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Coastal Processes and Shoreline Erosion
Photograph courtesy of Ellis Pickett, Chairman, Texas Surfrider Chapter
What Forces Drive Shoreline Erosion?
Wind-generated waves are important as energy transfer agents along Gulf Coast beaches as well as inland bays. Waves obtain energy from the wind, transfer it across expanse of the ocean or bay and deliver it to the nearshore coastal region. Here the energy is the focused and can cause erosion, generate a nearshore currents and sediment transport patterns. Although we focus on wave energy here transport by wind (Eolian transport?) and anthropogenic actions? may also contribute to coastal erosion.
Waves begin as micro ripples as the wind first touches the smooth water surface. Once micro ripples are formed the wind has something to work with and the resulting friction causes the formation of ripples and chop. If the wind has enough fetch and time over which to act a fully developed sea will be produced.
Understanding the offshore wave climate helps scientists to predict the strength of nearshore currents that the waves will generate. These nearshore currents have beneficial as well as destructive aspects. While nearshore flushing is beneficial to the water quality of the nearshore waters the same strong alongshore and offshore directed currents are often assoicated with coastal erosion.
In the nearshore, water and thus sediment movement are dominated by two wave-induced current systems:
A cell circulation system consisting of rip currents and associated longshore currents. Rip currents are strong narrow surface currents that occur when waves pile more water along the shore than can be returned to the ocean causing the water to rush straight out from the shoreline in a narrow band. Sediment transported by offshore directed rip currents is generally deposited offshore outside of the nearshore?.
Longshore currents, also referred to as littoral currents, occur in the nearshore? and are generated by an oblique (also called incident) wave approach to the shoreline. Waves breaking at appreciable angles (oblique) to the shoreline generate longshore currents that flow parallel to the shore within the confined region between the shore and breakers. The front part of the wave touches the shallow nearshore first and slows down. The remaining section of the wave bends as it approaches the nearshore creating a current that parallels the beach. Larger waves tend to strike the beach less often and at greater angles thereby creating stronger longshore currents. A good indicator that a longshore current exists along a shoreline is if sandbars are present. These longshore currents are most significant in causing a longshore movement of beach material which can consist of hundreds of kilometers of sediment. This shore parallel movement of sediment is called longshore transport.
Nearshore sediment is disturbed and suspended in the water by breaking waves is then transported either offshore by rip currents or alongshore by longshore currents. A different process is responsible for sand transport at the shoreline and subaerial (typically dry portion) of the beach. This process is called Beach Drift.
Beach drift occurs along the shoreline when waves reach the shore at an angle (not parallel) causing the swash and entrained sediment to travel obliquely up the beach. The returning backwash moves back to the ocean with a direction that is defined by the slope of the beach but roughly perpendicular to the shore. The differences in the direction of the swash and backwash cause a net longshore movement of sediment.
Longshore Drift is the movement of sediment along the coast in a preferred direction that is typically in response to a predominant wind direction. Longshore drift emcompasses the entire region of the beach and nearshore thus incorporates the action of both beach drift and longshore sediment transport.
Direct wave action and often assoicated elevated water levels and longshore currents produced by incident waves are the primary factors resulting in coastal erosion along the Texas Coast. Waves are the primary force causing erosion along the coast. Waves cause erosion in two ways, 1) Direct impact with the shoreline and 2) Formation of wave induced longshore currents. An observer can witness shoreline erosion as the forceful waves directly impact the shoreline. The second mechanism of erosion is less apparent to the naked eye. A longshore current develops when incident waves? approach the beach. As the front part of the wave hits the shallow water the wave slows down. The rest of the wave bends as it comes closer to the shore creating a current that parallels the beach. The current is directed alongshore away from the approaching wave direction (Figure #). The longshore current carries sediment that is suspended by breaking waves along with it as it flows alongshore. Larger waves, which strike the beach less often approach at greater angles, thereby creating stronger Longshore currents. Areas with developed longshore currents are often identified by an established system of longshore sand bars.