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The present invention relates to erosion control, specifically to the use of recycled log and stump waste to prevent loss of topsoil in locations where soil is temporarily disturbed by human activity. 
     Federal, state and municipal regulations are increasingly requiring erosion control around construction sites, placing limitations on dumping waste materials in landfills, and restricting the burning of waste timber materials. The present invention helps to solve these persistent environmental problems by providing a means of preventing topsoil erosion in areas where the soil is temporarily disturbed through construction, farming, or similar activities, while at the same time providing a means for disposing recycled log and stump waste without overburdening landfills or increasing air pollution burdens by burning. 
     The two most common methods of erosion control around construction sites are the use of silt fences or straw bales strategically placed to trap topsoil. Other erosion control methods include application of interlocking concrete and other slab arrangements, use of jute sacking pegged to the soil, and use of sand bags on dikes and revetments. Although all of these methods have proved effective for certain applications, they have certain disadvantages. 
     Straw bales have the disadvantage that they do not contour to the landscape, they must be removed from the site after construction is completed, and, because they do not permit water to wash through them, gullies or channels are often eroded around and under them. Any straw that is spread over the site after the bales have served their useful purpose is apt to contain weed seeds that are a nuisance to subsequent landscaping. 
     Silt fences also have the disadvantage of not contouring to the landscape, and in addition water washes under them and they can be easily knocked over by water surges. 
     The other common methods of erosion control are either too expensive or are physically unsuitable for controlling erosion around construction sites. 
     The present invention solves the disadvantages of the commonly-used erosion control methods. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a perspective view of a water-permeable erosion control bag constructed in accordance with the teachings of this invention. In the preferred embodiment the size-selected wood fragments are in the range of 3/8&#34; to 11/2&#34; and are bagged in 12&#34; diameter extruded high density polyethylene mesh of 1/2&#34; mesh size. 
     FIG. 2 is plan view showing placement of biofilter bags on a sloping surface to control erosion thereon. 
     FIG. 3 is a cross section of the lines of bags along the section A--A&#39; shown in FIG. 2. 
     FIG. 4 is a plan view showing placement of biofilter bags in ditches and swales to control erosion therein. 
     FIG. 5 is a cross section of the lines of bags along the section A--A&#39; shown in FIG. 2. 
    
    
     DETAILED DESCRIPTION 
     Preparation of Biofilter Bags. Biofilter bags are prepared by filling porous bags with hammer-milled wood fragments. In the preferred embodiment the bagging material is a porous fabric made of extruded plastic. The first step in the process involves using a standard hammer mill to mill stump and log waste into small fragments. Through experimentation it has been discovered that other forms of preparing wood fragments, for example by using a chipper, produce unsuitable material for this invention. Because wood chips produced by a chipper have smooth surfaces, they tend to stick together and prevent the necessary flow of water through the bags. Biofilter bags filled with chipped wood fragments block water flow and gullies or channels form under the bags, thereby diminishing the value of the invention. 
     Wood fragments are next transported from the hammer mill to a screening device. The preferred mode of transportation is by a conveyor belt, which receives the wood fragments from a surge bin that keeps the flow of wood fragments constant. 
     The wood fragments are next fed into a shaker with a plurality of screens to produce size-selected wood fragments. Commonly the invention uses a two screen system with the upper screen having openings of 11/2&#34; and the lower screen having openings of 3/8&#34;. It has been determined through experimentation that biofilter bags containing wood fragments larger 11/2&#34; are too porous, whereas biofilter bags containing wood fragments smaller than 3/8&#34; are not porous enough, tending to become clogged with sediment. 
     Next, the size-selected wood fragments are transported, commonly by a conveyor belt, to a commercially available bagging system. Extruded high density polyethylene of 1/2&#34; mesh size, forming a cylindrical tube of about 12&#34; diameter supplied by Norplex, Inc. of Seattle, Wash. is cut in the appropriate lengths to form bags of the desired length, commonly either 30&#34; or 60&#34;. One end of a length of bagging material is fastened closed with a first closing device, commonly with a hog ring or a tie, the bag is filled from the bagger, and then the other end is fastened closed with a second closing device. 
     Use of Biofilter Bags. Biofilter bags can be used to prevent erosion in a multitude of applications where flowing water might otherwise cause erosion. Three common uses are on sloping surfaces, in ditches or swales, and around catch basins. 
     To prevent erosion on an exposed slope, biofilter bags are placed in a plurality of lines across the sloping surface, approximately along contour lines. The ends of bags typically overlap adjacent bags by about six inches. The bags may be staked in place if the slope or the flow rate of runoff require it. The number of lines of biofilter bags, and their spacing, depend on the slope and the rate of runoff. In regions of steep slope and high runoff the lines of bags should be placed closer together than in regions of gentle slope and low runoff. In some instances a single line of bags will be sufficient to control erosion. 
     To prevent erosion in a ditch or swale, Biofilter bags are placed in a plurality of lines across the ditch or swale. The ends of bags typically overlap adjacent bags by about six inches. The bags may be staked in place if the grade of the ditch or swale, or the flow rate of runoff require it. The number of lines of biofilter bags, and their spacing, depend on the grade and the rate of runoff. In regions of steep grade and high runoff the lines of bags should be placed closer together than in regions of gentle grade and low runoff. In some instances a single line of bags will be sufficient to control erosion. 
     To prevent silt from a construction area from clogging storm sewers, a plurality of biofilter bags are arranged around catch basins so that runoff water approaching the catch basins must first flow through the biofilter bags. The biofilter bags so arranged trap most of the silt suspended in the runoff water, permitting only silt-free water to enter the storm sewers.

Summary:
A water permeable erosion control bag having a flexible mesh wall with openings through which water can flow. Discrete hammer-milled wood fragments within the bag.