Lab10_CoastlinesHurricanes_LLL

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Feb 20, 2024

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Lab 10–Coastal systems and hurricanes 15 Questions Lab goals Coastlines are fascinating and very dynamic places that lie at the juncture of the land (terrestrial lithosphere) and the sea (the hydrosphere), with the atmosphere above. They are also incredibly diverse in nature, and an interesting and fruitful place to apply an earth system science perspective. Finally, they are characterized by high population densities (of humans) and therefore, understanding shorelines is of distinct utility and consequence. This lab starts out by familiarizing you with some of the features and processes that characterize a chosen suite of shorelines, and then considers what happens during a defining event such as a hurricane. Because of the combination of a dynamic character and high density of human habitation, coastlines are also a place where issues of sustainability have significant implications and consequences. Recognition of coastal features (9 pts) In this portion of the lab you will first be introduced to a specific type of coastal feature by navigating to a given locality in Google Earth and then working through guiding questions. You will then be asked to find a similar feature of your choosing elsewhere along the adjacent coastline and answer some questions about it. Deltas : A delta is a body of sediment that forms where a river meets the sea (or other body of water), as the current weakens and loses the ability to transport sediment, which then comes to rest. As the river continues to supply sediment, with time the delta gets larger, and builds out. However, deltas vary incredibly because of the different degrees to which wave and tidal processes reshape the sediment supplied to the shoreline by the river, and because of the nature of the shoreline it builds against. The image to the right shows how a delta built out with time in the Puget Sound area of Washington State. In this particular case the delta filled an embayment, an indentation into the land. In other cases deltas build out from the shoreline and protrude into the ocean. This example also shows how deltas are connected to other aspects of earth systems – in this case to sea level rise due to the 1
end of the last Ice Age (which formed the embayment), and then to pulses of eruptions, which supplied very large volumes of sediment. Image taken from USGS site: http://walrus.wr.usgs.gov/geotech/pugetposter/model.html Navigate in Google Earth to 62 57.255 N 164 8.25 W You can copy and paste these coordinates directly into Google Earth search window to navigate there. Zoom in and out to get your bearings, then frame your window view so that it is about 100 km across (the ‘eye alt’ number which changes as you zoom in and out) . Orient N as shown in the upper right corner of your screen so that it is “up” (this is the default position and conventional view). What you are looking at is the delta for the Yukon River in Alaska where it enters the Bering Sea. The Yukon River is large and carries a lot of sediment, and this is a large delta. - Use the path tool to trace out the two major channels which presently reach the sea and mark them in bright green and make them wider so that your tracings are clearly visible. This is where the river is presently providing sediment to the front of the delta. - Note that there are many well developed channel forms on this delta, which are of significant size, but which no longer reach the sea. Find two of these and use the path tool trace them out and assign them a bright yellow color and increase the line thickness as you did above so that they are also clearly visible. Why do these channels no longer reach the shore? Channels get abandoned and plugged as a new channel develops elsewhere. Deltas are dynamic places where rivers are constantly changing their paths. - On the west side of the delta note the light-colored ridges that are parallel to the present-day shoreline but are presently inland of the shore. These are old beach ridges, formed as waves reshape the sediment supplied by the river to the shoreline area. As more sediment is supplied, new beach ridges form seaward to the older one and in this way the delta shoreline can build out. Trace the most prominent beach ridge as far as you can go, and assign that line a bright red coloration. Question 1: Copy and paste an image that shows the delta, with your colored line traces of active river channels, abandoned river channels, and beach ridges below. - Using either the line or path tool to make an estimate of how far in kilometers this delta protrudes out into the Bering Sea and has built out with time. Question 2: Estimate: _______ km Explore the edges of the delta in Google Earth. Where do you think the prevailing wind direction comes from and why? One thought path that may be able to help you answer this question is to consider that waves are a function of wind direction and strength, and then to consider how wave-related features are developed (or not) along different parts or sides of the delta, with the idea that the beach ridges will be best formed where the waves are the largest. The geomorphology of the delta is connected to climate in several ways. Since we see well-formed beach ridges on the Northwest side of the delta, that is a good indication that the prevailing wind is from that direction. 2
