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    Ideas | Courtney Humphries

    Boston vs. the rising tide

    Boston vs. the rising tide
    Wikimedia Commons/adobe/globe staff

    The sight of water filling streets in downtown Boston during this winter’s storms brought attention to the city’s profound vulnerability to flooding as sea levels rise. But it also brought renewed awareness to something that was once a more visible part of Boston life: the tides. The storms highlighted the fact that Boston’s vulnerability to flooding is very dependent on timing: where in the daily, monthly, yearly, and even decades-long tidal cycles a storm hits.

    The difference between Boston’s low and high tides — called the tidal range — is more than 10 feet, smaller than the highest tides in the world but much larger than those in most other coastal US cities. For most of Boston’s history, tidal cycles were a visible part of daily life, as Ralph Waldo Emerson described in his poem “Boston”: “And twice each day the flowing sea/Took Boston in its arms.”

    Generations of Bostonians sought to tame the tides by diligently filling in land and putting up seawalls. With sea levels rising, the cycles so familiar to Emerson could once again become part of the normal rhythms of the city. As future Bostonians face high tides invading their streets, they’ll have plenty to learn from the mistakes past leaders made — but also, perhaps, from the ingenuity they showed.

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    Boston was originally a tiny peninsula of less than 500 acres, sitting in a tidal estuary inside a shallow harbor. It was surrounded by water and shallow mud flats. The only dry connection to the mainland was a strip of land called the Neck, where Washington Street now runs, that narrowed to 120 feet wide at high tide. “Before people were so cut off from the water, I think they certainly were more aware of the tides,” says Nancy Seasholes, a historian specializing in Boston’s made land.

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    The tides weren’t just visible from the harbor; tidal water reached miles up the Charles and Mystic rivers. When Paul Revere made his midnight ride to warn American colonists of a British advance, he outpaced British troops by taking his horse upstream across the Charles in the direction of the tidal current; they crossed at a different point, headed downstream against the rising tide.

    As the city grew, the tides both helped and hindered as Boston struggled to metabolize its own wastes. Sewage piled up in pits and cesspools and poured into the city’s harbor and rivers. Robust tides were the city’s best source of relief — a twice-daily flush that carried some of the waste well offshore. But tidal dynamics also created problems. One reason Bostonians were aware of tidal cycles: everything smelled better at high tide. Low tide was notoriously noxious, when raw sewage and garbage on the mudflats were exposed.

    While there are a few places in the city where you can still see the dramatic rise and fall of the tides — Seasholes likes to take walking tours to the seawall on Atlantic Avenue — today the water “is either walled off or cut off, or not that many people live on the water now.” Dams, seawalls, and hundreds of acres of landfill have transformed Boston’s watery landscape to one that is clearly divided from water. The neighborhoods that were once marshes and mudflats now seem as if they’d always been on solid ground.

    But that may change whether we like it or not. As the sea level rises, many of the areas that were once tidal flats will be increasingly susceptible to flooding, bringing tidal rhythms unexpectedly into view. This time, Boston can do better at accommodating them.

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    Boston’s tidal range is mostly determined by the shape of land; inside the Gulf of Maine, tides resonate with one another, amplifying their height. In the past, these tides were impressive enough to be seen as a lucrative power source for the city. A tide-powered mill first appeared in the city in colonial times, when a dam was built across a cove near the North End in the 1600s, creating a mill pond. Such ponds collected water at high tide, then channeled it across a water wheel to power gristmills. A far more ambitious plan took shape in the early 19th century, when tide-powered mills were proposed to help turn Boston — always a center of shipping trade — into a major manufacturing center, as described in William Newman and Wilfred Holton’s book “Boston’s Back Bay.”

    Championed by local businessman Uriah Cotting, the original scheme was to build dams across Back Bay and South Bay to power up to 100 mills — grinding flour, sawing lumber, manufacturing nails, and spinning wool. Another proposal from Benjamin Dearborn, detailed in an 1814 map, called for further dams across the Charles River and in Charlestown. A report from a state legislative committee was equally grandiose, predicting that tidal power could provide jobs for 10,000 workers and generate millions of dollars.

    Eventually, Cotting and his associates formed the Boston and Roxbury Mill Corporation and constructed a scaled-back version of the Back Bay dams. The design was advanced for the time; dams created two basins, one kept constantly fuller than the other. Water poured into the Full Basin through sluice gates at high tide, and was let out of the Receiving Basin at low tide. The difference in water levels was used to power mills, making it possible to run a “perpetual” power source 24 hours a day. But the project cost more than expected and never yielded the promised power. Only a few mills were established. “It wasn’t very profitable,” says Seasholes.

