When denser river water flows into the ocean, it creeps along the bottom churning up sediments to create a turbidity current. For the first time, researchers caught a flood-triggered hyperpycnal flow in a murky, cloudy mess surging into the Gulf of Aqaba on video.

When a flashflood surged across the desert and poured into the Red Sea, it triggered a hyperpycnal flow, a phenomena where an influx of sediment-laden fresh water flows along the bottom of the salt water with mixing along the boundary between the fluids. The flow carries sediment from onshore and kicks up sediment along the ocean floor, creating a visibly sediment-laden flow. When the freshwater influx eventually runs out of energy, the sediments drop out to create turbidite deposits. The sudden dumping of sediment in the ocean basins can smother coral reefs and other creatures living along the sea floor.

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A research diver observing a hyperpcnal flow. Image credit: Gil Koplovitz

While this particular event was triggered by a flash flood, similar phenomena happen any time sediment-laden fresh water enters the ocean. Ephemeral rivers are another classic source, while steadier rivers can set up an unstable equilibrium of a freshwater wedge extending into the sea with a zone of brackish water where the layers mix. If the freshwater lacks in sediment to increase its density beyond that of salt water, it stays at the surface instead in a more stable stratified flow.

In the hyperarid Gulf of Aqaba, ephemeral streams and floods are a rare occurrence, making a hyperpycnal flow an even more exciting event most years. However, during the winter of 2012 to 2013, the gulf had an outrageous number of flashflood events, luring researchers into the region. They caught the hyperpycnal flow on video, the first time the phenomena was ever recorded. They also managed to collect other measurements, thoroughly characterizing the event.

The flood was a flow from a flashflood on January 27, 2013, entering the Kinnet canal. Starting at just 1 to 2 meters thick above the seafloor when it entered the canal, it quickly spread to 20 meters wide. Over the next four hours, the hyperpycnal flow grew at about 3 meters per hour, expanding until it was 13 meters thick. By the fifth hour, the plume reached a full kilometers off-shore in a tumbled, cloudy plume.

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Read Desert flash floods form hyperpycnal flows in the coral-rich Gulf of Aqaba, Red Sea. Tip via Brian Romans.