10 seems to play a minor role in this set of data in that it first shows up as weak or strong in combinations of 4 Xs at a time. It will be recalled from table 2 that X10 accounts for 1.19 % of the SS of Y, a con- tribution that seems to remain much the same throughout the analysis. summary Based on an analysis involving eleven independent variables, it is possible to say that the combination of six factors (table 5) that is most influential in the determination of longshore-current velocity in the study area at Virginia Beach is the combination made up of wave period, wave height, lower foreshore slope, wind velocity onshore, wind velocity offshore, and angle of wave approach, in that order. (When the derived variables of wave length and wave steepness are added_to the analysis, the six most- important factors are Sf, T, Lo, Ho/Lo, Uon, and Upp). The wave factors and the beach slope are variables agreed upon by workers in the field of longshore-current generation to be of fundamental importance. The signi- ficance of winds on and offshore is believed to lie in their ability to alter the wave form prior to breaking. The least-squares relations developed by this analysis appear to adequately represent the combinations of variables of most significance in nature. It is to be recalled that the framework of analysis is linear, but it is not uncommon, when a large number of variables is involved, that a- linear approximation yields reasonable results, even though some relations may be known to be non-linear. Moreover, non-dimensional ratios among some of the original variables, such as H [ils in this example, may rise to greater relative importance than the original variables themselves, as shown in table 2. This suggests that further analysis by use of non-dimen- sional variables alone (perhaps as derived from use of Buckingham's Pi theorem) may be useful and informative. We hope to extend the analysis in this direction. As a summary of the foregoing linear analysis, it seems fair to say that the least squares techniques used here have helped "sort out" the relative interplay of a group of variables as they affect longshore- current velocity on a particular beach during a given time-span. Statis- tically our model is "fixed", and extensions of our findings to general- izations about other beaches is valid mainly in that the underlying variables are perhaps the same, even though their relative rankings may vary from beach to beach, or on the same beach from time to time. That non-linearity is also a problem is discussed next. THE PROBLEM OF NON-LINEARITY The occurrence of non-linear relations among beach-ocean-atmosphere variables was touched upon above, and it is discussed here in connection 27 with the study of longshore-current velocity. Non-linearity can be detected in several ways, the simplest perhaps being the examination of scatter dia- erams of each pair of variables in the entire set. Another search method is to run the regression analysis first with the "raw" data and then with all or some Xs transformed to logarithms. In this example we shall use an extension of the linear model to seek for quadratic effects among the variables. When the linear regression of Y on some single X is studied, the computational model reduces to the following form: ¥ = Bo + B,X In terms of regression analysis, this involves the 2 x 2 matrix, vector-Y, and B-vector discussed in connection with equation (2), and yields the "sum of squares reductions" associated with one X at a time. However, this model can be extended as follows to include higher powers of X: seen “ “~ ) Y = Bo + BX + Box (3) where the coefficient B, is now associated with the quadratic form of X. The coefficients are still linear, and hence the same general technique may be used, even, if desirable, to include such powers as 1S, Mv SUSo The procedure used here is first to take each X by itself in terms of Y, to obtain the values in table 2. Then, for each X, its square is also included as in equation (3), to obtain a corresponding sum of squares of Y attributable to the combined linear and quadratic effects of the X. The difference between these two "sum of squares reductions" gives an estimate of the second degree non-linearity associated with each X. This was done with a computer program that computed the linear and quadratic relations between Y and each of the 13 Xs, as well as all interlocks between the Xs themselves. In this: latter analysis the order of entry of the Xs is involved, in that the expression: Nes ~ “~ D) ele Bo + Bj Xe + Boxe is not the same as the expression: 2@ = [i 2 eek > Bel even though, as was mentioned earlier, ryjy5o is the same as r ST ELAUISE the complete quadratic output for a problem involving 12 Xs is voluminous, and we shall here emphasize mainly the analysis in equation (3) that contains M% ChineCcwliyyrc Table 2A shows the linear and quadratic counterpart of table 2. It is apparent that in absolute terms, variables X5, X6, and X11, representing wave steepness, wind velocity onshore, and water density, have the largest quadratic components. In relative terms, the lower foreshore slope angle, 28 Table 2A.-Linear And Quadratic Relations Between Longshore Current Velocity And The Independent Variables In Table 2. Variable Positions %SS Reduc. %SS Reduc. 4% Added by Attributable Attributable Quadratic to Linear to Linear and Quadratic Mean Longshore Current Velocity Me -= -= —= Slope Angle Lower Foreshore X1 Fo dll AO), als} 5.02 Wave Period X2 21.98 2560 3.62 Wave Length (deep water) X3 26.46 27-43 0.97 Wave Height (deep water) x4 17.89 19.97 2.08 Wave Steepness (deep water) X5 Mire de) renal 25.30 Wind Velocity Onshore X6 6.92 25) OL 18.09 Wind Velocity Offshore Le 5.63 6.26 0.64 Wind Velocity Directed with Longshore Current x8 0.68 ig als} 0.46 Wind Velocity Directed against Longshore Current X9 0.00 0.00 0.00 Angle of Wave Approach X10 I.g19 2.38 1.19 Water Density X11 0.65 12.42 IL Tidal Current Velocity with Longshore Current X12 125 Ws Sk. 0.06 Tidal Current Velocity against Longshore Current X13 Oo23 Ooss/ o.14 X1, has doubled, from 5.11% to 10.13%. Thus, these several variables may enter the least-squares relations more effectively as logarithms or as variables raised to some power. Of particluar interest is water density, which in the linear sense is quite negligible (only 0.65%), but in the quadratic has risen to nearly 12%. This is an illustration of one limita- tion of a strictly linear analysis: some variables that have virtually no linear effect may become quite strong in a model that explicity includes non-linear effects. DEPOSITION AND EROSION ON THE LOWER FORESHORE Time Lag In Peak Interaction In a sequential multiregression analysis of the interaction of the Mission Beach, California, foreshore slope to the four independent variables.- of wave height, wave period, angle of wave approach, and longshore-current velocity, Krumbein (1961, p. 45) found that the maximum effect of these combined independent variables occurred (in a least-squares sense at least) sometime between 6 and 12 hours prior to the time of the measurement of foreshore slope. In regard to wave period, the analysis indicated that the greatest effect on foreshore slope was exerted some 30 hours prior to the time of measurement of beach slope. Because of the laboratory and field evidence for the delay in time in the peak interaction of certain dependent variables like beach slope and the independent variables that influence them significantly, the four interaction studies at Virginia Beach that follow were investigated over five or six "lag periods." In the case of modification of the segment of the lower foreshore that is covered and uncovered by the tide, it is necessary to standardize the times of slope measurement so that the measurement times are repre- sentative of similar dynamic conditions on the beach. lLow-tide time is a convenient reference time and was adopted here. Figure 2 shows the scheme adopted at Virginia Beach for making foreshore measurements in June and July of 1963. The ticks marks designated Pj, Po, P3,...-.-.--- Pjh9 represent the standard times during the 25-day period when the in- dependent variables were measured or for which Sie ae values were obtained. The tick marks designated Rj, Ro; R3 go00 0006 represent the times of low tide when measurements of the eats ete of — lower fore- shore stations were made. This measurement time (Ra) is seen (Gates 2) to progress through the times of measurement of the other variables. Because each lag period is 4 hours in length, the measurements for all of the independent variables for lag period 1 will have been made O to 4 hours prior to the foreshore-altitude measurements, for lag period 2 they will have been made 4 to 8 hours prior, and so on. Because the precise time of peak interaction between a dependent variable (Tp, Ke) measured at low-tide time and one or more independent variables is un- known, it is probably just as well that bias has not been introduced by setting up lag times of fixed numbers of hours prior to measurement of the dependent variable. 30 vooteetneees 2y ly) SINSW3TS 3SNOdS3Y ONY ( PSlqeee Od old) SINJW373 S$S3D0Nd JO LNSW3YNSVI3W JO S3WIL ONIMOHS WVHOVIG ‘2 3yNdIS SYNOH NI SWIL 006! ool! fele} x0) 0061 ooll lele}) and D) (tablies Bild and Bl). y Because variables X11 and xi contribute also to the weakest combinations (tables 39 Table 8.-The Five Strongest Per Cent Reductions in Net Deposition at Lower Foreshore Stations Sum of Squares Attributable to Combinations of Six Independent Variables (X3 and X5 of The Original 14 Xs Not Used) for Each of Six Lag Periods. Percent Independent Variable Combinations Reductions in SS hag al a] ae 6 9 10 13 73-31 iL. 2 6 9 aL 13 12.85 pete 6 9 13 ae DTT AO y 6 9 13 (AoE one y 6 9 10 12a 2B Lag 2 1 6 1©, ii iI2 14 69.40 iL 6 ©. IQ ia. 14 68.04 alt 6 da, 12 13) ah 67-76 1 6 9 ni 12 1h 67.62 a, 4 6 id 12 14 66.95 Lag 3 Alt y 9 ia ID [Po eh He fe y 9 al 14 70.96 lee 4 3 © ala! (OTS a2 Th iL, IB 14 70.56 2 y 6 I 2 70.45 Lag 4 2 6 lal, 13 ah 61.26 al 6 9 LAL 13 Wh 61.02 ie iM alah 2) ah 60.80 aL 6 AMO aL, is) as 60.39 2 y 8 ala 14 59.46 Lag 5 1 2 4 6 @ i© 68.98 ree 4 6 10 13 68.80 2 4 6 10 12 68.75 aoe iH 6 8 10 68.56 age 4 8 TAL 13 68.54 Lag 6 Tee y 7 8g 76.14 ie) 4 6 8 9 75-64 W 4 h 109 ala 72.36 IL ye 6 7% S99 72.34 al 4 Tt 3 9) Ae 72.06 40 Or °@ €0°0 gE"T 0°0 6S°0 TX a ysnady go doy, i y Z yydeq etaeL re7yeM 00°0 G9°0 ro) Sire! 00°O GS°T T6'T €TX Fu TeAsT TeVemM-TITIS TTed FO eqey 96°E€ 8S°0 98°0 GL°OT T8°0O 99°T 2TX “ath TeAsT Toyem=-TTTIS esty jo aqey og" € 6E°h eH °9 S0°0 OMX) S0°O TIX d Aqtsueq r07eM Ons 00°O Ont TLE 90°T?2 G2°0 OTX A queIIN) estoYssuoT OT’O 6E°H 9T°S Go°) 0g °€T 64°S 6X 7) yoworddy evem go eTsuy Z0°O T2°0 GEE Q2°T 05°98 €8°S QX dy eZoyg OF TeTTereg AqTooTeA PUTM tO QT °T OT’? €6°H 99 °¢2 99°L 1X Jon eLousssO AZTOOTAA PUTM INS Cilane Hey HL°9 HEE gS°0 9X ei aroysug AZTOOTEA PUTM qLt2 +0°0 EL°0 On'T 60°T 99°T GX Siva ssoudeeyg avey 1S°0O GO'T 62°0 9£°O GQ°e G9°T +X Set qZUSTOH oveM ETE gS°0 €9°0O GT°O Ht BES €x Si yqsueT oveM 00°€ eh °O 22°0 00°0 OT °H €9°0O ox a potsed aseM 6E°€T 9S°T gL° OT 69°OT 16° OT OL: TX as] aLOYyse toy JamMoT aTsuy edotg A Jy aTOYse1o7g JaMOT UOTSOTY J2N GB°ge ge * et €9°HS GLE’ 88°19 gL°9S uotzonpsy Ss % TeqOL potted G potsed + potted € potseg g potseg T potszeg spotieg Sey Ag gg UT UOTIONpSY AusodtEeg UOTATSOg Toquvg STOUSPT TEN "Q-T Spotseg Bey soy ‘ATTenpTATpUL ueyeL ‘seTqetsze, JUepusdeput ueeqmog 04 eTQeyngTtz44y Sserenbg jJo umg suoT e499 SLOYseto4 TamoT ye UOTSOAY JeN UT SUOTJONpeYy 4USD Jeg="°6 eTaeL 4 Bel and B22), it is difficult to assess their true influence. It appears, however, that these variables will logically combine with wave steepness (X5, tables B15 and B16) to erode the lower foreshore during falling tide. Variable X11, water density, will affect the rate of sand-grain transport by fluid drag. The variable D, X14, will be of importance in determination of the magnitude of erosion or deposition during rising and falling tides as it affects the infiltration of swashes and the ability of the backwash to transport sand back toward the breaker zone; i.e., the ability to scour the lower foreshore. The significance to net erosion of variable X/, wind velocity offshore, is believed to lie in three areas: 1) in its interlock with @ and V (table B9), noted earlier in the section on longshore-current velocity, 2) in the postulated mechanism whereby offshore wind of suffi- cient magnitude is capable of aiding in the offshore tranport of finer particles thrown into suspension in the breaker zone at the time (lag period 2) that high tide covers the lower foreshore, and 3) in its interlock with water density, wherein offshore winds cause denser water to move shoreward in the lower layers, as mentioned in the section treating (M,),. Angle of wave approach, X9, although interlocked to a degree with V, may influence circulation over the lower foreshore at high tide in such a way that rip eurrents are more effective in removing sand from the foreshore for certain values of Q. The rate of rise of the still-water level (X12) is of impor- tance to net foreshore erosion in lag periods 3 and 6, when the tide is rising. The rate of rise and fall (X13, table B15) of the still water level will determine the time over which any of the other independent variables will be able to act at a given point. Table 10 presents the results of the regression analysis for the strongest variables taken six at a time after redundant variables X3 (L)) and X5 (Ho/L,) have been removed. If the maximum %SS-reduction values for lag periods for correlative tide stages are added together, lag periods 1 and 4, 2 and 5, and 3 and 6 have values of 67.21, 84.82, and 58.09, re- spectively. Thus, the influence of variables acting about the time of high tide is highest on net erosion of the foreshore, as it is measured in this study. And the four variables most influential at high tide will be (table 10) Sp, Uor, @, and D. Summary.