Nowhere is the need for landscape and irrigation as critical as on recently graded construction projects. The exposed soil is highly unstable and, therefore, vulnerable to the forces of nature which can cause erosion and threaten the entire project. The growth in this aspect of the landscape construction industry has been spurred in part by revised local grading ordinances that seek not only to control erosion on construction sites, but to prevent sediment damage to local waterways and reservoirs as well.
Tremendous strides have been made over the years in the control of erosion and sediment. Tools and techniques have never been more sophisticated. Today, landscape contractors can ensure that a site will resist the stress of erosion almost indefinitely.
Real estate developers have also recognized that it is less expensive to prevent erosion than it is to pay for cleanup and regrading, not to mention the cost of any ensuing lawsuits. In addition, growth has been stimulated by state and federal mandates to reclaim vast areas disturbed by surface-mining operations. Another force in the growth of the landscape construction industry has been the influence of state highway departments. These agencies are responsible for extensive cut and fill slopes which threaten roadways and public safety until properly stabilized.
Erosion and sediment control plans for different kinds of projects have much in common. Foremost among these is the need to protect the soil surface from the erosive impact of falling rain drops. This can be achieved by growing a stand of grass, providing a protective layer of straw mulch or applying chemicals to form a crust over the soil surface. Another major element in erosion prevention is controlling the flow of rain water once it has fallen. This is done by using a series of diversion ditches and earth berms to direct the flow to a safe discharge point. Finally, erosion and sediment controls work to trap and remove the sediment in the flowing water. This is accomplished by using silt fences, sediment traps and sediment basins.
The landscape professional this has three main types of erosion and sediment control measures available to him: structural controls (diversion ditches and sediment traps), vegetative controls (grasses and mulches), and chemical controls (mulch tackifiers and soil sealants).
Structural controls can be constructed during grading operations without the need for additional equipment. Besides preventing erosion of slopes, ditches and berms are used to remove from flat areas, such as house pads They can also be used to prevent off-site water from entering and eroding the project itself, or to prevent sediment-laden water from leaving the site. Whenever feasible, erosion and sediment control plans should make use of permanent channels on the site. These channels can be lined with durable materials such as riprap, asphalt or concrete, and serve the essential function of diverting the flowing water to a specific location.
The second major function of structural controls is to trap and remove sediment before it leaves the site. This is usually done wither with a series of small sediment traps or with a large sediment basin. Slit fences make effective sediment traps. They are constructed with a length of geotextile material, often called filter fabric. There are a number of geotextiles available, each with a different strength, pore size and permeability. Straw bales are frequently used to form semicircular sediment traps. The bales must be securely anchored and set into the ground to form a watertight seal. Sand bags are also used to construct small sediment traps, particularly on roadways where straw bales cannot be used. The sand bags form a tight seal with the roadway and can trap quite a bit of silt if cleaned following each storm.
Basins are the last line of defense against sediment. A project will often have a large basin constructed at the lowest elevation for removing sediment before runoff leaves the site. Although the surface area of the basin, not the depth, determines its effectiveness in removing sediment, basins do need to have sufficient storage capacity to hold the trapped material. Otherwise, they must have easy equipment access for periodic cleanout. A method of draining the basin is also necessary to reduce the hazard of accidental drowning and to prevent mosquito breeding. The basin outflow is usually through a metal pipe riser. A basin should have an emergency spillway in case the riser?s capacity is exceeded. Because they require shaping and excavation of the earth, structural controls must be constructed before vegetative controls.
Vegetation plays a vital role in controlling erosion. A wide spectrum of plant materials is now available for erosion control, and designers have paid increasing attention to specifying plant mixes that are compatible with a particular site. In addition to the seed mix, the variables of this type of control include the irrigation system, the desired level of weed control, the permanent or temporary nature of the planting, and soil conditions. A soil test is essential in determining the correct seed mixture for a project.
An improper planting specification may achieve the short term goal of site stabilization, but could cause problems in the long run. Annual ryegrass, oats and barley were once highly regarded as temporary ground cover. After the site was stabilized, however, they can create a fire hazard and leave the site with a stubborn crop that must be eradicated before installing the permanent landscaping.
Weed control is a factor in re-vegetation plans, especially when one is transforming an area for aesthetic beauty as well as erosion control. Containing weeds in ornamental plantings is a major expense and can cost thousands of dollars. On the other hand, natural grasses and shrubs that are prevalent in the surrounding undisturbed areas may be desirable for erosion control. In this case, the contractor simply helps nature along at a greatly increased pace.
Temporary irrigation systems are often installed during the ground cover?s establishment period. The amount of water needed for germination varies with the plant mix. In some combinations, seeds are coated with a mucilage layer that breaks down only after several waterings, constituting a built-in protection system against premature sprouting.
The most common methods of applying vegetative controls are by hydraulic planting, hydroseeding and hydraulic mulching. These terms have been used interchangeably for years, but actually describe somewhat different techniques.
