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Healing a Headcut

Two headcuts had developed on a short stretch of the San Pablo Creek due to a single large culvert (12 ft diameter) that was placed too low. The two headcuts were approximately 30 and 15 inches tall, separated by just over 100 feet. The two separate cuts were advancing upstream, threatening reaches of the stream that were well vegetated and healthy. The Committee decided to take action to halt and stabilize the headcuts, so the area could begin healing and withstand the seasonal flash floods without further erosion.

What’s a Headcut?
What’s a Headcut?

Headcuts are characterized by:
  • a waterfall or abrupt change in slope of a streambed;
  • a fragile, cracked, or crumbling lip of the falls;
  • a bowl-shaped pool at the base of the fall (called a plunge pool);
  • undercutting;
  • rapid headward (that is, upstream) erosion during flood flows; followed by
  • drying, cracking, and sloughing during low-flow or no-flow periods.
Log Structures
Log Structures
During the design phase, the Committee decided to install a stepped log structure, with the logs running parallel to the stream channel. The log structures allow water to fall without causing further cutting. The logs provide a non-erodable surface for the water to flow over and splash on, while not impounding water.

The log structures stabilize the soil and help keep it wet, allowing vegetation to reestablish and grow. In the long run, the vegetation will hold the soil. Over time, the logs will decay, making it easier for roots and vegetation to grow around the structures, further anchoring the logs and soil in place. This type of structure only works for streams that are perennial, because a sustained source of water is essential for growing the vegetation that is the final solution for stopping active headcutting.

In this instance, it was necessary to build large structures. However, the larger of the two structures is actually too large to be practical or cost-effective. We do not recommend building structures this large. A good rule of thumb is that headcuts taller than 30 to 36 inches are probably not suitable for this approach.
Vegetation
Vegetation

Restoration at this site included building the log structures and supplementing the local willow and sedge populations by planting cuttings and plugs. It is important to note that the Fence-Out project surrounded these log structures. The remarkable boost in vegetation due to the Fence-Out played a critical role in establishing enough vegetation to protect the large log structures and hold the soil in place.