Hey guys:
I found this in the recently released Conowingo’s Environmental Impact Statement (EIS) for relicensing.
This could be very useful for setting minimum flow decision rules for the Conowingo in the Phase 6
simulation. Let’s be sure to tuck this info away in a safe place for when we’re ready to recalibrate the
Conowingo.
- Lew
2.1.3.3 Conowingo Project
The Conowingo Project is a peaking hydroelectric facility that uses a limited active
storage reservoir to generate during peak electricity demand periods. The project is
typically
operated semi-automatically as the generation setting (in MW) is programmed into the
control system; however, turbines are brought on- line manually by an operator to ensure
an
efficient start-up until the generation setting is reached. At times, the project is also
operated
in either full manual or automatic mode, and this type of operation is typically dictated by
the
prevailing river flow and system load conditions. The current Conowingo Project license
allows for the Conowingo Pond to fluctuate between elevation 101.2 feet and 110.2 feet.
There are critical water levels associated with water withdrawals that influence reservoir
levels in Conowingo Pond (see section 3.3.2, Water Resources, for details).
The current flow regime downstream of Conowingo dam is as follows:
Date Minimum Flow
March 1 - March 31:
3,500 cfs or natural inflow (as measured at the U.S.
Geological Survey [USGS] Marietta gage), whichever is less;
April 1 - April 30:
10,000 cfs or natural inflow, whichever is less;
May 1 - May 31:
7,500 cfs or natural inflow, whichever is less;
June 1 - September 14:
5,000 cfs or natural inflow, whichever is less;
September 15 - November 30: 3,500 cfs or natural inflow, whichever is less; and
December 1 - February 28:
3,500 cfs intermittent (maximum 6 hours off
followed by equal amount on).
The downstream discharge must equal these values or the discharge measured at
the Susquehanna River at the Marietta U.S. Geological Survey (USGS) gage (No.
01576000), whichever is less. The Marietta gage is located approximately 35 miles
upstream of Conowingo dam above Safe Harbor dam. The gage is generally considered
reflective of the lower Susquehanna River’s natural flow regime. During high electrical
demand periods with low inflow Exelon uses water from the available Conowingo Pond
storage (within its license constraints) to meet this demand. During non-peak periods of
electrical demand, some combination of turbine units is used to provide the minimum
flow requirements at the project. When inflows are below the minimum turbine capacity,
any additional water needed to meet minimum flow requirements would be taken from
storage.
During periods of regional drought and low river flow, Exelon has requested and
received Commission approval for a temporary variance in the required minimum flow
release from the Conowingo Project. As a result of a recent agreement with resource
agencies, beginning in 2012, when the minimum flow variance is in effect, leakage flows
(which total about 800 cfs) counts as approximately 580 cfs towards the minimum flow
discharge at the Conowingo Project. The remaining portion of the project leakage
(approximately 220 cfs) is credited to the PBAPS facility, as part of its consumptive use
agreement. This temporary variance is typically approved by the Commission and the
resource agencies (i.e., SRBC, Maryland DNR, Pennsylvania FBC, and FWS), as well.
The temporary variance allows Exelon to maintain an adequate pond level elevation and
storage capacity throughout a low-flow period, not only for electric generating capacity,
but also to ensure an adequate water supply is available for recreational interests and
consumptive water usage on Conowingo Pond.
During high flow events, typically all units are operated at maximum wicket gate
opening and crest gate operation is used to pass the remainder of the streamflow. Under
such conditions, Conowingo Pond is usually kept near elevation 109.2 feet to prevent
splashing of water onto the U.S. Route 1 roadway and debris from "floating over" the
closed crest gates. There is a skimmer beam, bottom elevation 109.2 feet, that provides
protection to motorists and prevents floating debris from going over the closed crest
gates, whose top elevation is 110.2 feet.
More Conowingo operating rules from the EIS. I think the most important to us is “Critical water
level elevations for Conowingo Pond include the following: 107.2 feet: The pond must be
maintained at this elevation on weekends between Memorial Day and Labor Day to meet
recreational needs.” Other than that it looks like the pool is not allowed to drop below 104.2
feet, as long as it’s possible to do this during drought.
- Lew
Section 3.3.1:
Conowingo Project
The Conowingo Project is the farthest downstream dam on the Susquehanna
River. It is located 10 miles upstream from the Chesapeake Bay and impounds the river
about 14 miles upstream to the Holtwood Project, creating a 9,000-acre reservoir that
contains 310,000 acre-feet of design storage capacity and 71,000 acre-feet of usable
storage. On a daily basis, the Conowingo Project operates on a run-of-river/peaking basis
and generates 574.54 MW at full generating capacity. Conowingo dam uses a limited
active storage in combination with the operation of the Muddy Run Project to meet peak
electrical demand. Two fish lifts at this facility (east and west) seasonally pass resident
and migratory fish upstream.
Conowingo Pond serves as the source of cooling water for the 2,186-MW PBAPS,
about 7 miles upstream of Conowingo dam, which has a maximum withdrawal capacity
of 3,450 cfs. The York Energy Center is permitted to withdraw up to 20 cfs for cooling
water. The pond is also a public water supply source for the city of Baltimore, Harford
County (Maryland), and the Chester (Pennsylvania) Water Authority. The permitted
range of water level fluctuation in the impoundment is 9 feet, from elevation 101.2 to
110.2 feet (figure 3-3). Exelon states that, although 9 feet of fluctuation is permitted, the
current operating regime typically restricts fluctuations to about 4.5 feet (between
elevation 104.7 to 110.2 feet) to minimize the potential for intake difficulties at PBAPS
and cavitation within the Muddy Run turbines. During summer weekends, the
impoundment water level is maintained at elevation 107.2 feet to provide a water level
suitable for summertime recreational use. Critical water level elevations for Conowingo
Pond include the following:
• 107.2 feet: The pond must be maintained at this elevation on weekends between
Memorial Day and Labor Day to meet recreational needs;
• 104.7 feet: Below this level, Muddy Run cannot operate its pumps due to
cavitation;
• 104.2 feet: PBAPS begins experiencing cooling water problems when the
elevation of the pool drops to this level;
• 100.5 feet: Below this elevation, the Chester Water Authority is unable to
withdraw water from the pond;
• 99.2 feet: The Nuclear Regulatory Commission license for PBAPS requires the
plant to shut down completely at this water level;
• 98.0 feet: The York Energy Center cannot withdraw water below this elevation;
and
• 91.5 feet: The city of Baltimore cannot withdraw water from the pond below this
water elevation.
Figure 3-3.
December
Conowingo monthly pond elevations, January 2004 through
2010 (Source: URS and Gomez and Sullivan,
2012b).
A minimum flow regime was established in a Settlement Agreement in 1989
between Exelon and several federal and state resource agencies. The Settlement
Agreement specifies that the flows represent turbine releases and exclude gate
leakage. Table 3-5 shows the 1989 negotiated minimum flow schedule agreed to by
all parties.
The Commission granted temporary flow variances to Exelon on six occasions
over the period 1998 to 2011(1998, 1999, 2001, 2002, 2005, and 2007) during summer
drought periods, permitting leakage to be counted toward the minimum flow
requirement. Flows released from Conowingo dam are measured by USGS gage no.
01578310 immediately downstream of the dam (table 3-6). Flows are generally highest
in March and April and lowest in July, August, and September. The peaking operation
of the Conowingo Project during lower flow periods often results in substantial changes
in the flow and stage (figure 3-4) in the river below Conowingo dam. Muddy Run
Project operation also affects the water level of Conowingo Pond as figure 3-5 shows.