FAILURE AVOIDANCE EFFECTIVE SILT FENCE

Abstract

A silt fence structure and method of controlling erosion are disclosed. The silt fence structure includes a water permeable fence, an apron having first and second edges, the first edge providing a buried apron toe, the second edge attaching to the water permeable fence, and at least one flow barrier attached to the water permeable fence and to the apron so as to impede lateral runoff flow along the water permeable fence.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is claims benefit of U.S. Provisional Patent Application No. 60/852,102 entitled “FAILURE AVOIDANCE SILT FENCE,” filed Oct. 16, 2006, the contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

This disclosure relates to soil retention methods and devices in general, and, more specifically, to a failure avoidance silt fence.

BACKGROUND OF THE INVENTION

Construction and large-scale landscaping operations often involve the removal or displacement of topsoil and plant matter serving to prevent erosion. Silt, clay, sediment, and other ground matter may then be swept away by runoff. The silt and other material may be carried suspended in runoff water until it is deposited in creeks, rivers, or still bodies of water. This can have a deleterious effect on wildlife and the environment. Traditional silt fences have been used as a measure for controlling runoff and erosion and to slow the displacement of silt and its subsequent deposition in undesirable locations. A traditional silt fence may be nothing more than a semi-permeable barrier placed in the flow of runoff.

A number of problems arise with conventional silt fences. These include, among others, failure of the silt fence to trap sediment due to scour from concentrated flow along the toe, resulting in undercutting of the toe and discharge of sediment underneath the fence. Failure to trap sediment due to failure of the posts and excessive stretching and sagging of the fence can allow sediment to flow over the fence. This phenomenon is known as overtopping of the fence. Failure to trap clay and fine silt due to inadequate settling time has also been a problem.

Attempts to solve the problem of post failure and excessive sagging have been focused on using stronger posts and adding a wire mesh backing to the fence. These solutions are problematic because of the added costs for materials and installation.

One solution for the failure of a traditional silt fence to trap clay and fine silt due to inadequate settling time was to provide an ox-bow shaped installation formed by extending the downstream end of a silt fence uphill. This creates an impoundment at the downstream end. This solution is also problematic since impoundment drainage areas are typically sufficiently large that a significant quantity of runoff water and sediment is concentrated at the point. As a result, the impoundment is likely to fill up, resulting in uncontrolled sediment flow overtopping the silt fence or flowing around the uphill end.

What is needed is a system and method for addressing the above, and related, problems.

SUMMARY OF THE INVENTION

The present invention disclosed and claimed herein, in one aspect thereof, includes a silt fence structure. The silt fence structure has a water permeable fence, and an apron having first and second edges, the first edge providing a buried apron toe, the second edge attaching to the water permeable fence. At least one flow barrier is attached to the water permeable fence and to the apron so as to impede lateral runoff flow along the water permeable fence. The water permeable fence may also provide a buried fence toe.

In one embodiment, the at least one flow barrier has a generally right triangular shape providing first and second edges attaching to the water permeable fence and the apron, respectively. In another embodiment, the at least one flow barrier has a curved triangular shape with first and second straight legs attaching to the water permeable fence and the apron, respectively, and a curved leg spanning between the water permeable fence and the apron. A plurality of flow barriers may attach to the water permeable fence at substantially the same location.

In some embodiments, the silt fence structure will comprise at least one fence post attached to the water permeable fence. A backing plate may be positioned along the fence post and attached to the fence post such that the water permeable fence and the flow barrier interpose the backing plate and the fence post where the flow barrier is attached to the fence.

In some embodiments, a floculant is applied to the apron so as to disperse into runoff over the apron. In another embodiment, at least one geo textile pouch containing floculant is attached to the apron so as to disperse floculant into runoff over the apron. In another embodiment, the apron is further comprised of first and second layers with a floculant placed therebetween so as to disperse into runoff over the apron.

The present invention, disclosed and claimed herein, in another aspect thereof comprises a method of controlling erosion on a sloped soil surface having an upstream and a downstream direction. The method includes planting a plurality of fence posts into the soil, attaching a silt fence between the plurality of posts, and providing an apron along the fence near the toe portion extending overground from the fence in an upstream direction to an apron toe. The apron toe may be buried in the soil. At least one flow barrier may be attached between the fence and the apron.