Hydraulic planting is a term for the application of planting materials by employing pressurized water as the primary carrier. The material is sprayed from a machine designed specifically for this purpose. The equipment can also be used to apply chemical controls such as straw mulch or soil stabilizers.
Hydroseeding is the application of seed (and optional fertilizers and chemicals) to the planting area using the same water spray technique. Hydraulic mulching, on the other hand, involves the addition of a mulch product to the seed, water, and optional fertilizer/chemical slurry in the tank. The same equipment is used in each case.
Mulches serve as protection for the germinating seed by retaining soil moisture, as well as providing protection from wind and rain. Hydraulic mulches may consist of virgin wood fibers, wood fiber and newspaper mixes, excelsior, grass clippings and even screened compost.
Thorough wetting and agitation of all materials in the equipment?s mixing tank is essential for uniform application. Mulch rates are specified in terms of pounds of mulch per acre, and typically range from 1,000 to 3,000 pounds per acre. The rates vary in relation to the protection necessary for the seed variety and with the prevailing conditions on the jobsite.
The application of straw to newly hydroseeded or hydraulically mulched slopes was once a practice reserved almost exclusively for big highway projects. Large expanses of slope, extending for miles on either side of new highways, were often covered with one to five inches of straw. Today, the practice is fairly common on smaller projects as well. Straw mulch forms a protective covering that absorbs the impact of raindrops. When anchored to the ground with a tackifier, straw provides additional protection from wind erosion. Asphalt, once commonly used to secure the straw, is now losing popularity in favor of chemical or organic tackifiers applied with a hydraulic planter.
Crimping is another method used to secure straw mulch. It involves using a large, weighted mechanical roller that literally punches the straw into the soil as it rolls along. A portion of the crimped straw stands upright among the horizontal straws, creating windrows and preventing migration in high winds. Crimping is very effective but cannot be done on hard soil or on slopes which do not have adequate access. Straw-blowing equipment is usually towed behind large flatbed trucks that carry the straw bales. Crews remove the baling wires and feed them into the machine, which cuts them into smaller pieces. Most straw-blowers can apply about seventeen tons of straw per hour.
Chemicals applied on construction sites for erosion control fall into two categories. The first includes materials used to tack down the mulches used in vegetative controls. The second category consists of those materials used to hold soil in place during and after site construction. This type of application can be expected to last from a few weeks to a year depending on the application rate and the severity of the erosion conditions.
Chemical controls are typically of the copolymer type, meaning that they bind soil particles together. The most popular are derived from a vinyl acetate, acrylic or styrene-butadlene base. Some have additional components to enhance the product?s performance in the field. The chemicals are packaged in liquid form and are diluted with water just prior to application. Dilution rates depend on the manufacturer?s specifications, and can vary from six to forty part of water per one part of the chemical product. Most copolymer control products can be applied with standard hydraulic planting equipment, a water truck or an agricultural spray unit. Chemicals are used to tack down seed and mulches when significant holding power is required due to slope steepness, soil conditions or pronounced erosion forces. The copolymer products which are most effective are those which penetrate the mulch and soak into the soil surface, creating an integrated vertical bond between mulch, seed and soil.
Application rates vary according to the product. Those incorporating an acrylic copolymer base enjoy lower application rates due to superior moisture and ultra-violet resistance. Typically these application rates will be forty to fifty gallons of concentrate per acre. The vinyl acetate and styrene-butadlene mixtures tend to require higher application rates of sixty to a hundred gallons of concentrate per acre. Products suitable for use as a tackifer can be applied either directly as part of the seed slurry or as a second application after the mulch, seed and fertilizer are already in place.
There are organic-based tackifiers that can be used in the same way as copolymer products. The organics come in powdered form and are derived from plant or vegetable products such as plantago, guar bean and the by-products of grain mill operations. They are low in moisture resistance and las as a covering over the surface of the mulch.
Application rates for soil sealants can vary widely depending on the products and job conditions. For an acrylic-based coploymer product, application would vary from 50 to 200 gallons of concentrate per acre. The vinyl acetate and styrene-butadlene products are generally applied at rates from 110 to 450 gallons of concentrate pr acre. For particular job site recommendations, consult with the manufacturer.
There is a curing period for chemical controls, usually 24 to 48 hours. This time is required for the ingredients to achieve their maximum bonding strengths. The application should be done when the air temperature is above 46 degrees F. To facilitate proper curing. If heavy rainfall or improper irrigation occurs prior to complete bonding, the application can be diluted and the crust weakened.
Once a chemical control is applied and cured, freezing will not be harmful to crust formation or tackifying action. Copolymer products, however, do not react well to freeze-thaw cycles before applications, and as a liquid it should be protected from freezing. Most chemical products can be cleaned up using plain water. Tools and equipment should be washed off immediately after the application in order to make cleanup easier.
By following certain procedures, erosion control can be efficient and effective.