“Charles River Dam, Design Memorandum No. 2”, The Department of the Army New England Division Corps of Engineers, 1972.
L. Other Plans Investigated
32. Flood Control - Several alternative plans for flood control were studied. Consideration was given to the possibilities of upstream reservoir storage, perimeter diking, diversion and the provision of pumping facilities to afford a reasonably constant level in the Basin at all times.
a. Upstream Reservoir Storage - A review of all of the recent major floods revealed that the runoff from the lower 56 square miles of the watershed contributed up to 90 percent of the total inflow to the basin, and also that the peak inflow from the lower watershed occurred within 2 to 3 hours after the storm, indicating the importance of local inflow to the Basin. It was evident, therefore, that upstream reservoir storage or diversion out of the watershed, would have little or no effect on reducing the Basin inflow. Reservoir storage on the lower tributaries which drain into the Basin will be effective in reducing floodflows, however, the widespread distribution, the large number of storage areas required and the highly urbanized nature of the basin make such plans economically prohibitive. b. Perimeter Diking - Local protection measures consisting of diking miles of riverfront and providing numerous small pumping stations to control the interior drainage were also studied. These studies were discontinued because the cost of providing protection of this nature exceeded $50 million. In addition, the construction of dikes in many areas would destroy the scenic and aesthetic values currently being preserved. c. Diversion of Flows - An investigation was made to divert additional Charles River flows to the adjacent watershed by way of Mother Brook which now diverts flows to the Neponset River. However, this brook is situated about 25 miles above the mouth of the Charles River and is too far upstream to be effective in reducing peak levels in the lower Basin. d. Flood-Proofing and Zoning Measures - Consideration was also given to the possibilities of using a combination of flood-proofing and zoning measures to decrease future flood damages in the area adjacent to the Basin. It was determined that such measures could not be readily and economically achieved except through the expenditure of great sums of money and through the complete disruption of city functions. This consideration would not provide protection to the extremely heavy vehicular traffic using the main arteries on both banks of the river. Further studies were discontinued.
e. Basin Level Control - In view of the more rapid filling of the Basin that is now being experienced following heavy rainfalls, control of the water level in the Basin through the provision of a pumping station was considered the one positive and economically feasible method of securing desired results. Five alternate locations for a pumping station at the existing dam were studied. Three utilized the existing lock as a discharge channel, and one required the installation of a discharge conduit through the existing dam. The fifth scheme used the existing lock as an entrance channel to the pumping station located downstream of the lock. This proposal required an extension to the lock and installation of a new lock gate. All five plans were either physically or financially not feasible owing to the unusually difficult and costly foundation conditions, undesirable hydraulic characteristics and other problems, such as, the interruption of navigation during the construction period. Further, these plans would not provide for existing and future navigation needs. All five plans were abandoned.
33. Locking Facilities - Consideration was given to improving locking facilities at the existing dam. However, the greater part of the top of dam, comprising of about 7 acres, is now occupied by the Museum of Science with an investment in facilities of about $15 million. Because of these facilities, the sole existing lock could not be enlarged and additional locks could not economically or physically be provided.
N. Project Formulation And Evaluation
47. Flood Control
a. Basin Level Control - The existing Charles River dam was constructed in 1910 to create a fresh water Basin known as the Charles River Basin. Primary objectives of the project included elimination of extensive mud flats and consequent nuisance at low tide, protection of large low areas in Boston and Cambridge from tidal flooding; stabilization of the ground water table in adjoining areas, and the creation of a significant water body of recreational purposes. A design objective was to maintain the basin at a permanent elevation of 108 feet MDC datum.
The depth of the Basin generally varies from 3 to 14 feet with a maximum of 30 feet. At the design elevation of 108 feet, the water surface area of 675 acres has a shore line of approximately 20 miles. Prior to the construction of the dam, the area along the Basin and the tributary streams had been subjected to frequent tides up to elevation 112-113 feet and construction of facilities vulnerable to such levels was restricted, both by ordinance and by consequences of encroachment below such an elevation.
