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Recent Storm Activity and its Effect on Turbidity Levels in Neversink Reservoir

Explore the effects of storm events on turbidity levels in Neversink Reservoir, factors contributing to elevated turbidity post-Irene, and long-term recovery rates.

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Recent Storm Activity and its Effect on Turbidity Levels in Neversink Reservoir

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  1. Recent Storm Activity and its Effect on Turbidity Levels in Neversink Reservoir Rich Van Dreason Watershed Water Quality Science and Research New York City Department of Environmental Protection NYC Watershed/Tifft Science & Technical Symposium September 19 , 2013 Thayer Hotel, West Point

  2. Introduction Neversink Reservoir not too long ago August 29, 2011 Neversink River just below dam September 19, 2012 Neversink River above reservoir September 19, 2012 Neversink Reservoir

  3. Objectives • Discuss factors associated with recent elevated turbidity in the Neversink Reservoir • Occurrence of recent large storm events • Increase in sources of turbidity resulting from Irene • Discuss factors that may be contributing to the slow recovery since Irene • Occurrence of small storm events • Particle size • Recent monitoring upgrades

  4. Land use in the Neversink Basin

  5. Hydrology and site locations in Neversink basin West Branch 10 NR4 20 NR3 East Branch NR2 30 NR1 40 Neversink Reservoir (cross-section) Aden Brook Kramer Brook

  6. Turbidity Characteristics Definition: • Measure of the light-scattering effects of suspended particulate material. • Nephelometer; results in NTU • Turbidity is related to suspended sediment concentrations • Also depends on the particle size distribution and refractive index which may change with turbidity source • Suspended particles that contribute to turbidity are generally in the 1-10 micron range • Examples: clay, fine silt and algae • SWTR Source Water turbidity limit = 5 NTU Turbidity=-0.01435 + 0.7135 (TSS) R-sq=85.7%

  7. Turbidity and Stream flow • Complex relationship • Available sediment supply • Location of sediment supply • High turbidity at onset => available material in channel • High turbidity later on hydrograph • Sediment sources faraway • New sediment sources become available

  8. Long-term turbidity at Neversink Reservoir and Streams Pre conditions Post 2. Mean Daily Flow (cfs) Neversink River @ NCG 1. 3. Reservoir Elevation taps (1-4) Mean Turbidity (NTU) October 1, 2010 Neversink River (NCG) Turbidity (NTU) Aden Brook (NK4) Kramer Brook (NK6) Year

  9. Long-term turbidity at Neversink Reservoir and Streams 2. Mean Daily Flow (cfs) Neversink River @ NCG 1. 3. Reservoir Elevation taps (1-4) Mean Turbidity (NTU) Neversink River (NCG) ? Turbidity (NTU) Aden Brook (NK4) ? Monthly stream samples not adequate for flashy mountain streams ? Kramer Brook (NK6) ? Year

  10. Details of recent flow events • Event 2 Aug 28, 2011 (Irene) • Daily mean flow was 7,220 cfs (largest on record) • Peak flow was 20,900 cfs (60 yr event, 2nd largest) • 4.8 inches Aug 27-28, 3.6 inches on Aug 15 • Event 1 Oct 1, 2010 • Daily mean flow was 6030 cfs • Peak flow was 16,400 cfs (about 20 yr flood, 6th largest) • 7.8 inches 5 days prior w/ 5.7 on Sep. 30 • Event 3 Sept 18, 2012 • Daily mean flow was 3410 cfs • Peak flow was 17,800 cfs • ( 45 yr. flood, 4th largest) • 5.3 inches Sept 17-18 • (localized). Mean Daily Flow (cfs) Neversink River @ NCG 162.5 NTU Reservoir Elevation taps (1-4) 59.5 ntu Mean Turbidity (NTU) 29.6 NTU

  11. Turbidity Recovery Rates Event 2 Event 3 Event 1 • Why the slow recovery after Event 3 ??? 162.5 NTU 124 days Mean Turbidity (NTU) Reservoir Elevation taps (1-4) 130 days 59.5 ntu 28 days 29.6 NTU 3.8 ntu 2.7 ntu 1.6 ntu 40% higher (3.2 vs. 1.9 NTU) Mean Turbidity (NTU) Low Range 2. 1430 cfs Mean Daily Flow (cfs) 1180 cfs 1270 cfs 1. 3. Historic 95th percentile (1.9 ntu) Neversink River (NCG) Turbidity (NTU) • Higher resolution stream data needed!

  12. Additional factors - particle size Neversink Results • 80% of turbidity caused by particles < 4 µm • Composition: 69% clay, 18% quartz Particle size Settling rate • Post Irene, Upstate Freshwater Institute (UFI) contracted to evaluate turbidity causing particles in the Delaware Reservoirs • Size distribution and composition • Scanning electron microscopy interfaced with Automated image and X-ray analyses From “Hurricane Irene Turbidity Studies” prepared by Upstate Freshwater Institute December 14, 2012

  13. Neversink Basin Surficial Geology • Most deposits contain very little fine sediment • Glacial till most likely source of small particles • Very abundant • Streams are often in close proximity to the glacial tills • Tills generally tightly packed; impermeable

  14. Eroding glacial till hillslopes • Hypothesis • High flows caused new hillslope failures (or exploited old ones) freeing fine sediment to become entrained • Currently 15 large failures in till (>1000 sq. ft. eroding bank) • Some old but enlarged, some initiated by Irene • How much fine material? How transportable?

  15. What about glacial lake clays? • Exposed glacial lake clays are rare in the main branches of Neversink River • Smaller tributaries still to be assessed Figure 5. Channel incision into lacustrine deposits post Irene. W. Branch Neversink upstream of Frost Valley (December 2011)

  16. Real time turbidity data is here! • Digital Turbidity, Temperature Sensors and Data logger • Forest Technology Systems and Campbell Scientific • Real-time data access via phone-line • Benefits • Early warning • Evaluate flow-turbidity relationship • Basin changes

  17. Vertical Profiling Systems • YSI vertical profiling system to be installed next year • Turbidity, temperature, conductivity, dissolved oxygen etc. • Real-time data access via radio modem to land line connection • Benefits • Early-warning • Track turbidity interflows • Monitor resuspension • Select appropriate intake

  18. Conclusions • Recent elevated turbidities in Neversink Reservoir related to large storm events starting on October 1, 2010 • And possibly to greater availability of fine sediment courtesy of Irene Elevation taps (1-4) Turbidity (NTU) Mean Daily Flow (cfs) Neversink River @ NCG

  19. Conclusions (continued) • Longer recovery periods post Events 2 and 3 associated with: • Occurrence of multiple storm events following initial major event 162.5 NTU 124 days Reservoir Elevation taps (1-4) Mean Turbidity (NTU) 130 days 59.5 ntu 28 days 29.6 NTU 3.8 ntu 2.7 ntu 1.6 ntu Mean Daily Flow (cfs) Multiple events Multiple events

  20. Conclusions (continued) • And possibly from an increase of clay-sized particles derived from eroding banks of glacial till Glacial till bank Exposed lake clay deposit

  21. Questions ?

  22. Conclusions • Recent elevated turbidities in Neversink Reservoir related to : • Large storm events starting on October 1, 2010 • And possibly to greater availability of fine sediment • Longer recovery periods post Events 2 and 3 associated with: • Occurrence of multiple storm events following initial major event • And possibly from an increase of clay-sized particles derived from eroding banks of glacial till and recent exposures of lacustrine clays

  23. Additional factors - particle size • Post Irene, Upstate Freshwater Institute determined the size distribution of turbidity causing particles in all Delaware Reservoirs • Scanning electron microscopy interfaced with Automated image and X-ray analyses • Particle cross-sectional Area per unit Volume of water (PAV) strongly correlates to turbidity. Key Findings • 80% of turbidity caused by particles < 4 µm, substantially smaller than other Delaware Reservoirs • 4 µm upper limit of clay-sized particles Other Delaware reservoirs Neversink Particle Size Distribution 4 Modified from “Hurricane Irene Turbidity Studies” prepared by Upstate Freshwater Institute December 14, 2012

  24. Conclusions (continued) • And possibly from an increase of clay-sized particles derived from eroding banks of glacial till Neversink Particle Size Distribution 4 Glacial till bank Exposed lake clay deposit

  25. Extreme flow event trends in Neversink basin Mean Daily Flows>95th percentile (579 cfs) Warm season (June-October) Cold season (November-May) All months • All major Catskill streams show similar trends • Schoharie Creek, Esopus Creek, E. and W. Branch of Delaware River Inspired by Matonse A. H. and A. Frei (In press)

  26. Additional Factors? • More resuspension after Events 2 and 3? • Unknown; no data • Did particles settle more slowly after Events 2 and 3? • DOC tends to prevent aggradation of particles • Settling rates decrease with decreasing water temperature, particle size • Slow recovery not related to DOC and temperature • Particle size?

  27. Turbidity interflow post Irene 4 3 2 1

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