Isabella Kennedy, Pre-Construction Manager/Estimator, GZA GeoEnvironmental, Inc.
Brendon Murphy, EIT, Senior Project Manager, GZA GeoEnvironmental, Inc.
Background Information
Environmental impacts caused by marine and coastal construction continue to be a growing concern to scientists, engineers, owners, and contractors. The National Oceanic and Atmospheric Administration (NOAA) reports that the construction industry is one of the largest contributors to sediment pollution, significantly impacting water quality within marine habitats. There are several marine construction operations that generate suspended particles, including vessel traffic (propwash), dredging, pile driving and removal, drilling, and beach nourishment/replenishment. The sediments released during construction activities can result in elevated turbidity leading to an array of environmental concerns.
The U.S. Geological Survey (USGS) defines turbidity as the measurement of scattered light caused by suspended particles, which is measured in Nephelometric Turbidity Units (NTUs). Turbidity is directly impacted by high levels of Total Suspended Solids (TSS), measured in mg/L, that can significantly reduce the clarity of the water. Research has shown that elevated levels of turbidity can impact water quality by reducing Dissolved Oxygen (DO) levels, affecting photosynthesis processes, and harming aquatic ecosystems. For example, several studies have found that high levels of turbidity can directly affect the early stages of Atlantic Salmon, with an impact on the development and survival rates of Atlantic Salmon larvae eggs due to low levels of DO. NOAA reports that egg/larvae can be affected by TSS concentration levels as low as 10 mg/L to 120 mg/L. Additional research has shown that high levels of turbidity can alter social, migratory, and foraging behaviors in marine life.
Based on the water quality data collected during construction of the Tappan Zee Hudson River Crossing Project, NOAA reports that pile driving can produce TSS levels upward of 10 mg/L above ambient conditions within 300-ft of the construction activity.
Project Details
The NOAA OMAO Ship & Supply Facility Relocation project, located on the Naval Station in Newport, Rhode Island, involved the construction of a new four-berth pier and trestle, floating dock, waterfront bulkhead, and an Administration Building. Skanska USA Civil NE. Inc. (Skanska) was the lead contractor on the project and was supported by GZA GeoEnvironmental, Inc. (GZA) to perform the project specified automated turbidity monitoring during pile driving. Coastal Marine Construction LLC (Coastal) was the subcontractor selected by Skanska to perform pile driving.
The NOAA OMAO project specifically included the in-water construction of a new 723′-9″ bulkhead set outboard of the existing seawall, with a 521′-3″ long by 28′-0” wide access trestle (consisting of nineteen (19) bents, utilities integrated) feeding two (2) 80′-0” long gangways. The primary structure is a 587′-0” long by 62′-0” wide pier on thirty (30) bents of 30-inch steel pipe piles with high-density polyethylene (HDPE) jackets, topped with precast concrete planks and a cast-in-place (CIP) slab, plus eight (8) substation platforms, a 50-ton bollard on top of a 3′-6″ utility trench, and a continuous fender system consisting of pipe piles, steel walers, and rubber arch fenders. Ancillary features include a 9′-0” wide by 12′-0” long concrete support platform and a 19’-8” wide by 65’-8” long concrete float.
Prior to construction, the U.S. Navy performed a Final Environmental Assessment (FEA) for the proposed project in accordance with the National Environmental Policy Act of 1969. The project area was classified as an Essential Fish Habitat by several government regulatory agencies and the Fishery Management Plans (FMPs). The FEA reported that the proposed in-water work could have an adverse effect on the marine ecosystem in Narragansett Bay. Due to potential impacts of the proposed work, the National Marine Fisheries Services (NMFS) recommended that Skanska install turbidity barriers enclosing the work area and perform monitoring while pile driving during critical fish spawning periods (February 1st – May 31st).
Turbidity Monitoring
The Specification required three (3) buoy-mounted automated turbidity monitors with three turbidity sensors each at various levels within the water column. Sensors were installed three (3) feet above the mudline, five (5) feet below the tidal water level, and at the midpoint of the water column. The buoys were to be secured to moorings approximately fifteen (15) feet outboard of the turbidity barriers to ensure there was no excessive sediment transport. Thresholds values were to be set at 10 NTUs above the values observed in the baseline period.
GZA designed and performed automated, buoy-mounted turbidity monitoring while pile driving was taking place during the aforementioned fish spawning seasons in 2024 and 2025. Turbidity measurements were collected at 10-minute intervals and notifications were automatically distributed to the Project team if NTU threshold values were reached or exceeded. The system was configured to upload data in near real-time to a monitoring website managed by GZA. This system eliminated the labor-intensive effort of manual data collection and helped to ensure regulatory compliance.
Prior to the construction activities in 2024 and 2025, GZA collected baseline data for five (5) days to better understand local and naturally occurring turbidity levels. During the baseline periods, the data showed low levels of ambient turbidity within the proposed work area with an average of 0.11 NTUs and 0.03 NTUs, respectively. No significant weather events or construction activity took place during the baseline period.
In the early stages of the 2024 and 2025 construction monitoring windows, the average turbidity levels were generally in compliance with the project requirements. During the later stages of the monitoring windows, turbidity levels peaked at 289 NTUs and 1495 NTUs, respectively. Most of these elevated readings were attributable to increasing marine growth on the sensors over time which restricted the sensor’s ability to accurately measure turbidity.
It should be noted that different marine environments such as salt water, fresh water, strong currents, stagnant water, and water temperatures can all have varying effects on the sensors and data accuracy over time. It’s important to take these factors into account when deploying this technology and to perform maintenance to the systems periodically. Figure 2 demonstrates the effect of marine growth on the turbidity sensors in a saltwater environment after approximately one (1) month without sensor maintenance. Elevated NTUs began in early May and continued to increase into early June. The marine growth was verified when the equipment was removed following the end of the monitoring window. There are some preventative measures that can be taken to slow or eliminate marine growth such as installing copper mesh on the equipment which will prevent organisms like barnacles and algae from attaching to the sensor.
Discussion
Based on the sensitivity of aquatic ecosystems, turbidity monitoring should be considered to ensure there is no excessive sediment generated as a result of marine construction activities, including pile driving. New technology allows contractors to automate data collection as a cost-effective alternative to manual monitoring methods. The automated approach reduces the overall labor effort required by the Contractor, provides consistent and accurate turbidity measurements, and allows for near real-time data collection and alert notifications. Project teams are then able to make informed decisions while maintaining compliance with local environmental regulations.