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