--Based upon an analysis involving 12 independent variables, it is possible to say that the combination of 5 variables that shows the most influence on net erosion of the lower foreshore is composed of vari- ables Sr, T, Vor; a, and D. This combination is most-influential about the times of high tide. During times of falling tide, variables Sp, T, Ho and Q@ compose the most-influential combination of variables. The rate of rise of the still-water level, depth to the water table, and beach slope are judged to be the most important variables during time of rising tide. 42 Table Lag 1 Lag 2 Lag 3 Lag 4 Lag 5 Lag 6 10.-The Five Strongest Per Cent Reductions in Net Erosion at Lower Foreshore Stations Sum of Squares Attributable to Combinations of Six Independent Variables (X3 and X5 of The Original 14 Xs Not Used) for Each of Six Lag Periods. Independent Variable Combinations BPRPPEPRP PRPRER PRPPER PRERPPRP PRRBPERB PREPHPPR @ 2 NM Po NM mw hw ~ P iS iS - Je Fr FHS ON ON ON ON ON ON ON ODN ON ON ON OD if SJ 4qaqg Od CO CO C ©0 CO OO \O \O 10 \O \O \O \0 10 \0 \O \O \O \O 10 \O \O \O \O \0 \O \O 10 10 \0 \O 1O 10 ale) 10 1O LO LO 10 10 10 43 at, ali ala ala dal ala aa aa ala aL ala dla 11 all, 12 1h 14 12 14 14 13) ah 12 1h 12 12 12 12 ae 12 14 14 14 14 14 12 14 12 14 12 1h 12 14 1A IS ale Percent Reductions in SS BOTTOM SLOPE AND PARTICLE SIZE IN THE SHOALING-WAVE ZONE Mean Slope in the Shoaling-Wave Zone (S,) Also studied was the interaction of 12 environmental variables with a portion of the beach slope roughly 250 feet beyond the breaker zone. In the case of measurement of the mean slope in the shoaling-wave zone (S,), or average mean grain size on the slope £ ™2)s 7; the measurements of the independent variables were nearly always made exactly ney, 2 US, ere BO hours prior to measurement of the dependent variable. This exact lag- time convention was considered more realistic than the 4-hour-long one used for lower-foreshore measurements, because the measurement of the de- pendent variable was not tied to a dynamic condition, such as obtained at the time of low tide for lower-foreshore measurements. Variables measured.--Recent laboratory work on the equilibrium char- acteristics of sand beaches beyond the breaker zone (cf. Eagleson, Glenn, and Dracup, 1963) commonly utilizes measurement or determination of the following variables, where one is concerned with the mechanics of slope alteration: bottom slope itself at the point in question, particle size on the slope, wave period, wave length, wave height, water depth, specific eravity of the sand particles and the fluid, and kinematic viscosity of the fluid. The variables described in the paragraphs below were measured in this study in an effort to approximate the variables found useful in laboratory studies. The subset of data used earlier in the description of the regression method was taken from the fourth lag period of this section of the study. The segment of the shoaling-wave-zone slope selected for study here was one which was reasonably flat and yet which exhibited a relatively large range of slope values during the periods of observation. As seen in figure 3, the slope was determined from soundings at four stations at the 15th Street pier, each of which was 25 feet apart. The slope was determined by plotting the sounding values for these four, and for several surrounding stations, on cross-section paper and then drawing an estimated mean slope through the four stations by eye. Still-water depth (a) aie the midpoint of this mean slope (fig. 3) was determined for each lag period from precision tide data or direct measurement by sounding line. Mean grain size, M,, determined as explained in appendix A, was obtained for each of the four stations used in slope determination. The four values were aver- aged to give (SE) values for the regression analyses. Wave parameters were again estimated from wave-spectrum-analyzer records furnished by the Coastal Fngineering Research Center. Specific gravity of the particles was assumed constant (2.65, or that of common quartz). Rather than deter- mine kinematic viscosity, we instead determined water density, from pre- cision temperature and salinity measurements (see appendix A for details). In addition to the above variables that are similar or identical to 44 those commonly measured in laboratory studies, we utilized measurements of wind velocity (onshore, offshore, and parallel to shore), angle of wave- front approach (which is not constantly parallel to the shoreline in nature, as it usually is in the laboratory), and tidal-current velocity. Finally, we inserted wave steepness as a variable in the least-squares analysis and ran five lag periods. Thus: Sg = 2 [7 We)eo Bo Bop Hos Holos Uso Uses Up» @ hy p, Cle. L Results.--Table 11 presents the results of the first stage of re- gression analysis. The highest %-SS-reduction is found at lag period 5, perhaps because of the dominance of x4 (Gels) and X/ (Gee) Ine, als} *ELLSO noted in passing that lag periods 2 and 5, which show the highest %-SS-re- ductions are lag periods that coincide with times of high tide for 6 of the 18 slope observations used in the analysis. Discussion.