The document summarizes the results of a bathymetric survey of Conowingo Pond conducted in 2011 and compares it to a previous 2008 survey. The 2011 survey found that the upper part of the pond has remained in dynamic equilibrium, while the lower pond experienced 1 to 3.5 feet of increased sediment deposition. Between 2008 and 2011, the pond accumulated approximately 5,870 acre-feet of sediment, representing a net deposition of 8.67 million tons or about 2.9 million tons per year. The survey provides baseline data on changes in the pond's sediment profile that can help inform future sediment management decisions.

1. Conowingo Hydroelectric Project (FERC No. 405)
Sediment Task Force Meeting
January 23, 2012
Marjorie L. Zeff, PhD., PG (URS)
Gary Lemay (Gomez & Sullivan)

2. CONOWINGO POND BATHYMETRIC SURVEY
 SURVEY OBJECTIVES
 Create a thorough bed elevation map of Conowingo Pond
 Determine where and to what extent Conowingo Pond’s sediment/bathymetric profile has changed since the 2008
USGS survey
 Establish a physical “baseline” benchmark to better inform future sediment management decisions
 WORK CONDUCTED
 Collected bathymetric and water velocity data in Conowingo Pond
 Week of October 24, 2011 (< 6 weeks after flows receded from Tropical Storm Lee)
 26 previously surveyed USGS transect locations
 33 additional transects, 5 longitudinal profiles
 Conducted QAQC on collected data
 Plotted 2011 data against 2008 data for each transect
 Calculated and compared 2008 vs. 2011 average depths, sediment
volume/load
 Provided data (along with other sources) to USACE for use in the
sediment model
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3. 2011 Transect Layout
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4. CONOWINGO POND BATHYMETRIC SURVEY
 FINDINGS
 USGS XC 19 and upstream are likely in dynamic equilibrium
 Some areas of deposition and scour, but generally balanced
 In the lower Pond (USGS XC 20 and below), deposition outweighed scour
 Average cross-section depths generally decreased by 1 foot to 3.5 feet
 Deposition occurred around banks/edges, scour in the main channel
 The river thalweg appeared to shift toward the dam’s spillway in the
farthest downstream cross-sections
 Conowingo Pond accumulated approximately 5,870 acre-ft of sediment
between the 2008 survey (fall 2008) and 2011 survey (late October 2011)
 Net sediment deposition between the 2008 and 2011 surveys was 8.67
million tons, which translates to approximately 2.9 million tons of
deposition per year
 For reference, historic deposition rates have ranged from 3.1 million
tons/yr from 1929-1958, to 2.5 million tons/yr from 1958-1993, to 1.5
million tons/yr from 1996-2008 (Langland 2009)
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5. Upper Pond
110
2008-2011 Cross-Section Comparison - Looking Downstream
100
Bed Elevation (ft NGVD 1929)
90
80
70
60
50
Data:GSE 2011. GSE XC3,USGS XC2
Data:USGS 2008. GSE XC3,USGS XC2
40
0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000
Distance from Left [West] Bank (ft)
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6. Middle Pond
110
2008-2011 Cross-Section Comparison - Looking Downstream
100
Bed Elevation (ft NGVD 1929)
90
80
70
60
50
Data:GSE 2011. GSE XC31,USGS XC15
Data:USGS 2008. GSE XC31,USGS XC15
40
0 1,000 2,000 3,000 4,000 5,000 6,000
Distance from Left [West] Bank (ft)
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7. Lower Pond
110
2008-2011 Cross-Section Comparison - Looking Downstream
100
Bed Elevation (ft NGVD 1929)
90
80
70
60
50
Data:GSE 2011. GSE XC41,USGS XC20
Data:USGS 2008. GSE XC41,USGS XC20
40
0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000
Distance from Left [West] Bank (ft)
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8. Channel Shift Close to Dam
110
2008-2011 Cross-Section Comparison - Looking Downstream
100
Bed Elevation (ft NGVD 1929)
90
80
70
60
50
Data:GSE 2011. GSE XC51,USGS XC25
Data:USGS 2008. GSE XC51,USGS XC25
40
0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000
Distance from Left [West] Bank (ft)
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9. Longitudinal Profile
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10. Questions?
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11. Longitudinal Profile Differences
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