When Silt and Sediment Controls Are Not Enough
by Mark Howland, Environmental Research Corps
While at FoxWoods, a subdivision under construction in North Andover, Massachusetts, I couldn't help but notice the sediment problem created by residue turbidity and silt in the runoff clouding entry waters to the adjacent wetlands.
Turbidity is not a silt or sedimentation problem, but in actuality is a problem unique to itself. Turbidity occurs when silt and or sedimentation has settled out of eroding water flows, leaving inside the flow small dissolved solids particles that continue to be carried with the movement of the water. In certain soils, particularly in soils with a substrate type called clay fines, the turbidity caused by these suspended solids creates intense "clouded" waters entering wetlands to clog and block plant pores, and interfere with wetlands functioning and development. Wetlands, functioning as nature's "lungs" and "kidney" require that the plant pores and soil pores remain unclogged in order to utilize their pollutant filtering ability. Dissolved and suspended solids, once coming out of their colloid state, form fine coatings over the pores.
As a wetlands and wildlife professional biologist, I realize that erosion control - such as hay bales and silt fence - cannot successfully remove small particles in the water runoff, such as those present when suspended fines are present in the runoff from a construction site. Our own siltation control product, Biofence, which has tested out 15 - 25% more effective than hay bales and plastic silt fence, still only rates at 80% effective in silt removal when suspended solids and fines are involved. The small particles were tested by us to account for most of the remaining 20%. This fence has an ability to change its mesh size, dependent on the rainfall and moisture regime, so it can trap more of the fines.
No silt fencing type, ideal for the usual regime of sands, gravels, and loam common to the bulk of erosion, can simultaneously remove all suspended solids as well. Obviously, suspended solids cause a different type of wetlands and erosion problem, which requires its own independent solution.
Metcalf and Parker Contractors, prime contractors for the FoxWoods project, are one of the North Shore of Massachusetts' largest road and utility construction outfits. They are very conscious about erosion control - utilizing erosion control mats, fencing, hydroseeding, dust watering, and advanced drainage design.
At the FoxWoods subdivision, one 1000-foot stretch of road passed through a section of clay fines soils. The sheet runoff coming off the hydroseeded slopes, along with the mud from the clay fines - which tracked on to the newly paved road - carried large amounts of small particle suspended solids, regardless of the erosion controls that were used. The major problem was that the runoff, carried into the drainage system, was brought to the detention structure close to the main road, Salem Street, which would quickly cloud up. Its drainage outlet then would carry the suspended solid laden waters out to the adjacent bordering vegetated wetlands. After each storm event, the nearby wetlands would look like a section of wooded swamp that had been coated with a fine layer of brown talcum powder.
The North Andover Conservation Commission asked Metcalf and Parker to address the problem. At a loss, Metcalf and Parker's site manager, Tom Sawyer, asked if our firm, Environmental Research Corps (ERC), could offer any suggestions while we were delivering BioFence to the site one day and noticed the problem. We offered, as a courtesy to a client, to look into the problem, which we quickly recognized was not an easy solution.
I come from one of the more unusual backgrounds leading into the aquatic biology field. At one time, I owned a large fish farming (aquaculture) collaborative in the Northeast with 16 separate farm locations. As a fish farmer, I had to be more concerned with water quality than most people, as species such as trout require water quality greater than drinking water quality to survive and flourish. Innovative and inventive designs needed to be developed to control silt and pollution entry into the fish farms.
One such design was used quite successfully to control fine silt which would clog the trout gills and choke the fish from a lack of oxygen. The cloudiness conditions were very similar as present on the Salem Street site. I pulled out my old files on that design and readapted that turbidity trap for use in a situation that was present on Salem Street's FoxWoods subdivision.
Essentially, turbidity traps require finer mesh sized screens than were currently being used on that site with BioFence, small peastone, hay bales, or silt fence. The particle size is too small to be trapped by those materials - they being designed for average erosive materials in the 10-50 mm particle size. If they were adapted to accommodate the 1-8 mm size commonly found as suspended solids by shrinking their mesh size, then the larger erosive materials would quickly block and clog pores of the erosion devices, creating dam effects and causing flooding during storm events. Thus it was decided to let the erosion fencing do the job they were designed to - filter out the sedimentation and silt, and pass the suspended solids laden flows on to a separate innovation.
Our initial modification of fish farming technology for the FoxWoods subdivision was a frame-supported mylar mesh which was coated by diatomaceous earth (DE) which some might be familiar with in the swimming pool filter industry. This use of DE brings a screen material down into the range of the particles to be removed - the mylar already being a smaller mesh size with good porosity. ERC adapted the old fish farming design to fit into the hollow in the ground that the Metcalf and Parker people had been directing the runoff into, in an attempt to control the fines and turbidity.
First we surface-coated the hollow with sod strips. Fully grown and dense grass such as sod, will trap some of the turbidity on its own. As sod is usually cut in 3 foot wide strips, the strips were laid horizontally across the hollow. Between each strip, we placed a wood frame supported mylar screen, coated with DE. Two-by-fours hold the frames upright and support the structure. Also, at the base of the channel that the flow travels, we placed oyster shells. These shells were historically used to trap the fine particles off of roadways, driveways, and ocean entry points for land runoff that caused the undesirable "muddy" taste of shellfish growing in turbid waters, filtering out these muds to create the famous clean shellfish and the "New England Seafood taste". They are very effective small particle removers using the carbonate powder found on the shells to attract and hold very small suspended solids and the dense packing of the crushed shells to trap the fines.
The conservation commission of North Andover allowed us to try this experimental design as an attempt to see if an old design from one industry can be applied to another. We greatly appreciated the commission's permission to try the concept, and assured the commission that close monitoring on the experiment would be done and the new experiment would not create any new problems.
As the project required no more than working in an approximately 4 foot by 10 foot area of the uplands (a small hollow) that already is in silt removal attempts, the experiment had everything to gain and nothing to lose.
The following is a description of the engineering used for the FoxWoods subdivision. Sod strips were laid over American Excelsior Company's Curlex erosion control matting to have the fine blades of grass attract and hold some of the fines found in the dissolved solids, as flows were directed through this natural hollow from the discharge point of the detention structure during storm events. To aid in the removal, oyster shells (found as a common feed supply store item to provide roughage for chicken and turkey raising) were placed over the sod in the direct flow path through the hollow. Intervening mylar screens coated with diatomaceous earth were spaced about two feet apart and held in place by two-by-fours, strapped to their sides. These were then placed into the hollow sunk into the side slopes to create slow permeable dams to slow the flow and force the waters through the screening or over the sod, erosion control blanket, and shell mixtures. This rough, rudimentary system worked fine for the next six months as the slopes became vegetated, the road crews finish paving and lot construction, and the drainage system cleared itself of accumulated silt materials, thus eliminating source materials.
ERC recently came across another site in Rowley, Massachusetts in 1997 where the same fine silt clays were encountered. We decided to refine the Turbidity Trap. For directed piped flow situations, we have devised an oyster trap unit which receives the turbidity laden water from a source through a riser pipe, encased inside an oyster shell-filled conical surrounding mesh. The perimeter of the trap was sodded over a base of bentonite clay or CETCO geotextile clay liners, and planted with woolgrass to assist in the filtering. We found this ideal for small turbidity situations.
For sheet flow situations carrying turbidity, we directed the sheet flow into a gradual concentrated sheet flow developing into a channel scenario. In this channel, the sides and base were lined with American Excelsior Company's High Velocity Curlex, which is then sodded and then over-coated with oyster or clam shells (in New England, clam shells are as readily available as oyster shells). We then took standard air conditioner filter frames and used BonTerra's BogMat inside the frames, coated with diatomaceous earth for fine filtering and slow permeability damming. The filter screens in the sheet flow turbidity trap and the oyster shell mesh in the circular oyster shell trap are the only points which require routine cleaning and maintenance, the remainder of the traps only require sporadic cleaning and checking.
Both new Turbidity Trap structures have been field tested successfully in New England, and our company hopes for the sake of wetlands that the re-adaptation of old fish farm technology continues to work to control the fine silt and turbidity problems that seems to be prevalent in flows off certain hills and hollows. L&W
For more information, contact Mark A. Howland, Chief Biologist, Environmental Research Corps, 15 Mohawk Ave., E. Freetown, MA 02717, (508)763-5253, fax (508)763-8781.
©2000, 1999, 1998 Land and Water, Inc.