A lot of attention has been given to the flooding that would result if rising sea levels lead to the over-topping of the dams at the mouths of the Charles and Mystic rivers. Until recently, no one was talking about what high water could do to the dams themselves.
The New Charles River Dam and the Amelia Earhart Dam are both about 50 years old. They serve similar functions for the Charles and the Mystic respectively. They regulate the water level in the lower basins of both rivers to a level that roughly equates to mid-tide in the harbor. Under normal conditions, gates close at high tide to keep the ocean from raising the water level in the basins. At low tide, the river flow in the basins is allowed to drain out.
The dams are both equipped with a set of massive pumps to throw the river flow over the walls into the ocean in case rain-driven flooding coincides with high tide. These pumps have been adequate to handle all the storms we have had over the past 50 years. I have seen the pumps in the Amelia Earhart drain the Mystic basin down to exposed mudflats in the middle of a heavy rainfall event.
The dams were both built to survive the highest harbor water levels that had ever been observed to date (the flood of 1851) plus another foot and a half. Their decks sit at about 8 feet above normal high tide level (“118 MDC Datum”), roughly 2 feet higher than the highest more recently measured storm surge levels (the Blizzard of 78 and the January 2018 storm).
That level seemed awfully safe when the dams were built, but with rising seas it is now recognized that by the latter part of this century there will be a material risk in any given year that the ocean will surge over and around these dams.
The flows associated with ocean storm surge are much too big for the pumps to handle, so a flanking/overtopping event would mean severe flooding in the Charles basin and in the Mystic basin, all the way out to Alewife. Discussions as to how to reduce the risk of flanking and over-topping are underway in both Boston and Cambridge. The risk analysis depend on complex modeling of how long the over-topping event lasts and so how much water actually flows upstream.
A few weeks ago, DCR educated the legislature about a much more proximate risk. Even a brief overtopping event could severely damage, even cripple the pumping systems of the dams. They are not built to handle overtopping. Salt-water could quickly flood into critical electrical and mechanical areas that are just not designed to get wet. The damage could reach $150 million to repair and the entire region would be vulnerable until the repairs were complete.
Fortunately, state engineers who lie awake worrying about these dams surfaced the risks up the chain of command and set in motion a pair of projects to harden the dams to handle over-topping events. The fix design is complete for the Charles and work will likely begin in this construction season on the Charles. The Mystic dam is a few months behind — as part of the project, they have to move the operating staff into portions of the facility that have hazardous building materials in them that need to be removed.
Once these fixes are in place, the conversation will turn in earnest to what it will take to raise these structures and the areas around them so that the point when sea level rise becomes an urgent threat to riverside neighborhoods can be pushed much further out into the future. Fortunately, it appears that even without increased elevation, the overtopping flood risk remains low for the next few decades.