Most of us were disappointed by how our regional infrastructure performed during the most recent flooding event. We had both severe sewer backups and severe storm-water flooding in many neighborhoods in Arlington, Belmont and Cambridge. While we have made progress over the last few years, it is clear that we have to keep at our efforts. Please do share your thoughts and observations on this event.
Question 1: Was it an off-the-page storm overall? Not a 100 year storm, but definitely big. The rain started falling in Belmont on Saturday morning and continued through Monday evening — in other words, roughly a 60 hour storm. Local rain gauges suggested that Belmont got 8.5 inches of rain from the 3 day storm. A 100 year storm is a storm with severity that has a one percent chance of recurring in any given year. According to the traditional official methods — which do not make any allowance for increased precipitation due to climate change — 6.5 inches of rain in 24 hours or 8.5 inches in 48 hours is a 100 year storm in this area. So, this storm was a little more spreadout in time but did approach a 100 year event locally. However, it was not quite as dramatic for the whole regional system. National weather service data from Logan put the rain fall in the 72 hours from Saturday morning through Tuesday morning at 6.98 inches, well shy of the 100 year mark. (By comparison in only 48 hours around Mothers Day 2006, 7.61 inches fell at Logan yet the flooding problems were not nearly as bad then.)
Question 2: Was the flood itself off-the-page by the numbers? Apparently so. The river flow coming down from Winchester on the Mystic was 140 times greater than its calm day level of 10 cubic feet per second. The mean discharge at the Aberjona guage up in Winchester was a huge 1420 cubic feet per second on Monday and still 1,120 cfs on Tuesday. The highest day of the Mother’s day storm was only 1,110 cfs. For those who remember the October 1996 storm, which is the last storm in my memory to have such severe local effects, the highest daily discharge in storm was only 1070 cfs. High river flow could be explained by locally heavy rainfall or by saturated soil that absorbed little of the rainfall. When the Mystic floods, the Alewife Brook tributary also floods. At the Alewife itself, the flow crested at about 120 cfs in the Mothers day storm, while in this storm, the flow exceeded 200 cfs.
Question 3: How often can we expect storms with this local flooding impact? Hard to say, but much more frequently than every 100 years. The replacement of absorbent soils with impermeable roofs and roads makes a big difference. They send water straight through the storm drain system into the rivers, increasing the peak flow in the rivers. High peak river flows drive up flood waters. