How Bedrock Fractures and Rock Armor Shape Waterfall Erosion

Waterfalls play an important role in river development and landscape evolution. Researchers are especially interested in knickpoints—sudden changes in the steepness of a river channel. Knickpoints can range in size from a few centimeters to features as large as Niagara Falls.

Researchers in Virginia hypothesized that waterfalls, a type of knickpoint, could influence how rivers erode solid rock, or bedrock. They investigated whether cracks and other pre-existing weaknesses affect how flowing water removes rock slabs from river channels through a process called plucking.

How Fracture Direction Affects River Erosion

Although bedrock fractures can form in many directions, the researchers focused on three orientations. A fracture parallel to a flat river bed was classified as a horizontal fracture. When the fracture surface tilted downward in the direction of water flow, it was classified as a downstream-dipping fracture. When it tilted downward against the direction of flow, it was classified as an upstream-dipping fracture.

First, the researchers tested how fracture direction affects erosion upstream of an established knickpoint. They used an artificial channel, called a flume, to simulate a natural river system. To create the experimental river bed, they glued a layer of tiny tiles—about the size of an adult’s thumbnail—to a plastic frame and placed the frame inside the flume. They then placed a layer of unbonded tiles upstream of the knickpoint.

At the start of each experiment, the researchers increased the amount of water flowing through the channel in steps every five minutes. They continued until the flow reached its maximum or all of the loose tiles had been eroded. The researchers expected the tiles to be carried downstream beyond the knickpoint.

They measured water depth, the number of tiles removed by plucking, and the amount of water at fixed locations upstream of the knickpoint. The team repeated the experiment eight times for each fracture direction.

Horizontal Fractures Were Most Vulnerable to Erosion

The research team found differences in both the amount and timing of erosion among the three fracture orientations. In particular, erosion began at lower water velocities in horizontal layers than in sloped layers.

Based on the observed erosion rates, the researchers concluded that horizontal fractures were the most susceptible to erosion. Upstream-dipping fractures were the least erodible, while downstream-dipping fractures had an intermediate level of erodibility.

How Loose Rock Armor Influences Knickpoints

The researchers then tested how knickpoints form and influence water flow when all of the tiles are unbonded and able to move freely. They individually stacked approximately 5,000 loose tiles across the flume. Before starting the experiment, they slowly filled the channel with water, submerging all of the tiles without moving them. The researchers then increased the water flow, allowing erosion to begin.

Instead of collecting the same measurements as in the first experiment, the team filmed the experiment. They observed how the knickpoint shape changed and how the loose tiles were removed.

The researchers found that the tiles pulled from the channel formed a layer of armor. In a natural river, this type of loose-rock armor can protect the river bed beneath a waterfall from erosion. The underlying rock can erode only when flowing water carries the protective armor downstream.

Unlike in the first experiment, the researchers did not find differences in erosion based on fracture direction during the loose-tile experiment. However, they did observe differences in the amount of armor that remained in different locations.

Upstream-dipping beds were the least armored because water quickly carried the tiles near the waterfall downstream. Downstream-dipping beds retained the most armor, and the eroded tiles took longer to move downstream.

Why Crack Direction Matters for Waterfalls

The research team concluded that waterfalls erode at different rates depending on both bedrock fracture direction and the development of protective rock armor. The orientation of cracks in bedrock also influences the shape of a waterfall and the surrounding river channel.

So the next time you see a large waterfall, consider that its shape may be influenced by a tiny crack—possibly no larger than a thumbnail.

Post views: 15