Sediment Task Force MeetingJanuary 23, 2012

Marjorie L. Zeff, PhD., PG (URS)Gary Lemay (Gomez & Sullivan)

Conowingo Hydroelectric Project (FERC No. 405)

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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2011 Transect Layout

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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Upper Pond

40

50

60

70

80

90

100

110

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

Bed

Elev

ation

(ft N

GVD

1929

)

Distance from Left [West] Bank (ft)

2008-2011 Cross-Section Comparison - Looking Downstream

Data:GSE 2011. GSE XC3,USGS XC2Data:USGS 2008. GSE XC3,USGS XC2

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Middle Pond

40

50

60

70

80

90

100

110

0 1,000 2,000 3,000 4,000 5,000 6,000

Bed

Elev

ation

(ft N

GVD

192

9)

Distance from Left [West] Bank (ft)

2008-2011 Cross-Section Comparison - Looking Downstream

Data:GSE 2011. GSE XC31,USGS XC15Data:USGS 2008. GSE XC31,USGS XC15

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Lower Pond

40

50

60

70

80

90

100

110

0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000

Bed

Elev

ation

(ft N

GVD

192

9)

Distance from Left [West] Bank (ft)

2008-2011 Cross-Section Comparison - Looking Downstream

Data:GSE 2011. GSE XC41,USGS XC20Data:USGS 2008. GSE XC41,USGS XC20

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Channel Shift Close to Dam

40

50

60

70

80

90

100

110

0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000

Bed

Elev

ation

(ft N

GVD

192

9)

Distance from Left [West] Bank (ft)

2008-2011 Cross-Section Comparison - Looking Downstream

Data:GSE 2011. GSE XC51,USGS XC25Data:USGS 2008. GSE XC51,USGS XC25

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Longitudinal Profile

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Questions?

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Longitudinal Profile Differences