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Guess What Runs Downhill? (and it’s probably not what you’re thinking …)
by Rick Stryker, P.E.
If you're a regular reader of Building Principles, you're probably expecting another literary treasure about wastewater. Believe it or not, there are other facilities that are affected by gravity. This month, we're going to look briefly at soil erosion. Engineers and regulators refer to the subject as "Sedimentation and Erosion Control." Although the fundamental processes are pretty much common sense, most people don't put these pieces together until there's a problem or an issue.
The freeze-thaw cycle, glacial movement, and the wind all break down the earth's surface. But these forces are generally very slow, occurring over millennia with effects so incremental that they're overlooked by everyone but a studying scientist. On the other hand, erosion and sedimentation are much more dramatic, resulting from water moving over the surface of the earth. The moving water's energy picks up tiny particles of soil (the erosion part) and keeps them suspended. When the water stream slows, it loses energy and the particles fall out of the fluid (the sedimentation part). With some im portant exceptions, the heaviest fall first and the lightest ones last. On a grand scale, this is visible from space in major river deltas. On a smaller, more camp-size scale, the inlet to your lake might come to mind. But the principles of both of these examples are largely the same, despite that enormous range in size.
"But if the energy in flowing water keeps the particles suspended," you ask, "Why are stagnant mud puddles often muddy and cloudy?" An excellent question, Grasshopper! It's because some of the individual particles are so small that gravity isn't the dominant force acting on it. The smallest particles of mineral soils are clays. They are so tiny that instead of gravity governing them, they respond instead to electro-magnetic forces between them and the water molecules, gluing them together until the water evaporates. This is why when clay soils get wet, they stay wet. This is also what makes muddy water (which doesn't clear up) cloudy, and it creates real problems for aquatic organisms. Follow this chain of events: Cloudy water warms more quickly and stays warm longer than clear water. Warmer water holds less dissolved oxygen than colder water, so in the end, native species of fish and other aquatic life can suffocate in a relatively short time. This seemingly short-term problem can have very long-term effects.
Strategies to Reduce Sedimentation and Erosion
So here we are at the crux of the problem: Gravity causes water to run downhill, running water picks up soil particles, moving and ultimately depositing those particles where we probably don't want them. What to do? The answer lies in a multi-pronged approach where you can use many strategies if you keep in mind what we've learned so far. In past columns, we've looked at the cost of earth moving and seen how expensive it is, relative to the other components of a construction project. For all of the same reasons, it's equally expensive to return soil from where it has landed. Over the long run, then, it is much cheaper to keep soil in place than it is to move it back and to separate it from its carrier, surface water.
Your first, best bet is to keep the muddy water from getting to a water way like a lake, creek, or stream. Where there is a construction project underway, temporary measures like a silt fence and straw bales can help to keep material inside the limits of construction. A "silt fence" is intended to screen the larger particles from suspension, acting much like a filter for water flowing from the bare earth around the project site.
The agency responsible for approving and monitoring these assemblies will provide standard construction details which show how this should be installed in your area. Generally speaking though, the fabric will be stretched between wooden or galvanized posts, with a roll of the material buried in a trench along the bottom. Some standard installations support the back of the fence with staked straw bales, and others have galvanized fencing.
A silt fence should be inspected frequently to prevent breaches and repaired immediately. If the system is placed and installed correctly, sediment will collect on the uphill side. This has to be removed and respread after each storm event. As soon as construction uphill of the fence is complete, those areas should be planted with an appropriate ground cover to hold the surface material in place. Once the vegetation is established, the silt fence is removed and taken to the landfill. This isn't cheap or easy. Properly prepared construction documents will ensure that the installing contractor is responsible for temporary measures from installation to disposal. Do not pay for this item until the work is complete, or you'll be left holding the bag (or waste fence).
Sediment Traps and Other Techniques
Other techniques may be required for large construction projects including sediment traps, detention facilities, silt sacks and others. Each of these has specific operating and maintenance instructions which are part of the plans and permits. Although your contractor is responsible for ensuring that they are installed, operated and dismantled in accordance with the permits, your organization or you personally, are financially responsible. Under most circumstances, you, not the contractor, will be responsible for paying any fines which are levied if the temporary measures aren't installed or maintained in accordance with the permit. Hiring a competent, experienced, and diligent contractor may appear to cost more initially, but it may save you from wasting money in the long run.