In one embodiment, the method includes spraying a floculant onto the apron. In another embodiment, the method includes imbuing the apron with a floculant.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and is not meant to be limited by the accompanying drawings, in which like reference numbers indicate similar parts:

FIG. 1 is a perspective view of one embodiment of a silt fence according to aspects of the present disclosure;

FIG. 2 is a perspective view of another embodiment of a silt fence according to aspects of the present disclosure;

FIG. 3 is a comparison chart illustrating the efficiency of a traditional silt fence versus a silt fence according to aspects of the present disclosure;

FIG. 4 is a perspective view of another embodiment of a silt fence according to aspects of the present disclosure;

FIG. 5 is a perspective view of a flow barrier attached to an apron and fence according to aspects of the present disclosure;

FIG. 6 is an exploded view of a flow barrier, fence, and fence post attachment according to aspects of the present disclosure; and

FIG. 7 is a close up view of a fence post attached to a silt fence according to aspects of the present disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a perspective view of one embodiment of a silt fence 100 according to aspects of the present disclosure. Also illustrated in FIG. 1 are the contour lines C of the prevailing landscape along with the arrows F indicating the direction of flow of runoff. In some embodiments, an apron 102 is provided to protect the soil 103 at a toe 104 of a fence wall 108. Flow barriers 106 may be provided to prevent concentrated runoff flow along the toe 104. Application of polyacrylamide or another floculant, as described in greater detail below, may be used to aggregate fine particles and enhance settling. The combination of the strength of the fence wall 108 and spacing D of the posts 110 keeps stretching and sagging to an acceptable level under anticipated water and sediment loading. The apron 102, fence wall 108, and flow barriers 106 may be made from various geotextile materials. The materials chosen may be selected based upon the desired strength, longevity, and overall cost of the fence 100.

Some embodiments described herein will use a different material for the apron 102 than the fence wall 108. The apron 102 may be constructed with a heavier and tightly woven geotextile material to control the flow of water beneath the apron 102. The heavier material will minimize the stretching of the fabric during mechanized installation and spread very well on the ground. This may also minimize seepage of the impounded flow through the apron 102.

In one embodiment, the materials of the fence wall 108 and apron 102 are attached by sewing the pieces together with at least one single seam using polyester thread. Hot glue may be applied continuously, or at intervals, on the stitch to prevent any unraveling of the thread. Some embodiments will use hot glue exclusively.

In operation, as runoff approaches the fence 100 in the direction of the arrows F, it flows over the apron 102, which protects the toe 104 of the fence wall 108. An uphill end, or toe 112, of the apron 102 may be anchored by laying it in a toe trench and backfilling it. The toe trench may be compacted and the apron 102 is folded back over the compacted toe trench protecting the buried apron toe 112 and preventing erosion. Thus, in one embodiment, the problem of toe scour and undercutting seen in the conventional silt fence design is addressed by locating an apron upslope of the fence wall 108.

Settling time can be increased by the introduction of the flow barriers 106 along the fence 100 and application of a floculant such as polyacrylamide. Polyacrylamide (PAM) aids in flocculation (aggregation) of fine particles and decreases the settling time, when mixed with the runoff flow. This enhances the trapping of clay and fine silt particles. Examples of application of polyacrylamide within the context of the present disclosure discussed further below.

A series of impoundments 114 (shown containing runoff) may be formed by the interception of the flow along the fence by flow barriers 106. This may prevent concentrated flow from forming along the toe 104 of the fence 100 and increase the detention time allowing more sediment to settle from the runoff. Additionally, the series of small impoundments result in settling sediment more or less continuously along the fence and may prevent the overtopping problem posed by forming an ox-bow shape at the downstream end.

With respect to sagging and post failure, the silt fence wall material 108 of the present disclosure may be made of a fabric that is stronger than conventional fence with posts 110 appropriately spaced so that unacceptable stretching does not occur. The final selection of fabric and post spacing may be based on computer and/or laboratory testing to determine the maximum post spacing for each fabric before unacceptable deformation occurs, along with an economic analysis of the trade-off between installing fewer posts with stronger (more costly) fabric or installing more posts and using lower-strength fabric. In one embodiment, stronger fabric will be constructed by reinforcing the polypropylene woven fabric (conventional silt fence material) with polyester fibers. This technique may mitigate the need to install a wire backing in addition to the fence itself and the cost associated therewith. A series of small impoundments 114 defined by the structure of the fence 100 may also aid in uniform load distribution along the fence 100 preventing excessive stretching and sagging at low spots. Runoff is shown in the impoundments 114 for illustration.

