Over the years, these attractive river ponds accumulated large amounts of sediment, to the point where the scenic open-water character of the landscape was being severely compromised by the formation of swampland. Adding to the silt accumulation was an estimated 4,500 cubic yards of sediment that was deposited in the ponds as a result of a tropically influenced storm. In order to restore the scenic beauty and the aquatic habit of the ponds, which serve to embellish the historic structures that were, and still are, connected with the Pocantico River, Historic Hudson Valley and Huskie Engineering embarked on a program to dredge the ponds. There were many challenges and considerations to be addressed on this environmentally and historically sensitive site. Huskie Engineering was retained to provide a full range of civil and environmental planning, permitting, design, and construction administration services for this project. 

Dredging the Pocantico River at Philipsburg Manor

HUSKIE ENGINEERING                                          Project Profile

Philipsburg Manor is an important 18th-century historic preservation property situated along the Pocantico River in Sleepy Hollow, Westchester County, NY. The site is a popular tourist attraction, educational resource, and National Historic Landmark. Philipsburg Manor is owned and operated by Historic Hudson Valley. The Pocantico River, which is tributary to the Hudson River, courses through the site and is segmented by a dam and foot bridge that were constructed in the 1960’s. The ponds on the upstream and downstream sides of the dam provide for a scenic open-water focal point in the landscape. At the north end of the dam, immediately adjacent to the ponds, is the historic core, which includes the meticulously restored early 18th-century Manor House and the recreated Mill House with grist mill. The Visitors’ Center is located at the south end of the dam.

Photo along the upstream side of the dam showing unsightly sediment accumulation and swamp-like conditions in the Upper Pond. The Mill House (brown) and Manor House (white) appear in the backdrop.

The photos above show the sediment accumulation in the Upper Pond at low water level after some of the sediment had already been removed as part of a previous flood restoration project. The Upper Pond is nearly three acres. Not shown is the one-acre Lower Pond, which is tidally influenced.  The Lower Pond also required dredging.

 (See Page 2 for restoration measures.)

Dredging the Pocantico River at Philipsburg Manor

HUSKIE ENGINEERING                             Project Profile  continued

One of the most challenging aspects of the project involved determining the fate of the 20,000 cubic yards of sediment to be dredged. The sediment sampling and testing program determined that the sediment contamination level was “Category 2”, meaning that it was moderately contaminated, but not considered “Category 3”. NYSDEC regulations required that the Category 2 sediment be disposed of in a controlled manner and properly contained. The regulations did not allow the dredged sediment to be used for surface fill material but did not require that it be handled as hazardous waste. A range of sediment disposal/re-use alternatives were considered; however, given the proximity of the site in urban surroundings, off-site disposal options were extremely costly.

 

Despite the fact that Philipsburg Manor is largely devoted to historic re-creation, including a fully functional grist mill, and livestock and farming presentations, there was one area within the property that had the potential for on-site sediment dewatering and storage. A portion of this area contained small areas of federal wetlands. The project was subject to a long list of federal, state and local permits and approvals and went through the full gamut of SEQR review, with the local planning board serving as lead agency. Being a historic site, it was important to the owner and the state historic preservation office that the sediment storage area not adversely affect visual resources. It was also important for the owner to keep the facility tours in operation during the dredging project and limit any downtime to January and February of the winter months.

 

In consideration of all factors, it was decided that the hydraulic dredging method would afford the least operational and environmental disruption.

 

Huskie Engineering evaluated the feasibility of various dredging and site design alternatives for consideration by the owner and followed through with preparing construction drawings and specifications, permit applications and performed construction contract administration. Included were the preparation of environmental assessment documents to satisfy local and state SEQR requirements and the acquisition of permits and approvals. 

(Above) Philipsburg Manor “Upper” Pond dredged and restored to scenic and environmental quality. (Below) The on-site sediment dewatering and storage basin was designed and constructed to receive the hydraulically dredged sediment from the Upper and Lower Ponds.

Capped, restored and reclaimed, shown above, is the sediment dewatering and storage area, viewing westerly from the east end. The sediment storage basin was designed to blend in with the existing topography, resulting in minimal impact to visual resources.

Functional improvements to the site included a restored parking area, a portion of which was constructed above the sediment storage area.

The dredging project also included dredging the Lower (tidal) pond, which is shown above in its restored condition. The Mill House stands prominently on the north edge of the pond.

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