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Waves and Beaches

  • Page ID
    10870
  • Review in progress by Kimberly Bowman (KB) - 5/28/20. Notes in red.

    Waves and Beaches

    Introduction:

            Beaches are a type of landform that typically occur along coastlines and are present alongside bodies of water. They usually experience waves or current flow behavior that deposits and reworks sedimentary grains as well as their geologic structure over time. The fluids at work in shaping this environment include water and air (laminar, turbulent, and transitional flow types).

    Sediment transport processes:

    Both laminar and turbulent flows (as well as all varying gradation in-between) are key players that shape this type of environment. Laminar flow, such as that in slow moving shallow water (uncommon in nature) is responsible for the transport and deposition of small-sized grains, while turbulent transport (more common in this environment) is responsible for the transport and deposition of grains anywhere from medium to very large sizes; all of this can of course be calculated using Reynolds' number (bedforms and flow velocity).

    Notes from Dawn: Laminar flows aren't important for beach deposits because the flows are almost always too fast. The flows are usually dominated by waves and storms, which don't allow mud-sized grains to accumulate. For the transport processes, it would be good to add more on what causes the flows along beaches (e.g. waves and storms) and how those flows vary from offshore to the berm on the beach. It's important to focus on the things that make waves and beaches different from other environments.

    I've posted several videos of wave transport of sediment in the Bahamas that might help you think about this: https://youtu.be/4EO43A8V6_c https://youtu.be/3QNwFgpmKB0

    *UPDATE BASED OFF DAWN'S COMMENTS*

                    The key transport processes that dominate in shaping this type of environment include waves and turbulent flows via storms, lower to higher energy, respectively. Flow speeds present in this environment are almost always too fast (experiencing higher Reynolds’ Number) to allow for mud-sized grains to accumulate. Rather, small, medium, and larger sedimentary grains tend to be transported with a very poorly sorted accumulation at the depositional site (1).impact-of-breakwaters-on-beach-development-at-Sunset-Beach.jpg

                    There are many different flow speed and turbulence variations present in waves. The ebb and flow nature of the tides at varying depths modulate the maximal and minimal flow speeds experienced among the waves (Reynolds’ Number). These factors all lead to a multitude of sedimentary variations along beaches.

                    At the breaker zone, wave turbulence of varying flow speeds is experienced. As the waves move in from offshore (greater depth) carrying sedimentary grains of varying sizes and shapes closer to the swash zone (shallower depths), the flow speed, and therefore turbulence, increases rapidly. The suspended grains transported by these waves promptly deposit medium-to-larger/coarser sized sedimentary grains at a varying gradient along the breaker zone (2). The accumulation of sedimentary grains at this site tends to be poorly sorted and sub-rounded to angular, forming planar laminae dipping towards the ocean at the contact site. As the waves move back offshore from the breaker zone (high turbulence and flow speeds) they tend to carry and transport some medium-to-heavy grains back as the tide recedes from the swash zone, others are left deposited at the base.

                    In contrast to the breaker zone, turbulence and flow speeds of waves located in the offshore zone tend to be lower in energy and more consistent. Flow energy (Reynolds’ Number) and turbulence decreases with increasing depths, this which is due to the difference in eolian influence present on the surface of the water versus the lack thereof at greater depths. Larger and coarser grains tend to be carried and transported closer to the surface in this zone with smaller grains experiencing transportation and deposition closer at the base (1).

                    During periods of high tides, such as storms, the behavior and location of grain transportation and deposition shift greatly. As the ebb and flow of high tides act along the shoreline, the tides will usually reach up and over the berm (maximum distance the waves can reach under normal non-stormy conditions) and deposit sedimentary grains further in along the shoreline. The presence of high winds under these conditions create eolian dunes along the drier portions of the beach face.

    Characteristics of deposited sediment:

    Common sedimentary structures that occur in this environment are ones produced by the processes of current ripples (sediment transport over a ripple), wave ripples, dunes, antidunes, mudcracks, planar/flat lamination,raindrop impressions, cross-bedding, bioturbation, high-sediment load/slope failure etc (go into further detail per each sedimentary structure). Grains of all varying sizes are deposited along the coastline (depending on type of flow/Reynolds' number experienced at the time of deposition). The principle of Original Horizontality is very much at play in this environment (include example sketch). Tectonic movements that result in the change of water-level or relocation of a beach coastline may cause the preservation of trace and/or body fossils (paleo-beach to form, limestones etc).

    Notes from Dawn: Current ripples, dunes, anitdunes are rare because they require a unidirectional flow. They can occur in streams flowing into the ocean near the beach, but you will need to explicitly say that they are due to the flow in channels and not due to the activity of waves on beaches. Similarly, mudcracks and raindrop impressions are not related to typical beach processes, although they may form in lagoons. Slope failures require slopes, which are usually reduced when there are a lot of waves. It would be better to focus on the features that are characteristic of BEACHES and the offshore areas, more than the associated environments.

