New Mexico Central

Woodchip Berms

by Dr. Christian Meuli, La Resolana

Wood chip berm at La Resolana

Woodchip berms are the easiest and most useful rainwater harvesting practice I use. The evolution of this method began in 2002 when the risk of wildfire in New Mexico became extreme and the pine-bark beetle became epidemic in our pinon-juniper forests.

At La Resolana in Edgewood, thirty miles east of Albuquerque, the forest had grown so thick that little snow and only some rain could reach the ground due to the extremely dense tree cover. These trees also shielded the ground from the sun so that few grasses and shrubs could grow beneath them.

By 1890, most fires in New Mexico were put out as quickly as possible, driving the “yellow wolf” (periodic natural wildfires) out of the area. In the past, wildfires would pass through our forests every 5-15 years. These fires would burn at 3-400 degrees Fahrenheit, prowling beneath the old trees in our high desert forest, burning the grasses and culling the seedlings without harming the tall, older trees. Now small diameter trees grow up beneath the large, old trees, compete with them for nutrients and water, and create fire ladders that cause hot crown fires (up to 2000 degrees Fahrenheit) that rage through the forests, killing the old trees as well. The overgrowth of too many trees in our forests has been a major factor causing the devastating infestation of the pine bark beetle.

Several years ago friends in the Back Yard Tree Farmers came over to thin my forest at La Resolana. Over 1500 trees per acre grew here now, where previously only thirty to fifty trees per acre thrived in a healthy forest. We thoughtfully used chain saws to thin and a massive Vermeer chipper to turn the small diameter pinon pine and juniper trees into firewood and a 10 cubic yard pile of small chips.
I used most of these chips to make paths to help negotiate the yearly mud challenge. With the remaining chips I experimented by laying berms of woodchips along the contour of the land, following a horizontal line perpendicular to the slope of the land.

Using a wooden triangle with a line level attached to its horizontal member, I found the land’s contours by poking surveying flags into the ground. I used a pitch fork to scoop the woodchips into a wheelbarrow, rolled them downhill, spilled them along the flags on contour, and raked them into a berm that snaked across the steep landscape. These berms are creating the densest stands of blue grama grass I have ever seen! They cause the water to show down, spread out, and soak into the soil.

I began experimenting with the best height and width to make these simple woodchip berms. I found that berms 8-12 inches tall and 14-18 inches wide at the base were optimal. They can vary from ten feet to over a hundred feet long, depending where the trees, shrubs, rocks, and paths lie in the landscape. I curve the berm ends uphill to keep water from doing an end run and creating new erosion pathways. I usually lay the berms 30-40 feet apart, but on a steep slope they can be as close as 10-15 feet from each other.

After receiving only three-quarters of an inch of precipitation over a seven month period in 2005-2006, we received an amazing 17 inches of rain over twelve weeks during the summer of 2006, including two 2.5 inch rain events. These huge rains put the woodchip berms to the test on soil that was already saturated with moisture. The woodchip berms performed beyond my wildest expectations! I was delighted to find water standing 8-12 inches deep uphill from the berms before it spilled evenly over 5-20 foot-wide overflows across the tops of these saturated berms.

Friends wondered why these woodchip berms worked so well, with no blowouts in dozens berms. Close observation revealed that the surface of the woodchip berm is moistened from incipient rain falling directly upon it. The exposed chips on the top of the berm soak up the moisture making them dense and heavy. As the chips swell they knit together to create a stable, heavy organic mass on contour. As the rain continues and surface flow begins, the bottom of the berm gets soaked, with capillary action drawing moisture up into the berm’s center. As the rain continues and the water “dams up” behind the berm, the remaining chips in the center get thoroughly saturated. Then the berm becomes “leaky,” allowing water to seep safely through along its entire length. Now the woodchip berm is completely saturated and heavy with moisture.

Once the berm is saturated, the chips mat together in an amazing way. The size of the chips seems to make little difference. The berms take a very long time to dry out. Arina Pittman, the curious director of the Permaculture Institute in Santa Fe, visited several months after the last rain in 2006 and dug down with her fingers to center of the berm finding it still juicy wet!

Of the several dozen woodchip berms on my twelve acre site, only two have displayed any overflow issues. Each had minor “washouts” that were less than a foot wide and cut down only one to two inches below the brim, where the berm became more compact and wide enough that the overflow stabilized. These were easily repaired by raking chips back up over the washout area, taking just a minute to fix.

The heavy summer rains of 2006 filled soil and detritus (tree needle, plant and animal droppings) to the tops of some of the berms. These berms were easily upgraded by laying more woodchips on their top, level edge along their entire lengths.

The first berm should be laid at the top of a slope, so water has not built up enough volume or velocity to wash out the berm. If water comes rushing onto a site via a channel, barrow ditch or arroyo, more intensive interventions should be used to deal with these strong and potentially berm-damaging flows (see Brad Lancaster’s Rainwater Harvesting for Drylands and Beyond, Volume II: Water-Harvesting Earthworks).

Woodchip berms require no digging and are a permanent intervention with results that will last indefinitely. As these berms age their moist bases decompose making wonderful soil and their tops shrink down evenly, maintaining a level brim and conserving their water harvesting function. Eventually terraces develop, enhancing water-slowing and silt-harvesting plant growth, further stabilizing the soil and inviting forest development.

A woodchip berm places chipped slash in direct contact with the ground, keeping it moist for months at a time. It safely adds organic matter to the low-humus soil in our high plains desert. The woodchip berm prevents erosion and wildfires, is fun to design, and is easy to implement. It is a passive rainwater harvesting method that works while you do nothing! It transforms the liability of densely-growing trees and pinon-bark-beetle-infested wood into a soil and forest enhancing asset.

Woodchip berms require little or no maintenance. Woodchip berms are beautiful interventions that meld naturally into the landscape as plants grow vigorously above and below their meandering edges.

Woodchip berms can also be used as drains that guide water downhill. I pondered for years how to safely invite huge water flows from two roads and a culvert into La Resolana. I debated how to “dig a deep drain” from this culvert so I could direct this water downhill into a large swale in the valley below. This approach would require a lot of work and would involve cutting tree roots and digging up rocks to deepen the drain so it would flow evenly and easily. I got tired even thinking about the digging it would require and the trees it would harm! I eventually envisioned a woodchip berm on top of the ground and off contour accomplishing the same goal with minimal work (no digging!) that could safely guide the water downhill into the swale. Laying woodchips on top of the ground and raking them into a gradual curve (so detritis would not slow down and collect) was simple to implement and has worked exceedingly well! I check its flow during and after heavy rains to make sure debris does not dam up a section and cause an unwanted breakout of this rapidly flowing water.

Friday, October 29th, 2010, by John Weckerle and is filed under "Ecology/Environment, Edgewood, Permaculture ". You can leave a response here. Pinging is currently not allowed.