Lower Susquehanna River Reservoir System Bathymetry

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 bottomsurface profiles
• Provide updated estimates of remaining capacity for
sediment storage
• Provide information useful to consider appropriate
management options
Lower Susquehanna River Reservoirs
Lake Clarke
Lake Aldred
Conowingo
Reservoir
v
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
Estimated Sediment Loads to Reservoirs 1900-1999
Significant decrease in sediment loads
120
Loads to Reservoirs
Sediment Load (million tons)
100
80
60
40
20
0
1900’s
1910’s
1920’s
1930’s
1940’s
1950’s
YEAR
1960’s
1970’s
1980’s
1990’s
Current “trends” in sediment loads
From 2000-2008 (avg 3.1 Mt in, 1.2
Mt out, 55% trap w/scour)
Longer-term AVG (1980-1999) (3.2 Mt
in, 1.2 Mt out, 55% trap w/scour)
9
8
Reservoirs (IN)
Sediment load (million tons)
7
Reservoirs (OUT)
6
5
4
3
2
1
0
2000
2001
2002
2003
2004
2005
2006
2007
2008
Estimated Sediment Loads to Reservoirs 1900-2008
140
Load to Reservoirs
Sediment Load (million tons)
120
100
80
60
40
20
0
1900
1910
1920
1930
1940
1950
YEAR
1960
1970
1980
1990
2000
Streamflow Hydrograph
600000
01578310 - Susquehanna River at Conowingo, 1996-2008
Hurricane Ivan
Flow (cubic feet per second)
500000
400000
390,000 cfs (scour threshold)
300000
200000
100000
0
DATE
Predicted Sediment Scour vs. Flow
Predicted scour (millions of tons)
30
25
Scour estimates
95th percentile
95th percentile
Expon. (Scour estimates)
1972
20
15
1996
10
2004
Ivan
5
1975
0
300
400
500
600
700
800
900 1,000 1,100 1,200
Instantaneous Flow (times 1,000)
Wendy McPherson (USGS) - 9/20/04
Hurricane Ivan – September 2004
Susquehanna River
Bay Bridge
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)
• 1 M tons deposited
• Total 14 M tons
• Deepest areas near
sides of natural
narrow channels
(hydraulic scour
areas)
• Notice sediment
depths near dam
Current
Bathymetry
2008
Conowingo Dam
(Conowingo Reservoir)
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
0
10
Pa/Md state line
Average Depth (feet)
20
30
2008
40
1996
1993
50
Upstream
60
26
Dam
24
22
20
18
16
Transect number
14
12
10
8
6
Loss of sediment-storage capacity with time
Conowingo Capacity Change 1929-2008
0
Vertical Cross-sectional Area (feet2 times 100,000)
50
100
150
1929
200
1959
250
1990
1993
300
1996
350
2008
Sediment Storage Capacity
400
0
5
10
15
20
25
30
35
Distance Upstream (feet times 10,000)
40
45
50
55
60
Distance below surface, in feet
Conowingo 3-D Longitudinal Profile
Pa/Md state line
0
5
10
15
20
25
30
Distance upstream (feet times 10,000)
35
40
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