Waves

Left barrel in Sumba, Indonesia. Photo: Nancy Opitz
View Waves Gallery

Contents

What causes waves?

Waves are generated at sea by the wind. Small ripples form on the water as the wind blows across the ocean’s surface. The size of waves depends on three things:

  1. The duration of the wind;
  2. The strength of the wind, and;
  3. The fetch, or the distance over water across which the wind blows.

The longer the wind blows the bigger the waves; stronger winds mean higher waves; and the greater the fetch, the bigger the waves. Thus the biggest waves of all occur in the storms that last the longest with the most energetic winds with hundreds of miles between the storm at sea and the beach.

Waves do not actually consist of water traveling from where the wind is blowing all the way to the beach. Instead of moving water, waves are moving energy that was transferred from the wind to the water. This energy propagates, or moves, through the ocean to the beach in the form of a wave. But the water itself is not moving forward as in a current. Instead, the energy rolls through the water in a circular motion called a wave orbital. The crest of a wave is the top of a wave orbital, and the trough of a wave is the bottom of a wave orbital. When the waves reach the shore they expend their energy by breaking and then moving sand and shaping the beach.

What causes waves to break on shore?

As waves move towards shore, they begin to "feel" the ocean floor. In a process known as shoaling, this causes the wave orbitals to flatten as the bottom shoals. When waves feel the bottom they slow down and bunch together (decrease their wavelength); but the time between wave crests (period) does not change. The height of the wave will initially decrease when it feels bottom, but then will steadily increase until the wave becomes unstable and breaks near the beach. The water literally falls over. Waves expend the energy they gained from the wind by transferring that energy to the beach when they break.

Measuring waves

The dimensions of a wave are measured both by crest height and the distance between crests. Wave height is the vertical distance from the crest (highest part of the wave) to the trough (the lowest part of the wave). Most untrained observers at sea tend to greatly overestimate wave height, which is quite understandable because they do not have any stationary reference points. And then there is the terror factor. A person holding on for dear life on a rolling, bounding vessel is easily convinced of the giant size of the waves.

Standing on a beach, a good way to estimate wave height is to assume that the surfer out there is 6 feet tall! In many cases, the amplitude is also used as a measurement of the wave’s size; wave amplitude is one-half of the wave height. The wavelength is the distance from one crest to the next crest, or from one trough to the next.

Waves travel at different speeds, and the speed is typically measured as the wave period or wave frequency. Wave period is the number of seconds it takes two successive wave crests to pass a given point. Wave frequency is the inverse of the period, or the number of waves that pass a given point during a given time period. As the length of a wave increases, so does its speed. In a general way, the higher the wave period the greater the wave height. Big storms in North Carolina may produce waves with 12 to 15 second periods, while calm weather wave periods are more likely to be 3 to 5 seconds.

Types of waves

Three main wave types
Figure 1.7. The three main wave types on open ocean beaches. Drawing: Charles Pilkey

The way each wave breaks depends on the slope and shape of the bottom. In general, the wave will break in one of three ways: as a spilling, plunging, or surging breaker (see Fig. 1.7). Where the beach is relatively flat and wide, spilling breakers will form. Spilling breakers look like they are crumbling as they move along. With a slightly steeper beach slope, the crest curls over and creates a plunging breaker (Fig. 1.8). These breakers actually form a tube of air trapped beneath the curl. The tube of air is forced into the bottom when the wave breaks and this air helps to stir up the bottom sediment. A plunging breaker is the sensational, curling type commonly sought after by surfers. Because the energy of the plunging breaker is concentrated in a small or narrow area of the seafloor, it is able to move large amounts of sand.

On the steepest bottom slopes, the wave often does not break before reaching the beach. Instead, a surging breaker is formed where the wave surges up the beach and is reflected back to sea. These types of breakers may look just like a series of bubbling mounds of water moving ashore. All three wave types may be found on any beach at different times but often a beach has a characteristic or commonly occurring wave type.

Surfer
Figure 1.8. A surfer catches a calm-weather plunging wave at South Nags Head in September 2002. Waves on the Outer Banks are higher, on average, than those of southern North Carolina because the narrower continental shelf off North Carolina offers less friction to incoming waves from the open Atlantic.

As waves move from deep water and approach the shore at some angle, the part of a wavefront that enters shallow water first will begin to slow as it feels bottom. This portion of the wave slows down while the deep-water part keeps moving at its original higher velocity, causing the wave crest to bend or refract. Along most beaches, by the time the wave breaks, the refraction is so great that the wave crest is typically much closer to paralleling the shore orientation than it was in deep water. Since waves can, in theory, approach the beach from any direction, the amount and type of refraction varies widely. Waves that form offshore may arrive at the beach with an orientation identical to that of the beach. Such waves will undergo little refraction. But waves generated by storms may approach the shore at varying angles and are sometimes strongly refracted. The wave refraction picture is further complicated because shorelines may be oriented in many directions.

