In February, the New York City Department of Environmental Protection (DEP) announced the results of a scientific study that examined changes in reservoir storage capacity as a result of sediment deposition in the city’s six reservoirs in the Catskill Mountains. A three-year study by the U.S. Geological Survey (USGS) found that natural erosion of mountains and streams in the watershed has reduced total storage capacity in the reservoirs by approximately 2.4 percent.
Importantly, this sediment has collected in the reservoirs where it does not affect the city’s drinking water intakes, nor does it reduce DEP’s ability to deliver drinking water during the worst drought on record. The new storage numbers are the first comprehensive data collected by scientists and engineers since the reservoirs were constructed from 1913-1964.
“The data gathered by USGS serve as an important baseline for DEP to track the rate of sediment deposition in our reservoirs in the decades and centuries ahead,” DEP Commissioner Vincent Sapienza said. “The amount of sediment in the reservoirs thus far does not affect the quantity or quality of drinking water that DEP provides to the city, nor does it affect our downstream releases.”
The USGS study, which gathered data from 2013-2015, sought to understand the shape and the depth of each reservoir, and the extent to which sediment deposits reduced reservoir storage over time. Scientists collected data by using sonar to send a signal to the bottom of the reservoirs and measure their depth. Guided by a GPS system, the boat used for the survey traveled almost 700 miles as it traced dozens of shore-to-shore cross sections at each reservoir. Scientists collected millions of data points, including more detailed measurements near intake chambers, dams, and other infrastructure that are critical to the water supply’s operation. The data were used to make 3D models of each reservoir, allowing scientists to understand their storage capacities.
Overall, combined storage in the six reservoirs decreased 2.4 percent, from 489.66 billion gallons to 478.06 billion gallons (see Table 1). The decrease in storage capacity by percent at each reservoir varied from 9 percent at Schoharie Reservoir to 0.7 percent at Pepacton Reservoir. These data were also interpreted to measure the rate of sediment deposition by watershed area and by the age of each reservoir. Those interpretations help DEP understand how each reservoir’s age and the size of its drainage basin affect the amount of sediment it collects. USGS is currently performing an identical study of New York City’s reservoirs in the Croton System.
The study found that sediment is primarily collecting in portions of the reservoirs known as “dead storage.” These portions of the reservoirs are deeper than the lowest intakes that send water to New York City, which makes them inaccessible for water supply purposes. That’s why the reduction in storage, at this point, has no negative effect on the quantity or quality of drinking water that passes through the water supply.
The new data also did not affect the water supply system’s safe yield — a measurement of the maximum amount of drinking water that the system could provide under the most severe drought. In addition, the findings of the USGS study do not significantly affect cold-water volumes that are available for downstream releases.
Based on the data collected by USGS, DEP has determined that no action is needed at this time because the reduction in storage capacity is relatively small and it does not affect water supply operations. The findings do not justify the cost and risk of dredging within the reservoirs, especially because such work would yield no water-supply benefits. However, DEP might look toward building expertise within its science and engineering staff to conduct similar studies more frequently in the future. This would allow the city to collect more data and better understand the rate of sediment deposition over time.
Tracking reservoir storage is one of many scientific efforts that DEP has undertaken to gather data for long-term planning, a principle that has guided the New York City water supply since it was established in the 1800s. DEP collects and regularly examines similar data on the effects of climate change, numerous water quality parameters, and more.
The new USGS data are also complemented by studies through DEP’s Stream Management Program. Working with its county partners, the Stream Management Program is currently studying the rate of sediment transport and deposition for storm events of different intensities. Data show that recent stream projects — which aim to reduce the transport of sediments by correcting bank failures, reconnecting streams to their natural floodplains, and more — have reduced the amount of sediment moved by streams in the Catskills Mountains during typical storms.
Download a copy of the final USGS report at www.nyc.gov/html/dep/pdf/reports/catskill-delaware-bathymetry-study.pdf.
Information provided by the New York City Department of Environmental Protection (http://nyc.gov/dep).