Snead Pond Restoration

Transcription

1 Snead Pond Restoration Project overview Location: Crewe, VA Date of Project: Summer 2016 Scope of work: Construction of check dam & hydraulic dredging of 530 cubic yards of in situ sediment utilizing geotubes for material dewatering.

2 Snead Pond Restoration Page 2 of 8 Introduction and overview: Pajic Construction was contacted initially to construct a check dam for a farm pond in Crewe, VA. The owner was working with Aquatic Biological Monitoring Services, LLC to restore the pond. The plan included building a check dam at the pond inlet to control and isolate the incoming sediment, and adding an aeration system to help maintain depth and water quality. While consulting with the owners they raised the question of also removing the sediment deposits on the upper end of the pond. Pajic Construction was ultimately contracted to construct a check dam at the pond inlet and hydraulically remove 530 cubic yards in situ sediment from the upper end of the pond. Site Study & Planning: The project began with a site study to better understand where dredging would be most beneficial and at what rate the in-situ sediment would dewater. This study consisted of three parts, a site sounding, a sediment dewatering study, and a polymer lab report. This information allowed us to determine the size needed for the laydown area and the capacity of the Geotubes that would be required for the dredged material. The site sounding consisted of manual soundings at 38 locations. From this data the water depth, hardpan depth, and sediment depth were determined and plotted. Sediment core samples were taken for us to complete the dewatering study and polymer lab report. The dewatering study uses a representative sediment sample in a small Geotubes to determine the rate at which the sediment dewaters and the volume of material remaining once dried to a haul away consistency. This information is what determines the specifications required for the full size Geotubes given our removal goal of in-situ sediment. A portion of the core samples are also sent to a laboratory for testing to determine the appropriate organic polymer to use for the project. The polymer is an environmentally friendly solution that is injected into the dredge slurry passing in the pipe line to flocculate the smaller solid particles. This flocculation makes the solid particles large enough that they will not pass through the filtering material of the Geotubes rendering a clean water filtrate from the bags. Once the results of these studies were processed we had the information necessary to determine the required volume of polymer & capacity of the Geotubes.

3 Snead Pond Restoration Page 3 of 8 Because the hydraulic dredge pumps at a rate of 850 to 1000 gallons per minute, and the pond inflow was much less, we needed to place the spoils site in an area where we could divert the filtrate water back into the pond. Otherwise we would pump the pond water level down, significantly reducing the amount of time the dredge could operate on any given day. A wooded area above the pond, that would allow us to direct filtrate water back into the pond while also being aesthetically pleasing was selected as the best option. During the planning stages, we were approached by the owners and asked if we could remove a small island that was left in the upper end of the pond during the original construction. The island had become overgrown and unsightly with brush and trees up to 3 in diameter. While the dredge is designed primarily to remove sediment deposits, we have found during previous projects that it can effectively remove hard pack though at a much slower rate. We agreed to excavate as much of the island as possible while staying within the project budget. Project commencement: Equipment deployed to the site included our 6 hydraulic dredge and associated pipe line, polymer injection system, backhoe, excavator, and single axle dump truck. We began the project by constructing the check dam at the pond inlet. The check dam was constructed using #2 and #3 crushed stone placed with equipment, and by hand, over a geo-cloth under-laminate. Upon completion of the check dam we began prepping the island for excavation by grubbing the vegetation and saplings. We opted to pull out as much of the growth as possible in order to remove the root balls that would slow the dredge progress. Utilizing an island to shore rigging system we pulled the larger trees and bushes, uprooting and removing as much of the root system as possible. This also helped to loosen the top layers of hard packed soil allowing as to dredge the material more efficiently. After removing as much growth and debris as possible the dredge was deployed and rigged, the pipe line was constructed, and the laydown area was prepared and leveled. Two fifty foot by forty-five foot circumference Geotubes were installed over a 6 mil plastic scour apron. The scour apron was constructed to divert the filtrate water back into the stream feeding the pond. Dredging began with a primary focus on removing the island. This was a much slower process than removing traditional silt as the island had significant areas of hard pack and debris. Excavating the island from 1.5 above the water line to a depth of approximately 3.5 took three days of operation. Once the island was excavated we focused our attention on the

4 Snead Pond Restoration Page 4 of 8 areas of the pond that had the highest deposits of sediment based on our sounding study. Dredging of this area continued for three additional days until the capacity of the geotubes were reached, resulting in approximately 550 cubic yards of material being removed from the pond. Once all dredging operations were concluded we completed all the necessary site restorations and repairs. This included seeding and laying straw over any areas of the shore line that had been disturbed by the heavy equipment, as well as spreading sand in a beach area the owners use for swimming. Conclusion: This project was completed within budget and schedule with minimal surprises along the way. The owners in this case had a comprehensive plan that included both restoration and upgrading the existing system to allow for easier maintenance. By constructing the check dam future sedimentation will be largely captured before it can enter the pond. The forebay area above the check dam is easily accessible by a backhoe or excavator for cleanout and maintenance, and the aeration system will help control sediment deposits and maintain depth in the pond. Removing the island eliminated the need for vegetation control on the island and shallow water surrounding it. From the owner: Dear Joe, I love it!! I have named it the Rockyford Resort and retirement center. You and Nick did everything we talked about and then some. This weekend I sat on the beach and pretended I was in the Bahamas. The beach looks incredible. Taking the Island out really improved the size and looks of the pond. Everybody doubted you but me and said it could not be done. I know it took more time and effort than you had planned but you two stuck with it and got it done. I am extending you and Nick an invitation to come swim in the pond Summer My next project is to improve the picnic area. Sincerely, Gin Snead

5 Snead Pond Restoration Page 5 of 8 Additional photos: Island Removal - Before Island Removal - After

6 Snead Pond Restoration Page 6 of 8 Geotubes

7 Snead Pond Restoration Page 7 of 8 Completed beach area (top), check dam (center), and seeding & erosion control (lower) Check Dam and spillway

8 Snead Pond Restoration Page 8 of 8 Beach and spillway (left), Seeding & Erosion control (right center, far bank) Beach and spillway (left), Seeding & Erosion control (right center, far bank)