by Mohammad Kheirallah, February 2000

As part of this study, our team completed an Inflow Design Flood (IDF) analysis to evaluate if a less severe storm than the ½ PMF may be acceptable as the spillway design flood (SDF). The IDF is the flood flow above which the incremental increase in water surface elevation below the dam (due to dam failure) is not considered to present an unacceptable threat to downstream life and property. In the case of Arlington Reservoir Dam, a flood less than the ½ PMF may be adopted as the SDF if the consequences of dam failure at flood flows larger than the selected SDF are acceptable (i.e., no increased damage to downstream areas is created by dam failure). In general, the consequences of failure are considered acceptable when the incremental effects (depths) of failure on downstream structures are approximately two feet or less.

The IDF analysis was conducted under the scenarios with and without dam breach for both the ½ PMF and 500-year flood conditions. The National Weather Service (NWS) DAMBRK computer program was used to predict the hypothetical dam break wave formation at Arlington Dam and the wave’s downstream progression along Mill Brook to its confluence with the Lower Mystic Lake, located about 2.9 miles downstream from the dam.

Under both 500-year flood and ½ PMF scenarios, lateral inflow from the adjacent tributary was added to the stream flow 0.10 miles downstream of the dam. The channel was assigned a Manning’s coefficient of 0.045, and an overbank Manning’s value of 0.10. The breach opening was assumed to have a trapezoidal shape with an average maximum width of 40 feet and side slopes of 0.5H:1V. The time to maximum breach was set at 0.5 hours.

For the 500-year flood, the peak discharge through the Arlington Reservoir Dam breach opening is approximately 2,800 cfs; combined peak flow just downstream of the unnamed tributary is 3,000 cfs. Peak flow beyond the tributary for the 500-year, without dam failure, is about 1,230 cfs. Flood depths along Mill Brook range from about 4.6 to 10.5 feet for the 500-year flood without dam breach, and from 5.1 to 12.1 feet for the 500-year with dam breach scenario.

Under the ½ PMF, the peak breach discharge at Arlington Reservoir Dam is about 5,100 cfs. The difference in breach flow between the two flood events occurs because under the 500-year flood the reservoir is not completely filled, while under the ½ PMF the dam overtops. Combined peak ½ PMF flow, just downstream of the unnamed tributary, is about 6,300 cfs. Flood depths along Mill Brook range from about 6.4 to 15.6 feet for the ½ PMF with dam breach scenario. Computer input and output summaries for the various DAMBRK simulations are included as Appendix C.

The results of the above-described IDF analysis were inconclusive with respect to adopting an SDF less than the ½ PMF established per MADEM dam safety regulations. The incremental increase in 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 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 standpoint.

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