--It is seen in table 11 that when the variables are considered individually, mean grain size and water density (Xisaa arid) generally have the greatest effect on shoaling-wave zone slope, out-rank- ing the wave parameters considerably. The significantly high effect of (M,) s on ise could easily have been predicted from considerations of the mechanics of slope formation and from the known relation between slope and particle size, usually found on foreshore slopes (cf. Bascom, MEHL) A difference exists in the mechanics of alternation of the lower foreshore slope, however, where permeability of the sloping surface assumes con- siderable importance in the process of transportation and deposition. Fluid drag forces are of greater significance to grain movement on the shoaling-wave slope. Water density, a factor of unexpected importance, undoubtedly in- fluences the shoaling-zone slope through its effect on threshold drag velocities and turbulence at the bed. Wave tank experiments on beach slope modification at the Coastal Engineering Research Center, using warm and cold water, have revealed that under constant incoming wave energies the slope modification was more rapid under cold-water conditions. (The final slopes under both conditions were closely similar). The strongest combinations of variables taken six at a time make an interesting study. Referring again to lag periods 2 and 5 (tables B28 and B31), which show the greatest percent reductions in total SS, it is seen that average grain size, wave period, wave steepness and tidal-current velocity, appear in the two combinations of six variables for these lag periods. Absent, but occurring in all of the other strongest combinations of six (tables B27, B29, and B30) is wave height (x4), while wind velocity offshore and parallel to shore appears in two of the other combinations. This may be evidence that the slope in this region is shaped by tidal currents in conjunction with wave characteristics at high tide (lag periods 2 and 5), but more by wave height and winds during half tides or low tide. More observations for lag periods keyed to times of low tide will be need- ed to settle this point. 45 UG? J, (ane) LIS 68°S 01° 0 otX 2) AgTooTaA yuEerM) TeptL 69° 1S GS°6S G9 °6S 98°1S 15° 0S TD fc) Aqq~sueg 107eM 9S°6 T2e°S ee GH*S Gee O—TX y yydeq reyemM-TTT99 2S°0 19°S ie peat, 2G°ch = 9° OT 6X fe) yoeorddy eAeM JO eTsuy TO"0 6'2 66°) 95°), 09'2 x orn arog 04 TeTrerted AATOOTSA PUTM GQ°OT = 99“ 66° L0°0 Oly 1x i) eLoussJO AYTOOTSA PUTM 9z°0 CLS 48°8 9S°0 T6°T gx Een ezoysug AYTOOTeA PUTM SSS « @E°S 99°T SG°OS GO GX ony oH ssoudeeqg ovem TG se GOAN, EI°OG ——-- GIEPE HX °H qUSTSH eAeM org GT LL Ep°rr 1@°O ex Tip ygsueT oveM G6°O JOE 99°T 9S°9 00°0 2X ah potted eveM 26°99 Grea) GWG) SPE) 02° €9 ie © (7i) su0Z eAeM-SuTTeoys aZTS uUTeIy uesW eon-- ----- ----- ----- wo--- i aC SuOZ sASM=SUTTeoUS eTsuy edots €T° 16 90°26 28 °6 26 °S6 GT°&6 uotyonpey ss % TetOL G potted 4 potted € potted g potseg T potaeg Spotdeg Seq Ag ag ut uoTIONpPeY queosZeg uoTITSOg Toquiks aTqetre "G-T Spotted sey coz ‘ATTenpTATpuL ueyey, ‘seTqet ze, qjUepusedepult SATEMT, 09 SeTQeIAnGT744y Serenbg Jo umg sdoTg suogy aAeM-SuTTeoug UT UOCTJONPaYy 4UEeDO Jeg="TT eTaeL 46 Removing the contribution of the dominant variable (X1) from the analysis (tables B32-B36) reveals the new dominance of another geometric variable, X10 (water depth), which may be considered as a mediator for the various process elements in the environment. Thus, ignoring X10, it is seen that lag periods 2 and 5 are still the most-influential on shoaling-zone slope, when the variables are taken six at a time. Because variables X3 and X5 exhibit data redundancy with those of X2 and X4, as explained earlier, it is seen that wave period is the important variable for measurement times weighted with those of high- tide measurements (lag periods 2 and 5; tables B33 and B36), while wave height (X4) is most important in altering the slope for measurement times weighted with those of low-tide measurements (tables B32, B34, B35). This result of the least-squares analysis, if truly representative of high and low-tide conditions, would be in keeping with the knowledge that waves of a given period and height will exert a greater effect at a fixed point on the bottom when the tide is low than when it is high (cf. Inman and Nasu, IHG, We 30). The wave-height effect might then be diminished at high tide to the extent that the only effect that is felt is the effect of wave length (wave period) as it influences drag over the bottom. A study of the weakest combinations of the variables taken six at a time shows (tables B37-B41) that wave steepness and tidal-current veloc- ity, which are both present in four of the strongest combinations for the five lag periods are also present in these weakest combinations. The only variables that are present in the strongest combinations of six at a time (tables B27-B31), and present in none of the weakest combinations for the corresponding lag periods, are mean size (X1), wave length (X3), wave height (X4), water depth (X10), and water density (X11). Finally, reference to frequency tables B42-Bh46, for variables con- sidered in combinations of six at a time, reveals that the distributions of numbers of combinations by %-SS-reduction classes are polymodal for lag periods 1, 3, 4, and 5, but essentially unimodal for lag period 2. Lag periods 3 and 4 exhibit four modes, while 1 and 5 exhibit three each. One inference is that sub-groups of variables that influence the slope to dissimilar degrees are somewhat segregated during lag periods 3 and }. These sub-groups, however, if they are in fact semi-discrete in a physical sense, each seem to influence the shoaling-wave-zone slope to about the same degree during lag period 2. If true, it might be said that the com- (gama caloMN NG), Xo XO) KelOnmvandexale) (railte B33) is the most-significant combination of variables to influence the foreshore slope and that this influence is exerted between 4 and 8 hours prior to slope measurement. Additional work with such frequency tables is planned. They are presented in appendix B for the interested reader and for future reference. Summary.--The bottom slope of the beach at 15th Street, some 250- feet seaward of the breaker zone, may be thought of as being controlled mostly by the following combination of six variables: average mean grain size of the bottom materials, wave period, wave length, wave steepness, 47 water depth, and tidal-current velocity. This combination of variables 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. This delay may reflect the influence exerted on the slope during the time of the previous two high tides. Wave height, angle of wave approach, and water density become more influential through delays in time amounting to O-4, 8-12, or 12-16 hours, and coinciding with or approaching times of low tide. Average Mean Grain Size in the Shoaling-wave Zone (M,) Variables measured.--The measurements used in this analysis were the same ones as were used in the previous analysis for Sg, average mean grain size being interchanged with S, as the dependent variable. Thus: (M) nat (Ss, T, lo, Hos Ho/Lg, U on? Voges Up, a, hy Pp, Oe =») The analysis was run for five periods, as was done for Sg. Results.