The risk of enhanced storm surge due to sea level rise is an issue that I am taking seriously as a state senator representing low-lying areas in the Charles and Mystic watersheds. I’ve been talking with a lot of people in the region about how to develop some consensus about the risk levels and move the necessary conversations about infrastructure investments.
Hurricane Sandy has recently increased perceptions of urgency about the issue but a number of groups of have been thinking about the issue for several years. See generally this review of other efforts prepared for this website in September by Erica Mattison. The National Oceanic and Atmospheric Administration recently produced a new report estimating likely sea level rise by 2100. The Boston Harbor Association — using roughly consistent numbers — has produced a set of maps comparing possible storm surge levels to ground elevations in coastal areas. The Massachusetts Department of Environmental Protection is leading an ongoing study of the challenges created by likely climate change. The Office of Coastal Zone Management is also pushing communities to think about the issue.
NOAA estimates with 90% confidence that sea level will rise at least 8 inches and not more than 6.6 feet above 1992 levels by 2100. Sea level rise is a result of global warming through (a) the expansion of warming ocean water and (b) the melting of great ice sheets. Ice melt in Greenland and Antartica is especially difficult to project. Given the wide uncertainty, NOAA’s approach is to create four different scenarios.
- The Highest scenario, 6.6 feet, assumes the “maximum possible glacier and ice sheet loss by the end of the century”. The report acknowledges that there are some higher estimates of ice sheet loss, but discounts them as implausible. The Highest scenario does, however, project much greater loss than the last international panel report, IPCC4, which excluded “future rapid dynamical changes in ice flow”. The Highest scenario is over three times greater than the highest suggested rise in IPCC4 (.59 m or 1.94 feet).
- Two intermediate scenarios at 3.9 and 1.6 feet make more modest projections of ice sheet loss, with the lower of the two defining the risk primarily from ocean expansion due to warning using optimistic assumptions about global efforts to reduce emissions.
- The low scenario, 8 inches, simply extrapolates observed 20th century sea-level rise with no acceleration due to climate change.
The scenarios above pertain to global average sea level rise. There is some evidence that sea level rise may be more dramatic along our coast line than in some areas of the world.
Later in this century, when elevated sea levels combine with storm surges like Hurricane Sandy, areas that have been previously safe from inundation will face substantial risk of flooding. It may be that the greatest exposure will be to sub-surface infrastructure like subway and road tunnels and sewer systems. Inundation of these structures could incapacitate the Boston area for extended periods. The investments necessary to adequately control these risks may be large and it seems wise to start to try to bring them into focus, given the lag times involved in major structural investments.
The issues for Back Bay, the Fenway, East Cambridge and low-lying areas further up river are particularly difficult to evaluate. These areas are protected by damsthat keep sea water out. These dams are equipped with huge pumps sufficient to keep pumping the river out, even when harbor tides rise above river level.
According to a 1972 design memorandum from the Army Corps of Engineers (extracted and reproduced as part of MIT course curriculum here), the topography around the dam prevented a dam height of more than approximately 1.4 feet above the highest previously recorded high tide.
According to the design memorandum,
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[s], the pumps will be operable and could handle interior runoff and overtopping without causing a significant rise in the basin level.
In the scenario envisioned, water would be flowing over the dam from the harbor into the river basin but the pumps could keep up with it. With sea level rise of as much as 3 feet by mid-century and possibly, in addition, higher storm surges due to bigger storms, the probability of a greater overtopping than envisioned seems possible. Evaluation of this risk requires an updated hydrological model — a complex technical effort. Additionally, some careful topographic analysis is needed to understand under what conditions the dams could be bypassed entirely by overland or subsurface flows.
Getting a process going to fully assess the risks will be a high priority for me over the next couple of years. Most likely, the basic risk assessment process should be driven by state agencies, but we may need to put legislation in place to fully empower them to address the issues. Of course, cities and towns will have a central role as to their own infrastructure. In particular, the City of Boston has been monitoring the issue of sea level rise.
Addendum: Here is a link to follow Senator Brownsberger’s bill S.344 on flood planning: