In A. McLachlan and T. Erasmus, (eds~), Sandy Beaches as Eco­ 63 sy~tems, D.W. Junk Publishers, ' The Hague, p. 63-85, 1983. BEACH CHANGES ON COASTS lHTH DIFFERENT WAVE CLHIATES D. G. AUBREY (Department of Geology and Geophysics, Woods Hole Oceanographic Institution) severe wave climate) and the lowest / vari abi 1i ty along protected coasts (1 east severe wave climate). All open coast ,r-· locations studied had a seasonal variability SYNOPSIS "'".,. __..-, Seasonal and longer-term oeach which accounted for at least50%of th~ beach variability is quantified for seven U.S. variability. Protected coastal locations beaches exposed to widely varying wave had less pronounced seasonal signatures. climates. One U.S. west coast location These seasonal and aseasonal beach responses (southern California) and six U.S. east mirror corresponding seasonality (or lack coast locations (from North Carolina to thereof) in wave and storm climates. The Massachusetts) form the basis of this study re-emphasizes the need for careful study. Wave exposure varies from complete measurement or estimation of coastal wave exposure to open ocean waves, to partly climate to enable predictive modelling of sheltered locations, and finally to nearly shorelin-e behaviour, and discusses different complete sheltering where locally-generated analysis techniques for analyzing changes in waves dominate. Beach response was beach profiles through time. documented with beach profiles distributeq INTRODUCTION along each of the seven coastal locations, Quantification of spatial and temporal spanning a minimum of 'five years of observa­ scales of beach change is vital to a wide tion. Frequency of measurement was at least variety of scientific and engineering once per month, with periods of more intense investigations of nearshore environments. weekly sampling lasti~g for up to two years Vertical elevation changes of 2.5 metres, (southern California location). Wave climate mean shoreline transgressions on the order was either measured directly or estimated of 50 metres, and volume changes on the 3 from hindcast and/or compilations of ship order of 102 m /m of beach length can observations. Consequently, wave informa­ occur on time scales of hours, drastically tion varies in detail from joint statistics altering the physical and biological of wave height, frequency, and direction, to characteristics of beaches (Fig. 1). compilations of local storm history (and Intertidal benthic communities must be able hence inferred wave behaviour). Magnitude to respond quickly and efficiently to these of annual beach variability ranged from 3.3 m3 profile readjustments, since habitat, oxygen per metre of beach to 0.2 m3 per metre of levels, nutrient retention, and other beach, with the greatest variability in environmental factors can be significantly regions exposed to open ocean waves (most ; 64 storms in February, 1980, caused marked RANGE 2 erosion along the beaches in Santa Barbara, exposing underlying beach material which had , 28 80 2 24 80 not been disturbed in the preceding decade ..._ (Fig. 1). During the later stages of the .,"'"' ~ storm, an oil-impregnated horizon which had , .... "' ~ r-------------~-----------.~~/~---10 ~ been deposited during February, 1969 was .. § .... _, Gj exposed, and eroded from the beachface. In t this instance, the residence time of the oil ~-- -2 was of the order of 10 years, in contrast to 90 60 30 0 the residence time of months for oil in OFFSHORE .DISTANCE (m) beach sands emanating from local, natural oil seeps in the Santa Barbara Channel. Presence of a persistent hydrocarbon horizon 1) Beach erosion resulting from a series of limits the vertical mobility of biota, and storms battering Santa Barbara, affects the transport of nutrients and California, in February, 1980, causing oxygen through normally permeable beach vertical cuts in the beach of up to 2.5 sands. metres, and hori zonta 1 . beach retreat of The importance of beach variability in up to 60 metres. engineering studies is well-known. Seasonal and aseasonal beach changes can affect the altered in a short time (Steele, Munro and lifetime of coastal structures, and the Giese, 1970; Parr, Diener, and Lacy, 1978). design of beach protection devices. Proper The degree of seasonality in these changes set-back requirements for near-shoreline similarly may affect the viability of development is dependent on long-term trends nearshore benthic communities, since the in coastal change as well as natural seasonal timing of beach changes interacts with the fluctuations in beach level.