Ocean Lecture & Educator’s Night May 16, 2012 Rip Currents: Nearshore Fundamentals Below is an overview of the activities Rip Currents: Nearshore Fundamentals (University Corporation for Atmospheric Research, COMET modules) to incorporate information learned from Dr. Herrington’s presentation and subsequent discussion. Lesson Overview Students learn how nearshore circulation and wave dynamics are involved in the formation of rip currents. Lesson Rationale Students will participate in an online exploration of the influence of nearshore circulation and wave dynamics on the formation of rip currents. They are exposed to the science behind nearshore waves, from the transfer of energy to the loss and dissipation of energy in waves. After gaining insight into how waves form and act in the nearshore environment, students will further explore the science behind rip currents. First, students will gain a deeper understanding of the different characteristics of rip currents. Second, students will learn what a forcing mechanism is and how longshore variations in waves, wave-beach interactions, and tides can all influence the formation and strength of rip currents. Students understanding will be assessed throughout with small quizzes to check for comprehension and application of the knowledge they are gaining. Key Concept Students are exposed to the science behind how rip currents form. Rip Currents: Nearshore Fundamentals - 2012 Update » Print Ve... http://www.meted.ucar.edu/marine/ripcurrents/NSF/print.htm Rip Currents: Nearshore Fundamentals2012 Update Produced by The COMET® Program Print Version Table of Contents Introduction Nearshore Terminology and Circulation Zones Circulation Onshore Flow Longshore Current Rip Currents The Surf Zone Characteristics Variable and Fixed Bathymetry Multi-spectrum Waves Wave Height Slope of the Beach Nearshore Waves Wave Energy Transformation Shoaling Refraction Due to Depth Difraction Wave Energy Loss and Dissipation Reflection Breaking Rip Currents Characteristics Circulation Visual Clues Wave Angle Location Spacing Duration Velocity Rip Current Forcing Mechanisms Longshore Variations in Incoming Waves Wave Set-up Wave-Wave Interactions Other Temporal Modulations Wave Boundary Interaction Surf Zone Bathymetry Coastal Structures Longshore Sandbar Tidal Forcing Modulation by Tides 1 of 51 5/15/12 1:26 PM Rip Currents: Nearshore Fundamentals - 2012 Update » Print Ve... http://www.meted.ucar.edu/marine/ripcurrents/NSF/print.htm Summary 2 of 51 5/15/12 1:26 PM Rip Currents: Nearshore Fundamentals - 2012 Update » Print Ve... http://www.meted.ucar.edu/marine/ripcurrents/NSF/print.htm Introduction This module provides insight into how nearshore circulation and wave dynamics are involved in rip current formation. (Wave characteristics, types, and life cycles are discussed in greater detail in other COMET modules.) 3 of 51 5/15/12 1:26 PM Rip Currents: Nearshore Fundamentals - 2012 Update » Print Ve... http://www.meted.ucar.edu/marine/ripcurrents/NSF/print.htm Nearshore Terminology Nearshore Terminology Zones Waves absorb energy from the wind. That energy is transmitted across the water surface by waves. At the shoreline, waves break and unleash that energy on the beach. There are four defined zones in the nearshore environment. When a wave passes from deep to shallow water, it enters the shoaling zone. In this zone the depth of the water is less than half the wavelength. The vertical orientation of the wave changes as it begins to feel the efect of the lake or ocean bottom. As the wave nears the shore, its steepness increases and the wave eventually breaks. Where the wave begins to break is known as the breaker zone. Not all waves begin to break in the same location due to wave height variation. The breaker zone is where the majority of waves reach their steepness limit for a given wave spectrum. After the wave begins to break, it enters the surf zone where water is transported toward the beach in the form of smaller, broken waves known as bores. These bores can be thought of as continually breaking waves. As bores reach the beach, water particles are pushed onshore and then retreat seaward. This area of run-up and backwash of water is known as the swash zone. 4 of 51 5/15/12 1:26 PM Rip Currents: Nearshore Fundamentals - 2012 Update » Print Ve... http://www.meted.ucar.edu/marine/ripcurrents/NSF/print.htm Question In which zone is the depth of water less than half of the wave length? (Choose the best answer.) a) Shoaling zone b) Breaker zone c) Surf zone d) Swash zone Done The correct answer is a. 5 of 51 5/15/12 1:26 PM Rip Currents: Nearshore Fundamentals - 2012 Update » Print Ve... http://www.meted.ucar.edu/marine/ripcurrents/NSF/print.htm Nearshore Terminology Circulation Nearshore Terminology » Circulation Onshore Flow In the deeper water beyond the shoaling zone, water particles of non-breaking waves have a decreasing orbital motion to a depth equal to half the wavelength, but little to