Given how many rivers there are in this country, it comes as no surprise that a rainy season rarely passes without a river overflowing its banks. As this planet continues to undergo climate change, and weather conditions become more extreme, this phenomenon will occur even more frequently.
Erosion occurs when streams and rivers become inundated with more water than they can hold. This happens due to increased stormwater runoff or because of actual rainfall events. The extra water results in scouring, which is when water eats away at the banks of a river or stream. So riverbank and stream bank erosion control is about protecting banks from the water that runs between its shores.
One of the most common ways to protect riverbanks and stream banks is by using what is known as hard armor solutions. These solutions include the use of riprap, gabions and segmental retaining walls. Although each of these methods is different, the science behind them is the same: a durable material, such as rock or concrete, is placed on the face of the riverbank to protect it from eroding and eventually collapsing.
Riprap is simply a collection of rocks and stones placed along the bank to increase its resistance to erosion. Prior to installation, the bank is graded to a predetermined angle. After the grading is complete, the stones are placed at the toe of the bank, on its face or everywhere in between. Where you place the stones will be determined by the needs of the particular project.
Riprap is always buried into the bed to some degree. When high flows occur, the stream will cut into the bank and take away large chunks of it. So if the riprap isn’t keyed into the bank, it is likely that it will be undercut and crumble.
It may be helpful to think about installing a fence post. You don’t just leave the post sitting on the surface. You bury it deep enough so that it doesn’t get knocked over every time the wind blows. Well, the same principal applies when working with riprap.
Stone selection is an important consideration. In climates where freeze/thaw cycles are common, it’s a good idea to choose a stone that can stand up to the elements. Less durable stones will decompose quickly and need to be replaced frequently. Granite and most other igneous stones are strong enough to withstand even the harshest conditions.
Function matters more than form, but it’s important to think about aesthetics. “If we’re going to use riprap, we typically use fieldstone,” says Jeff Weiss, water resources engineer at Barr Engineering Company, Minneapolis,
Minnesota. “It’s just rounded rock that is harvested from a gravel pit, but it looks really nice and it blends well into a lot of different areas. With other stones, it’s often clear that they are alien to the environment, so they can be eyesores. You don’t want to use anything that’s going to look flat-out ugly.”
Before the riprap is installed, filter fabric is often placed under neath the rocks to ensure that the finer bits of sediment don’t escape through the gaps in between the stones. Filter fabric is usually some sort of geosynthetic material. If the smaller soil particles don’t remain in place, the bank will eventually collapse. “Without filter fabric, you’re not really going to be able to stop erosion,” says Weiss. “You’ll slow it down, but sooner than later, the bank is going to fail.”
Gabions are another commonly used hard armor solution. Gabions are multi-celled wire or mesh boxes that are filled with stones, rocks or other fill material. They are linked together to form a monolithic structure and can be placed on the toe, face or lip of a bank. Gabions come in various sizes and are commonly stacked on top of each other to form retaining walls. Although typically constructed of galvanized wire, they are also available with PVC coating for use when dealing with caustic surfaces such as high alkaloid soils.
As is the case with most methods of shoreline erosion control, the first step when installing gabions is to grade the area to be protected to the desired slope. Typically, after this step is completed, installers will lay down a filter fabric. As is the case with riprap, if soil is allowed to migrate away from the bank, it will collapse and all your hard work will have been pointless. Try to avoid laying the filter fabric beneath the baskets. Doing so will reduce the gabions’ ability to hold onto the soil surface and can lead to slippage. Instead, place the filter material behind the cages.
After the filter material is installed, the gabions are assembled, set into place and laced or tied together.
You’re probably familiar with the phrase “A chain is only as strong as its weakest link.” When working with gabions, it’s a good idea to keep this saying in mind. If one gabion basket is lost because it is poorly connected to the adjacent unit, it’s very likely that the whole structure will be rendered ineffective.
After the gabions are connected, filling can begin. Be sure to place the rocks and stones carefully. Haphazard filling can damage the baskets, leading to failure down the line.
Gabions are also available as mattresses. Mattresses consist of wire mesh baskets as well, but they are thinner and longer than traditional gabions. Mattresses are placed inside the water, six to eight feet in front of gabion baskets to help combat scouring.
