Engineering is concerned with the meeting of science with action. What does our knowledge of matter, force and energy allow us to predict about the behavior of a given system? How will our constructions or modifications to a natural system perform? The task of the engineer is to make those predictions, and so help outline the most worthwhile plan to accomplish stakeholders’ goals. (The worthiness of the goals is a separate consideration.) Coastal engineers are in the business of predicting sediment behavior, both due to natural influences and after human intervention, with the goal of building new, or preserving existing, structures.
In the case of Kiribati professor Paul Kench of Simon Fraser
University holds the opinion (borne out by evidence from a number of Pacific
islands, though far from all) that atolls need not necessarily sink beneath
rising ocean levels, but that they will continue to be rebuilt at slightly
above sea level. As evidence he cites the fact that coral islands and atolls
tend to have a very consistent 1.5-2 m height above sea level wherever they do
occur. As natural features they are very consistent in appearance, and indicate
consistent natural processes in their formation and maintenance.
This is a valid observation but for the islands to continually be rebuilt above rising sea level, two main assumptions are made. First, that the living coral reef—the foundation of the entire structure—can keep up with sea level rise. This is not necessarily the case. NOAA scientists have estimated that living coral, especially with warming seas, will have trouble growing to match increasingly rapid sea level rise around the world in coming decades.
Second is that there is sufficient sediment supply to accomplish the rise in land level to match the sea. This is not necessarily wrong, but is a still weaker assumption. Barrier islands and beaches attached to continents have the uplands to migrate upward across as the sea transgresses. Islands have no such retreat, and do not supply sediment to one another.
A factor as simple as sediment’s natural angle of repose
becomes important here. For a given amount of sediment, its natural angle of
repose determines the height which it can achieve, in air or in water. To grow
in height, either fresh sediment must be supplied, or it must be reallocated
within the pile, resulting in a steeper slope in several locations. In those
locations, gravity will act to pull the sediment down back toward its natural
angle of repose and reducing the pile’s overall height.
This is the converse of the problem in dredging harbors and shipping channels. The deepened channel will fill over time, since the walls of the channel are much steeper than the angle of repose of the (usually) silt and sand. Regular re-dredging is required to prevent backfill. In the case of Kiribati, dredging the central lagoons and depositing the dredge spoils (as the dredged sediment is called) on top of the islands is a temporary solution, since the dredged portion of the lagoon will slowly backfill and regain its natural slope.
The sediment added to the top of the island will tend to wash
down to the lower elevation leaving the island lower again. Like all
unconsolidated sediment in or near active marine environments, the Kiribati
atolls are in a state of dynamic equilibrium with the surrounding sea. Added
sediment will not be stable: it will not remain in place indefinitely. Dr. Kench
acknowledges this, in saying that dredging the lagoon and adding 0.5 to 1 m to
the islands’ average height will solve the problem for about 50 years. It is by
no means a permanent solution.
Very broadly, in our fight against nature, nature always
wins.
Tomorrow: sea level rise in the Solomon Islands.
Be brave, and be well.
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