CEP_28_Taylor_Layout 1 2/13/2014 4:02 PM Page 1 Project Considerations Rehabilitation of the Otis Reservoir Dam: Improving Cost Effectiveness by Including Bridge Placement was found to be in “Poor” condition due pri- Combining efforts to address marily to the deteriorating/leaking spillway dam safety issues with access and downstream masonry wall and the erodi- bility of the “emergency spillway” over the and transportation issues main embankment section of the dam. provided a win-win situation To address most of the main dam safety concerns, a reconstructed spillway with a for all of the involved hydraulically actuated crest gate was chosen parties. as the preferred alternative. The crest gate option provided additional hydraulic capacity while also improving the DCR’s ability to MATTHEW A. TAYLOR & JOHN G. DELANO manage the reservoir level, most importantly, he Massachusetts Department of Con - during the annual winter drawdown. ser vation and Recreation (DCR) owns During the planning stages of the project, Tand operates the Otis Reservoir Dam in maintaining access to the west side of the Otis, Massachusetts, for recreational purposes. reservoir during construction was identified In 2006, the 145-year-old, 31.5-foot-tall, earth- as a critical component in the viability of the en embankment dam with downstream project. At the time, the bridge over the spill- masonry wall and stone masonry spillway way was a one and one-half lane, temporary CIVIL ENGINEERING PRACTICE 2013 1 CEP_28_Taylor_Layout 1 2/13/2014 4:02 PM Page 2 “Bailey bridge” installed in 1995. It was sup- ered by about 3 feet to create an “emergency posed to have been replaced with a permanent spillway,” although no erosion protection, bridge in 1998. Given that the dam is a large, other than the asphalt pavement roadway, high hazard structure, the dam repairs needed was provided. The dam was originally built to move forward to protect the public safety. with a 38-foot-long stone masonry primary To expedite the project, the DCR elected to spillway located near the dam’s left (west) incorporate the bridge replacement into the abutment. The spillway was divided into two, dam rehabilitation project. The dam rehabili- 19-foot-long segments by a stone masonry tation project was transformed, by necessity pier. and creative planning, into a combined dam In 1995, MassHighway replaced the deteri- and bridge replacement project that ultimate- orating original bridge over the spillway with ly benefited the DCR and the town of Otis. a temporary one and one-half lane Bailey bridge. This bridge was intended to be in serv- Project Background ice for three years. A permanent, two-lane Otis Reservoir Dam is located within Tolland bridge was slated for construction in 1998. State Forest on the Fall River in the town of However, the project was delayed and post- Otis, in Berkshire County (see Figure 1). The poned, apparently due to MassHighway’s dam was originally constructed in 1866 by the bridge project prioritizations. By 2009, the per- Farmington River Water Power Company in manent bridge replacement at the Otis order to provide supplemental water to power Reservoir Dam had been officially postponed mills on the Farmington River in Connecticut. until 2014. The Commonwealth acquired the reservoir in Prior to the recent dam rehabilitation, there 1967. Otis Reservoir is a 1,000-acre impound- were two gatehouses/outlet structures at the ment with a 16-square-mile drainage area that dam. The original gatehouse, located in the encompasses portions of three towns (Otis, middle of the upstream side of the embank- Tolland and Blandford). Having a storage ment, was taken out of service in 1984 when capacity of 22,000 acre-feet (7.1 billion gallons) the outlet works at the dam were rehabilitat- at normal pool, it is the largest recreational ed. The original sluice gates, stems and opera- body of water in the Commonwealth of Mas - tors were removed and a 4-foot-high by 8- sa chusetts. foot-wide intake conduit, with an upstream Tolland Road, owned by the town of Otis, steel trash rack, was installed within the reser- passes over the dam and provides the only voir. The intake conduit extends into the reser- access to the western side of the reservoir dur- voir about 40 feet upstream of the dam. A ing the winter months. The majority of the res- downstream outlet tower was constructed in idents on the western side of the impound- 1984 with a second gatehouse opposite to the ment are seasonal, with only four full-time original upstream gatehouse. The outlet tower residents present during the winter. Because has two, 48-inch-square slide gates. One gate of the recreation industry supported by the serves as a low-level outlet and the other reservoir, the population in