There may be slopes and embankments that are part of the final project, but are too steep to keep ground cover seed in place long enough to germinate and establish. In these cases, other devices may be required. Among a host of others, manufacturers produce mats of different materials that will act like a blanket through which rain can water and the vegetation can grow. At the same time, it slows the flow of surface water that would otherwise wash away seed and mulch. Roadway drainage ditches are often "armored" with mats in this way. Modern blankets or mats are often biodegradable, disappearing in months as the vegetation becomes established. However, these must be installed in accordance with the manufacturer's instructions, or they simply will not work correctly. Most often, the installer fails to use enough anchors or staples to keep the mat in place. Where this happens, what's left at the bottom of the hill is often a gloppy, goopy clump of mud, seed, (and money) that looks a lot like erosion control matting. As is often the case, manufacturer's "recommendations" are more of a mandate if you're going to get what you're paying for, so this is no time to skimp or cut corners.
Unraveling the Mystery
But what about where there is no construction and there's erosion and sedimentation happening already? Let's look at where you're likely to find that and think about how you might unravel the mystery.
Flats of sediment will accumulate at the bottom of grades where water ends its run down hill and its energy is dissipated. Here's the first indicator of a problem, because the material came from somewhere uphill. If you can't identify the channel immediately, perhaps a walk in the next rainstorm is in order. Begin at the pile of sediment at the bottom of the hill, and work your way upstream. In short order, it's likely that you'll encounter a channelizing feature of some sort like a culvert pipe or downspout. These structures are usually quite smooth inside, engineered that way to ensure a fast flow and quick passage for water inside. Think of them as "water cannons" because that's exactly how they act. They're causing erosion because the energy of the water leaving the pipe is more than enough to cut soil particles from their matrix, suspend them, and carry them away. The solution to this is to slow the water leaving the pipes. The first thing that you should see is that the end of the pipe is fit with an outlet that contains the soil around the pipe end, called a "headwall." Another mechanism at the outlet of the pipe changes the shape of the discharging water from a round, cylinder to a wider, flat flow called a "flared end." This spreads the energy over a less efficient shape. The larger contact surface between the water and the structure increases the friction and slows the rushing water. You may see a "discharge apron," comprised of large rocks called "riprap." Individual rocks can vary from the size of a closed fist up to the size of a VW Beetle.
To function correctly, these energy dissipaters are constructed so that the top of the rock bed is even with the elevation of the inside bottom of the discharging pipe (known as the "pipe invert"). Beneath the rocks, a geotextile protects the soil from the rushing water as it tumbles through the riprap. The rocks cause the water to splash and lose energy, and the geotextile protects the soil particles from the erosive water moving above. Careful design is every bit as important as careful installation. For the system to work correctly, the storm flow rate, the slope and dimensions of the pipe, determine the dimensions of the apron, the depth of the stone bed, the size of the rocks, and the slope of the discharge area combine to produce an integrated system which is neither too big nor too small. A "one-size-fits-all" solution isn't really a solution anyone can afford since any size but "just right" costs more.
Finally, what is this "ground cover" that we've been talking about? Most often, it's a shallow rooting, vigorously spreading vegetation that quickly develops a thick root mat, creating a fibrous matrix with the soil itself. In some regions, rye grasses provide a great temporary cover while other slower growing grasses take hold. A hybrid plant called Crown Vetch was developed at Penn State University many years ago. The plant has been the highway median and shoulder stabilization across the country. This might be a great solution for some areas of your property, but its tendency to spread and tenacious root system may damage concrete and may make it a very poor selection in other places.
So how do you know what's right for you? Probably the very best place to find answers to this question is your local (usually county) agricultural extension office. Among other services, they're in the business of helping farmers keep their soil in place and often have connections to others in your area with similar topography and soil conditions. They're very likely to already have an extensive list of seed mixes that thrive in your area. Moreover, they can probably take you to visit neighbors and other sites nearby where folks have had a range of success. There's nothing like seeing and comparing things done right and working with failing projects installed or planned to help you make the right decision for you and your facility.
By now, you're probably surprised to have learned that the "control" part of "erosion control" is a misnomer, since erosion is a fundamental process shaping our world. Erosion management better describes a successful project. So, instead of living with the effects of uncontrolled runoff, get a handle on the existing problem areas by ensuring that your next project includes comprehensive, well planned temporary and permanent erosion management plans.
Rick Stryker, P.E., is a professional engineer, serving camps and conference centers nationwide with consulting services from site planning and design through general consultations on a wide range of infrastructure issues, including water supply, sewage disposal, and construction administration. He can be reached at 570-828-4004, or by e-mail at firstname.lastname@example.org.
Originally published in the 2009 September/October issue of Camping Magazine.