no net flow in the wave direction. As waves move into shallower water, the circular orbits become progressively more distorted and upon breaking are highly disrupted. Following the wave breaking, water particles still oscillate moving landward with the wave crest and seaward with the trough. A rise in the mean water level above the still water level, known as wave set-up, is determined by the average of the onshore currents. The other parts of the nearshore circulation system stem from this onshore flow of water. Click to see an onshore flow animation. (media/flash/onshore_flow2.swf) 6 of 51 5/15/12 1:26 PM Rip Currents: Nearshore Fundamentals - 2012 Update » Print Ve... http://www.meted.ucar.edu/marine/ripcurrents/NSF/print.htm Nearshore Terminology » Circulation Longshore Current When waves break on the shoreline, they create currents parallel to the shoreline called longshore currents. Longshore currents occur most often when waves approach the shoreline at an angle. The angle of the incoming wave causes a progressively breaking wave that moves along the shoreline and a longshore current that moves in the same direction as the breaking wave. The longshore current spans the entire width of the surf zone. It reaches maximum strength in the middle of the surf zone and diminishes in strength farther ofshore. Larger waves create faster longshore currents. The angle of wave approach at breaking also afects the speed of the current. Peak currents occur when the wave approaches from 45 degrees. Higher or lower angles produce slower currents. Waves breaking parallel to the shoreline will have no longshore current generated by the wave angle. Like rip currents, longshore currents are subtle but can be seen or felt while standing in the surf zone. Longshore currents will always be present with rip currents as part of the nearshore circulation system. Large-scale currents moving at slower speeds nearshore can also be generated from persistent synoptic-scale winds as opposed to locally-breaking waves. Question 7 of 51 5/15/12 1:26 PM Rip Currents: Nearshore Fundamentals - 2012 Update » Print Ve... http://www.meted.ucar.edu/marine/ripcurrents/NSF/print.htm Which of the following will always be present with rip currents as part of the nearshore circulation system? (Choose the best answer.) a) Shoaling zone b) Submarine canyons c) Longshore currents Done The correct answer is c. 8 of 51 5/15/12 1:26 PM Rip Currents: Nearshore Fundamentals - 2012 Update » Print Ve... http://www.meted.ucar.edu/marine/ripcurrents/NSF/print.htm Nearshore Terminology » Circulation Rip Currents Rip currents are jet-like currents of water that typically extend from near the shoreline out past the line of breaking waves. Rip currents can be caused by several wave phenomena that will be presented later in this module. These include ofshore flow through channels in sandbars, natural variability of breaking wave heights, and longshore current interaction with man-made structures. Rip currents are a natural part of the dynamic nearshore circulation system. A portion of the longshore current enters into "feeder currents," which are the segments on the shore-side of a rip current. A rip current also has a neck and a head, as illustrated in the drawing. Question Rip currents are _____. (Choose the answer that best completes the sentence.) 9 of 51 5/15/12 1:26 PM Rip Currents: Nearshore Fundamentals - 2012 Update » Print Ve... http://www.meted.ucar.edu/marine/ripcurrents/NSF/print.htm a) a part of the daily nearshore circulation that occurs naturally at surf beaches b) an unnatural part of the daily nearshore circulation system due only to man-made structures c) a natural part of the longshore current Done The correct answer is a. 10 of 51 5/15/12 1:26 PM Rip Currents: Nearshore Fundamentals - 2012 Update » Print Ve... http://www.meted.ucar.edu/marine/ripcurrents/NSF/print.htm Nearshore Terminology The Surf Zone Nearshore Terminology » The Surf Zone Characteristics Of the four zones defined in the nearshore environment, the surf zone is the most important for the marine forecaster’s analysis of rip current potential. There are several aspects of the surf zone that must be assessed, including: Variable and fixed bathymetry Multi-spectrum waves Wave height Slope of the beach Question In which zone do rip currents originate? (Choose the best answer.) 11 of 51 5/15/12 1:26 PM Rip Currents: Nearshore Fundamentals - 2012 Update » Print Ve... http://www.meted.ucar.edu/marine/ripcurrents/NSF/print.htm a) Shoaling zone b) Breaker zone c) Surf zone d) Swash zone Done The correct answer is c. Nearshore Terminology » The Surf Zone Variable and Fixed Bathymetry Beaches exist because of the erosion and recovery of sand as shorelines adjust to the forces that shape them. Varying weather and storm patterns that occur every year cause seasonal fluctuations in the beach width and the bathymetry (data derived from measurements of water depth are compiled in a topographic map).