Survey of Oceanography
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27SedimentMovementbyWavesd.pptxSediment Movement by Waves
By: John Van Leer
October 6, 2019
a) Plunging Breaker b) Spilling Breaker
Breaker just at moment of beginning to plunge. Note the spray from the portion of the wave which has already broken. There will be a bubble cloud underwater and a cloud of suspended sand as well. Once the sand is suspended it can easily be transported by currents.
Surfer riding a plunging breaker. Note: The crest is in free fall and is no longer in a wave orbit. If a component of the freefalling crest velocity is parallel to the beach it will contribute to an along shore current drift.
As waves approach a beach they undergo shortening, steepening and eventually breaking. The orbital motion becomes rectified through the breaking process. Part of the steady current created by breaking moves parallel to the beach. In the case below it would move from right to left. That steady alongshore current drift, can transport sand once it has been suspended by a plunging breakers impacting the bottom.
Dominant storm waves come from the north east along the east coast of Florida. Jetties are perpendicular to the beach, north and south of Government Cut, in Miami Beach. These barriers interrupt the alongshore transport of sediment. So the average sand transport will be southward. This causes a buildup of sand on the north side of the north jetty, and sand erosion/starvation on down stream of Government Cut.
Note: The along shore transport in the upper figure is concentrated in the surf zone. And that there is zigzag motion of material in the Swash zone with each wave. In the lower figure, waves erode a cliff and the alongshore transport carries it until it is no longer suspended by wave motion in the quiet bay.
Converging alongshore transports produce a rip current.
Change in wave climate between Summer and Winter change the amount of sediment on a beach.
In Summer gentle waves deposit fine sand in a more gently sloping beach. In Winter large waves scour fine sand away leaving a courser sediment and a steeper beach.
Fine particles take a very long time to settle once they have been suspended by dredging, propeller wash or natural processes. Note: Extremely slow settling velocity of tiny spherical bodies of specific gravity = 2.6 like quartz sand shown in the figure to left.
Oscillating water movement of shallow waves above the bottom, stirs up puffs of sand. This suspended sand can be moved by alongshore drift currents.
A buoy can be designed to generate power.
Sediments tend to be trapped between headlands, with escape down canyons or into basins, where they accumulate.
Additional Issues regarding Sediment Transport by waves: 1) Size of sediment and the steepness of a beach are related to wave climate. Steep beach covered with large boulders means harsh wave climate, gentle sloping beach with fine silt means gentle wave climate. 2) Waves are refracted by currents and water depth changes, so they can focus or defocus depending on the bathymetry and current patterns. 4) Dredge and fill operations can release fine sediments by dredging which can smother adjacent reefs or grasses. 5) Sea Grasses and Mangroves hold fine sediment, while sponges, oysters and other filter feeders remove sediment. 6) Global sea level rose about 1”/decade during the last century has a already produced major effects on coastal development and delta agriculture.
Discussion of beach re-nourishment and Hurricane Sandy Damage.
Tanker bow plunges downward into a large head sea pushing water outward to both sides. Deck is already awash from previous waves. If there were a strong headwind, the spray would be carried back to the camera in drenching sheets.
