Lower Susquehanna River Reservoir System

Bathymetry

In cooperation with - Pennsylvania Dept. of Environmental Protection - Susquehanna River Basin Commission - Lower Susquehanna River Keeper

Problem / Background

• Reservoirs filling with sediment and associated nutrients reducing capacity for sediment storage

• Approximately 55-60% of the sediment and 40% of the phosphorus is being trapped

• Revised estimate in 2001 of 25-30 years of remaining sediment storage capacity

(SSC)

• At capacity sediment and phosphorus loads may increase significantly to the upper Chesapeake Bay

Project Objectives

• Collect bathymetry data to update and map bottom- surface profiles

• Provide updated estimates of remaining capacity for sediment storage

• Provide information useful to consider appropriate management options

v

Lower Susquehanna River Reservoirs

Lake Clarke

Lake Aldred

Conowingo Reservoir

Courtesy Exelon Corp.

Susquehanna River Sediment Transport

• Historical and current trends

• Reservoir Transport and Dynamics

- importance

- implications for predicting reservoir

capacity

• Reservoir Filling and Remaining Capacity

0

20

40

60

80

100

120

1900’s 1910’s 1920’s 1930’s 1940’s 1950’s 1960’s 1970’s 1980’s 1990’s

YEAR

Se

dim

en

t L

oad

(m

illio

n t

on

s)

Loads to Reservoirs

Significant decrease in sediment loads

Estimated Sediment Loads to Reservoirs 1900-1999

Current “trends” in sediment loads From 2000-2008

(avg 3.1 Mt in, 1.2 Mt out,55% trap w/scour)

Long-term AVG (1980-1999) (3.2 Mt in, 1.2 Mt out, 55% trap w/scour)

2000 2001 2002 2003 2004 2005 2006 2007 20080

1

2

3

4

5

6

7

8

9

Reservoirs (IN) Reservoirs (OUT)

Sedi

men

t loa

d (m

illio

n to

ns)

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 20000

20

40

60

80

100

120

140

Load to Reservoirs

YEAR

Se

dim

en

t Lo

ad

(m

illi

on

to

ns)

Estimated Sediment Loads to Reservoirs 1900-2008

1996

1004

1997

0207

1997

0613

1997

1017

1998

0220

1998

0626

1998

1030

1999

0305

1999

0709

1999

1112

2000

0317

2000

0721

2000

1124

2001

0331

2001

0804

2001

1208

2002

0414

2002

0818

2002

1222

2003

0428

2003

0901

2004

0105

2004

0511

2004

0914

2005

0118

2005

0525

2005

0928

2006

0201

2006

0608

2006

1012

2007

0215

2007

0622

2007

1026

2008

0229

2008

0705

0

100000

200000

300000

400000

500000

600000

DATE

Flo

w (

cu

bic

fe

et

pe

r s

ec

on

d) 390,000 cfs (scour threshold)

Hurricane Ivan

Streamflow Hydrograph

01578310 - Susquehanna River at Conowingo, 1996-2008

Predicted Sediment Scour vs. Flow

0

5

10

15

20

25

30

300 400 600 700 800 900 1,000 1,100 1,200Instantaneous Flow (times 1,000)

Pre

dic

ted

sco

ur

(mil

lion

s of

1972

1996

1975

Scour estimates

95th percentile

95th percentile

Expon. (Scour estimates)

500

ton

s)

2004 Ivan

Wendy McPherson (USGS) - 9/20/04

Susquehanna River

Bay Bridge

Hurricane Ivan – September 2004

Turbid inflow

Delta(sand)

Floating DebrisWater Surface

Relatively clear water(clay)

Density current(Silts)

Fine sediments(silts & clays)

Sluiceway

Idealized Reservoir Sediment Dynamics

Safe Harbor Dam(Lake Clarke)

CurrentBathymetry 2008

Safe Harbor Dam(Lake Clarke)

• 1.7 M tons deposited• Total 92 M tons• Deepest areas near

the dam (turbine and spill gates) and

along an old canal

Holtwood Dam(Lake Aldred)

CurrentBathymetry 2008

Holtwood Dam(Lake Aldred)

• 1 M tons deposited• Total 14 M tons• Deepest areas near

sides of natural narrow channels (hydraulic scour areas)

• Notice sediment depths near dam

Conowingo Dam(Conowingo Reservoir)

CurrentBathymetry 2008

QA lines

Conowingo Dam (Conowingo Reservoir)

• 12 M tons deposited, all in lower

third• Total 174 M tons• Total storage 204 M

tons (SSC)• Approx 30 Mt

remain• Deepest areas near

the dam (turbine and spill gates) and

natural hydraulic scour areas

Change in depth by transect - Conowingo

26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6

0

10

20

30

40

50

60

2008

1996

1993

Transect number

Av

era

ge

De

pth

(fe

et)

Upstream

Dam

Pa/Md state line

0 5 10 15 20 25 30 35 40

Conowingo 3-D Longitudinal Profile

Distance upstream (feet times 10,000)

Dis

tan

ce b

elow

su

rfac

e, in

fee

t

Pa/Md state line

0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0 27.5 30.0 32.5 35.0 37.5 40.0 42.5 45.0 47.5 50.0 52.5 55.0 57.5 60.0

0

50

100

150

200

250

300

350

400

Conowingo Capacity Change 1929-2008

1929

1959

1990

1993

1996

2008

Sediment Storage Capac-ity

Distance Upstream (feet times 10,000)

Ver

tica

l C

ross

-sec

tio

nal

Are

a (f

eet2

tim

es 1

00,0

00)

Loss of sediment-storage capacity with time

SSC Predictions

• From 1996-2008, 14.7M tons deposited• Approximately 30M tons remain before SSC• Predictions at steady state -- 15-20 years

• Predictions with future change- decrease transport (3.2 to 2.5), add 5

years- statistically expected scour removal,

add 5 years- trapping efficiency of 55%, add 0

years

• 25-30 years with future change

Summary

• From 1996-2008, sediment loads approximate 20 year average

• Less scour producing events resulting in slightly lower sediment transport to upper BAY

• Remaining capacity of 30M tons

• Under current conditions, 15-20 years to SSC

• Could be extended to 25-30 years considering future scenarios

• Encourage bathymetry be conducted more frequently

Where Do We Go From Here

• Work with other Federal and State agencies and other partners considering solutions

(USACE, USGS USEPA, PADEP, MDDNR, SRBC)• USACE new study• Reconvene the Sediment Task force (STF)• Some solutions under consideration include

(from STF)

- reservoir sediment removal

- reduce sediment transport from source areas (BMP’s stream stabilization, wetlands, buffers, etc.)

- use of floodplains to dissipate stream energy and store sediment

The ultimate DESTINY of all reservoirs is to fill with

sediment

- Linsley and others, 1992

Thank You