Reservoir Consultant's Report

by Mohammad Kheirallah, February 2000

SECTION 6:   CONCLUSIONS AND RECOMMENDATIONS

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 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 PMF, 58% of the 500-year flood, and 95% of the 100-year event. The dam will overtop by 0.68 feet during the 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 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 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 PMF. The incremental flooding along Mill Brook is less than two feet under the PMF scenario and may be considered inappreciable from a downstream hazard standpoint.

7. We recommend that the town petition MADEM to lower the SDF from PMF to 500-year event due to the inappreciable incremental flooding along Mill Brook during the 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 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 larger).

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.

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