Rogue waves

Rogue waves are spectacular and dangerous waves. These poorly understood waves are rare on beaches, but they do occur occasionally. A few years ago, a rogue wave struck a south Florida beach, rearranging some cars, but no one was injured because it occurred in the middle of the night. As wave trains travel across the ocean from various storms, they frequently meet each other. When this happens, the waves will either cancel each other out or reinforce each other. If the wave crests coincide with other crests, they will have positive interference, which really means that the two intersecting waves will be one wave, with the combined height of the two. Likewise if the waves of two intersecting trains are out of phase, the troughs of one set can cancel out the crest heights of the other. When several waves intersect at just the right time and phase, a rare rogue wave of immense height can form. Such a wave can bury a ship. Currents can also increase or decrease the heights of a wave train depending on the local conditions. If a wind is blowing against a current, wave height will increase. Experienced sailors fear a strong wind from the north when sailing in the north-flowing Gulf Stream.

Tsunamis

Tsunamis, sometimes called tidal waves, are not generated by the wind like other ocean waves. They are caused by sudden underwater disturbances such as earthquakes, volcanic eruptions and submarine landslides. The catastrophic release of energy during any one of these events forms a sudden wave of immense wavelength (sometimes hundreds of miles!). Traveling at very high speed, these waves spread out from their source until they reach an obstacle such as a shoreline

Tsunamis are extremely dangerous because they travel so fast that there is little time to warn people of their impending arrival. A tsunami formed in Hawaii will take only hours to reach Japan, Alaska and Washington. Reaching heights of 100 feet (30 meters), tsunamis can remove all the water away from the beach as they approach the shore. Boats unfortunate enough to be in that water will be grounded by the tsunami wave, and unwary beach dwellers are likely to run out to the newly exposed land to gather stranded fish. When the wave comes ashore, it does so with such force that it literally destroys everything in its path. And people and debris caught in the wave often get transported out to sea when the water returns to the ocean.

Learn how to survive a tsunami. Lessons from Chile, Hawaii, and Japan.

What are long shore currents?

Breaking waves form longshore currents that carry sand grains (and swimmers) along the beach. Longshore currents form in the surf zone because waves approach the shoreline at an angle (Fig. 1.9). When a wave breaks, a portion of the energy is directed laterally along the beach and this forms the current. Even a very gentle current can carry a lot of sand because the breaking waves kick sand up into the water column as evidenced by the discoloration of the water in the surf zone.

For a given wave size, the greater the angle between the waves and the shoreline (up to 45 degrees), the stronger the longshore current. This current, sometimes called littoral drift, is responsible for most of the sand movement along beaches. Other things are involved in the genesis of surf zone currents. For example, winds can either decrease or increase current velocity depending on whether they blow with or against the wave formed current. Tidal currents can also be important.

Rip currents

Wave refraction
Figure 1.9. Wave refraction. Wave crests approaching a typical North Carolina shoreline bend or refract, causing the waves to strike the shoreline almost head-on. As the wave breaks, a portion of the energy flows in the direction show by the arrows, forming the longshore current. The longshore current transports sediment. Structures sucn as the one shown here, interrupt sediment transport, causing downdrift erosion. Drawing: Charles Pilkey.

A special type of surf zone current, one that everyone who swims in the ocean should be familiar with is the rip current, sometimes called rip tides. These are strong seaward flowing currents set up by the return flow of water held onshore by the longshore currents, waves and storm surges. Often they flow through gaps in offshore bars and thus they may occur repeatedly at the same location. The details of the genesis of these currents are beyond the scope of this chapter but it is important to note that they are a significant hazard to swimmers. While standing on the beach, watch for the telltale band of seaward flowing water and if you see such – stay out of the water. If you have the misfortune to caught in a current, swim laterally rather than directly toward shore. Be cautious and know what you’re doing when you swim at the beach.

Back to Top

The beach ecosystem is made up of living and non-living parts.

Plants and animals and sand and water influence each other, often amidst breathtaking scenery. Greater than the sum of its parts, beaches sustain major portions of global biodiversity. With over half the world’s population living within 50 km of the coast, human influence on that biodiversity is inevitable, making the study of beaches even more important.


In This Section

  • Beach Basics

    Beach Basics

    Learn how a beach is defined, why beaches are different colors, and the parts of a beach.

    Read More

  • Exploring The Sand

    Exploring The Sand

    There is no debate: sea level rise is happening right now and threatens all of our beaches.

    Read More

  • Flora and Fauna

    Flora and Fauna

    The beach is home to scores of amazing plants and animals. Learn just a few of these and make your next visit to the beach a treat.

    Read More

  • Safety

    Beach Safety

    Follow these safety tips to reduce risk of danger at the beach.

    Read More

  • Sand Dunes

    Sand Dunes

    Sand dune formation, types of dunes, and where they exist.

    Read More

  • Seashells

    Seashells

    Seashells are an important part of biological and geological beach processes as well as an important part of human culture.

    Read More

  • Shoreline Engineering

    Shoreline Engineering

    Shoreline engineering is a general phrase that refers to any method of changing or altering the natural shoreline system in order to stabilize it.

    Read More

  • Tides

    Changes In Sea Level

    Learn about tides, storm surges, and sea level rise.

    Read More

  • Waves

    Waves

    What causes waves to break, different types of waves, and rogue waves.

    Read More

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928