Installation and use of facilities prior to dam construction were also subject to low tide levels of elevation 100 feet or lower, and the then contemplated future occasional prelowering of the Basin possibly to elevation 105 feet obviously presented no problems. However, the relatively consistent regulations of the Basin within close proximity to elevation 108 feet for many years led to extensive construction of adjacent facilities at appreciably lower elevations than had been considered in the original design. Therefore, the restrictions and ordinances in effect prior to construction of the dam appeared to become less important and were accordingly violated. It appears that most of the facilities constructed had been made on the assumption of continuous control of the Basin at elevation 108 feet. Again, over the years, the sustained basin elevation normally in close accord with elevation 108 feet has led to uses which are dependent upon limited drawdown of the Basin, with consequent objections to elevations at or below 107 feet for any length of time.
b. Top of Dam - The selection of the height of dam was predicated on both the tidal and river flooding conditions and the physical land features at the dam site. The history of hurricanes and other severe coastal storms in Massachusetts goes back to 1635. Past hurricanes have resulted in serious tidal flooding along the coast of Massachusetts south of Cape Cod. However, the problem of hurricane tidal flooding in Boston Harbor has not been serious because of protection afforded by surrounding ground from high sustained winds from the south. Slow moving severe coastal storms, commonly called “northeasters”, have caused the highest tides in Boston Harbor. These storms with prolonged periods of easterly and northeasterly winds result in the greatest tidal flood levels.
The dam site is sheltered from ocean waves, and has only a short tidal fetch of about 1,300 yards; therefore, wave action is not considered to be of consequence at the project. The highest recorded tide at Boston, adjusted to 1970 levels, occurred in April 1851 with an elevation of 11 feet, msl, or 116.6 feet, MDC datum.
The upper limit in height of dam was governed by surrounding land elevations. The topography south of the site is generally between elevation 117 and 118 MDC datum. Thus, raising the dam above 118 would not be realistic without extensive filling or diking to prevent tidal flooding around the right abutment. Such filling or diking in the center of urban Boston for the purpose of preventing this extremely rare event from occurring is not economically or aesthetically feasible. The minimum top of dam was selected at elevation 118 feet, MDC, which is 1.4 feet above the highest recorded harbor tide.
The amount of overtopping that could safely be handled was determined on the assumption that a tidal flood occurred which was 2.5 feet higher than any flood level previously experienced and coincident with a 10 year rainstorm over the Charles River Basin. For these condition, the pumps will be operable and could handle interior runoff and overtopping without causing a significant rise in the basin level.
During the freshwater standard project flood and with pumps in full operation, the Basin would rise to elevation 110.5 feet MDC. With the top of dam at elevation 118, there is ample freeboard from river flooding.
O. Corrosion Mitigation
32. General - The proposed project will be located at the mouth of the Charles River in an area of brackish and salt water. Corrosion is considered to be a serious problem at this site and corrosion characteristics of the area and its effect on the proposed structures, studies and surveys have been made of the water and soil at this site. These studies and surveys also included an inspection of the Mystic River Locks and Dam facility which is similar to the proposed project located in brackish water environment less than 3 miles from the project site.
S. Environmental Quality
58. Architectural And Engineering Treatment
a. General - Architectural design of facilities and structures required for this project will be based upon the development of a harmonious relationship between structures and contiguous visual amenities, including landscaping. The design will provide an aesthetic value and enhance the environment of the urban waterfront site. Locus of the principal structures places them in close proximity to a network of elevated highways and bridges. The complex will become a scenic attribute in an otherwise depressing environment. Nearby structures, in general, lack aesthetic or historical value. A modern architectural concept for this project should be an incentive to improve the entire area. An architect perspective rendition is shown on the following page.
59. Environmental Impact - The project will enhance the aesthetic and recreational potential of the area, provide a flood free environment, eliminate extensive flood losses, provide improved vehicular and boat traffic flows, enhance water quality to the Charles River Basin and improve fish passage. A detailed five-point statement of the impact the project will have on the environment was prepared as required by Section 102 (2) (c) of the National Environmental Policy Act of 1969.