Resuspension as a Source of Turbidity in a Water Supply Reservoir
Emmet M. Owens, Rakesh K. Gelda, Steven W. EfflerUpstate Freshwater Institute, Syracuse NY
Donald C. Pierson New York City Dept. of Environmental Protection, Kingston NY
Watershed Science & Technical ConferenceNew York Water Environment Assoc.
West Point, NYSeptember 2009
New York City Department of Environmental ProtectionBureau of Water Supply
Water Quality
Schoharie ReservoirDiverts water from
Schoharie Cr. (Mohawk R. basin) into Shandaken Tunnel, Esopus Cr.,
Ashokan Res., and Catskill Aqueduct
Schoharie Reservoir long, narrow shape; steep
bottom slopes deep, thermally stratified short residence time; function
is primarily diversion large watershed; 9/17/99
(Hurricane Floyd) reservoir rose 9.8 m (32 ft.) in 24 hrs
episodes of elevated turbidity driven by runoff events, exacerbated by reservoir drawdown
SchoharieCreek
ManorKill
BearKill
Intake toShandaken
Tunnel
Scale in km0 10.5
15 m
30 m
Dam
Modeling Goals
understand factors leading to historical turbidity events
contribute to design features of potential structural turbidity control alternatives
allow evaluation of turbidity control alternatives: structural and operational
Monitoring Program
stream inflows: USGS
reservoir outflows & operations: NYCDEP
local meteorology: NYCDEP
routine temperature and turbidity monitoring: tributaries, water column and withdrawal: NYCDEP
event-based monitoring: Schoharie Creek (robotic); water column (robotic and manual gridding): UFI (Sept. 2002 – Dec. 2005).
Historic Reservoir Drawdown
Minimum Annual WSE (m)
320 325 330 335 340 345
Cu
mu
lativ
e P
rob
ab
ility
0.0
0.2
0.4
0.6
0.8
1.015 10 5 02025
Maximum Annual Drawdown (m)
2005 2002 2004 2003
All years
prior to
1980
Median annual drawdown = 17 m (56 ft.)
2002-05 monitoring period: 2 full reservoir years, 2 with significant drawdown
Schoharie Reservoir Turbidity Model
state variable is turbidity Tn (an optical property)
while there is no conservation principle for Tn, it is treated as if it is mass (good empirical evidence for doing so)
turbidity model considers following processes: turbidity loading, deposition, transport, export, and resuspension
Model Framework: CE-QUAL-W2 (W2) two-dimensional approach assumes that
temperature and turbidity are uniform over width of the basin
hydrothermal component of model previously applied by UFI
model enhanced by UFI to simulate turbidity and resuspension (W2Tn)
Distance from dam (m)02000400060008000
Ele
vatio
n (m
)
300
310
320
330
340
350
Early Model Testing assumed that stream loading is the only source
of particles and turbidity
resulted in underprediction of observed Tn in water column and withdrawal during certain runoff events
underprediction was greater during periods of reservoir drawdown
Resuspension Relationship
Focus on field measurements to validate model predictions of shear stress
Two Sources of MotionCausing Resuspension
Stream Inflow – high current velocity near mouth of Schoharie Creek during runoff events
Waves – oscillatory motion associated with wind-driven surface waves
Full Reservoir, Low Streamflow: Large A, Small Q Small V
(Deposition)
Drawdown, High Streamflow: Small A, Large Q Large V
(Resuspension)
A
A
Drawdown Reservoir cross
sections near creek
mouth under two conditions
Velocity (V) =Streamflow (Q)
Area (A)
ResuspensionDue to
Stream Inflow
Shear Stress relationship
22UB
gU
C
22UB
gU
C
22UB
gU
C
22UB
gU
C
= g V2/CB2
g = acceleration of gravity
ResuspensionZones
Intake
Schoharie Cr.Bear Kill
1. Resuspension in inflow regiondue to Schoharie Cr runoff events
HydrodynamicMonitoring
E.A. Cowen, Cornell Univ.
T-RDI 1200 KHz Workhorse Monitor ADCP
Nortek Vector ADV
Temperature loggers Aug. – Sept. 2004
Observed and Predicted Bed Stress
Wave-Induced Resuspension
fetch < 1500 m; wave heights < 30 cm (small) due to small waves, wave-induced bed stress
and resuspension occur where depth < 1 m (narrow strip along lee shore)
effect of drawdown: sediments that are in a depositional environment at full reservoir are exposed to resuspension during drawdown
ResuspensionZones
Intake
Schoharie Cr.Bear Kill
1. Resuspension in inflow regiondue to Schoharie Cr runoff events
2. Wave Resuspension at Shoreline (SW, W, NW winds dominant)
October 2001Severe Drawdown (19 meters; 62 ft.)
Gatehouse andIntake Structure
Surface Wave Model
Donelan/GLERL model used to simulate waves and associated bottom motion and bed stress
Previously applied to coastal ocean, large estuaries, Great Lakes; first application to small lake or reservoir1
measurements of wave height and period made with submerged pressure sensors were used to validate the model
1 Owens, E.M. 2009. Observation and simulation of surface waves in two water supply reservoirs. Jour. of Hydraulic Engr. 135(8): 663-670.
Surface WaveModel
ValidationOct.-Nov.
2002W
ind,
m/s
ec
0
2
4
6
8
28 29 30 31 1 2 3 4 5 6 7 8 9 10 11 12
Pre
ssur
e
diffe
renc
e, m
b
0
4
8
12ObservedPredicted
Wav
e
heig
ht, c
m
0
5
10
October November
Dire
ctio
n
N
E
S
W
Drawdown Conditions – 2002
Water Surface Elevation
Ele
vatio
n, m
305
310
315
320
325
330
335
340
345
Schoharie Cr. streamflow
Jul Aug Sep Oct Nov Dec
Flo
w, m
3/s
ec
0
50
100
150
200
250
Example simulationsfollow
Model Performance: 13-16 Oct 2002Red: no resuspension White: all resuspensionGreen: inflow resuspension
Probability that Withdrawal (Tunnel) Turbidity is less than X
(days Tn > 10 NTU) Sept 2002 - Dec 2005
Conclusions
tributary input is generally the dominant source of turbidity to Schoharie Reservoir
resuspension near creek mouth caused by runoff events can be an important contributing source, particularly during drawdown
wave-driven resuspension is source to surface waters, and is a minor contributing source of turbidity
turbidity model accurately represents these two resuspension processes