Referring now to FIG. 2, a perspective view of another embodiment of a silt fence 200 according to aspects of the present disclosure is shown. The silt fence of FIG. 2 is similar to the silt fence of FIG. 1 with some modification as described below. In order to further minimize the disturbance caused to the parent soil during installation, the toe trench installation method can be eliminated. Instead, the toe 112 of the apron 102 is tucked into a narrow slit in the ground and compacted. A vibratory plow or high speed concrete cutting saw may be used to make the narrow slit in the ground wide enough to insert the silt fence material. To prevent the wind from blowing underneath the apron during heavy storms, the toe 104 of the fence wall 108 may be buried along with a downhill apron toe 201.

In the present embodiment, the shape of the flow barriers 202 is changed from straight triangular (as in FIG. 1) to a curved triangular structure to facilitate trapping. The curved shape functions as a bag of sorts and intercepts and stores sediment. Additionally, to accommodate the change in flow contours (topography) along the construction site, the flow barriers 202 may be installed as dual barriers as shown in FIG. 2. This will enhance the trapping of the sediment as the runoff flow will be intercepted by either of the flow barriers at each post.

The silt fence 200 of FIG. 2 is illustrated as being installed along two distinct slopes as illustrated by the placement of the contour lines C and the arrows F indicating runoff direction. However, it is understood that the fence 200 could be installed along a single slope. Furthermore, any of the embodiments described herein may be useful for installation along single slopes or multiple slopes on the prevailing landscape as dictated by the needs of the end user.

As described, a floculant may be used with various embodiments of the silt fences disclosed herein to aid aggregation of fine soil particles. Anionic polyacrylamide (PAM) is one possible choice of floculant. PAM is soil specific and needs to be mixed well with the runoff flow for effective flocculation. Anionic PAM is available is several physical forms with various formulations. In one embodiment, liquid emulsion anionic PAM will be sprayed onto the material of the fence wall 108 and/or apron 102 and dried. As the runoff flow approaches the fence PAM is mixed at the impounding areas 114 and flocculates fine particles.

In another embodiment, the apron 102 may comprise at least two layers (not shown). Anionic PAM in crystal form may be spread on a non-porous layer of the apron 102 and then covered with a layer of porous polypropylene woven material. The bottom apron material and the porous top woven material are sealed at the edges to form a bag structure and hold the PAM crystals. As the runoff water approaches the fence, water seeps through the porous woven material and mixes with the PAM crystals. Thus, PAM is released at the impounding area for aggregation of fine particles. In yet another embodiment, anionic PAM wafers are stuffed in a meshed geo textile tube (not shown) and laid along the apron toe. As the runoff flow approaches the fence, it passes through the meshed geo textile tube and mixes with the PAM wafers. Thus, with any of these methods, PAM is released into the flow and flocculation takes place in the impounded area.

Referring now to FIG. 3, a comparison chart illustrating the efficiency of a traditional silt fence versus a silt fence according to aspects of the present disclosure is shown. A silt fence constructed according the principles described above (without the use of PAM) was evaluated at the silt fence testing site (SFTS) at USDA ARS Hydraulics Lab. The fence was evaluated under a wide range of testing conditions. Tests were performed with the aid of an artificial rainfall simulator specifically designed for the silt fence testing site. In every test it was observed that the scouring of the toe and excessive stretching of the fence were substantially eliminated. Detention time and trapping of sediment drastically increased due to addition of the flow barriers.

FIG. 3 illustrates the performance of a silt fence (denoted “FAEST”) built as described herein when compared to a conventional silt fence. Conventional silt fence performance data was obtained from the same silt fence testing site. Results of six field tests with identical testing parameters were considered here. The test parameters covered a combination of three different soil textures and two slopes along the fence. The ratio of total sediment load from the source area plot (EP) to total sediment discharged with the flow along the fence (UF) was calculated for a side by side comparison. For clarity, the ratios are presented as percentages in FIG. 3.

A value of 100% or greater would indicate that there was more sediment discharged with the flow along the fence than there was sediment generated from the source area. In a conventional silt fence design, this would imply that additional sediment was generated by scouring of the toe trench which would lead to the failures described above. With a silt fence designed as described in this disclosure, the runoff flow from the source area would have to first fill the impoundments before it starts flowing around the barriers and flows along the apron toe. Thus, the total sediment load discharged in the flow along the fence is less than the sediment load discharged from the source area. These behaviors can be observed with the field test results depicted in FIG. 3.

It can be seen in FIG. 3 that with a conventional silt fence, 5 out of 6 tests resulted in more sediment load discharge than what was generated from the source area. This increase resulted from erosion along the fence. Also, conventional silt fence test results indicated that the fence failed with 2 out of 3 soils at 13% slope along the fence and sediment trapping efficiency was very poor.

