Lower Susquehanna

River Reservoir System

Background / Problem

• Reservoirs filling with sediment and associated

nutrients thereby reducing storage capacity

• USGS previously collected bathymetry and cores in 1990, 1993, 1996, 2001, and 2008

• Approximately 55-60% of the sediment, 40% of the phosphorus, and 2% of the nitrogen is being trapped

• Revised estimate in 2008 of 15-25 years of remaining sediment storage capacity (SSC)

• At capacity, sediment and phosphorus loads may increase significantly to the upper Chesapeake Bay impacting TMDL’s and allocation issues

Susquehanna River Sediment Transport

• Historical sediment trends

• Reservoir Transport and Dynamics

• Reservoir Filling and Remaining Capacity

v

Lower Susquehanna River Reservoirs

Lake Clarke

Lake Aldred

Conowingo

Reservoir

Courtesy Exelon Corp.

Estimated Sediment Loads to Reservoirs 1900-2010

Streamflow Hydrograph

01570500 - Susquehanna River at Harrisburg, PA, 1900-2010

Wendy McPherson (USGS) - 9/20/04

Susquehanna River

Bay Bridge

Hurricane Ivan – September 2004

Turbid inflow

Delta

(sand)

Floating Debris Water Surface

Relatively clear water

(clay)

Density current

(Silts)

Fine sediments

(silts & clays)

Sluiceway

Idealized Reservoir Sediment Dynamics

Generally, more sand, less silt and clay as upstream distance increases. (19 locations in Conowingo)

Sediment Core Data

DOWN STREAM UPSTREAM

Safe Harbor Dam

(Lake Clarke)

2008 Bathymetry

Holtwood Dam

(Lake Aldred)

2008

Bathymetry

Conowingo Dam

(Conowingo Reservoir)

2008

Bathymetry

QA lines

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 capacity with time - Conowingo

0 60,00010,000 20,000 30,000 40,000 50,000

DISTANCE UPSTREAM FROM CONOWINGO DAM, IN FEET

50

400

100

150

200

250

300

350

VE

RT

ICA

L C

RO

SS

-SE

CT

ION

AL A

RE

A, IN

SQ

UA

RE

FE

ET

(´

1,0

00)

1928

APPROXIMATE LEVEL OF MAXIMUM SEDIMENT-STORAGECAPACITY

0

REMAINING STORAGE CAPACITY

Change in capacity with time - Conowingo

0 60,00010,000 20,000 30,000 40,000 50,000

DISTANCE UPSTREAM FROM CONOWINGO DAM, IN FEET

50

400

100

150

200

250

300

350

VE

RT

ICA

L C

RO

SS

-SE

CT

ION

AL A

RE

A, IN

SQ

UA

RE

FE

ET

(´

1,0

00)

EAPPROXIMATE LEVEL OF

REMAINING STORAGE CAPACITY

MAXIMUM SEDIMENT-STORAGCAPACITY

1928

1959

1990

1993

1996

SEDIMENT DEPOSITION

0

Change in depth by transect - Conowingo

Pa/Md state line

Downstream (Dam) Upstream

Average of

10 foot loss

Brown – deposition

Red – scour

Resulting from the

January 1996 flood

event.

Brown – deposition

Red – scour

Resulting from the

January 1996 flood

event.

Summary

• From 1990-2008, USGS has documented decreasing sediment storage capacity using bathymetry

• Remaining capacity (2011) of 30M tons

• Under current conditions, 10-20 years to SSC

• Need increased sampling supplemented with turbidity installation

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 “Lower Susquehanna River Watershed Assessment” includes Reservoir Modeling and sediment alternatives

• Some alternitives under consideration include

- reservoir sediment removal / beneficial use

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

- use of floodplains to dissipate stream energy and store sediment