More and more frequently, segmental retaining walls (SRWs) are being installed along shorelines where erosion is a concern. SRWs are retaining walls that rely on their own weight to stay in place. They are constructed of individual concrete units that are small enough to be installed with relative ease. Installation can typically be performed by one worker without the aid of heavy construction or installation equipment.
Unlike traditional retaining walls, they are not constructed using mortar. Instead, they are “dry stacked,” and are connected together mechanically, using devices such as nylon pins. Because they are fixed in place using a pin, the structures are flexible, which allows them to withstand minor ground movement.
Another common approach to riverbank and stream bank erosion control is the use of soft armor solutions. These methods function almost identically to hard armor solutions; however, instead of using hard materials, they rely on the use of softer substances such as synthetic fabrics, grasses and other vegetation. On the whole, soft armoring is less expensive and more environmentally-friendly than hard armoring.
Geotextiles are one of the more popular options. Geotextiles are porous fabrics, which are constructed by weaving or bonding fibers made from synthetic materials such as polypropylene, polyester, polyethylene and nylon. The fabrics, which come in a myriad of shapes and sizes, are applied to the face of the distressed bank and then vegetated.
The idea behind using geotextiles is to reinforce the root strength of the vegetation being applied to the bank, Photo courtesy: Stormwater Environmental Compliance Alliance allowing it to withstand heavier and faster water flows. “Essentially, geotextiles act as anchors, firmly affixing the roots of the plant material to the subgrade,” says Lee Pierce, director of the stormwater group for Propex, Chattanooga, Tennessee. “Without geotextiles, plant material can withstand water flowing at roughly five to six feet per second. When you add a geosynthetic into the mix, the plants can handle velocities in excess of twenty seconds.”
Geotextiles encompass a wide range of materials and products, but when it comes to river or stream bank erosion control, turf reinforcement mats (TRMs) are becoming increasingly popular. “What we’re seeing used today are what are called second-generation or high performance TRMs,” says Pierce. “These are woven products, with incredibly high tensile strength and superior UV-resistance, so they have a design life of up to fifty years.”
There are a couple of important things to remember when working with TRMs. Be sure to use only second-generation TRMs. First-generation TRMs are nowhere near as durable and will not perform well in stream or riverbank applications. “Because they’re not woven, they tend to get torn up pretty easily,” says Pierce. “When working on or near the water, you’re going to want to stay away from the first-generation products.”
TRMs are often used in conjunction with other geotextile fabrics. At the toe of any river or stream bank, the wave action, or the water that hits the shoreline, will disturb the soil and displace the finer particles of sediment. To prevent these particles from migrating through the voids in the TRMs, a thin layer of fabric is placed underneath the mat.
There are a number of advantages to working with TRMs. Because they promote vegetation, these products can drastically improve water quality. Remember, vegetation is Mother Nature’s natural filter system, so anytime you add plant material to a body of water, you’re going to reduce contaminants and other impurities. In a country that’s become so environmentally conscious, that’s a really nice selling point.
Vegetated retaining walls (VRWs) are also commonly used. VRWs are constructed using a series of geotextile bags which are linked together using a connector spike. Each bag is filled with a special soil mixture and can be planted with native plants or hydroseeded. “The vegetated retaining walls are very efficient products,” says Todd Strand, president of Versa-Lok Retaining Wall Systems, Oakdale, Minnesota.
“Essentially, they allow you to simultaneously implement both longand short-term erosion control solutions. The bag structure handles the short-term duties, keeping the soil in place until the vegetation is established and can take care of the long-term erosion concerns.”
There are a couple of methods of erosion control that fall into a gray area: they are considered to be neither hard nor soft armor. Rather than using material to create a physical barrier between the water and the shoreline, these methods employ various techniques designed to divert the flow away from the bank. Among these methods are the use of root wads and rock veins.
Root wads consist of the root mass or root ball of a tree and a portion the trunk. A trench is dug along the toe of a bank, and the tree material is placed into the ditch, with the roots dangling into the water. The roots deflect water away from the bank, keeping it safe from erosion. Root wads also provide structural support to the stream bank, reducing the likelihood of collapse.
Rock veins function almost identically to root wads. From the top of the bank, a row of rocks is placed at an angle going down into the stream. The rocks divert stream flow away from the bank, which allows sediment to stay in place. The size of the rock used will depend on the size of the stream, but stones are typically heaver than one to two tons.
Whether we like it or not, erosion is here to stay. Learn all you can about it and help keep rivers and streams in their place.
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