Barrier islands : A barrier island is an elongate body of predominantly sandy sediment that forms off shore from the mainland. They typically occur on low-relief and sediment-rich coastlines. Waves help to form the sediment body in the first place, but as it becomes vegetated, the vegetation traps and stabilizes sediment and plays a crucial role in the building and maintenance of the barrier island. People love to live on barrier islands, and have significantly altered their behavior in some cases. Barrier islands are very common along the southern Atlantic coast of the U.S. and along the Gulf Coast. The map to the left shows two barrier islands off mainland Maryland. The oblique air photo above shows the Ocean City inlet portion of the map area with highly populated Fenwick Island to the north (farther away) and Assateague Island to the south with the open ocean to the right. Assateague is where wild horses live, and has been protected from development. The two islands used to be aligned, but are now offset, and the map shows how Assateague has retreated almost a mile since 1849. This retreat is due to coastal engineering that has kept Fenwick Island in place, while Assateague has continued to move. In addition, note in the oblique air photo image, the fan shaped bodies of sediment on the back side (bay side to the left) of the island. How did these get there? Image source: http://pubs.usgs.gov/circ/c1075/conflicts.html Navigate to 28 4.7 N 96 53 W Zoom in and out to get your bearings, then frame the horizontal distance in your window view to be about 35-40 km across . You are looking at part of Matagorda Island in Texas, a barrier island. Note the white strip on the seaward side of the island. This is the shore face, the sandy beach, and if you zoom in you can see the waves in the image. Now note the delta-like feature with channels that fan out into the lagoon between the island and the mainland to the left. Where did this sediment come from? The answer can be found in the area the delta channels converge on, where the beach is wider. This used to be a tidal channel, moving sediment back and forth and forming the delta, but it has been “plugged” 3
as the waves moved sand into that area. The tidal currents move sediment. When the tide rises it moves the sediment in toward the lagoon, which since that area is more protected (from waves), forms what is known as a flood tide delta on the inside of the tidal channel. When it moves sediment out to sea an ebb tide delta can also form on the seaward side, but often the waves redistribute this sediment (since it is in the open ocean), and so ebb tide deltas associated with tidal channels are often missing or are well less developed and harder to see. - Navigate along the length of Matagorda Island and find another flood tide delta (there is one just north). Zoom in a bit and use the path tool to trace 2 or 3 of the relict channels that distributed sediment to form the delta. Question 3: Insert a Google Earth image of that flood tide delta with traced channels below. Question 4: For this image, describe whether it is an active flood tide delta or an inactive one where the tidal channel has been plugged with sand. Navigate to 32 34.774 N 80 8.693 W Frame your window so that the field of view is 2 km wide . You should be viewing the very end of Kiawah Island, South Carolina, with the Atlantic Ocean to the right. A fair-sized tidal channel marks the southern end of the island here. Note the white shore face and the distinctive curved or hooked ridges of white sand that merge with the shore face to the northeast. This end of the island is a distinctive landform feature known as a spit . A spit forms due to a process known as longshore drift , which in turn is a function of prevailing wind directions and the resulting waves. Longshore drift describes the movement of sand along a beach; when the waves come in at a consistent angle to the beach, sand is washed up the shore face by the waves at the same angle of the incoming waves, and then as the waves retreat, the sand is carried back down the shore face straight out to sea. The net effect is that the sediment is transported parallel to the shore (as shown in the diagram to the left). Since this is a dynamic process, it often helps to see the process animated. Watch this short YouTube video explaining this process at: https://www.youtube.com/watch? v=U9EhVa4MmEs . It is difficult to understand shorelines without considering longshore drift which shapes and reshapes beaches constantly. Image to right and above is a diagram showing how longshore drift works. The zigzag arrows on the beach indicated the movement of sediment (sand). Image source is USGS site http://pubs.usgs.gov/circ/c1075/change.html In the case of Kiawah Island, the longshore drift is from the northeast towards the southwest. When sediment comes to a tidal channel where the waves bend around (refract), it enters deeper water and 4
the sand builds out on the one side forming distinctive curved sand bodies called spits . If you look at the Google image of Kiawah you can see older sand spit ridges where vegetation has had a chance to be established. The other side of the tidal channel is typically characterized by erosion. With some more thought you can see that the above processes suggest the tidal channel here has moved to the southwest with time, and will continue to do so. Image to right is an oblique aerial view of a spit that formed along the end of a South Carolina barrier island. Note the multiple curved whitish sand bodies. At one time each one of these was the active shore face and new sand ridges have extended the spit towards the viewer with time. The oldest part of the spit is farthest away. In this particular case with a careful look one can also see how the waves are oblique in a manner consistent with a transport direction toward the viewer, and consistent with the explanation given above. Image source is the USGS website: http://pubs.usgs.gov/circ/c1075/barrier.html Images above showing a bit of what Kiawah Island is like on the ground. To the left one can see how the sea grass (tolerant of salt) is vegetating and stabilizing the wind-blown sand that has formed small dunes. Dunes are a common component of barrier islands. Wind is also an important sediment transport mechanism in these shore line settings with the beach shore face providing the source of the sand . To the right is the sandy expanse of wave ripples exposed at low tide on the spit of Kiawah. Navigate to 32 17.964 N 80 31.843 W Frame your window so that it covers 3-4 km of image , which is of Pritchard’s Island, South Carolina. As you progress from right to left across your view (with N in the standard position) you should see: a) the open Atlantic Ocean, b) the thin white sandy shore face, c) the vegetated backbone of a barrier island, and d) an array of meandering and branching tidal channels with tidal flats in between the 5
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