Short answers, pulled from the story.
An alluvial fan is a cone-shaped accumulation of sediment that spreads outward where a confined channel, such as a mountain canyon, exits onto open flat ground. The sudden reduction in flow velocity causes the water or debris to drop the sediment it was carrying, building a fan-shaped deposit that can range from less than 1 square kilometer to nearly 20,000 square kilometers in area.
Some of the largest alluvial fans on Earth are found along the Himalayan mountain front on the Indo-Gangetic Plain. The Koshi River alone has built a megafan covering approximately 15,000 square kilometers below its exit from the Himalayan foothills onto the plains of India.
Alluvial fans have been confirmed on both Mars and Titan. On Mars, fans inside craters such as Saheki, Gale, and Holden Crater confirm past fluvial activity, with the Curiosity rover physically verifying sediments in Gale Crater. On Titan, the Cassini-Huygens mission detected fans at the mouths of methane-ethane rivers using synthetic aperture radar.
Alluvial fan floods are unusually dangerous because the flow path is unpredictable: channels fill with sediment and the water breaks out in a new direction without warning. Floods are typically short, lasting only a few hours, but carry high sediment loads and can transition between clear water, hyperconcentrated flow, and debris flow. Raising buildings on fill by up to a meter is not sufficient to mitigate the risk.
Debris flow fans receive most of their sediment as thick, concrete-like slurries of water and particles ranging from clay to boulders, and they tend to be steep and poorly vegetated. Fluvial fans are built by stream flow, including flash floods and sheetfloods that carry between 20 and 45 percent sediment by volume, and they can grow far larger, with gentler slopes.
In August 2008, high monsoon flows breached the Koshi River embankment, diverting most of the river into an unprotected ancient channel across the central part of its megafan. The flooded area had been densely settled and stable for over 200 years. More than a million people were rendered homeless, approximately a thousand died, and thousands of hectares of crops were destroyed.