Results of the field tests for the fence designed according to aspects of the present disclosure indicated that the new design eliminated the failure modes of a conventional silt fence and performed superiorly in trapping sediment. With 4 out of 6 comparison tests the ratio of total sediment load at the source area to sediment load upstream of the fence turned out to be much less than 1, indicating significant trapping of sediment. The overall trapping efficiency of the designed fence as shown in the present disclosure was estimated to be around 90%. Also, due to increased detention time behind the fence, the trapping of fine silt and clay particles increased when compared to a conventional silt fence. Testing under extreme conditions also demonstrated that the flow across the apron does not disturb the soil under the apron and that fabric installed with a vibratory plow stays anchored.

FIG. 4 is a perspective view of another embodiment of a silt fence 400 according to aspects of the present disclosure. In this embodiment, the shape of the flow barriers 402 is changed to a straight triangular shape and is attached such that it is always perpendicular to the fence wall 108. This embodiment can result in lowered construction costs while retaining substantially all of the benefits described in the present disclosure. A computer simulation model of the hydraulics and sediment trapping was developed and used to simulate the impact of physical and hydrologic variables that determine the efficiency of the fence. These showed that the high trapping efficiency of the previously described designs may still be achieved with the modified flow barriers 402 of FIG. 4.

The flow barrier 402 may be constructed out of the same material as the fence wall 108. The flow barrier material 402 may be permeable and filter the impounded flow downstream to avoid overtopping of the barriers 402 and fence wall 108. Silt fence material (e.g., polypropylene) may be cut to the required dimensions and shape. It may be pre-creased (e.g., with a hot iron) to make a two inch flap on the vertical and horizontal side of the triangular flow barrier 402. The flow barrier 402 may be attached to the fence wall 108 and apron 102 with hot glue, rivets, or other means. If hot glue is used, immediately after attaching the flow barrier 402, the hot glue may be spread evenly on the two inch flap with the help of a roller.

Referring now to FIG. 5, a perspective view of a flow barrier attached to an apron and fence according to aspects of the present disclosure is shown. It can be seen that to further secure the barrier to the apron, plastic snap-on rivets 502 may be installed, possibly using a backing plate 504. Another embodiment uses an industrial sewing machine to sew the barrier to the fence and apron. Selection of the attachment method can be based on cost and ease of operation.

Referring now to FIG. 6, an exploded view of a flow barrier, fence, and fence post attachment according to aspects of the present disclosure is shown. The posts 110 may be metal posts with stabilizers to support the fence wall 108. The spacing of the posts 110 can be fixed or allowed to vary depending on the site and local climate. In some embodiments, computer simulations may be used to determine post spacing. The fence and the vertical flap of the flow barrier (glued area) may be attached to the fence post using a metal backing plate 602, with plastic push type rivets 604 as shown in FIG. 6. The surface of the metal backing plate 602 and post 110 may be roughened to prevent pulling of the fabric from one post interval to another, thus preventing excessive stretching. Securing the flow barrier 402 to the fence post with roughened post, metal backing plate, and plastic rivets, will give additional strength for the fence material to withstand the impoundment load.

Referring now to FIG. 7 a close up view of a fence post attached to a silt fence according to aspects of the present disclosure is shown. The silt fence 700 of FIG. 7 is similar to those previously described but illustrates another method of attaching the fence posts 110 to the fence wall 108. No flow barrier is illustrated in FIG. 7 (for the sake of simplicity) but it is understood that the present method of fence post attachment may be appropriately adaptable for locations along the fence wall 108 where a flow barrier is attached to the fence post 110 see, e.g., FIG. 6).

The post 110 attaching to the fence wall 108 may be a steel post. The post 110 may be at least as tall as the fence wall 108 to securely retain the fence wall 108, and be securely implantable into the ground. In the present embodiment, the fence wall 108 is sandwiched between the post 110 and a tubular securement 702. The post 110 may be shaped to conform to the contour of the tubular securement 702 in order to increase holding strength. This also isolates the individual fence sections such that stretching, sagging, or other deformations in one section will not necessarily affect adjacent sections.

The tubular securement 702 may be an electrical or mechanical tubing piece. In one embodiment, the securement 702 is sized in length substantially similarly to the height of the portion of the post 110 that is above ground in order to fully secure the fence 108 to the post 110. The securement 702 may attach to the post with sheet metal screws 704 or other fasteners. In one embodiment, the length of the sheetmetal screws 704 will be chosen such that the screws do not extend completely through the securement 702. Part or all of the fence 700 may be preassembled before being installed, or the post(s) 110 may be inserted into the ground and the fence wall 108 attached to the posts 110 with the securement 702 using hand tools the site of installation.

