Lab 10 – Coastal systems and hurricanes
Earth System Science Lab
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 their 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 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 they are also a place where considerations of sustainability have significant implications and consequences.
Recognition of coastal features.
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.
A delta is a body of sediment that forms where a river meets the sea (or other body of water), and 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 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 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 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) and frame your window so that it is about 100 km across. 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 the 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 shore line 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 river channels and beach ridges (and with a scale bar) in 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 ->
Question 3: This is a thought question. 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.
A barrier island is an elongate body of predominantly sandy sediment that forms off shore 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 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 and have the horizontal distance in your window frame be about 40 km. 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. It is a delta feature, but where did the 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” 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 the area is more protected (from waves) forms what is known as a flood tide delta. When it moves sediment out to sea an ebb tide delta can also form, but often the waves redistribute this sediment (since it is on the seaward side), and so ebb tide deltas associated with tidal channels are often missing or are well less developed.
– Navigate along the length of Matagorda island and find another flood tide delta. Use the path tool to trace 2 or 3 of the relict channels that distributed sediment to form the delta.
Question 4: Insert a Google Earth image of that flood tide delta below.
Question 5: 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.
Question 6: How many other flood tide deltas (active or inactive) do you see along the length of Matagorda island?
Navigate to 32 34.774 N 80 8.693 W. Frame your window so that the field of view is 2-3 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 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. When the waves come in at a consistent angle to the beach then as the sand is washed up the shore face by the wave, and then back down the shore face, 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 and a YouTube schematic animation explaining this process can be found at https://www.youtube.com/watch?v=U9EhVa4MmEs . It is difficult to understand shorelines without taking into account longshore drift.
Image to right and above is a diagram showing how longshore drift works. 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 to the southwest. When the sediment comes to the tidal channel the waves bend around (refract) and as it enters deeper water 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 suggests the tidal channel 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 of what a bit of 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 and 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 (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 an array of meandering and branching tidal channels with tidal flats in between the channels. Further west is the mainland. One can note in the vegetated area (trees here) of the island linear features that are approximately parallel to the modern shoreline. These are old beach ridges that become stabilized by the vegetation with a new beach being formed on the ocean side. In this way the island can grow with time, building out in a seaward direction. At present the island is being eroded. Note the nice spit that has formed at the south end of the island. Tidal flats and tidal channels found on the landward side of the island result from the interplay between tides, sediment and vegetation. The tidal channels distribute sediment and the vegetation traps it. Tidal flats and channels are areas of very high biologic productivity.
Navigate to 32 17.614 N 80 34.454 W and frame the window so that it covers about 10-12 km from east to west. This is Capers and St. Philips Island area of the South Carolina coast.
– Use the path tool and what you have learned above to trace examples of the following features using the colors indicated: a) tidal channel in light blue, b) shore face in yellow, c) vegetated beach ridge in brown, d) spit in red.
Question 7: Insert a copy of the image below.
Question 8: What is the direction of longshore drift in this case and on what do you base your answer?
Emergent shorelines are ones dominated by high relief and erosion. Features that you will learn about as you go through this section are sea cliffs, sea stacks, sea arches, wave cut benches, and wave cut terraces. Perhaps the best introduction here is with some photos from coastal California.
To the left one can see sea cliffs that characterize much of the coastline of northern California. Note the flat, now vegetated top of the sea cliffs, which is a wave cut terrace (how they form is explained below). Also note the many rocks poking up in the surf zone out to sea. In a protected bay to the very left a sandy beach has formed at the foot of the sea cliffs. To the right is a feature known as a sea stack (note it also has a flat top in this particular case, not all sea stacks do). Also note the surf zone which indicates the water is shallow that surrounds the sea stack. As the waves erode at the base of the sea stack causing the overlying rocks to fall and get ground up and carried away the sea stack is gradually getting smaller.
To the left is an offshore island with a rather large sea arch eroded into it. Note also the surf zone around it, indicating somewhat shallower water. This submerged area of shallow waters is due to wave erosion and is known as a wave cut bench. To the right is an example of a typical wave on a windy day. Armed with boulders and sediment the waves hammer against the shoreline and are a significant erosion agent.
Navigate to 39 18.455 N and 123 48.405 W, which is the Mendocino, California coast line area, so that the image captures a 2-3 km wide window. Once again, note all the sea cliffs here. These are one indication that this is an emergent coastline, created in this case by uplift of the crust relative to sea level. To have emergent shorelines in California makes perfect sense, because of the active tectonics and earthquakes that characterize this part of the world, and which are related to juncture between the Pacific plate and North American plate (which the well known San Andreas fault is part of), each moving their own separate ways.
The cliffs you see here form as the waves cut at and undermine the base, and then the overlying rocks fall into the sea, where they are ground up and carried away by the continued wave action. With time the waves cut a notch into the coast and the cliff retreats. An example is given below.
Image to left is of coastline at Pacifica, California before 1997. During larger storms the waves cut at the base of this cliff made up of soft and relatively loose material and trigger mass wasting. The large rocks and earth moving equipment were from an attempt to protect the cliff from the waves. The 30 meter high cliff failed as during the 1997 to 1998 El Nino storms the cliff retreated some 10 meters, and the front row of houses you see here were a loss. Most sea cliff retreat is episodic and associated with larger storms. Image source is USGS website http://pubs.usgs.gov/pp/pp1693/. Image to the right is for comparison purposes this is the same stretch of coast line along Pacifica, California, but in 1998. Image source is http://coastal.er.usgs.gov/hurricanes/elnino/ . A YouTube video of the Pacifica case can be found at https://www.youtube.com/watch?v=Hw6sDulj7GA
A rather amazing YouTube video of mass wasting triggered by sea cliff erosion can be viewed at https://www.youtube.com/watch?v=ZVjr4mii3cE .
Note the extensive areas of white, foaming, crashing waves that extend from the cliff out to sea in your Google Earth scene of Mendocino. This area is a wave cut bench, the base of the notch cut as the waves etch their way into the land. The rocks are of different types and are fractured to different degrees, so that some are easier to erode than others, which accounts for much of the irregularity you see in such coast lines. This is what produces the very irregular shoreline with all the small bays. The bays are the area where the softer and/or more fracture rocks allowed the waves to erode further back into the land. Sometimes, the waves erode right around a more resistant section of rocks and leave a small island or rock spire behind as the cliff retreats. These are known as sea stacks.
Especially striking is the fairly flat surface that the town of Mendocino sits on. It contrasts with the rather hilly topography further in. This used to be a wave cut bench, but was uplifted above sea level by the tectonics of an area, to form a feature known as a wave cut terrace. Old beach deposits can often be found covering the very top of such wave cut terraces. To really see the nature of this terrace use the navigation tools so that you have an oblique view.
– On average about how high is the terrace above sea level? This is the amount of tectonic uplift relative to sea level that has occurred since the wave cut bench formed. In that sea level globally has been increasing in the last 10,000 years, it is clear that this terrace is due to crustal uplift.
Question 9: Height of wave cut terrace? ->
– Navigate along the coast (south is best direction to go) here until you find another example of a wave cut terrace. Use the path or polygon tool to trace and identify examples of the following features using the colors indicated: a) sea cliff – brown, b) wave cut terrace – yellow (polygon tool can work well here), c) and sea stack – red. Again, make the lines wide enough they are easily seen.
Question 10: Insert your resulting map here.
These are very different coastal systems, one where biologic activity dominates. A great variety of organisms in the reef environment, including corals and calcareous algae, make homes and skeletal elements out of the ions in the seawater. This hard mineral material gets broken up and reworked after the organisms die resulting in abundant carbonate sediment. Most of southern Florida is made of such carbonate sediment. The reef itself where the interlocking corals create a rigid framework acts as a wave barrier creating a sheltered lagoonal environment behind where sediment collects and with a host of organisms adapted to these conditions. If you ever get the chance snorkeling or diving on these reefs is an incredible experience.
Image to right is of New Caledonia to the east of Australia in the Pacific Ocean. The fringing white line here is a framing coral reef complex that creates sheltered (and lighter blue) waters, with much deeper waters further offshore. Image is from NASA Visible Earth http://soundwaves.usgs.gov/2006/07/awards.html .
