Tag Archives: sediment spill

Saving beaches at the expense of coral reefs?

Southern Florida, especially Miami Beach, has a problem with beach erosion. Since the city is built on a sand bar on the outside of Biscayne Bay, this is no surprise to geomorphologists. But it is an economic issue when a beach resort lacks a beach. Which is why Miami Beach, several decades ago, became the first place to create a new beach by sand replenishment, i.e., adding a large amount of sand taken elsewhere.

From an economical point of view this has been a complete success. The problem is the environmental side effects.

The world’s third largest barrier reef is located off southern Florida (the largest is the Great Barrier Reef, and the second largest is in the Gulf of Honduras). There are also nearshore reefs off Florida. These coral reefs may be harmed by beach replenishment.

Beach replenishment is done by dredging sand offshore, and releasing it on the beach. It is either pumped or transported in barges. The dredging areas, known as “borrowing areas,” are often located between the three lines of coral reefs outside the beach. When dredging, a certain proportion of the material will inevitably be lost in the water as sediment spill, or dredging spill, creating a cloud of dirty water that drifts with the current. The fraction that is most easily stirred up is silt, finer than sand.

Coral polyps are very sensitive to particles in the water, including this so-called siltation. If covered by just 1 mm of silt they die. That’s just over 1/32″ for you who prefer inches.

Normal beach sand is practically devoid of silt, since waves soon would wash it away. In contrast, the offshore sand can contain many percent of silt. This means that once the sand has been put on the beach, the beach starts leaking silt every time there is some wave action, which harms any coral reef nearby. Consider that the beach is gradually eroding, otherwise it would not have needed replenishment of sand. This means that the beach will continue leaking silt until all the replenished sand is gone (or until all has been reworked, but that will take almost as long).

Here is thus the problem. First, there is a big siltation event offshore during the dredging. Second, there is a slow and long-lasting siltation even nearshore. The latter has in effect, according to a group studying this, killed the nearshore reef on all locations where beach replenishment has been made.

There are of course also other sources of sediment spill. Dredging inlets, such as harbor entrances, comes to mind, as does the dumping of dredged material offshore.

So this is the problem of siltation in a nut shell. I’ll write another blog about what to do about it.