A Turbidity Model For Ashokan Reservoir

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A Turbidity Model For Ashokan Reservoir

• Rakesh K. Gelda, Steven W. Effler
• Feng Peng, Emmet M. Owens
• Upstate Freshwater Institute, Syracuse, NY
• Donald C. Pierson
• New York City Department of Environmental
  Protection

2009 Watershed Science Technical
Conference September 14th-15th, Thayer Hotel,
West Point, New York
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• network of 19 reservoirs
• three controlled lakes
• Croton, Catskill, Delaware systems
• watershed 1930 mi2
• storage 550 BG
• unfiltered supply
• 1.2 BG/day
• Ashokan Reservoir
• watershed 257 mi2
• storage 130 BG
• Catskill Aq. 600 MGD
• Turbidity lt 8 NTU (90th percentile 1987-2008)

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Ashokan Reservoir
West Basin
East Basin
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West Basin
Bridge and Dividing Weir
Upper Gate Chamber
East Basin
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Gates (4)
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East Basin
Diversion Wall
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Upper Gate Chamber
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Intake Structure
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Turbidity Problem

• stream channel and banks erosion glacial and
  fluvial sediment Esopus Creek 85 of the inflow
• turbidity in waters leaving Ashokan Reservoir can
  be high following major runoff events
• alum treatment before it enters Kensico Nine
  alum events, 524 days during 1987-2007
• turbidity model to evaluate management
  alternatives

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Features of Turbidity Model

• Two-dimensional (longitudinal-vertical),
  laterally averaged transport framework
  (CE-QUAL-W2)
• State variables Temperature (T) and turbidity
  (Tn)
• Three size classes of Tn
• Source of Tn external loading
• Sinks settling, export (via withdrawal, spill,
  waste channel diversion)
• Two basins simulated separately

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Model Grid West Basin
Esopus Creek
27 segments (330 m avg) 47 layers (1 m) 1 branch
dividing weir
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Model Grid East Basin
dividing weir
37 segments ( 300 m avg) 26 layers (1 m) 1 branch
spill
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Model Grid Vertical Layers
dividing weir
west basin
east basin
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Turbidity (Tn)

• primary metric of quality for water supplies
• measure of light scattering by particles at 90
  collection angle, units of NTU

Tn a
90
1
1
light scattering coefficient (b, m-1)
incident beam
0
scattered light

• Tn a b supported in peer-reviewed literature
• b, Tn f (particle concentration, size
  distribution, composition, shape)

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Scattering (b) and Turbidity (Tn) Behaves Like
Intensive Properties

• mass balance calculations can be done
• well-established in optical literature
  (Davies-Colley et al. 1993)

example
Q1, b1, Tn1
Q, b, Tn
Q Q1 Q2
Q2, b2, Tn2
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Turbidity As the Model State Variable

• Tn is the regulated parameter
• disadvantages of TSS (a gravimetric measurement)
  as an alternative (would have to rely on Tn k
  TSS)
• differences in particle size and composition
  dependencies of Tn and TSS
• Tn, b (scattering) and c (beam attenuation)
  measurements more precise
• limitations in temporal and spatial resolution
  e.g., robotic and rapid profiling capabilities
  for Tn and c
• pore size for TSS measurements too large (1.7 µm)
• variation in relationship between Tn and TSS in
  time and space (i.e., k is not really a constant)
• Tn, and c supported in peer-reviewed
  literature, without published critical comments

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Model Inputs

• Model testing period 2003-2007
• supported by UFIs intensive (Robohut on Esopus
  Creek, in-reservoir robots) and DEPs routine
  monitoring data
• constrained by the availability of operations
  data
• Additional (secondary) validation period
  1995-2002
• Operations data
• Hydrologic inputs/outputs
• Loading of turbidity
• Creek temperature
• Meteorological data

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In-Reservoir Robots Example, 2007
April November (June in 2007) depth-profiles
every 6 hours depth interval 1 m
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In-Reservoir Rapid ProfilingExample, 11/30/2006
after major runoff events depth interval 0.25 m
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Example of Driving Conditions and Reservoir
Response June 2006
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Turbidity-Causing Particles

• Four Features
• number concentration
• size distribution
• composition
• shape

April 2005
Individual Particle Analysis (IPA) Technology

• 75-80 clay
• Tn associated with 1-10 µ
• sub-µ particles unimportant
• TSS filter pore size 1.7 µm misses some
  turbidity causing particles

bm(660) minerogenic particle scattering
coefficient, m-1
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Turbidity Size-Classes for Model
Esopus Creek
Class size (µm) Size range vel (m/d)
1 1 lt 1.75 0.075
2 3.14 1.75-5.75 0.75
3 8.11 gt 5.75 5.0
Fractions in Esopus Creek
Stokes Law
Class Q 40 m3/s Q gt 40 m3/s
1 10 10
2 65 45
3 25 45
coefficient specification constrained by reality
of particle characteristics as obtained from IPA
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Hydrothermal Model Performance
withdrawal temperature (Tw) importance of
withdrawal depth information
2003-2007
1995-2002
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Turbidity Model Performance
withdrawal turbidity (Tn,w) importance of
detailed monitoring of forcing conditions
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Turbidity Model Performance
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Turbidity Model Performance
East Basin 6/30/2006
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Performance Summary
Alum treatment events
Alum Start Alum End RMSE (NTU) Event Peak Tn (NTU) RMSEN ()
1/22/1996 6/21/1996 58.1 158 37
1/14/1997 1/29/1997 1.4 9 17
1/10/2001 2/2/2001 10.0 12 83
4/5/2005 6/20/2005 28.5 150 19
10/13/2005 11/23/2005 5.5 29 19
12/1/2005 4/10/2006 6.5 45 14
5/15/2006 5/23/2006 2.5 10 25
6/28/2006 8/2/2006 20.4 140 15

Normalized RMSE (Gelda and Effler, 2007)

• performance for well monitored years consistent
  with that reported for Schoharie Reservoir (Gelda
  and Effler, 2007)

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Summary

• 2-D model CE-QUAL-W2 as transport framework
• Turbidity as a state variable
• Characterization of turbidity-causing particles
• Three size classes
• Model performed well in simulating in-reservoir
  and withdrawal temperature and turbidity
• Model is suitable for evaluating management
  alternatives
• Future research resuspension, particle-based
  modeling including aggregation

Gelda, R. K., S. W. Effler, F. Peng, E. M. Owens
and D. C. Pierson, 2009. Turbidity model for
Ashokan Reservoir, New York Case Study. J.
Environ. Eng. 135 885-895. e-mail
RKGelda_at_UpstateFreshwater.Org
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Ashokan Reservoir East Basin Spillway 4/2005