--Table 12 presents the results of the first stage of the analysis where it is seen that the most-influential lag period is number 3, which occurs 8-12 hours prior to measurement of the independent variable. Lag periods 2, 4, and 5 are of about equal influence, while lag period 1 is the least influential. Variable Xl is seen to be among the dominate variables when they are taken individually, and this is to be expected in- asmuch as a high interdependence between S, and (My) s was noted in the pre- vious analysis. Because of the possible masking influence of this dimen- sionless variable, Xl, two sets of tables have been prepared, both of which show the strongest combination of independent variables influencing (M, ) One set (tables B7- B51) includes 411 12 Xs; the other set (tables B52- B56) does not include Xl. Finally, a set of tables for the combination of independent variables showing the weakest influence (tables B57-B61), and a set of frequency tables (tables B62-B66) have been prepared. s* Discussion.--Turning to table B49, for lag period 3, one sees that the most-influential combination of variables taken six at a time consists of Se aE lile/dbas UO ” Q@, and C. Variables ay Ale tp and C also enter into the weakest combination, when they combine with Lo, Ujp, and h (table B59). Recognizing the importance of slope in its influence on particle-sizes transported, we turn our attention to the other variables in this strongest combination of six variables. The reaaer is reminded that T is redundant with Telesis through the relationship Lo = 5.12Te, which relationship was used in this study for obtaining L,. At any rate, the fluid forces at the water-sediment interface that are induced by varying wave periods and wave steepness will tend to move grains of various sizes over the shoaling- zone slope. Superimposed upon this mass transport by the wave-drift cur- rent will be the transport induced by tidal and wind-driven currents. For the bottom slope in question (fig. 3), the major current will be the tidal 48 QS ° On Th 8d €o°S HE "tS Ted? dh oTX 0) Aq4TOOTEA JUetINO-TePTL gE*HT Sore Goes «UOC C« "ULE TIX d Aytsueq 107eM 99°T E€"0 SOx OL°O 9g°T OLX Yy yydeq 107eM-TITIS 16°? 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BH €£°09 0Z* €9 aK aS) QUOT SeABM=SUTTEOUS eTsuy edoTs SESS eooon eocee= cene- ----- k = (2M) SUZ aAeM-SuTTeoys aZzta uTery UesW 6g°€6 g9°h6 SL°g6 Ot°€6 = ST “98 uoTzonpey SS % TedOL G potted + potted € potted g potsed T potsed Spotdzeg Seq Ag ga UT UOTZONpeYy yused.TEg UOT TSOg Toquxts STAeTIe/\ sab SOE ke en eee Pes Ente as ee Ea ee ee ee ee ee ee eS eee "G-[ Spotdeg Sey Jog “AT TeNPTATPUT ueyey, ‘seTqetsze, Juepuedspurt eATEM], 09 eTaegnqTz49y Sezenbg jo umg (eucZ eAeM=SUTTeOYS) ezZTS UTerH UeeW UT UOTJONpSY JUEeD 4ed-"*cT PTAPL 49 current and it will react a complex way with wind-generated bottom currénts and wave-drift currents. Thus, it is possible to conceive of a situation in which the angle of wave approach, the direction and velocity of the wind, and the direction and velocity of the tidal current at a given time will interact to reinforce or impede one another. A wind moving opposite in direction to a tidal current, for example, will produce an increase in the tidal-current velocity near the bottom (Reid, 1957) and, should the wave fronts be traveling in the direction of flow of the tidal current, the net current velocity could have a velocity up to about 16 percent (cf. Collins, 1964) greater than the simple algebraic summation of the wave- induced current and the reinforcing tidal current. As noted by Collins (1964, p. 1051), "the effects of even very small currents on the mass transport ios fine material / by waves could be very large." D. R. Tuck (oral communication) has studied the interrelationships of wind velocity and direction, angle of wave approach, and tidal-current velocity on the average mean-grain-size values for the beach slope in question. In instances where the wave-drift current was reinforced by the tidal current, water velocities of 21-29 cm/sec were obtained near the bed. Mean particle sizes actually observed at the bed under these circustances, and for the slopes in effect at the time, were within the range of particle sizes predicted by theory as being moved by the net current. ey Thus, there is every reason to believe that a combination such as Sg, I, Ho/Lo, Up, G, and C, mentioned at the outset as being the most- influential Pemdmacson of six variables in the most-influential lag period, is in fact a valid combination for this area of the beach at Virginia Beach. Table B54 shows that the most-influential combination (Xl ignored for 6 Xs at a time) includes: T, Lo, Up, a, h, and p. Variables X10 and X11 take the place of variables X5 and X12 of the analysis in which X1 was included. Water depth merely acts to mediate the various process elements, while water density will affect fluid drag at the bed. As seen in all of tables B52-B56, water density, X1l, is a dominant variable in the strong- est combinations. Of further and more general significance is the rather persistent contribution of variables X6, X7, X8, and X11 and X12 in the strongest combinations of Xs taken six at a time (tables B52-B56). Wind velocities onshore (X6) and offshore (X7) have also been seen to be of importance in net erosion on the lower foreshore. In that section of the study it was postulated that when the offshore wind reaches a certain velocity it is important in producing a weak surface current beyond the breaker zone that is capable of transporting fine particles put into suspension in the break- ers out into the shoaling-wave zone. The onshore wind, however, may pro- duce a seaward return flow of water on the bottom that will transport somewhat coarser particles out of the breaker zone and onto the slope in the shoaling-wave zone. Winds parallel to shore will probably interact significantly with tidal currents, which are generally parallel to the shoreline in the area of investigation, and augment or decrease the current velocities in the lower layers. 