Opossum Lake is a 59-acre Commonwealth-owned impoundment located near Carlisle in Lower Frankford Township,
Cumberland County, Pennsylvania. A dam that is located on Opossum Creek, a tributary of Conodoguinet
Creek, creates the lake. The impoundment is very popular with outdoor recreation enthusiasts in southcentral Pennsylvania.
DAM AND APPURTENANCES
The Opossum Lake Dam consists of an earth embankment approximately 310 feet long with a maximum height of 38 feet
from the downstream toe and a crest width of 12 feet. The flood discharge facilities for the dam consist of a combined
primary and emergency spillway located on the left abutment (looking downstream). The spillway structures of the dam
consist of a concrete overflow section and a trapezoidal concrete-lined spillway discharge channel that terminates in
a concrete-lined plunge pool at the toe level of the dam near the left abutment.
The overflow structure of the spillway is an 86-foot-wide concrete weir, trapezoidal in cross section, located at
an elevation approximately six feet below the dam crest. The outlet works consist of a reinforced concrete box culvert
and a control tower located at the center of the embankment. The conduit is equipped with trash rack devices on the
upstream end with an endwall at the discharge end. Stop logs located in the control tower control the flow through the
conduit. The stop logs divide the control tower into inflow and outflow chambers. The stop logs in the control tower
extend from the invert elevation of the box culvert to the normal pool elevation. The flow entering into the intake
chamber of the control tower through the upstream portion of the outlet conduit rises to the normal pool elevation,
spills over the stop logs into the outflow chamber, which in turn discharges into the downstream portion of the outlet
conduit. The lake can be drawn down below the normal pool elevation by removing the stop logs. The control tower is
not equipped with any mechanical device for removing the stop logs. This outlet system constitutes the emergency drawdown
facility for the dam.
Downstream from the dam, Opossum Creek generally flows south, meandering for approximately ¼ mile, and joins
Conodoguinet Creek. It is estimated that failure of the dam would pose a serious public safety threat.
Approximately 50 residents and 14 homes could be inundated if a failure were to occur.
On September 12, 2005, PFBC maintenance staff were performing routine maintenance to the emergency spillway when they
observed seepage from the spillway drains (or weep holes). A clayish material along with drain material
was flowing with the seeping water. This is an early indication of erosion that will weaken the dam if
not addressed. Maintenance then reported their findings to PFBC engineering. A close inspection of the
facility was then conducted by PFBC engineering staff, in close consultation with DEP dam safety staff.
It was determined after this close inspection that that the lake must be lowered for public safety.
PFBC must limit and control the erosion beneath the spillway because voids can form under the spillway
slabs from migration of soil particles with the seepage. Furthermore, the sediment under the slab will clog the drains
allowing uplift pressure on the spillway slabs, which can damage the spillway. An example of damage caused by uplift
pressure is Dutch Fork Lake Dam in Washington County. See picture at right.
September 19, 2005, the Commission publicly announced that Opossum Lake was to be drawn down. On September 21, 2005
the Commission started the drawdown by pulling two stop logs. The drawdown continued pulling two logs twice per week
(weather permitting) as PFBC engineers monitored the seepage. The seepage stopped when the lake was lowered to an elevation
ranging from 6 to 8 feet below normal pool. On October 6, 2005 the PFBC drilled seven ten inch cores in the spillway
to evaluate the sub-base beneath the spillway slabs. Cores numbered 1, 4, 5 and 7 were clear of sediment and exhibited
no undermining. This indicates the seepage/piping path is not traveling beneath the spillway weir nor does it extend
to the second column of weep holes. Cores numbered 2, 3 and 6 exposed sediment laden drainage material along with some
undermining, thus indicating that the seepage/piping path emanates below the upper retaining wall and continues to the
bottom of the spillway.
With the approval of the Department of Environmental Protection (DEP) Division of Dam Safety, the PFBC stopped the
draw down at 14 feet below the normal pool elevation. The lake will remain at that level until further problems arise
and/or corrective measures are completed.
HYDRAULIC AND HYDROLOGICAL ANALYSIS
The dam/spillway was constructed in 1962 consistent with the regulatory standards
of the time. Due to an increase in the Spillway Design Flood (SDF), the Dam/spillway are not up to current
regulations. In developing the Emergency Action Plan (EAP) for Opossum Lake, a hydraulic analysis of
the spillway showed that the dam can only pass 22% of the SDF flow. At the SDF, the Opossum Lake Dam will overtop the
embankment at a depth of 6.84 feet.
REPAIRS TO THE DAM/SPILLWAY
Current dam safety regulations require embankment overtopping protection in addition to
the repairs to address the spillway seepage issues. Any repair plan without embankment overtopping protection
will not be permitted. The most cost-effective solution to remediate the dam would be to rebuild/update
the existing spillway along with armoring the embankment with Articulated Concrete Blocks (ACB’s). The Rebuilt
spillway will correct the seepage problems and the ACB armoring will allow the entire embankment to act as an emergency
spillway during a SDF event. The estimated cost of this project is approximately $3.05 million.
The Commission does not currently have funding available or allocated to completing the estimated $3.05 million repair
project. Furthermore, once the Commission secures the necessary funding it will taken approximately two (2) years for
Design/Permitting and an additional two (2) years for construction. The PFBC anticipates the repairs will be functional
for 50 years.