Navigate to 18 26.120 N 87 45.505 W and zoom until your view window is about 2 km across. You should be seeing part of the southeastern coastline of the Yucatan Peninsula, and the imagery here shows shallow features. There is a lot that can be seen here, but it may not be obvious at first. Start by finding the line of white about 700 meters off shore. These are breaking waves due to shallowing of the sea bed, and they mark the location of the main barrier reef underneath which is a linear feature roughly parallel to the shore with distinctly deeper water offshore. Note the slightly different coloration of the shallow coral reef with a brownish tinge that can be seen. This reflects the color of living coral. The coral carbonate material is bleached white once the coral that used to live in it dies. Note how as you trace the reef there are breaks and/or interruptions. These are tidal channels that cut through the reef. Between the reef and the beach sand is a white expanse with brownish dots. The white area represents carbonate sand material made from the waves breaking down the coral and other skeletal elements. The brownish dots are small patch reefs that grow in the protected lagoonal area. Different types of corals favor the patch reefs over the main reef. Along the mainland shoreline one can see the thin sandy beach with the jungle on the landward side. If you navigate along the beach at points you will see small islands of vegetation along the shore. This vegetation must be able to tolerate saltwater, and much of it is populated by mangrove, which are small trees that can grow in salt water. Mangroves are also an important biologic component of this shoreline system (in addition to the corals). Sediment gets trapped in amongst the roots of mangroves, and thus these small islands are there because of the mangroves. Finally, note on the seaward and deeper side of the main reef (the fore reef area) a streaked character where the streaks are almost perpendicular to the reef. These are storm related features, where, during large storms (and this is an area where hurricanes occur on a regular basis) strong currents move coral debris and carbonate sand down the ocean-side slope and deeper offshore.
– Now you should navigate along the coast to find another area where you can see many of the same elements. Use the path tool to trace out the main reef with a red line. Use the path tool to circle one or two patch reef areas with a purple line. If you can find any mangrove islands, use the path tool to circle one or two examples with a bright green line. Use the path to trace a tidal channel that cuts through the reef with a light blue line.
Question 11: Make a copy of your image and paste it below along with an explanatory caption.
The effects of hurricanes.
You have already been exposed to the idea that the rate of coastal processes is not constant. Sea cliff retreat is accelerated during storms. One can contemplate the relative contributions of day-to-day events versus the rare but more powerful events, such as a hurricane. More sea cliff retreat often occurs during big storms than during all the time between big storms. More change occurs on a barrier island during a hurricane than all the time between. Therefore it is appropriate to focus on hurricanes in particular. Hurricanes are tropical cyclonic storm systems that originate in equatorial Atlantic waters and have winds in excess of 74 mph. On the west side of the Pacific they are known as typhoons. Sandy and Katrina are hurricanes fresh in many minds at the time of writing this lab book. There were many before them, and these names will retreat in memory as they are replaced by new destructive hurricanes. One of the critical processes during a hurricane is the storm surge.
What is storm surge and what does it do?? Storm surge causes most of the risk and devastation associated with hurricanes, and can also move a large volume of sediment. As the hurricane comes on shore, on one side of the eye (to the north or east for the Atlantic and Gulf coasts of the U.S. respectively) the winds are blowing onshore, and this pushes the water on shore. Remember that these are winds in excess of 70 mph. It is a bit like blowing on a bowl of soup strong enough that you push it up and over the bowl edge. What can be surprising is the magnitude of storm surge – up to 30 feet in extreme cases. For Sandy at the Battery in New York City in 2012 it was a bit over 13 feet and associated waves reached heights of 32 feet above mean sea level. For more details visit NOAA’s (National Oceanic and Atmospheric Agency, federal) website on storm surges – https://www.nhc.noaa.gov/surge/ .
In this simplified image from NOAA note how the storm surge can be added on top of the normal tide to cause a higher storm tide. Also note that the storm waves are above (on top of) the storm tide level. The highest elevation on some barrier islands can be less than the height of the storm surge and thus the water can cover the island during the storm. Image source: https://www.nhc.noaa.gov/surge/ .
This image from the same source shows how the pushing of the hurricane winds causes most of the surge, but a small percentage is also due to the atmospheric low pressure, which causes an increase in the local sea level. Naturally, the larger the hurricane the larger the potential surge, but other factors such as the profile of the sea bed can also make a significant difference.
One way to get an appreciation of storm surges is to see examples and the following YouTube video which you are encouraged to watch is of Hurricane Sandy storm surge – https://www.youtube.com/watch?v=hBsvNfrUN0I. There are plenty of other videos of storm surge for your further education. You can also explore the storm surge risk map from NOAA – https://www.nhc.noaa.gov/surge/risk/ . There are also quite a few YouTube video clips of the storm