50 Also of considerable significance in consideration of the role of onshore and offshore winds is their interlock with water density, X1l. If the wind blows offshore long enough and strong enough, the wind stress at the water surface produces a seaward movement of the upper layers of the ocean and a shoreward return flow in the lower layers. (The procedure is more or less reversed with an onshore wind.) This sort of "pseudo-upwell- ing" results in the shoreward movement of colder, more-saline water in the summer months and relatively warmer, more-saline water in the winter months. This type of turnover of the continental-shelf waters has been undergoing documentation by the Virginia Institute of Marine Science for over two years, using sea-bed drifters and sea-surface drift bottles. In addition, the temperature and salinity measurements made at the 15th Street pier correlate well with observed offshore and onshore wind movements. Thus, there is a clear interlock between Upor, Upon» afd P. And water density will clearly influence fluid drag velocities and turbulence at the bed and, thereby, the particle-size distribution in the shoaling-wave zone. Summary.--Based upon an analysis involving 12 independent variables, average mean size on the shoaling-zone slope at 15th Street, 250 feet beyond the breakers, is most-influenced by the combination of variables (taken arbitrarily in a combination of six) composed of mean slope, wave period, wave steepness, wind velocity parallel to shore, angle of wave approach, and tidal-current velocity. This combination of variables is most influential on (Mz), after a lag in time of 8-12 hours. Mean slope is the most-dominant independent variable at all times. When removed from the analysis, it is in the main replaced by water density, and this variable is found to be of considerable importance in all of the strong combinations of variables. The importance of wind velocity onshore and 6@fshore in the analyses is found in their interlock with water density as it will change when shelf water in the lower layers is moved to the shore during offshore winds and out to sea under onshore winds. The shelf water in the upper and lower layers is stratified as to density, especially during the summer months, and the density affects the sizes of particles moved. Wind velocity parallel to shore is significant in its ability to reinforce or decrease tidal-current velocities near the bottom, because tidal currents in the study area flow generally parallel to shore. The angle of wave approach is probably of significance as it interlocks with tidal-current velocity. Wave-drift and tidal currents interlock in their effect on particle movement at the bed. FUTURE STUDY A major aim of any search for significant interactions in a natural system is to provide a basis upon which more formal models may be erected. The authors are continuing their efforts in this direction, using the pre- sent data set. Additional techniques (for example, factor analysis and discriminant functions) will be examined. The data set will also be SI augmented with additional field measurements in the study area. Develop- ment of relatively informal "process-response" models is underway, while construction of stochastic process (simulation) models will be attemped in the near future. Expression and analysis of the present variables in non-dimensional form will also, it is hoped, lead to better integration of field and wave-tank studies of the beach-ocean-atmoshpere system. 52 REFERENCES Bascom, W. N., 1951, The relationship between sand size and beach face slope: Trans, Am. Geoph. Union, v. 32, p. 866-874. Brebner, A. and J. W. Kamphuis, 1963, Model tests on the relationship between deep-water wave characteristics and longshore currents: Dept. Civ. Eng., Queen's Univ., Kingston, Ontario, Canada, Rpt. No. 31, 25 pp. Bruun, P., 1963, Longshore currents and longshore troughs: Jour. Geophys. Res., v. 68, p. 1065-1078... Collins, J. I., 1964, The effect of currents on the mass transport of progressive water waves: Jour. Geophys. Res., v. 69, p. 1051-1056. Eagleson, P. S., B. Glenne and J, A. Dracup, 1963, Equilibrium character- istics of sand beaches: Jour. Hydraul, Div. Amer. Soc. Civ. Eng. Proc., v. 89, p. 35-57 Harrison, W..,, M. L. Brehmer and R. B. Stone, 1964, Nearshore tidal and non-tidal currents at Virginia Beach, Virginia, U. S. Army Coastal Engineering Research Center Tech. Memo. No. 5, 20 pp. Harrison, W. and W. Wilson, 1964, Development of a method for numerical calculation of wave refraction: U. S. Army Coastal Engineering Research Center Tech. Memo. No. 6, 64 pp. Harrison, W., W. C. Krumbein and W. Wilson, 1964, Sedimentation at an inlet entrance: Rudee Inlet, Virginia Beach, Virginia, U. S. Army Coastal Engineering Research Center Tech. Memo (in press). Harrison, W. and R. Morales-Alamo, 1964, Dynamic properties of immersed sand at Virginia Beach, Virginia: U. S. Army Coastal Engineering Research Center Tech. Memo. (in press). Harrison, W. and K. A, Wagner, 1964, Beach changes at Virginia Beach, Virginia: U. S. Army Coastal Engineering Research Center, Misc, Paper (in press). Helle, J. R., 1958, Surf statistics for the coasts of the United States: U. S, Army, Corps of Engineers, Beach Erosion Board, Tech. Memo, 108, 22 pp. Inman, D, L. and N. Nasu, 1956, Orbital velocity associated with wave action near the breaker zone: U.S. Army, Corps of Engineers, Beach Erosion Board, Tech. Memo. 79, 43 pp. 53 Inman, D. L. and R. A. Bagnold, 1963, "Littoral Processes" in Hill, M.N., The Sea: v. 3, pp. 529-553 (Interscience Publishers, New York). Inter-Agency Comm, Water Res., 1957, Measurement and analysis of sediment loads in streams: Subcomm. on Sedimentation, Rpt. 12, U. S. Gov't. Printing Office, Washington, D. C., 55 pp. Kemeny, J. G., J. L. Snell and G. L. Thompson, 1958, Introduction to finite mathematics: Prentice-Hall, Englewood Cliffs, N.J., 372 pp. King, C. A. M., 1959, Beaches and coasts; Edward Arnold, London, 403 pp. Krumbein, W. C., 1961, The analysis of observational data from natural beaches: U. S. Army Corps of Engineers, Beach Erosion Board, Tech, Memo. 130, 59 pp. Krumbein, W. C. and L. L. Sloss, 1953, Stratigraphy and sedimentation: Freeman, San Francisco, 660 pp. Krumbein, W. C., B. Benson and W. B. Hempkins, 1964, A computer program for sequential linear multiple regression: ONR Tech. Rpt., Contract Nonr-1228 (26), ONR Task No. 389-135, Northwestern University, Dept. of Geology (in press). Miller, R. G., 1958, The screening procedure, Studies in statistical weather prediction: Final Rpt., Contract No. AF(604)-1590, Hartford, Conn., Travelers Weather Research Center, p. 86-95. Panofsky, H. A. and G. W. Brier, 1958, Some applications of statistics to meteorology: Univ. Park, Pa., The Pennsylvania State University, 224 pp. Putnam, J. A., W. H. Munk and M, A. Traylor, 1949, The prediction of long- shore currents: Trans. Amer. Geophys. Union, v. 30, p. 337-345. Rao, C. R., 1952, Advanced statistical methods in biometric research: Wiley, New York, 390 pp. Reid, R. O., 1957, Modification of the quadratic bottom-stress law for turbulent channel flow in the presence of surface wind stress: U. S. Army, Corps of Engineers, Beach Erosion Board, Tech. Memo. 93 33s pps Stahler, A. N., 1964, Tidal cycle of changes in an equilibrium beach, Sandy Hook, New Jersey: Tech. Rpt. 4, ONR Contract 266, (68), Dept. of Geology, Coiumbia Univ., New York, 46 pp. U. S. Congress, 1953, Virginia Beach, Va., beach erosion control study: 83rd Congress, 1st Session, House Doc. 186, 45 pp. 54 U. S. Navy Hydrographic Office, 1962, Tables for sea water. density: H. O. Publ. No. 615, 265 pp. Whitten, E. H. T., 1964, Process-response models in geology: Geol. Soc. Amer. Bull., v.. 75, p. 455-464. Zeigler, J. M. and Barbara Gill, 1959, Tables and graphs for the settling- velocity of quartz in water, above the range of Stokes' Law: Woods Hole Oceanographic Inst. Ref. No. 59-36, 13 pp. Zeigler, J. M., G. G. Whitney and C. R. Hayes, 1960, Woods Hole rapid sediment analyzer: Jour. Sed. Petrology, v. 30, p. 490-495. Zeigler, J. M., and S. D. Tuttle, 1961, Beach changes based on daily measurements of four Cape Cod beaches: Jour. Geol. v. 69, p. 583-599, 55 LIST OF FIGURES Maps showing area of investigation and transects at which measurements were taken. Diagram showing times of measurement of process elements (Py, Possess Pi 5) and response elements (R), Ro,......-- - Ros) Diagram showing average beach profiles at the 15th-Street pier (north- ern transect) and Camp Pendleton property line (southern transect), and segments of the beach for which slope and mean grain size were deter- mined. Diagram showing envelopes of altitudinal values at lower-foreshore stations for 25 profiles each at the 15th-Street pier and Camp Pendleton line during the months of June and July. 56 APPENDIX A Descriptions of the variables used in this study and explanations of how they were measured are given in this appendix, together with the ranges in the values that were employed in the regression analyses. Addi- tional details of the measurement techniques may be found in two other re- ports in this series on Virginia Beach (Harrison and Wagner, 1964; Harrison and Morales-Alamo, 1964). 57 Ba Depth of Water at Wave Breaking The average vertical distance, in feet and hundredths, from the water surface immediately in front of a breaking wave to the bottom. This is an average of ten measurements, recorded four times each day. Values used ranged from 0.35 to 3.80 feet. CS Con © Tidal-current Velocity, C3; Opposed to Longshore Current, Co; And in Same Direction as Longshore Current, Cg. The mean tidal-current velocity was measured one meter above the bottom at a point 850 feet from shore. A Price current meter was used, and measured or interpolated values were expressed to hundredths of feet ‘per second. The respective ranges of values used were: C=0.00 to 0.68 ft/sec, Co=0.00 to 0.66 ft/sec, and C,=0.00 to 0.36 ft/sec. Some tidal-current- velocity values were estimated from a relationship established between the tidal range and the tidal-current velocity at the point. D Depth of Water Table at Top of Uprush The depth to the water table at the top of the swash line was measured to feet and tenths four times daily. Values used ranged from 0.01 to 1.83 feet. h Still-water Depth The still-water depth was determined from tide-gage data. The values used are for the depth from the still-water surface to the mid-point of the slope (fig. 3) in the shoaling-wave zone that was investigated for changes in inclination and sediment size. Values used in the analyses ranged from 10.6 to 16.1 feet. Hy, Height of Breaking Wave The breaker height was measured directly with a graduated rod and was taken as the average trough-to-crest distance of ten successive break- ing waves. Breaker heights were measured four times daily. Values used in the analyses ranged from 0.69 to 4.50 feet. 58 Ho Deep-water Wave Height (Significant) Wave-height walues were determined from wave-spectrum-analyzer re- cords furnished by the Soastal Engineering Research Center, as they were obtained from the relay-type wave gage in 20 feet of water at the end of the 15th Street pier. The peak value on the linear-average curve for wave heights of the dominant wave train was multiplied by 2.22. This was done because J. M. Caldwell had found (written communication, 1963) the follow- ing relationship in a study of 92 simultaneous wave recordings made by both magnetic-tape and paper-tape methods: Average height on analyzer record _ 45 Significant height on chart record Height values were converted to deep-water ones by entering tables. Thus conversion to deep-water wave height values did not involve consideration of wave refraction or special shoaling effectS. Values for deep-water significant wave heights ranged fran 0.56 to 12.97 feet. The absolute validity of the wave-height values so determined is not of importance, as all that was needed was a consistently objectively-determined measure of this variable. Ho/Lo Deep-water Wave Steepness Values of wave steepness, expressed in terms of the ratio for deep- water waves Ho/Lo, ranged from 0.00100 to 0.07226. Je Net Deposition at Lower Foreshore Stations in Previous 24.5-hour Period Net deposition was determined from measured changes in altitude of stations occupied on successive days at low tide. Net deposition was measured over a distance of 125 feet, or at six stations (fig. 3, 4) at 15th Street and at five stations (Fig. 4) at the Camp Pendleton property line. Values used ranged from 0.00 to 1.50 feet. Ke Net Erosion at Lower Foreshore Stations in Previous 24.5-hour Period The net erosion was determined in a like manner to that of the increment of deposition, Jp. Values used ranged from 0.00 to 1.30 feet. 59 L fe) Wave Length in Deep Water Values for wave length (expressed as a deep-water value) ranged from 49.20 to 989.24 feet. (Mz) 5 Average Mean Nominal Grain Diameter Over Bottom in Shoaling-wave Zone Mean-size values were averaged for four samples taken 25 feet apart in the shoaling-wave zone (fig. 3) where MLW depths ranged between 10.1 and 14.1 feet. The sampling device, a pipe dredge, permitted taking an integrated sample over a 15-foot distance parallel to shore, and so the average value for the four samples is presumed to represent the average mean size of a 15 x 100 foot area of the bottom. Individual (M,) 5 values were determined using a Woods Hole Rapid Sand Analyzer (Zeigler, and others, 1960; Zeigler and Gill, ng5@))e and the procedure outlined in Harrison