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Building Principles: A Pig Fell in the Mud . . .
There’s an old joke that starts, “A pig fell in the mud . . .” and while mud at camp can provide a fine program opportunity, there’s nothing funny about mud tracking into all of the indoor program and living spaces. Also, mud in waterways and streams can quickly lower water quality and attract the attention of all sorts of uninvited guests (like insects and regulators). This month, by examining how mud is created, we’ll be able to devise strategies to reduce or even eliminate it.
Muddy Snow Globes
We all know that “mud” is simply a mixture of soil particles and water. What you may have never considered is that the physical properties of the soil vary not just across your property but by depth. Generally speaking, there are two layers of earth we need to consider: The top layer (hence the name “topsoil”) contains a large percentage of organic materials from decomposing vegetation. It’s usually a dark color and somewhat spongy. Beneath the topsoil is the mineral layer comprised of varying degrees of decomposing rock, usually with the finest material near the surface and the more massive and cohesive stuff in the deeper layers above bedrock.
Soil scientists classify the material above bedrock through three descriptors that refer to the size of individual particles. Those are sand, silt, and clay. The most intuitive way to identify the soil matrix components is by touch. Just like at the beach, sand feels gritty; silt feels powdery and soft; and clay feels sticky.
You may have heard the term “loam.” That refers to a mixture of the three par¬ticle sizes with about 40 percent sand, 20 percent clay, and 40 percent silt. You might guess that sand can be a fairly large particle. But it may surprise you to learn that clay, and not silt, is the smallest of the three particles. In fact, the illustration on the right (see Figure 1) really helps make that size difference clear. The clay particle is represented by the tiny dot, and the other particles are shown relative to that dot!
Larger particles (sands) will settle out of a well-mixed solution first — usually in about a minute. Silt-sized particles will fall out of suspension in a couple of hours. Clays can take days because they are so small that electrical forces are stronger than gravity at that scale. The Colorado State pamphlet referenced in Figure 1 even shows a simple way to determine the relative amounts of each component by shaking a tall jar (with a tight lid) that contains a soil sample. (Sounds like a neat environmental science program element to me!)
The point here is that where clay gets into suspension from running water (off a road, in a ditch, or from a stream bank), that material can cause cloudiness long after the rain event. When the clay does settle out of the water (say, in a lake or eddy in a stream), it takes almost nothing at all to stir things up and have cloudy water again. Cloudy water gets warm faster and stays warm longer. This drives oxygen out of the solution and stresses, even smothers, aquatic life. Keeping silt and clay out of runoff — prevention — is the key to healthy surface waters.
Laying the Mountains Low
Erosion, defined by the movement of soil by glacier, wind, or water, is a natural process that’s literally shaping the world around us today, as it has for countless millennia. Understand, then, that the effort to control it is monumental. It’s like the riddle: “How do you eat an elephant? One bite at a time!”
Making mud requires water, soil, and energy. You can’t stop the rain, and soil is everywhere. So the only thing that’s left is the energy component. As the shaker jar experiment illustrates, once the energy from the muddy water dissipates, all of the soil components will eventually settle out. The harder the jar is shaken, the more thoroughly the particles are suspended. More system energy means longer settling time, and we’ve seen that very little system energy will allow clay particles to stay suspended a long, long time. In a stream, that can be miles and miles.
Slowing the Changes
So what can we do to prevent erosion? We can slow the water (reducing the energy), cover the soil, or both. How can we slow the water? We know that smooth surfaces allow water to move faster than rough ones. That’s why a water slide works, right? So it follows that rougher surfaces will hinder water flow.
If you were to follow a drop in a rainstorm, it would look like this: The raindrop hits a road, and gravity pulls it downhill immediately. On a properly graded road, that would be away from the center line toward the shoulder. If the road is paved, it moves quicker than it would on a soil or gravel road. When the raindrop gets to a thickly grassed shoulder (a much rougher surface than a road), it will stack up, “looking” for a path downhill.
