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Internal Swash and Surf (Water Movements/Marine Sediments/Continental Shelf) K Proc. Nat. Acad. Sci. USA Vol. 70, No. 8, pp. 2379-2380, August 1973 Internal Swash and Surf (water movements/marine sediments/continental shelf) K. 0. EMERY AND C. G. GUNNERSON Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543; and Regional Office, International Joint Commission, Windsor, Ontario Contributed by K. 0. Emery, May 29, 1973 ABSTRACT 114 temperature sections, each containing about 13 km/hr over deep water and of less than 2 km/hr an average of 10 bathythermograms, show that internal across shelf depths. Plots show that the wave of the waves commonly become altered on crossing the shoaling length bottom of the continental shelf in Santa Monica Bay, internal waves ranged from 3 to 24 km and averaged 9 km. Calif. The alteration takes the form of internal swash (in The maximum amplitude of isotherms in each section ranged which the lower isotherms reach farther landward than from 5 to 40 m and averaged 14 m. Both the lengths and the would be expected from their depth above the outer shelf amplitudes have approximately log-normal distributions. or above deeper bottom) and of internal surf (that is, de- noted by temperature inversions, isolated boluses of colder The nearly symmetrical forms of most of the internal waves water at the bottom, and complex short wave-length varia- above the outer shelf were distorted in the inner shelf where tions of isotherms). pronounced shoaling of the bottom occurred. The depth at which this change in form of isotherms took place was mainly Evidence of internal swash and surf produced by internal between 25 and 50 m. Examination of the 114 thermal sec- waves crossing the continental shelf was found in the 1950s tions showed that the isotherms could be grouped into seven during studies of water characteristics needed for the evalua- main classes (Fig. 3). Classes 1 and 2 indicate no discernable tion of potential effects of sewage disposal at Santa Monica effect of the bottom slope on the internal waves, except that Bay, Calif. These studies were conducted at the Allan Han- class-2 waves were superimposed upon a general upward slope cock Foundation, University of Southern California, under of isotherms that probably was due to upwelling. Classes 3 the general direction of R. E. Stevenson (1) and at the Bureau and 4 reveal internal swash, whereby the shallow crests and of Sanitation, City of Los Angeles, under the direction of troughs preceded the deeper ones; and class 4 additionally Gunnerson (2). The data on internal waves and their effect shows definite run-up across the shoaling bottom. Classes 5, upon suspended sediment were considered important and use- 6, and 7 indicate the breaking of internal waves above the ful enough to justify further work perhaps as doctoral dis- slope. Evidence for class 5 is in the form of temperature in- sertations. Unfortunately, local student interest did not occur, versions comparable to the presence of the cylinder of air be- perhaps because of changes in the organizations. Accordingly, neath the curved wedge of water of a breaking surface wave. the writers have prepared this summary from data nearly 20 Class 6 shows an advanced position of a bolus of cold water as years old in the renewed hope that others may continue the though it had been driven shoreward by the collapse of the effort on different continental shelves of the world. internal surf. Class 7 has extreme short wave-length irregulari- MEASUREMENTS AND RESULTS ties in the isotherms, particularly near the bottom, considered as evidence of incomplete mixing after the passage of internal The best series of data on the internal waves, swash, and surf surf. This class grades into water made isothermal by repeated on the continental shelf during this study was obtained for 114 thermal sections across the shelf between Santa Monica and Redondo submarine canyons (Fig. 1). These measure- ments were made during a nearly 2-year period, August 1954 to May 1956. Most sections ranged from 10- to 15-km long usually between the shelf-break at about 90 m to the near- shore zone. They required 20-70 min, or an average of 45 min. Along each section were 5-20 bathythermograph lowerings, an average'of 10 per section, or a total of about 1150 lower- ings. Plots of isotherms with depth between the position of each bathythermograph recording along the sections revealed the presence of broad undulations in virtually every section (Fig. 2). These undulations are considered evidence of internal waves, as discussed by Summers and Emery (3) and LaFond (4). They have about the same apparent wave length whether the section was made in a seaward or a shoreward direction, indicating that the speed of the internal waves was slow com- pared with the 14-18 km/hr speed of the ship on which the FIG. 1. Santa Monica Bay, Calif. The 114 sections on which bathythermograms were made. Probably they are of tidal the investigation was based were nearly perpendicular to the shore period, like those described from the same region by Summers in the area between the two wide lines. The dashed line denotes and Emery that proceeded from west to east at a speed of the shelf-break at about 90-m depth. 