and Morales-Alamo (1964). The statistics used to estimate the mean nominal diameters were 12) ap 12 dp JP) O O Mo = e ? ee (for summer data) 5 12) ap Je) cr iP) ap dee Pp Ie 10 O O NO = 2 2 (ec oe (for winter data) D Average (M,) values used in the analyses ranged form 0.234 to 0.843 mm. R Range of Tide Over One Tidal Cycle The measured tidal range at 15th Street as taken from records furnish= ed by the U. S. Coast and Geodetic Survey. Values used ranged between ~* 2.4 and 4.7 feet. This was the only independent variable whose values were held constant over several lag periods, a practice which is to be discour- aged. Se Mean Slope Over Lower Foreshore of Beach The mean slope was determined for a 200-foot distance of the lower foreshore (fig. 3) from measurements at nine stations that were made daily 60 at low tide. This measurement was for Sp taken_as a dependent variable. Values ranged from 1.40 to 4.55 degrees. When Sp was taken as an independ- ent variable, the slope was measured over the distances shown on figure }. Bs Mean Slope of Beach Over Inner Portion of Shoaling-wave Zone The mean slope was determined for a 100-foot length of beach in the shoaling-wave zone (fig. 3) where MIW depths ranged between 10.1 and 14.1 feet. Measurements of the altitude of five stations were made daily at low tide in the summer, and every four hours in the winter. Mean slope values used in the analyses ranged between 0.50 and 2.22 degrees. aly Wave Period Wave period was determined from wave-spectrum-analyzer records fur- nished by the Coastal Engineering Research Center. The period used was the one corresponding to the peak value on the linear-average curve of wave heights for the dominant wave train. (Period values thus obtained were nearly always larger than those obtained in a "significant-wave" chart analysis.) Values used ranged from 3.10 to 13.90 seconds. Une Us Upp U U wy Us Mean Wind Velocity Against the Direction of Longshore Current, U8 In An Offshore Direction, Vor; In An Onshore Direction, U,,3 Parallel to Shore Up: And in the Same Direction as the Longshore Current, Ugo Mean wind velocities and directions were recorded at 2-hour intervals at the Cape Henry weather station (fig. 1B), some 7.5 miles north of the study area. The deviations in wind directions permitted when describing the direction as onshore, offshore, and so on, are given below: (+ LO of wind vector directed opposite to longshore- eurrent flow) Uoe (+ 80° of wind vector perpendicular to shore and in an offshore direction) G 80° of wind vector perpendicular to shore and in an onshore direction) 6 U (Ge 10° of wind vector directed in either direction along trend of shoreline) (+ iO of wind vector directed parallel to longshore- current flow, and in the same direction) values ranged from 0.00 to 16.00 M.P.H. Uo¢ values ranged from 0.00 to 34.00 M.P.H. values ranged from 0.00 to 26.75 M.P.H. U. values ranged from 0.00 to 17.50 M.P.H. Us values ranged from 0.00 to 18.00 M.P.H. Vv Mean Velocity of Longshore Current The current velocity and direction was measured four times daily by timing the movement of 2 or 3 fluorescene dye patches moving over (usually) a 100-foot distance. Values used in the analyses are absolute values, a zero-velocity value representing either a no-current condition or a rip- current condition. ‘Values used ranged from 0.00 to 3.20 feet per second. Qa Angle of Wave Approach The angle of wave-front approach of the dominant wave train as measured with a pelorus in a zone 1000 to 1300 feet from shore in water depths of 20-26 feet. Measurements were made 4 times daily. Values used in this study were absolute values only and ranged between 2 and 75 degrees. Tye Rate of Rise of Still-water Level The instantaneous rate of rise of the tide; in hundredths of feet per hour, as determined from observed times and magnitudes of high and low water. Values used ranged from 0.00 to 0.67 feet per hour, and were computed for the mid-points of the 6 lag periods. Because times of low tide in nature did not always coincide with anticipated times (CURE ries 2) che Nr and Ne computations sometimes resulted in values for rising tide during a lag period when it should only have been falling, or vice versa. 62 Te Rate of Fall of Still-water Level The instantaneous rate of fall of the tide, JOSIP Inoehe,, low water. in hundredths of feet as determined from observed times and magnitudes of high and Values used ranged from 0.00 to 0.60 feet per hour, and were computed for the mid-points of the 6 lag periods. fe) Water Density The "sigmatee" values for the sea water as determined (U. S. Navy Hydro. Office, 1962) from temperature (+ 0.05°C) and salinity (+ 0.02 °/oo) data once daily at noon time in the surf zone, for the summer measurements, and three times daily in the shoaling-wave zone, for the winter measure- ments. 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Generally, the significance of the improvement attained at each step of the screening is tested and the screening discontinued when the amount of improvement is found not to be significant. Near that point the addi- tion of many more predictors usually lowers the predictive ability of the system on independent data. As pointed out by Panofsky and Brier (1958), however, objective standard significance tests may be misleading on data such as those of this study, because the underlying assumptions may be violated. The predictors used here are certainly interdependent in time and space. Often the most practical and convincing test of significance can be an application of the result to an independent set of data. This will be the method used by the present investigators for "significance" tests in future work with predictor equations for data of this sort. RESULTS Predictors Selected As mentioned earlier, variables X3 and X5 (Lg and Ho/Lo) were not used when screening for predictor equations, because of their redundance with each other and with T (wave period). The results of the screening procedure for 1 through 4 predictors are given in table Al, where the order of the first four selected predictors is shown along with the lag and correlation coefficients. For example, in run 1 the first predictor selected by screen- ing was (T) with a lag of 0-4 hours and a correlation of 0.47. The second predictor selected CHE, with lag of 0-4 hours) increases the correlation to 0.64. Four predictors bring the correlation to 0.72. Predictor equations, each containing the four predictors of table Al, are presented below for the five screening runs. 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