Managing Storm Flows in Boston and Brookline’s Historic Emerald Necklace

Sept. 1, 2002
In October 1996, the city of Boston and the town of Brookline, MA, experienced one of the worst floods in their history along the Muddy River. The flood, caused by a major storm, resulted in more than $70 million in damage to residential, commercial, and public property, including hospitals and educational institutions. Eight to 12 in. of rain fell during just the first two days of the storm, roads were washed out, and cars were trapped in several feet of water. Kenmore Square, one of the busiest subway stations on Boston’s Green Line, was completely underwater, forcing the Massachusetts Bay Transportation Authority (MBTA) to shut it down temporarily. The flood conditions also posed a hazard to public health and the immediate environment in Boston, as raw sewage discharged from the city’s sewer systems onto the local streets.The chief source of the flooding was the Muddy River, a 3.5-mi.-long river that drains 7.5 mi. of highly urbanized watershed in both Boston and Brookline. The river is very flat, with a drop in elevation of less than 1 ft./mi. in its lower reaches near its confluence with the Charles River. The river flows through three ponds–Ward’s Pond, Willow Pond, and Leverett Pond–and then through the Riverway, Back Bay Fens, and Charlesgate before it meets the Charles. Leverett Pond receives the largest outflows of stormwater entering the Muddy River. Because of the river’s low gradient in the area between Leverett Pond and the Charles River, the pond lacks sufficient capacity for handling stormwater, resulting in flooding. During the October 1996 storm, the peak elevation of Leverett Pond was 19 ft.–11 ft. higher than normal. A National Treasure
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.
Dredging was considered in terms of partial and full dredging of various sections of the river, with the intent of increasing hydraulic capacity. Four dredging alternatives involved dredging the Charlesgate area only, dredging a channel through the Back Bay Fens, dredging the Riverway, and bank-to-bank dredging. The first three alternatives, which consist of partial dredging, involve only the removal of accumulated sediments under the water in those areas. Bank-to-bank dredging, by contrast, consists of sediment removal and total removal of the phragmites root mass. In the end, the team learned from the hydraulic modeling that only a combination of three flood-control alternatives–dredging the Charlesgate area, partially dredging the Back Bay Fens, and removing flow constrictions in the culverts though infrastructure improvement–would fully achieve the objective of flood control. Bank-to-bank dredging would be beneficial in the Back Bay Fens and sections of the Riverway where the phragmites must be removed to ensure that their future regrowth does not constrict the flood-control capacity. Bank-to-bank dredging also would help achieve objectives other than flood control, such as water quality and habitat enhancement. The team used similar screening processes to achieve the other objectives.In the Charlesgate area, the last portion of the Muddy River before it empties into the Charles River, the project team recommended dredging to remove 3,300 yd.3 of sediment, as well as removing waterway obstructions. In the Back Bay Fens area, the Riverway, and Leverett Pond, bank-to-bank dredging will include the removal of dense thickets of phragmites. The dredging of the fens is to be accompanied by daylighting. The two daylighted areas of the Back Bay Fens will be restored as closely as possible to Olmsted’s original plan, obtained from the historical archives. Replanting with vegetation selected from the Olmsted list will replace the removed invasives and other disturbed areas throughout the length of the project.Water-Quality ImprovementsWater quality in the Muddy River has declined, having been affected by urban stormwater runoff carrying sand, sediment, and pollutants from streets and parking lots into the river. Moreover, the river’s low rate of flow during dry weather has limited the flushing of the channel. In the Back Bay Fens, a combined sewer overflow still exists where a mixture of storm drainage and sewage is discharged during large storms. Dredging to reduce flooding also addresses the issue of water quality by removing contaminants and reducing the oxygen demand of the sediments.The BMPs employed here are designed to improve the quality of the stormwater that reaches the river. In the Muddy River, BMPs are of paramount importance not only for their water-quality benefits but also because they protect the investment of the flood-control improvements. Average annual sedimentation loads to the river have been reduced by almost 40% between 1974 and 2000, and the goal of the project is to reduce these loads by another 30%.A selection of BMPs have been identified for implementation, including improved street sweeping, a catch-basin data management system and improved cleaning program, some additional particle separators, instream sedimentation areas, a waterfowl control program, and a public education program aimed at reducing pesticide and herbicide use and runoff into the river. Building a CoalitionThe collaborative process that was the genesis for the rehabilitation efforts remains as important as the rehabilitation itself. Over the past decade, Boston and Brookline, along with their state partners–the EOEA, the Department of Environmental Management (DEM), and the Massachusetts Emergency Management Agency–have been implementing improvements to the park using a project vision that demonstrates a renewed appreciation of the Emerald Necklace and reinforced political will to commit resources to rehabilitate a world-class park system. A memorandum of understanding codifies the commitment to collaboration and cooperation among the municipalities and the state in project planning, permitting, funding, and implementation. The municipalities aggressively have pursued partnerships with private-sector and cultural institutions. The project also received funding from the Federal Emergency Management Agency, Office of Housing and Urban Development, DEM, and Boston Water and Sewer Commission.The project includes a 29-member citizens advisory committee (CAC), whose purpose is to advise in the preparation of the environmental reports. Members of the CAC include individual residents, experts in landscape preservation, and representatives from community groups, environmental associations, and educational and cultural institutions. Among the groups represented are the Charles River Watershed Association, National Park Service, Fenway Alliance, Brookline Conservation Commission, and Emerald Necklace Conservancy. The CAC has been a strong and passionate voice for the historical significance of the Emerald Necklace to ensure that engineered solutions do not compromise the historic integrity or value of the park system.One important area in which the CAC played a considerable role was selecting staging areas for the project for dredging equipment. The CAC reviewed and advised on the criteria used in the selection process, which included bank disturbance, construction-related truck traffic, ecological and habitat impacts, neighborhood traffic impacts, and pedestrian circulation. Moving Forward
As the first-phase improvements are completed, measures will be taken to ensure that the Muddy River does not revert to its previous condition. These steps include a revitalized management and maintenance program to maintain and enhance the improvements to the park and river areas. Part of the maintenance program will include monitoring and conditions inventories, such as periodic water-quality sampling, seasonal inspection of plantings to prevent the return of the phragmites, periodic measurements of sediment deposition, and other inventories of the health and condition of plantings and park elements. The draft environmental impact report was submitted in January 2002, and the immediate next step for Boston and Brookline will be to dredge the Charlesgate area using funding already in place. Funding for the subsequent construction will be obtained from a combination of federal, state, and local sources. Proponents of the project have been working with the US Army Corps of Engineers to undertake design and construction elements of the next portion of the project.Subsequent phases described in the Emerald Necklace Master Plan include additional historic landscape plantings and other improvements, traffic circulation improvements, and park and water-body improvements in the remaining areas of the Emerald Necklace outside of the Muddy River. The ultimate goal of the master plan is to restore the Emerald Necklace to its former status as an urban jewel, improving the diversity of vegetation and wildlife and expanding the opportunities that the park provides for passive and active recreation.The rehabilitation of the Emerald Necklace is one of the most ambitious and comprehensive landscape preservation projects in the nation, meeting multiple goals of environmental protection and historic preservation. According to Robert Durand, state secretary of environmental affairs, “This effort will be a national model, the beginning of a great public-sector project that will encourage private-sector efforts.”

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