Click here for larger viewThe Muddy River is part of the Emerald Necklace, a series of parks and riverways designed by renowned landscape architect Frederick Law Olmsted. It is the oldest remaining linear park system in the United States and the largest in Boston. Created more than 100 years ago, the Emerald Necklace was to be a special “green retreat” for city dwellers and was intended to unite diverse urban neighborhoods. In the past century, the area surrounding the Muddy River has undergone extensive urbanization. As the populations of Boston and Brookline have grown, they have brought with them the attendant expansion of buildings, roadways, traffic, and congestion, all of which have had a detrimental effect on the watershed. The watershed has been widely paved, leaving little natural storage. In addition, sediment and debris have washed into the river, constricting flood-carrying capacity and causing the water quality to deteriorate. Non-native invasive species of plants, such as phragmites, knotweed, and buckthorn, have overtaken parts of the river. These flora push out native species, limiting the diversity of wildlife that can live within the watershed, and they grow in large, dense stands that further constrict flood-carrying capacity, literally choking the river and blocking the river views Olmsted created. Other flow restrictions have been created over the years as public agencies have filled sections of the river and installed pipes or culverts that restrict flow during major storms.Tailored SolutionsBoston and Brookline community leaders knew that they needed to address the flood-control and water-quality issues, but they also wanted to restore the beauty of Olmsted’s historic park. The need for rehabilitation of the Emerald Necklace parks was first addressed in 1989, and again in 2001 with the comprehensive Emerald Necklace Parks Master Plan, which identifies more than 52 improvements aimed at rehabilitating and preserving the Emerald Necklace. Boston, Brookline, and the Executive Office of Environmental Affairs (EOEA) incorporated elements of the master plan, also addressing flood control and water quality to comprise the first phase of the Muddy River project. The specific objectives of the first phase are to improve flood control and water quality, enhance the aquatic and riparian habitat, restore the landscape and historic resources, and institute best management practices (BMPs). Boston and Brookline enlisted the help of Camp Dresser & McKee to serve as the project’s engineering consultant. At first glance, some of the goals of the project appear mutually exclusive, but the project team took inspiration from the original Olmsted design concepts and developed solutions that meet the project goals and simultaneously preserve the historic integrity and characteristics of the Muddy River system. Because of the interrelated nature of some of the project objectives, some proposed solutions advanced several objectives concurrently. Potential solutions were screened by comparing their relative benefits, and the team selected the solutions it judged to be more technically suitable, environmentally advantageous, consistent with historic preservation goals, and cost-efficient. The recommended plan of action was tailored for each geographical area of the river and included a list of basinwide BMPs recommended for the Muddy River watershed. Considering the Alternatives: Flood ControlThe overall goal of the flood-control objective used in the screening process is to increase flow capacity so that the hydraulic grade line of the river does not rise above a given elevation during a storm event of the same magnitude as the October 1996 storm, which was used as the design storm for the plan. Flood analysis done on the watershed showed that the flooding is caused by insufficient culvert capacity in one particular area of the river, as well as sediment accumulation and reeds that have grown into the watercourse. To relieve flooding in the upstream part of the river, these constrictions of the river’s flow must be removed. Surface-water modeling indicated that by performing specific improvements on the upstream sections of the river, the city and town would prevent a repeat of the severe flooding that took place six years ago.The project team evaluated the alternatives to determine the effectiveness of each for fulfilling the flood-control objective, as well as their ability to fulfill the collective goals of removing flow restrictions, restoring as much of the original Olmsted design as is practical, removing contaminated sediments and increasing the volume of the aquatic habitat, increasing the diversity of the riparian habitat, and stabilizing eroding banks. The team considered the following alternatives: take no action, make improvements to the infrastructure of the river, increase upstream storage, floodproof properties, dredge different sections of the river, and dredge the river from bank to bank. The first alternative–taking no action–involved no removal of flow constrictions and no alteration of conditions along the Muddy River; however, certain municipal projects, such as locating and correcting illegal sewer connections, street sweeping, and catch-basin work, would be ongoing. Because these projects have little or no effect on the flood levels, this alternative was rejected as having no ability to fulfill the flood-control objective, let alone any of the other objectives.The second option–infrastructure improvements–centered on increased culvert capacity and a process called “daylighting,” whereby the closed or covered portions of a channel are opened, restoring a natural open channel. Daylighting increases hydraulic capacity and provides additional storage capacity. When the water surface elevation no longer is restricted by a culvert, it can rise in the open channel during high-flow periods. Daylighting works only in areas where the water surface can be exposed–it cannot be used where there are roads, buildings, or other structures over a channel. The project team analyzed the feasibility of daylighting two portions of the river and later concluded that daylighting and various infrastructure improvements, in conjunction with dredging alternatives, would eliminate the flow constriction in the Muddy River.The next alternative–increasing upstream storage–would reduce the volume of floodwater in the Muddy River drainage basin that reaches the sections of the river that have insufficient flow capacity, such as the culverts. The upstream storage would provide capacity to hold stormwater until it can be released to flow downstream at a later time, thereby reducing flooding impacts. The problem with this solution is that in an urban watershed such as the Muddy River, there are not enough locations available to store sufficient volumes so that storm flooding could be changed effectively at problem structures or problem river sections. Thus, the project team eliminated this option.Floodproofing was proposed as a way to construct dikes or modify storm pipelines and protect individual buildings to prevent damage. The team considered floodproofing the MBTA’s subway system, including the Kenmore station. But because floodproofing does not lower the flood stage, it does not address the other flood problems experienced by residents, institutions, and businesses in Boston and Brookline, nor does it prevent significant park areas and roadways from being flooded or damaged. Moreover, it keeps floodwaters out of protected areas, but only at the expense of flooding other properties. Therefore, the team determined that floodproofing does not address any of the other project objectives: water-quality improvements, habitat enhancement, or historic rehabilitation of the park or BMP implementation.