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(THE FULL REPORT IS LOCATED IN "MINUTES & DOCUMENTS-2014)


         FINDINGS
The information review and site inspection resulted in the following findings:
• The dam was found to be in poor condition
• The following long term improvements should be undertaken:
o The upstream and downstream slopes should be cleared of vegetation and
regraded to uniform stable slope.
o The upstream slope should have additional riprap placed where needed.
o The spillway should be replaced or overtopping protection provided with
adequate capacity to safely pass the SDF.
o The corrugated metal pipe on the left downstream slope providing drainage
for the roadway should be replaced with a more suitable drainage system.
The results of the slope stability analysis indicate that both the upstream and downstream
slopes do not meet established stability criteria for any of the loading conditions
analyzed. In order to meet the required stability criteria the slopes will need to be
strengthened, flattened, or buttressed in some way.Based on the hydrologic, hydraulic, and dam break analyses the full PMF is considered
the appropriate storm for the Spillway Design Flood (SDF). The existing spillway has
capacity to pass a storm that is somewhere between the 50-year and 100-year storms just
prior to embankment overtopping, indicating that the existing spillway is seriously
inadequate.


         REHABILITATION ALTERNATIVES
Based on the evaluations performed for Centennial Lake Dam, the existing spillway
capacity is deficient and the slopes do not meet the required criteria. Rehabilitation of the
dam is necessary in order to bring the dam into conformance with the NJDEP regulations.
To improve slope stability often times the slopes are flattened or buttressed with
additional earthfill or rockfill (riprap), which results in an expanded footprint of the
embankment. The upstream slope could be flattened or buttressed, but would require
draining the lake to perform the construction (see Appendix A: Figure 4: Upstream
Slope Strengthening). Some modifications to the principal spillway may also be
required to support and retain the added material. The estimated quantities for flattening
and buttressing the upstream slope are nearly the same. Therefore, we developed the
conceptual level construction cost estimate for just the flattening option (see Appendix J:
Construction Cost Estimates).
Flattening or buttresses is not a viable solution for the downstream slope unless Taunton
Lake can be lowered and the work can be contained within the Centennial Lake property.
The outlet end of the culvert would have to be extended through any added material.
Sheeting could be placed along the downstream toe to retain material placed to flatten the
slope.
Typically for existing dams with insufficient spillway capacity, the most cost effective
remediation is to armor the downstream embankment slope to allow overtopping during
extreme floods. The armored embankment serves as an auxiliary or secondary spillway
to help pass the SDF. The armoring resists erosion which prevents the earth embankment
from breaching during overtopping. Table 6.1 indicates that the crest of the embankment
overtops by nearly 5 feet of water during the SDF. Only roller compacted concrete and
reinforced cast-in-place concrete can withstand that amount of overtopping depth. Both
of these concrete materials would require steel sheet piling along the downstream toe to
prevent undermining of the concrete armoring material (see Appendix A: Figures 5 and
6).
The conceptual level construction cost estimates contained in Appendix J include a 40%
contingency to account for unanticipated items. The two alternatives include providing
overtopping protection to increase spillway capacity by armoring the downstream slopewith either reinforced cast-in-place concrete or roller compacted concrete. The
conceptual level construction cost estimates for both alternatives include flattening the
upstream slope with riprap. The conceptual level construction cost estimates are as
follows:
• Reinforced cast-in-place concrete overtopping protection - $1,639,715 (see
Appendix A: Figure 5)
• Roller compacted concrete overtopping protection - $1,569,365 (see Appendix A:
Figure 6)

          RECOMMENDATIONS
Centennial Lake Dam is a Class I High Hazard Dam, a failure of which could result in the
loss of life and significant property damage. The appropriate spillway design flood
(SDF) is the full Probable Maximum Flood (PMF).
Centennial Lake Dam does not meet NJDEP Dam Safety Standards. Both the upstream
and downstream slopes do not meet minimum required stability criteria and the spillway
cannot safely pass the required spillway design flood (SDF). The existing slopes are
overgrown with trees and brush, the upstream slope wave protection is sparse, the
existing roadway drainage pipes need improvement.
The dam should be remediated to bring it into compliance with NJDEP Dam Safety
Standards. The upstream slope should be flattened using riprap to improve the slope
stability. Two alternatives were evaluated to improve the stability of the downstream
slope and provide overtopping protection to safely pass the SDF. The conceptual level
construction cost estimates for both alternatives are within 4% of each other, given the
conceptual level of the estimate, they are essentially identical. Either alternative can be
permitted and constructed.
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