The Mill Brook Project:
The Forest Society Conservation Center located at 54 Portsmouth Street in Concord, NH has won numerous awards for the innovative design of their building and grounds. In the late nineties the gravel parking lot was expanded and renovated with the addition of a drainage system designed to infiltrate storwater. Five catch basins collected the Stormwater and dispersed it through a series of perforated pipe and stone infiltration cells. During large rain events, excess water was directed to a culvert at the top of an
existing ravine with a large scrub shrub wetland at its base.
Over time, the ravine saw some minor degradation resulting in sediment deposits in the wetland below, from a combination of factors including insufficient infiltration area and gravel fines entering the system from the parking area. During the spring of 2005 New Hampshire experienced an intense 3 day rain event now known as the "Mother's Day Flood of 2005". This event destabilized the ravine and the adjacent side slopes causing a minor slide and significant deposition of sediment over a 4000 square foot area within the scrub shrub wetland adjacent to Mill Brook As soon as was practical CEC installed a series of Filtrexx SiltSoxx check dams to reduce velocity, filter sediment and begin to stabilize the ravine. By utilizing blower trucks to install the filtersoxx CEC-NH was able to complete the work without any heavy equipment entering the destabilized area, sometimes working as far as 400 feet from the nearest access point. This kept the area of disturbance as small as possible and allowed a minimalist approach while still controlling sediment and allowing the ravine too naturally begin to stabilize itself. During the spring and summer of 2005 CEC-NH worked closely with West Environmental (http://www.westenv.net/) to monitor and modify the ravine and wetland area to insure that no additional degradation occurred.
In the fall of 2006 work began to replace and improve the drainage system beneath the parking lot. CEC was contracted to upgrade the existing drainage system with lager pipe sizing and infiltration cells. The outflow of the system was directed to a more stable area
of the property with a large level spreader to disperse and reduce the velocity of water exiting the system.
Mill Brook Wetland Restoration
Now that the drainage issues in the parking lot had been improved and the ravine had been temporarily stabilized, the Forest Society
could turn its attention to repairing the damaged wetland and completing final stabilization in the ravine. Earle Chase from West
Environmental Inc. identified 4033 square feet of scrub shrub wetland impaired by a sediment layer ranging from a few inches to over
two feet in depth. Much of the woody vegetation within the affected area had survived, removing this vegetation would be costly and
add years to the time it would take for new planting to colonize the site. Working closely with The Society and West Environmental
CEC introduced a plan that would use technology from Streamside Systems (http://www.streamsidesystems.com/), Filtrexx and Aqua Dam (http://www.waterstructures.com/). CEC-NH would isolate the area with an 18" Aqua dam and then utilize a combination of traditional mechanical methods and Streamside systems hydraulic technology to remove sediment from the affected area. Blower trucks would then be used to install seeded Filtersoxx and compost blankets to stabilize the restored wetland. The Forest Society and West Environmental Inc. began working with the NH Department of Environmental Services to approve the plan and CEC was contracted to oversee and complete the work.
The thickest sediment layers were removed by CEC using a mini excavator and tracked loader. The excavator took the grade down to
within a few inches of final grade removing approximately 250 yards of material. Care was taken on the part of the operators to protect as many plants as possible. Woody vegetation was tied into clumps so that the excavator could reach out and extract the sediment surrounding the plants.
Streamside Systems® Removal:
As the conventional removal continued, CEC began to prepare the site for Streamside Systems by impounding the wetland using an 18" Aqua Dam and creating a series of filter cells made of filtersoxx to dewater the sediment after it was extracted by Streamside Systems. The filter cells consisted of concentric rows of Filtrexx SiltSoxx filter media, a proprietary blend of compost, and wood fiber that allows water to rapidly filter through a three dimensional matrix created by the media. This allows SiltSoxx to removes up to 78% of total suspended solids and reduces nutrient levels in the water being returned to the wetland. Three 12" SiltSoxx made up the outer ring with five chambers to filter the water as it left the site. The structure was constructed so that water leaving the site had to pass through at least five 12" SiltSoxx before leaving the containment area.
The area was then flooded with approximately 6" of water to provide a media to carry the sediment out of the wetland. Streamside
systems pump was used to run two 2" Sandwand™ heads in the impounded area of wetland. The heads are designed to inject water into the sediment loosening the material and remove it through a suction hood that pumps the slurry of water and sediment away to the dewatering area. The Sandwand™ allowed sediment to be removed and return the wetland to original grade without damage to existing vegetation and root mass. Earle Chase from West Environmental monitored the site and utilized soil types and root crowns of trees to provide the best indication of where the original grade of the wetland had been.
As final grades were established, Earle Chase of West Environmental directed the planting of additional woody vegetation. The ravine
that carried the original stormwater runoff into the Wetland was covered in a 2" blanket of CEC compost growing media and planted with cinnamon fern and witch-hazel by West Environmental. After the sediment and dewatering area were removed, the entire area was blanketed with a 2" layer of compost growing media. Several Filtrexx SiltSoxx were left in place to provide storm water control until the site is fully stabilized. Within several years these SiltSoxx will, by design, slowly integrate into the wetlands soils and become part of the pit and mound landscape of the wetland. Several additional Soxx were installed with growing media under the direction of West Environmental Earle Chase. The final step was to close the historic woods road and trail that lead to the site. The road was planted with woody vegetation and blanketed with the CEC compost growing media.
The Mill Brook Wetlands restoration project has been deemed a success by all parties concerned, including the NH Department of
Wetlands Bureau. By using the Streamside Systems Sand Wand technology, vegetation that would have been destroyed through a purely conventional removal was able to be preserved. This has the potential to greatly accelerate the recovery of the site.