         *UPDATE BASED OFF DAWN'S COMMENTS*

               The common sedimentary structures that are characteristic of this environment are produced by the processes of wave ripples, planar lamination, cross-bedding, and bioturbation (3). Sedimentary grains of all varying sizes and shapes are deposited along the coastline - depending on the turbulence experienced by the waves’ ebb and flow (Reynolds' number at the time of sediment deposition). The principle of original horizontality (Might be a good idea to remind the reader what this is), as is the case with sediment and stratigraphic units, is very much at play in this environment.components-of-a-sandy-marine-beach.png

               Grains deposited along and past the berm zone of the beach tend to be finer and lighter in size due to ease of transportation (Finer and lighter in contrast to what? You may want to clarify and specify the range of grain sizes). Under non-stormy conditions, this zone experiences aeolian behavior and produces wind ripples, crossbedding, and dunes as sedimentary structures. Sedimentary grains tend to be well sorted with varying grain shapes present (1).

              Grains deposited at the breaker zone tend to be poorly sorted, medium to coarser/larger in size and range ranging from subrounded to angular in shape. This is due to the gradual decrease in depth as the wave approaches the breaker zone and thus increasing in turbulence. The sedimentary structures present at this zone tend to be wave ripples and planar laminae with a dip towards the ocean (6).

              In the offshore zone, the transportation and deposition of sedimentary grains vary in size, sorting, and shape depending on the depths of the current. This is due to the circular motion of waves caused by the behavior of wind blowing across the surface. This aeolian influence across the ocean surface decreases gradually with increasing depths until being eliminated entirely just prior to reaching the ocean floor. The sedimentary structures present within this zone include cross-bedding lamination, planar lamination, and bioturbation via marine organism behavior.

    Typical vertical sequence of facies:

    Use Walther's Law to predict sequence of sediment deposition at a typical beach/coast-line under the processes of laminar flow and turbulent flow/combination of both (IMPORTANT - grains/rocks of the same type are not necessarily deposited at the same time, BIG difference between correlating rocks based on having same lithology vs rock being deposited at the same time). Draw and/or use stratigraphic columns to present the aforementioned sequence of facies experiencing varying gradation of flow. Use flow type w/grain size and grain assortment to distinguish between turbulent environmental flows and laminar (combinations of both included).

    Again, you will want to focus on the various aspects of beaches, for example going from the berm to the swash zone to the breaker zone to the wave rippled zone as you go offshore or as sealevel goes up.

    *UPDATE BASED OFF DAWN'S COMMENTS*

            Walther’s Law of Facies states that the vertical depositional succession of stratigraphic facies reflects the lateral changes in sedimentary layers witnessed within the environment. It also states that when a depositional environment shifts laterally, stratigraphic layers will be deposited on top of one another (6). Walther’s Law is very much at play in a beach environment. As the overall sea level rises and falls, the present stratigraphic layers will be altered in accordance with Walther’s Law. Going from the beach, to the breaker zone where shallow marine rocks reside, to the offshore zone where deep marine rocks reside, all these layers will shift laterally reflecting the behavior experienced by the tide. Parts of the aforementioned lateral layers will also be deposited on top of another in accordance with the ebb and flow of the waves.

    Sequence of Facies - Beach.jpg20200528_103247_HDR.jpg

    *Quick note regarding Walther's Law. If erosion, non-deposition, or a break in deposition have taken place in the stratigraphy - Walther's Law will not work in these cases (4).

     

     

     

     

     

     

    *Walther's Law - before/after*

     

     

     

     

    *Stratigraphic Column (7)*

    Summary: 

    Recap laminar, turbulent and translational flow types as well as key characteristics between each (grain transport/deposition and assortment). Distinctive sedimentary structure formations produced via each type of flow (wave ripples, dunes, planar lamination etc). Include a graphical representation (sketch or otherwise) of the facies of each depositional environment INCLUDING the varying flow types and their effects on the deposition of said facies.

     

    Keywords:

    Laminar flow, Turbulent flow, Depositional environments, Grain sizes, Beaches, Paleo-beach, Principle of Original Horizontality, Grain transport, Coastline, bioturbation, Wave ripples, Current ripples, Dunes, Planar lamination, Stratigraphic columns, Trace/body fossils, Tides, Translational flow.

     

    References:

    1) https://geo.libretexts.org/Courses/University_of_California_Davis/UCD_GEL_109%3A_Sediments_and_Strata_(Sumner), Dawn Sumner UCD

    2) https://www.youtube.com/watch?v=U9wtyfwlhCc&feature=youtu.be, "Underwater in the breaker zone - small waves in the Bahamas", Dawn Sumner UCD

    3) https://www.youtube.com/watch?v=CHZtSPZcdz0, "Beaches and Waves", Dawn Sumner UCD

    4) https://www.youtube.com/watch?v=UG1WuSUoCcw, "Walther's Law", Dawn Sumner UCD

    5) https://journals.lib.unb.ca/index.ph...icle/view/3154

    6) Nichols Sedimentology and Stratigraphy

    7) Clifton H.E. (2005) Coastal Sedimentary Facies. In: Schwartz M.L. (eds) Encyclopedia of Coastal Science. Encyclopedia of Earth Science Series. Springer, Dordrecht