The Water Street area within the Town of Plymouth was subject to flooding, leading the Town to examine alternatives. Adding complexity to the situation, Town Brook, which flows beneath the Water Street Bridge, supports herring during their annual migration.
The Town selected Environmental Partners (EP) in the Summer of 2013 to provide construction administration and management associated with the construction of the Water Street Bridge replacement. This project includes the replacement of the existing single span bridge on Water Street over Town Brook which carries water into Plymouth harbor, giving this project an overall drainage area of 3.9 square miles with an anticipated design storm discharge of 130 cubic feet per second.
The existing structure is a 10-ft high by 30-ft long dam with a 6-ft high fixed concrete weir acting as a spillway. While the culvert opening remains the same, the old Dam was removed and a new rock ramp was installed to allow fish to pass through. Other project elements include new wing walls and barrier walls, drainage improvements, sidewalks, ornamental fence, a river rock wall veneer, and final paving. Bids were received and were within the estimate.
The proximity of the Water Street culvert to Plymouth Harbor subjects it to a range of marine backwater conditions related to the tidal elevations in the harbor. Based on the FEMA FIS for the Town, downstream water surface elevations for the 10-year storm fall above the low chord of the existing culvert, thus restricting positive flow toward the harbor. The traffic usage of Water Street required that the crossing be designed to convey the 10-year design flood with a minimum freeboard of one foot between water surface elevation.
Other construction challenges include a number of live and abandoned utility lines that spanned the crossing, some of which had to be supported and maintained during the culvert replacement, while others were cut and capped. Protecting an existing 24-inch PVC sanitary sewer main and a 12-inch ductile iron water main across Town Brook while driving in sheeting for foundation work and water control was a major challenge.
One of the innovative approaches by EP was to bypass the Town Brook flow by gravity through two 60-inch ADS pipes. The pipes were installed beneath Water Street and conveyed the flow into Plymouth Harbor, thus eliminating the need to bypass the flow through a pump system. The Department of Marine Fisheries was thrilled with this approach as it provided a safe fish passage from the brook into the harbor.
EP’s construction oversight resulted in 0% change orders on a large project with an aggressive schedule. The accelerated construction schedule included pre-stressed concrete components to be utilized to expedite the schedule and allow the downtown area to be open for the annual Thanksgiving Day parade.