2379 Downloaded by guest on October 2, 2021 2380 Geology: Emery and Gunnerson Proc. Nat. Acad. Sci. USA 70 (1978) DISTANCE IN KM V) w w x z a- w a FIG. 3. The seven classes into which thermal sections from Santa Monica Bay were grouped: 1. Internal waves with no effect by shoaling bottom: isotherms similar at different depths. 2. Internal waves with no effect by shoaling bottom: wave pattern superimposed on general temperature slope due to upwelling. 3. Internal swash: shallower'isotherms precede deeper ones; marked steepening of wave front. 4. Strong internal swash: definite wave run-up. 5. Internal surf: temperature inversion. 6. Internal surf: discrete bolus of cold water along the bottom. 7. Internal surf: FIG. 2. Examples of thermal sections across the continental extreme irregularity in wave form near the bottom. shelf and adjacent ocean floor off Santa Monica Bay. These sec- tions, made by bathythermograph lowerings during 1955-1956, il- On the basis of admittedly incomplete information, we be- lustrate classes 3, 4, 6, and 7 of internal swash and surf, as defined lieve that the swash and surf of internal waves may be a in the text. large factor in causing the bypassing of continental shelves by sediment that is contributed by streams and shore erosion, so mixing. In practice, classes 5, 6, and 7 are rarely pure and that this sediment does not remain on most shelves as bottom simple, but many sections contain evidence of two or even three of these classes. deposits. Conceivably, the internal surf leads in turn to in- ternal rip-currents that could have a direct function in Analysis of the 114 thermal sections indicate that 40% are transporting seaward the sediment that is put into suspension characterized by classes 1 or 2 (no effect of shoaling bottom), by the internal surf. 39% belong to classes 3 and 4 (internal swash up the bottom We hope that others who are interested in these problems slope), and 21% to classes 5, 6, or 7 (breaking internal waves- will participate in efforts to learn the effects and limits of the internal surf). No evidence of seasonal preference for one or sloping bottom of the nearshore zone on internal waves and another class could be detected, nor was there any evident the geological roles of internal swash and surf. control by size of the surface waves. Similar sections over the deep water of Redondo and Santa Monica canyons revealed We thank the Office of Naval Research, which supported this internal waves but no swash or surf, presumably because the summary under Contract N00014-66-C0241. This is Contribution no. 3123 of the Woods Hole Oceanographic Institution. bottom depths are too great to affect the isotherms in the overlying water. About a dozen other similar temperature 1. Stevenson, R. E. (1967) "Appendix to cruises for an oceano- cross-sections were made as far as 150 km west and 150 km graphic survey of Santa Monica Bay, California," Allan Hancock Foundation, Univ. Southern California, Los Angeles, southeast of Santa Monica Bay; some of them showed in- 388 pp. ternal swash and surf, but the number of examples was too 2. Bureau of Sanitation, City of Los Angeles (1954) "An ocean- small to justify a comparison of percentages with those in the ographic investigation of Santa Monica Bay, Aug. 25 to Sept. area of Fig. 1. 15, 1954;" (1955a) "Progress report no. 2, oceanographic in- vestigation of Santa Monica Bay;" (1955b) "Progress report SIGNIFICANCE no. 3, oceanographic investigation of Santa Monica Bay;" (1956) "Summary report, oceanographic investigation of Santa Internal swash and surf on the continental shelf may be Monica Bay." strong mixing agents in the nearshore zone, probably capable 3. Summers, H. J. & K. 0. Emery (1963) "Internal waves of tidal of rapidly dispersing pollutants that are introduced there. period off southern California," J. Geophys. Res. 68, 827- The run-up of cold water across the shoaling bottom also 839. 4. LaFond, E. C. (1961) "Internal wave motion and its geologi- changes the density structure in such a way as to tend to pro- cal significance," Mahadevan Volume: A Collection of Geolog- duce longshore geostrophic currents. More important from a ical Papers (Osmania Univ. Press, Hyderobad), pp. 61-77; sedimentologist's point of view is the probably very effective LaFond, E. C. (1962) "Internal waves," in The Sea, Ideas and turbulence caused by internal surf in resuspending bottom Observations on Progress in the Study of the Seas, ed.
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