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The mechanics of sand transport on beaches

In summer, there's nothing like feeling the sun on your face and the sand between your toes. But rising sea levels and stronger coastal storms associated with climate change pose a threat to the sands that make up our beaches. A common approach to combatting erosion at U.

The Movement of Beach Sand: An Analysis Using Fluorescent Grains - James C. Ingle - Google книги

The question is, can beach nourishment keep up with the ever-increasing forces of climate change or, like Sisyphus forever pushing his boulder up the hill, is adding sand to beaches an expensive, temporary fix to a long-term problem? We're learning as we go. A study published by Ludka in the June issue of the journal Coastal Engineering, examines exactly what happened to sand deposited on four beaches in San Diego County in California. She and her colleagues used jet skis, ATVs and other tools to continuously monitor sand levels and sand movement at the beaches over a period of about 10 years.

The research received funding from the U. Among their findings, the team learned that the entire amount of sand added to San Diego's Torrey Pines in was washed away during a single storm.

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At another beach, the addition of Olympic swimming pools-worth of sand contributed to the clogging and eventual closure of a nearby estuary. Among the team's more positive findings was that larger-grained sand appeared to have better staying power than finer-grained sand and, in some cases, the amount of sand deposited and removed from a beach by natural forces was higher than any mechanically added sand. The pattern of erosion that Ludka's team observed at ground level is what's alarming to researchers assessing the long-term future of U.

With climate change, sea level has risen by about 8 inches 20 centimeters since , according to the Intergovernmental Panel on Climate Change , and could rise 3 to 5 feet 0. More frequent coastal storms , also associated with climate change, take a further toll on beaches by unleashing rough waves that eat away at the shore. Western Carolina University's Program for the Study of Developed Shorelines hosts an interactive database of beach nourishment projects across the U.

As the data show, beach nourishment today is not cheap. As the demand for sand for beach nourishment increases, the cost will rise even higher, Orbach predicts. In addition, quartz is chemically very stable. Other minerals disappear rapidly due to chemical and mechanical destruction before they reach the beach. Another durable mineral is the mica, muscovite. Although not as abundant as quartz and feldspar, mica flakes are very visible in beach sand, even when they form less than one percent of the grains.

The flat flakes will lie on the surface of the beach, and on a sunny day when the grains catch the light angle off the beach, a sparkle is produced, especially in the wave swash zone. Mica is the sparkle in the beach. However because mica grains are light and flat, they are easily suspended by waves, and are usually removed from the beach by waves to be deposited in deep water beyond the edge of the continental shelf.

Quartz and feldspar are light colored and translucent, and if they are not iron-stained, the beach will be gray in color. But on some beaches, the sand grains are iron-stained oxidized like rust , departing a light yellow to yellow-brown color to the beach. A large part of the brown coloration of our beaches also comes from brown-stained shells that have broken down into small sand-sized fragments in the surf zone and have become mixed with the quartz sand grains.

Beaches come in many colors: white, black, tan, yellow, red, and more. Basically, beach material is small pieces of rocks that have eroded and have been carried to the beach via the wind and water.

Beach Basics

The geology of the surrounding area determines the color of those rocks. This determines what material is available for a beach. In Hawaii, much of the rock there is black basalt making the sand black on some beaches. In other areas, coral or white quart may provide the sediment supply for white beaches. Most beaches are some shade of brown or tan, but when the source is uniform, the colors can be extreme.

Look closely at beach sand and you will probably see many different colors, each one representing a different parent rock or source rock. Two things determine the size of sand on beaches: the energy height of the waves, and the size of the material furnished to the beach.

Other things being equal, the higher the average waves on a beach, the coarser the sand. This is because higher waves tend to move the smaller grains offshore to quieter water.

Sandshed: The Sand Is on the Move!

Dune sand is consistently finer grained than the adjacent beaches even though the dune sand is derived from winds blowing across the beach. The difference in size is due to the fact that winds generally only pick up and carry the finer grains of beach sand and leave the heavier, larger sand grains behind. Grain size determines the slope of the beach. Beaches with finer grain sizes tend to be flatter with gentler slopes between the high and low tide line than beaches with coarser grains.