- Finally, developmental stage of the benthic quantification of beach variability and its community. The seasonality and magnitude of statistical relationship to driving forces beach changes also pl,ay a direct role in can serve as useful input to nearshore retention of hydrocarbons in beach sands sediment transport models, particularly as a with subsequent impact on biota, a test of variation in beach response as a consideration in many beaches exposed to function of different sediment types (grain naturally-occurri,ng or man-induced size, sorting). Empirical guidance for hydrocarbons, in the shallow near.shore. modellers can also be provided through well­ Rapid beach-changes of large magnitude will constructed statistical studies of driving help rid the beaches of oil naturally; force/beach response, when constructed using longer-liyed_b~ach_hydrocarbons may _limit insight gleaned from dynamical considera- · · benthic diversi;ty or density. An example of tions (e.g., Aubrey et al., 1980). this longer time, scale for hydrocarbon The basic problem addressed here ii the residence was observed along beaches in quantification of seasonal ,and aseasonal Santa Barbara, California, by the author patterns of beach change along coasts with (unpublished data). A series of major 65 different wave climates, and for beaches Analysis procedures for most of these with different sediment characteristics. studies have varied considerably, with Rigorous statistical technique~ for little uniformity in treatment of the data. quantifying these changes must be developed Consequently we are left with many to allow for meaningful comparison of beach observations of beach change, of highly response at different sites, providing a variable quality, and no capability for " statistical basis for defining differences readily comparing changes at one location in beach behaviour. The ultimate goal is to with changes at another location. The develop a capability for predicting beach resulting lack of comparison leaves us with changes on many spatial and time scales, but a disturbing inability to synthesize these this goal is to be achieved only with data into a meaningful set of observations, careful statistical methods combined with which might provide valuable insight into dynamical (both analytical and numerical l causes and patterns of beach variability. modelling. Work reported in this paper represents Observation of changes in beach planform an attempt to take data from different have been made for the past century, and coasts of the United States,. exposed to relations between these beach changes and widely different wave climates, with the driving forces postulated. For instance, different sediment types, and synthesize it Davies (1964) related beach characteristics in a rigorous fashion to allow quantitative to global patterns of waves (swell coasts, intercomparison of magnitude of seasonal and storm coasts, and protected coasts), using aseasonal beach changes at these different not direct measurement but compilations of locations. The work represents a plea for winds and wave behaviour observed from ships some uniformity in analyzing beach data to and shore. Davies (1964} pointed out that provide results useful to a variety of the major drawback in obtaining statistical disciplines studying this active nearshore relationships between beach behaviour and environment. driving forces is lack of knowledge of the STUDY SITES driving forces, specifically wave activity •. Seven locations were selected for this.· This is still true at the present, although study (Fig. 2), six along the U.S. east progress has been made in the last couple of coast (Fig. 3) and one on the U.S. west decades in measuring nearshore wave coast (Fig. 4). The beaches span a spectrum characteristics (e.g., Pawka et al., 1976; of grain sizes, and range from open ocean Seymour and Sessions, 1976; Thompson, 1977; beaches, to those partly sheltered by Seymour, 1979). offshore shoals and islands, to completely Beach profile monitoring programmes sheltered beaches. A brief description of generally have had the following character­ each study site follows. istics: limited duration of sampling;, Torrey Pines, California: This southern inadequate sample frequency; inadequate California site (Fig. 4) is a long sandy. spatial coverage, particularly for beaches beach, extending for more than 40 km with no with much longshore variability; inadequate. man-made structures to impede longshore.sand· spatial density of sampling; and poor transport. The beach profile locations are documentation of the driving forces. backed by 100m high sea cliffs, composed of. 66 LOCATIONS OF STUDY AREAS : NEVADA ' ' ' ' ' ' ' .... CALIFORNIA ' ·.··. ' ··:: :::: .. ·.! N 2) Location map for seven beach study sites distributed along the U.S. east coast t 0 500 1000 (6) and the U.S. west coast (1). ... I .. METERS 0 2000 4000 FEET OEPTH IN FEET 4) Location map for Torrey Pines, California, with profile·