Post-Glacial Sea Level

Objective:

To explain why these changes occur, particularly in relation to post-glacial sea level rise.

In this section, we respond to a set of questions that may have come up in the course of working on the preceding exercise. Or, you may wish to discuss these questions in class as a summation.

1. Why and how did Misener's Island erode? Intuitively, we can say that large storm waves cause the erosion by hitting the cliffs with lots of energy (Fig. 39A). The till is carried as eroded particles by the wave as it flows back across the beach. Over the short term, this is true; over the long term it would cause a problem. The more sand that was eroded from the cliffs, the more sand would have to accumulate on the beach at the cliff base. Although sand is transported laterally by longshore drift and out to sea by currents, the accumulation of sediments on the beach would eventually stop waves getting to the cliffs. Any sand that was transported out to sea would eventually be brought back onshore by waves. Thus, there would be a natural feedback process that stopped the erosion and the beach would build out to sea (Fig. 39B).

This is observed in some parts of the world - often in regions where sea level is stable or gradually dropping. However, rising sea level prevents this happening by providing "extra space" out at sea for the beach sand to accumulate (Fig. 39C). Most of the material that is eroded from the cliffs can then be carried offshore by currents and stored in deep water where the waves no longer have any effect.

This is why post glacial sea level rise has led to erosion of our coasts. Because in many areas sea level is still rising, this erosion continues today, as clearly seen at Harris Misener's island.

2. How did the barrier beach migrate? We have already mentioned that the barrier beach was supplied with sediment from the eroding drumlin cliffs by the process of longshore drift. Much of the erosion and longshore drift happen during large storms when the energy of waves and currents, or their capacity to move the sediments, is much greater than usual. At the same time, large waves strike the barrier beach, which is only 3 to 4 metres high, and completely wash over it, moving large amounts of sediment from the beach into the bay behind (Fig. 40A).

The barrier beach is therefore being recycled constantly, with beach sediments being moved from the front of the beach to the back side. The net result is a migration of the entire barrier beach inland (Fig. 40B).

This is also a response to sea-level rising gradually. As sea level rises, the tendency for waves to wash over the beach would gradually increase.

If sea level dropped, the tendency for waves to overtop the beach would decrease and eventually stop. As shown in Figure 39B, the beach would then tend to build seaward.

Figure 39. (A, B, C) Coastal erosion under different conditions of sea level change.

Figure 40. (A, B) Migration of a barrier beach.

3. Why did the spit break up into separate barrier islands?

The spit is subject to the same action by waves during storms. Sometimes the washover is concentrated in particular areas and is so strong that the waves break through the barrier entirely. These areas are referred to as washover channels.

Often, the longshore drift will eventually move enough sediment to repair the washover channels created during a storm. However, if the amount of sand and gravel being eroded from the cliffs is insufficient, or the longshore drift is not strong enough, the channels may not fill up and the barrier islands may become permanently separated from the spit.

4. What will happen in the future at Story Head? Although predicting the future is hazardous, we can make some suggestions based on the above observations.

Sea level will probably keep rising for some time to come; it is predicted that increased melting of glaciers caused by greenhouse-effect warming will make sea level rise considerably faster over the next hundred years than it has over the last few thousand years. The tendency for cliffs to be eroded by the waves and the barrier beach to migrate landward will continue.

That barrier islands formed on one side of Story Head suggest the sand and gravel being supplied by the eroding cliffs may not be enough to maintain the barrier beach as a continuous structure. It will become more and more difficult for the cliffs to supply enough sediment to keep the barrier from being destroyed. Since the video was made, Don Forbes reports that an inlet (channel) has developed across the barrier beach, very close to Story Head.

Erosion of Story Head will continue until, like Harris Misener's island, it will disappear. At the present rates of erosional retreat, this may take over 100 years. But, if sea level starts to rise faster due to climate warming, it may happen within our lifetimes!

Last Modified: 2004-12-10