The reason for this has to do with the ability of sand to absorb water porosity and permeability from wave swash. Fine sand absorbs relatively little water and most of the water that flows up the beach also flows back down the beach. This backwash tends to move sand in a seaward direction and thus flattens the beach. Coarse sand or gravel on a beach absorbs much more of the wave swash and more water moves up the beach when a wave breaks than moves back down toward the sea. Less backwash means fewer sand grains moving back.

In other words in a coarse sand beach the tendency is to move sand landward and pile it up in a relatively steep slope. Minerals of this size, where individual grains cannot be identified by the naked eye, are generally the weathering products of rocks found far inland away from the beaches. Mud is rarely seen on beaches because the waves suspend such fine-grained material and quickly move it either seaward into deeper water or landward into the sounds and marshes behind barrier islands.

Sometimes layers of mud appear on beaches after storms.

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  8. These layers of mud usually contain fragments of vegetation indicating that they are of salt marsh origin. They got to the beach as the island migrated over them.

    Beach Basics

    These lumps are derived from beach nourishment projects that used material with too much mud in it. There are many sources of sand on beaches including the continental shelf. During fair weather, waves slowly push ashore sand and shells from water depths as great as 30 to 50 feet.

    Enter sand

    Even though the same sand may be partially lost back to the shelf in a seaward direction during storms, the continental shelf remains the largest source of sand for many beaches. Longshore transport is just one of several sources of beach sand, bringing sand from updrift beaches, sand bars, or tidal deltas. If there are bluffs or cliffs backing the beach, waves will inevitably erode them and provide sediment to the beach.

    Low bluffs or cutbanks can form the shores of some estuaries and also provide sand to their narrow beaches. Dunes along barrier islands supply a lot of sand to the beaches. This occurs when strong winds blow sand in an offshore direction or when storms erode and cut back the dunes.

    Rivers can also furnish sediment to beaches and are the major source of beach material in California. Dams on rivers have a huge impact on California beaches and lead to increased erosion. The shells on a beach also supply sediment as they are broken up by the waves or by scavenging organisms such as skates or rays or beach buggies. An increasingly important sand source on an ever increasing number of beaches is nourishment sand, dumped there by humans.

    Many communities now nourish their beaches bringing in sand from inland sand pits, or dredging and pumping sand from local inlets or from offshore on the continental shelf. Beaches come in a variety of sizes, shapes, and compositions. Some are wide, some narrow, some steep and others gentle, some with considerable shell content, and some with no shells. If you revisit a beach over a period of time you will find that the beach width changes and that the slope steepens or flattens in response to some seemingly invisible force of nature.

    If you know the right places to go, you may find tree stumps exposed in place on the beach, outcrops of mud layers, or rocks on the beach. These fascinating beach forms, fragments, and fossils are clues to the history of the beaches and how they formed. There are four main types of beaches. These shifting shoals of sand are generally migrating towards the mainland around the world, possibly due to sea level rise. Spits are similar to both types of beaches in that they are connected to the mainland at one end, but end in the water at the other end. When a large storm comes and cuts a new inlet in the spit, a new barrier island is formed.

    A pocket beach is a unique type of beach that is associated with exposed bedrock nestled between rocky coastal outcrops. They are found around the world and in abundance throughout the northern coast of the eastern United States and Canada. The recreational beach that we all know and love is the strip of sand extending from the low-tide line to the base of the first dune Fig.

    The submerged beach or shoreface extends seaward from the low-tide line to a depth of 30 to 60 feet, where the true continental shelf begins.

    Beach: A River of Sand

    The shoreface is steep and a distinct flattening of the sea floor marks its boundary with the continental shelf. The beach itself, both the emergent and submerged sections, is not featureless. Sandbars and wavy features of huge variety, called bedforms, cover the surface of the beach and shoreface. Not only are there an infinite variety of bedforms on beach surfaces, they are ever changing — each tidal cycle reveals a whole new set of features. Human-made and therefore artificial subdivisions of the beach for descriptive purposes are shown in Figure 2.