Erosion Control Structures Hold the Line on Habitat Health After Deluge

Posted on August 20, 2015 by Borderlands Restoration in Restoring Natural Process
This weir withstood a rain of almost 4 inches in 3 hours, with water levels up to 4 feet deep. This is the most the landowner had seen in 20 years of residence.

This weir withstood a rain of almost 4 inches in 3 hours, with water levels up to 4 feet deep. This is the most the landowner had seen in 20 years of residence.

In early August, parts of the Patagonia Mountains experienced extreme rains that reconfigured existing watercourses, formed new destructive arroyos (or stream channels), destroyed driveways and roads, and carried away valuable habitat components such as organic matter and soil. Luckily for some habitats and landowners, pre-emptive restoration work that improved habitat resilience was already in place and ready to withstand the storm.

Successive weirs change the path of water, allowing it to slow & drop sediments and debris, instead of carrying them away.

Successive weirs change the path of water, allowing it to slow & drop sediments and debris, instead of carrying them away.

In one instance, a landowner had hired Borderlands Restoration to restore healthy natural processes on his property. The owner was concerned about habitat degradation, caused by increased erosion from stormwater.  Water tends to find the lowest path of least resistance, cutting deeper and deeper into the ground and carrying more sediment, nutrients, and moisture out of the area. This results in erosion channels, plants whose roots no longer reach groundwater, and therefore loss of habitat for wildlife, including threatened and endangered species.

The Borderlands’ Restoration Crew set to work installing weirs in eroded arroyos on the property. Weirs are sets of individual wooden posts driven or dug into an arroyo channel bed, from 10-30 posts wide.  A waterway will receive a network of these weirs, so that when flood waters start their destructive cascade, they hit a weir, and “pin ball” off it into the next weir. This effectively transforms the water’s corrosive path into a zig-zag pattern, just like in the pin ball game. The goal of this work is not to earn points, but to stabilize the channel bottom and prevent further down-cutting and erosion.

The property owner was pleased with the new weirs, stating they are, "another example of Borderlands perfection." BR had previously completed other habitat-enhancing projects at the site.

The property owner was pleased with the new weirs, stating they are, “another example of Borderlands perfection.” BR had previously completed other habitat-enhancing projects at the site.

Wait, you might be saying, why doesn’t the water just pass through gaps between posts or uproot the posts? When a weir is installed, the goal is to sink the posts deep enough to reach the scour depth—the depth at which particle sizes increase dramatically, indicating little or no movement of the channel bed at the given depth. When posts are anchored at this level, a channel can be stabilized at its foundation, while the posts that rise above the channel bed capture organic matter and keep it in place, contributing to the formation of rich soil.

We are always ready to consult, plan, and execute contracted projects like these on public and private lands, with permitting, surveys, monitoring, recommendations and plans all available in customized fashion at reasonable rates. Give us a call if you’d like to fix or protect a driveway, arroyo, pond, or other habitat challenge, and we’ll teach you what’s possible on your property and get to work improving habitat for flora, fauna, and people alike.

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    The mission of Borderlands Restoration is to reconnect wildlife, land, and people in the Arizona/Sonora Borderland region by involving people in restoring the ecosystem on which we depend.

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