the town of Otis serves as a mid-level outlet. The outlet tower swells from a few thousand in the winter to also has a 10-foot-wide by 3-foot-high over- near 10,000 during the summer months. flow weir at the same elevation as the primary The Otis Reservoir Dam consists of an spillway crest (with the flashboard in place). earthen embankment with a downstream The two original 30-inch-square stone sluice- stone masonry wall. The dam has a maximum ways through the dam had been retrofitted structural height of about 31.5 feet and a with 26-inch-square steel conduits and then length of about 630 feet. The exposed portion grouted in place. of the downstream masonry wall of the dam is The Otis Reservoir is operated by the approximately 480 feet in length. Tolland DCR’s park staff according to an order of Road, a paved public roadway, traverses the conditions from the Otis Conservation top of the dam. In 1955/1956, after Hurricane Commission, which defines the target sea- Diane, the top of the embankment was low- sonal reservoir levels and governs the rates 2 CIVIL ENGINEERING PRACTICE 2013 CEP_28_Taylor_Layout 1 2/13/2014 4:02 PM Page 3 FIGURE 1. The location of the Otis Reservoir and dam. of reservoir drawdown and refill. In October for an annual weekend canoe race. The DCR of every year, the DCR opens the low-level subsequently dials back the release to lower outlet gate to provide a flow of around 300 the reservoir to about 2 inches per day until cubic feet per second (cfs) into the Fall River the winter pool elevation (6 to 8 feet below CIVIL ENGINEERING PRACTICE 2013 3 CEP_28_Taylor_Layout 1 2/13/2014 4:02 PM Page 4 FIGURE 2. View of the deteriorated and leaking conditions at the spillway prior to rehabili- tation. the primary spillway crest elevation) is ice. An excessive water release could cause the reached. The purpose of the annual draw- development of a void between the bottom of down is to allow for the inspection of the the ice and the surface of the reservoir. The dam structure, to prevent ice damage to the presence of such a void would be a major safe- shoreline docks and piers, and to provide ty concern, given the year-round recreational meltwater storage and discharge control use of the reservoir, which includes ice fishing during the spring freshet. Beginning in and snowmobiling on the reservoir. The January of each year, the DCR begins to raise DCR’s staff would use a chain saw to cut holes the reservoir level. The reservoir is raised to through the ice in order to evaluate the reser- within 4 feet of the summer pool level by voir level. Obviously, this practice was not an April, depending on the ice conditions on ideal approach. So, the DCR indicated that the the reservoir. The goal is to have the reser- dam rehabilitation project should also include voir level restored to the summer pool level provisions to improve the reservoir operation by July 4th each year. The 1984 outlet works procedures. configuration requires constant adjustment to manage the reservoir level, especially Engineering Evaluations & prior to or after storm events. Alternative Analyses With the winter pool being maintained by Following an inspection of the dam in May releasing water from the low-level outlet in 2006 by an engineering consulting firm, the the downstream outlet tower, the DCR could dam was judged to be in “poor” condition. not easily monitor the water level below the The primary deficiencies of the dam were: 4 CIVIL ENGINEERING PRACTICE 2013 CEP_28_Taylor_Layout 1 2/13/2014 4:02 PM Page 5 • The deteriorated masonry and leakage (PFMA) was performed with several members conditions at the spillway (see Figure 2) of consultant’s dam engineering team and and the downstream masonry wall; and, representatives from the DCR who were • The erodibility of the “emergency spill- responsible for the operation and maintenance way” over the main embankment section of the dam. The objective of the PFMA was to of the dam. assess possible failure modes and to deter- mine the most likely failure mechanisms, In 2007, the engineering consultant per- thereby allowing the design of the rehabilita- formed a Phase II Engineering Evaluation and tion project to address each of these deficien- Alternatives Analysis of the dam. This study cies. included field investigations covering wet- Using the information obtained from the lands delineation, a topographic survey of the detailed engineering evaluations and the dam and nearby areas, rare species determina- PFMA, an alternatives analysis was per- tions, diving inspections of the intake conduit formed to evaluate the repair/rehabilitation and interior of the downstream outlet tower, options that could address the dam safety subsurface explorations with test borings and issues.