Thus, the present invention is well adapted to carry out the objectives and attain the ends and advantages mentioned above as well as those inherent therein. While presently preferred embodiments have been described for purposes of this disclosure, numerous changes and modifications will be apparent to those of ordinary skill in the art. Such changes and modifications are encompassed within the spirit of this invention as defined by the claims.

Claims

1. A silt fence structure comprising:

a water permeable fence;

an apron having first and second edges, the first edge providing a buried upstream apron toe, the second edge attaching to the water permeable fence and providing a buried downstream apron toe; and

at least one flow barrier attached to the water permeable fence and to the apron so as to impede lateral runoff flow along the water permeable fence.

2. The silt fence structure of claim 1, wherein the water permeable fence provides a buried fence toe proximate the buried downstream apron toe.

3. The silt fence structure of claim 1, wherein the at least one flow barrier has a generally right triangular shape providing first and second edges attaching to the water permeable fence and the apron, respectively.

4. The silt fence structure of claim 1, wherein the at least one flow barrier has a curved triangular shape with first and second straight legs attaching to the water permeable fence and the apron, respectively, and a curved leg spanning between the water permeable fence and the apron.

5. The silt fence structure of claim 1, wherein the at least one flow barrier comprises a plurality of flow barriers attaching to the water permeable fence at substantially the same location.

6. The silt fence structure of claim 1, further comprising at least one fence post attached to the water permeable fence.

7. The silt fence structure of claim 6, further comprising a backing plate, the backing plate being positioned along the fence post and attached to the fence post such that the water permeable fence and the flow barrier interpose the backing plate and the fence post where the flow barrier is attached to the fence.

8. The silt fence structure of claim 1, further comprising a floculant applied to the apron so as to disperse into runoff over the apron.

9. The silt fence structure of claim 1, further comprising at least one geo textile pouch containing floculant attached to the apron so as to disperse floculant into runoff over the apron.

10. The silt fence structure of claim 1, wherein the apron is further comprised of first and second layers with a floculant placed therebewteen so as to disperse into runoff over the apron.

11. A silt fence structure for controlling erosion on a sloped ground surface having an upstream direction and a downstream direction, the structure comprising:

a water permeable fence providing a fence toe buried in the ground surface;

a plurality of flow barriers;

an apron having first and second edges, the first edge providing an upstream apron toe and the second edge providing a downsteam apron toe; and

a plurality of fence posts, each with a first end buried in the ground surface and extending generally upward therefrom;

wherein the apron is attached to the fence proximate the second edge of the apron and extends from the fence in the upstream direction, the fence is attached to the apron proximate the fence toe, and the upstream and downstream apron toes are buried in the ground surface;

wherein each of the plurality of flow barriers is attached to the filter fence along a first edge of the flow barrier and attached to the apron along a second edge of the flow barrier; and

wherein at least one of the plurality of fence posts attaches to the fence proximate a location connecting to at least one of the plurality of flow barriers.

12. The silt fence structure of claim 11, further comprising at least one backing plate positioned on a side of the fence opposite at least one of the plurality of fence posts and positioned such that at least one of the plurality flow barriers and the fence are sandwiched between the backing plate and the fence post.

13. The silt fence structure of claim 11, wherein at least one of the plurality of flow barriers has a generally right triangular shape providing first and second edges attaching to the water permeable fence and the apron, respectively.

14. The silt fence structure of claim 11, wherein at least one of the plurality of flow barriers has a curved triangular shape with first and second straight legs attaching to the water permeable fence and the apron, respectively, and a curved leg spanning between the water permeable fence and the apron.

15. The silt fence structure of claim 11, wherein at least two of the plurality of flow barriers attach to the water permeable fence at substantially the same location.

16. The silt fence structure of claim 11, further comprising a floculant applied to the apron such that the floculant disperses into runoff on the apron.

17. A method of controlling erosion on a sloped soil surface having an upstream and a downstream direction, the method comprising:

planting a plurality of fence posts into the soil;

attaching a silt fence between the plurality of posts;

providing an apron attached along the fence, the apron forming a downstream apron toe proximate the fence and extending overground therefrom in an upstream direction to form an upstream apron toe;

burying the downstream and upstream apron toes in the soil; and

attaching at least one flow barrier between the fence and the apron.

18. The method of claim 17, further comprising spraying a floculant onto the apron.

19. The method of claim 17, further comprising imbuing the apron with a floculant.