Reservoir Consultant's Reportby Mohammad Kheirallah, February 2000 SECTION 5: DISCUSSIONThe dam under its current operations plan will potentially overtopped during the ½ PMF design storm, as well as the 500-year and 100-year storm events, in the summer months when the reservoir elevations are maintained at 159 feet. This overtopping ranges between less than 0.1 feet to 0.7 feet in depth, and might result in a dam failure, which ultimately results in the flooding of some neighborhoods in Arlington. However, if the reservoir is permanently maintained at elevation 153 feet with the spillway gate always opened, then the overtopping potential will be eliminated during the 500-year and the 100-year floods. Because of the magnitude of the peak inflow during the ½ PMF, estimated at 3,100 cfs, there is no attenuation effect from the dam, and the peak outflow is also estimated at 3,100 cfs for both gated and ungated conditions. Although the results of the IDF analysis (described in Section 4) were inconclusive with respect to adopting an SDF less than the ½ PMF established per MADEM dam safety regulations, we believe that the 500-year event is an acceptable SDF. As stated in Section 4, 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. 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. Thus, the town of Arlington may want to weigh the added risk/liability in adopting the 500-year flood criteria versus the incremental costs required to modify the spillway to safely pass the ½ PMF. We have evaluated the following issues related to the required repairs at Arlington Reservoir:
As stated, the existing spillway is insufficient to pass the design storm of ½ PMF without overtopping the dam. To meet the requirements of the MADEMD, the emergency spillway has to be expanded to 180 feet (from its current 20-foot width) to pass the ½ PMF with the principal spillway opened (ungated). For the 500- and 100-year floods, the emergency spillway has to be expanded to 30 and 60 feet, respectively. However, if the spillway can safely pass the 3,100 cfs flood, the remaining issue for residential neighborhoods downstream from the dam is the current 5-foot diameter culvert of the Brook. This culvert controls the flow passing through the spillway into the Brook, and under current conditions, the culvert does not have the capacity to pass the 3,100 cfs. Therefore, even with the dam safely passing the design flood, the area downstream from the dam will be flooded due to the restrictive culvert flow. Although the seepage observed at the toe of the dam did not contain any fines, seepage in general is detrimental to dam’s stability and safety. Seepage generally results in creating a pipe-like void within the earthen dam, and a progressive failure starts to occur. This progressive failure, called piping, will ultimately result in a dam breach and sudden release of water currently stored within the reservoir. Based on our field observations, we believe that seepage could be controlled in one of two ways. The first would allow the seeping water to go through the dam section; however, it would provide a filter drainage to control the migration of fines and thus the development of a void. This option is considered economically advantageous to the second option, the cut-off wall option. This second option involves the installation of a cut-off (barrier) that will prevent water from seeping through the dam section. Several cut-off wall types could be constructed at the site, but all are considered to be highly expensive for Arlington Reservoir. Steel sheeting, slurry mix, and soil mixing are considered to be acceptable types for the dam. The wall has to extend to about 35 to 50 feet from the top of the dam, and along the entire western section of the dam. This solution is prohibitively expensive, and does not provide an extra safety factor at the margin. Both the upstream and downstream slopes of the dam need to be stabilized by either adding riprap or planting grass cover to prevent further erosion from taking place. In the event the town of Arlington select to replace the principal spillway completely, there will be no need for structural repairs. However, if the existing spillway remains, then the owner needs to complete some repairs at the training walls and the bridge. The one recreational issue that is important at this site is the maintenance of the beach area that currently exists at the northern edge of the reservoir. The beach area is separated from the main reservoir by a rock fill berm. The berm, in its current condition, does not hydraulically separate the beach area from the reservoir. Lowering the water elevation in the reservoir will also lower the water elevation in the beach area, and might result in losing the use of the beach. If the owner selects to permanently maintain the water in the reservoir at elevation 153 feet, then either the beach area needs to be hydraulically isolated from the reservoir or a substitute for the beach are needs to be constructed. The separation of the beach area is beyond the scope of this study. During the design of repairs for the dam, the designer should place materials at the top of the dam that are suitable for hiking trails to support the current use of the dam. Crushed stone, pavement, or other course cover will provide both protection against erosion in the event of overtopping and also a trail for public use. If the town selects the use of a grass cover, then an extensive maintenance plan needs to be implemented to maintain the grass cover. We have developed the following recommendations based on our field observations, results of hydraulic analysis, discussions with the design team, and experience with similar dams. We have evaluated four options for Arlington Reservoir: Option No. 1: Do nothing. Option No. 2: Comply with MADEM requirements. Option No. 3: Modify Dam (Limited Repairs) with Operation & Maintenance Plan. Option No. 4: Isolate Swimming Area and Reduce Design Requirements. 5.5.1 Option No. 1: Do Nothing Under this option, the owner will maintain the current use of the dam with minor repairs on the slopes and clearing of the channel downstream. As a minimum, the owner will be required to:
However, if the owner selects this option, there is a high risk of increasing the flooding impact of the dam on areas in Arlington due to the high risk of dam overtopping during the 100-year storm and events greater than the 100-year storm. If the dam breaches due to overtopping, an additional volume of water will be released toward residential neighborhoods. The estimated cost for Option No. 1 is $150.000. 5.5.2 Option No. 2: Comply with MADEM Requirements Under this option, the owner will complete the following:
The estimated construction cost for Option No. 2 is $585,500. 5.5.3 Option No. 3: Modify Dam (Limited Repairs) with Operation & Maintenance Plan Under this option, the owner will complete the following:
The estimated construction cost for Option No. 3 is $327,500. 5.5.4 Option No. 4: Isolate Swimming Area and Reduce Design Requirement Under this option, the owner will complete the following:
The estimated cost for Option No. 4 is $160,000. |
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