    Moreover, tidal dams created an environmental mess as ponds filled with sewage. In the early 1800s, the polluted colonial mill pond was filled to make Bulfinch Triangle (near today’s North Station). An anonymous letter to a local newspaper predicted the Back Bay dams would cause similar problem, “converting the beautiful sheet of water which skirts the Common into an empty mud-bason [sic], reeking with filth, abhorrent to the smell and disgusting to the eye.”

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    In fact, the Back Bay dam sparked immediate sanitary complaints, and railroad bridges constructed across the bay exacerbated the problem. By 1849, a city report described Back Bay as a “cesspool” covered with “greenish scum,” and its waters “bubbling like a cauldron with the noxious gases that are exploding from the corrupting mass below.” In the end, the dams made it far more attractive and lucrative to turn the water into real estate.

    Whether or not Boston ever profits from its tides, it will have to reckon with them as it rebuilds itself for a watery future. Today, we see the effects of Boston’s tides every time big storms hit.

    Later, the same problem helped propel the damming of the Charles River at the turn of the 20th century. “The real reason for building the dam was to create this basin to keep the polluted flats always covered with water,” Seasholes says. The move transformed the tidal estuary at the heart of the city into a sedentary river.

    * * *

    Could Boston ever harness its tides again? Maybe — with more modest ambitions. Tidal power has sparked increasing interest as a renewable energy source, but most of that interest is in developing freestanding turbines that can harness tidal currents without the need to build dams with potentially serious environmental impacts (not just “greenish scum” but harm to natural ecosystems, something our forebears gave little thought to). The turbines look much like underwater wind turbines anchored to the sea floor that generate power as ocean currents flow across them. But to be cost effective, they must be large and placed in very strong currents. “At this point we’re looking at the highest-energy sites,” says Levi Kilcher, an engineer at the National Renewable Energy Laboratory. As technology improves and costs go down, he says, “it’s going to open up new sites that aren’t quite as energetic, like Boston Harbor, to be developed.”

    Modern tidal power dams, called barrages, have been built a few places around the world, but at high cost and with environmental impacts. They can serve a double function of acting as seawalls to areas threatened by flooding.

    There’s been at least one proposal for Boston to revisit tidal power. Somerville-based architect Paul Lukez proposed a “Hydroelectric Canal” for the city-sponsored Boston Living with Water competition in 2015. The design calls for constructing a canal through Columbia Point (his firm calls it the Morrissey Canal) that would generate hydroelectric power for its neighborhood by stopping tidal water with a dam and releasing it through turbines along the canal. Lukez envisions a neighborhood combining tidal, wind, and solar power to be self-sufficient.

    Whether or not Boston ever profits from its tides, it will have to reckon with them as it rebuilds itself for a watery future. Today, we see the effects of Boston’s tides every time big storms hit. “It really matters when in the tidal range that they come to shore,” says Stephanie Kruel, senior environmental planner at engineering firm VHB. Low tides ensure that flood events are rarer than they could be. Hurricane Sandy could have caused major damage to Boston if it had hit at high tide. Some of the city’s biggest flood events have happened at noon and midnight, when bimonthly spring tides happen to occur. The flooding this winter resulted from a combination of daily, monthly, and yearly tidal peaks. There’s even a 19-year cycle based on the alignment of the Earth and moon — we’re roughly in the peak of the current one.

    Storms in combination with tides are the most immediate threat to Boston. But over the longer term, tides themselves will become a source of flooding as seas rise. A study led by Richard Ray, a geophysicist at NASA’s Goddard Space Flight Center, found that tide-only nuisance flooding began in Boston in 2011 and will become increasingly frequent over time, potentially occurring hundreds of times a year by 2050, depending on how high the oceans rise. “I’m sure there will be cases where, based upon the time of day, you don’t park your car in certain areas,” Ray says.

    Nuisance flooding will pose a long-term challenge for city planners. “We’ll have buildings and roadways in the intertidal zone,” Kruel says. Infrastructure can be designed to withstand occasional storms, she says, but “the materials can’t take being inundated by saltwater daily.” It would take permanent fixes — like building up land, erecting seawalls, or moving out of flood-prone areas — to deal with regular incursions. But at least some of those strategies could find new ways of accommodating Boston’s tides rather than walling them off.

    Courtney Humphries is a freelance journalist and an environmental science PhD student at UMass Boston. Follow her on Twitter @cehumphries.