Feedback in Action: Sea Level Rise and Coastal Subsidence

Sea Level Rise, Positive Feedback Loops and Coastal Subsidence

As you have just learned, sea level can fluctuate across a range of scales, from water level change due to tides to longer term changes driven by climate and the storage or release of ocean water in glaciers and ice sheets. You also learned that a wide range of datasets and observations indicate that, globally, sea level has been rising for the last 18,000 years and that the rate of rise appears to have accelerated in historic times. This change in sea level is known as an absolute change in sea level because it is caused by a change in the volume of ocean water in the ocean basins, which leads to a change in the height of the sea level.

In many places, this rise in absolute sea level is leading to the inundation of coastal systems such as barrier islands and saltwater marshes. This means that as sea level rises in the hydrosphere, other systems such as barrier islands and saltwater marshes are subjected to higher water levels. Thus, for their survival, these systems must somehow maintain and adapt their elevation above the rising absolute sea level. In some cases, these coastal environments are unable to adapt to the change and the total area of land above sea levels diminished. As such, environments such as barrier islands and saltmarshes become progressively inundated by the higher sea level and ultimately are lost to the sea by erosive processes. Consider a barrier island system that is subjected to a rise in sea level. It must have a steady supply of sediment to build vertically to maintain its height above sea level. If, however, there is no additional sediment available to the barrier island, then there is no way that additional sediment can accumulate to build the elevation of the barrier island. Without additional sediment, it will eventually be overwhelmed by the increase in sea level and drown in place.

There are also other mechanisms that can cause changes in the height of sea level at a given geographic location. For example, across the Mississippi River delta, similar to many other deltas, the land surface is known to be sinking. As the land surface sinks or subsides, there is an apparent rise in the elevation of sea level, even if the absolute elevation of sea level has not changed. Across the Mississippi River delta, there are several mechanisms known that can cause the land surface to sink. One of these is the slow, regional subsidence of the earth's crust because of loading of the crust by the weight of thick layers of sediment. Heavy loads of sediment act to slowly depress the crust and lower the land surface, especially if new sediments are not continually added to maintain the shoreline elevation. Another process is the compaction of buried sediment. When a layer of water-rich sediment becomes buried by additional sediment accumulating above, buried sediment layers undergo compaction because water is squeezed out of the pore space within the sediment by the weight of the overlying sediment. This is especially a challenge in organic-rich sediments or peat layers that accumulate in marsh and swamp environments that can lose a significant amount of water due to compaction. This loss of water means that the thickness of the buried sedimentary layer is reduced, which causes a reduction in the elevation of the land surface. For example, consider what would happen if you were to take a wet sponge and compress it with your hands. The water is squeezed out of the pore space of the sponge, and the force imposed by your hand causes the elevation of the top of the sponge to be lowered.

In the photos below, taken at Wallops Island, Virginia, barrier island roll-over due to storm overwash has acted to bury the salt marsh that grew in the quiet water behind the island. When first buried, the organic-rich muddy salt marsh sediment is thick and spongy, but over time, it becomes compacted and subsides. As the beach sands of the barrier island are eroded and pushed back, the buried compacted marsh will often become exposed and eroded on the ocean side of the barrier island. Hurricane Sandy, in the fall of 2012, produced both the overwash fan on the bayside of the island (image on the left), and also eroded and exposed a wide, compacted marsh shelf on the seaside that was exposed only at extremely low tide. Within days after the storm, normal wave action fully eroded the compacted marsh layers and the underlying bay muds, leaving the southern end of Wallops Island more susceptible to future storms and continued sea level rise.

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Figure 4.39: Above: Photograph after Hurricane Sandy showing the southern tip of Wallops Island where beach sediments were pushed back across the island and buried the salt marsh as part of a overwash fan. Below, the same location, but viewed toward the ocean side (east). Hurricane Sandy eroded ocean side sediments and exposed the old buried salt marsh that had been buried underneath the island for decades. Hurricane Sandy and earlier storms have caused the barrier island to lose elevation, and erode landward resulting in both a loss of the salt marsh behind and the dune systems that help stabilize the island from the erosive impacts of storm surges.
Credit: Sean R. Cornell

So, there is a potential for a very complex feedback between systems in an environment such as the Mississippi River delta or barrier islands. The delivery of sediment to the delta results in some land building and helps to maintain the elevation of the delta surface at or slightly above the elevation of sea level. The accumulating sediment however also causes the compaction of underlying sediment layers, which leads to a reduction in elevation of the delta surface and subjects the coastal marshes and barrier islands of the delta to greater marine inundation. Thus, as sediments accumulate in an area that is experiencing marine inundation because of a rise in global sea level, the accumulation of the sediment can positively reinforce the inundation because additional sediments may lead to a reduction in the land surface elevation. In the case of barrier islands like Wallops, which is located on the Mid-Atlantic Coast, much of the island building processes was prehistoric, when the beach systems were well-supplied with sediment from long-shore currents. More recent engineering projects updrift near Ocean City, Maryland, and natural processes have resulted in a reduced budget load, so that the barrier islands are not able to maintain critical elevation to minimize overwash. Thus, overwash processes continue to smother productive upward-building salt marsh ecosystems and actually act to produce even more subsidence.

In both cases, the interaction of the different coastal systems are so complex that relationships between each component and the feedback processes are not yet fully understood. Many different coastal geologists are continuously attempting to monitor, document, and quantify the feedback loops and interactions between the diverse biologic and geologic systems operating in these coastal settings. Major coastal events such as those to be discussed in the next module (Module 5), although destructive, provide significant opportunities to further consider how these coastal systems continue to respond and evolve in the face of short-term and long-term processes.