We prevent road-edge erosion by sloping the shoulder more steeply so that the water crosses it instead of running down the road edge. Since we want to be able to walk comfortably, the shoulder can’t be too steep — about 4 percent. The right combination of shoulder material (grass and topsoil or gravel) and slope will allow the water to move across without picking up soil particles. The ditch sides are steeper than the shoulder (with ratios from 3:1 to 2:1), so the grass needs to be thicker and more deeply rooted to keep the accelerating water separated from the soil.
Along the ditch bottom (which is seldom as steep as the ditch sides), water will flow deeper and move slower. Silt and sand particles that have been suspended can fall out, leaving only clays to flow along. The process changes again when the water gets to a crossing pipe (“culvert”). Because the materials inside the culvert (including concrete and most plastic pipe) are usually much smoother than the ditch on either end, the water accelerates through it. This is why culvert outlets require extra protection from erosion.
Keeping Things Separated
By avoiding a waterfall at the outlet (even a small one) and discharging the pipe to the ditch bottom, erosive forces are kept to a minimum. But depending on the steepness of the pipe and the amount of water flowing, you may need to do even more to keep the soil in place. Large, uniformly sized rocks (in size ranges from two inches to twenty-four inches), called “riprap,” are used to dissipate the high velocity at pipe outlets. Each state’s department of transportation has charts that will help you pick the right size and shape for the dissipater apron.
One element that you should never skip is the need for a nonwoven fabric underneath the riprap directly on the soil. This special material provides a physical barrier between swirling water and the erodible soil and keeps it in place. Properly sized and constructed riprap outlet aprons will discharge very clear runoff, or at least no more cloudy than what went into the pipe. If the water gets dirtier after a pipe outlet, your apron is not right, and it should be fixed immediately.
What about areas that need good vegetation but need to convey water before it’s established? There are a number of products on the market that will help stabilize shoulders, ditch sides, and bottoms until the grass is fixed in place. Generally, these come as rolls of mat that are anchored to the ground with long U-shaped pins. Some of these products are expected to decompose over time, as the grass takes over. These are usually made of natural materials like coconut husks. Others, though, will remain indefinitely, so make sure that you know the properties of the mat that you purchase. Oddly enough, the decomposing mats tend to cost more than their “forever” cousins.
Keep a couple things in mind when using stabilizing mats. First, the mats are marked with locations for the anchors to be pushed through. Each and every mark must get a pin, or the mat won’t stay in place. Putting it down new isn’t hard, but it’s not easy. Re-laying bunched up mat is nearly impossible to do right or well. Next, the pins are usually made of steel. Those will play havoc on mowing equipment (and sandaled feet) long after the mat is dissolved. Consider investing in another program-usable tool — an inexpensive metal detector — so that you can either recover the pins or drive them deeper when the grass is established. If your soil isn’t particularly rocky or hard, there are also plastic alternatives, but those bring some of the same concerns.
Doing It Right . . . The First Time
Finally, if you’re a regular reader of “Building Principles,” you surely remember hearing what an asset your state university’s agricultural extension office is when planning and maintaining your facility and its natural features. This topic is no exception, and in fact, the extension office may be your very best resource — their main gig is growing things and keeping soil in place. From analysis of your soil’s chemical and physical properties to recommending seed mixtures that will grow fast, thick, and hardy, those experts can help you get this job done right the very first time. And as far as I know, that expertise is available to you for free. All it takes is a telephone call, but you have to ask for help.
All of these factoids and techniques can be used across your property — anywhere soil is moving and creating a muddy problem downhill or down¬stream. Next time runoff is happening, take a walk in the rain and follow the problem uphill. Observe where water gets clearer and take some notes. When the sun comes out, you’ll be ready to make some changes to keep it from happening again — protecting the environment and your infrastructure at the same time.
Rick Stryker is a professional engineer with a particular passion for helping camps with infrastructure, planning, and regulatory issues. He can always be reached at email@example.com or 570.828.4004.
Originally published in the 2013 January/February Camping Magazine