1. Based on WSE’s interpretation of Massachusetts Dam
Safety Regulation (302 CMR 10.00), Arlington Reservoir Dam is classified as
Intermediate Size with a High Hazard Potential. Based on these
classifications, the spillway test flood adopted for the dam is the ½ PMF.
2. Based on field observations made during our site visit,
the dam is judged to be in fair/poor condition. The dam upstream
and downstream slopes show signs of erosion. The concrete inlet structure is
cracked, and the top of the dam is eroded due to foot traffic.
3. Currently there is no formal Operation & Maintenance
Plan. The town of Arlington maintains the reservoir elevation at 159 feet in
the summer months and at 153 feet between September and May. Water elevations
in the reservoir are controlled via a gate on the principal spillway.
4. Under the assumed operating conditions, where the water
is at elevation 159 feet, Arlington Reservoir Dam can pass only 18 percent of
the ½ PMF, 58% of the 500-year flood, and 95% of the 100-year event. The dam
will overtop by 0.68 feet during the ½ PMF, 0.14 feet during the 500-year
event, and less than 1 inch during the 100-year flood.
5. If water is maintained at elevation 153 feet, Arlington
Reservoir Dam will overtop only during the ½ PMF by 0.37 feet. During the
500-year event, the dam will have 1.7 feet of free board, and during the
100-year event, the dam will have 2.9 feet of free board.
6. Results of IDF analysis indicated that the incremental
increase in the downstream flood depths (i.e., with versus without dam breach)
was found to be less than two feet for both the 500-year and ½ PMF flood
scenarios. Incremental increases in flood depths range from about 0.6 to 1.8
feet for the 500-year flood and between 0.3 and 1.1 feet for the ½ PMF. The
incremental flooding along Mill Brook is less than two feet under the ½ PMF
scenario and may be considered inappreciable from a downstream hazard
7. We recommend that the town petition MADEM to lower the
SDF from ½ PMF to 500-year event due to the inappreciable incremental
flooding along Mill Brook during the ½ PMF. The town needs to weigh the added
risk/liability in adopting the 500-year flood criteria versus the incremental
costs associated with modifying the spillway to safely pass the ½ PMF.
8. Engage the services of a qualified professional
engineer, licensed in the Commonwealth of Massachusetts, to prepare and
implement a formal Operation & Maintenance Plan. Due to the impact of the
initial water elevation in the Reservoir at the onset of the storm (elevation
159 feet vs. 153 feet), it is crucial that the plan clearly specifies lowering
the water elevation in the reservoir prior to major storm events (25-year or
9. Engage the services of a qualified professional engineer
to prepare a final design study to establish the actual configuration of the
spillway weir. The study should be based on adopting the 500-year flood for
the design of the spillway after obtaining the approval of MADEM. The study
should take into account the limitations imposed by the existing channel and
culverts downstream from the dam to prevent flooding of the Mill Brook banks.
The study should also include recommendations for structural improvements to
the headwall and the training walls at the outlet structures. This work could
be completed as part of the overall dam improvement project.
10. We recommend that the town engage the services of a
licensed land surveyor to complete a detailed location and elevation survey of
the existing features including the dam crest, embankments, spillway,
emergency spillway, low-level structure, spillway channel, normal pond
elevation, and Mill Brook. This will be needed to confirm the assumptions made
in this study.
11. Engage the services of a professional engineer to
develop a formal Emergency Action Plan (EAP). The EAP should set forth basic
procedures, duties, and responsibilities to be implemented by the dam
operators (Arlington DPW) and other State and local emergency management
agencies in the event of an emergency situation at the dam. The EAP should
also include a formal Operation & Maintenance Plan for regulating the
seasonal pond level to meet recreational and flood control requirements.
12. Complete a study to evaluate options for permanently
isolating the recreational beach area from the reservoir to maintain water
elevation within the beach area at 159 feet regardless of the water
fluctuation within the reservoir.
13. Complete a study to evaluate environmental impacts of
maintaining water elevations in the reservoir at 153 feet. The study should
evaluate the impacts on the environment and wildlife habitat.