TABLE 1: Storage capacity, billion gallons As built New USGS Change % Reduction
Reservoir
* Years in Service
Ashokan East Basin Ashokan West Basin Ashokan Total Cannonsville Neversink
80.68 49.42 130.10 98.62 37.15 149.80 52.44 21.55 489.66
78.53 46.82 125.35 96.00 36.65 148.69 51.77 19.60 478.06
-2.15 -2.60 -4.75 -2.62 -0.50 -1.11 -0.67 -1.95 -11.60
2.6 5.3 3.7 2.7 1.3 0.7 1.3 9.0 2.4
99 99 99 47 61 60 63 88
Pepacton Rondout Schoharie Total
* Years in service are based on date of completion of USGS study.
tity 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. Scien- tists 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 mil- lions 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 depo- sition by watershed area and by the age of each reservoir. Those inter- pretations 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
In February, the New York City Department of Environmental Protec- tion (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 ca- pacity 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 quan- USGS study of sediment deposition in Catskills reservoirs reveals minimal decrease in storage capacity. Comprehensive survey confirms NYC water supply
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may 2018
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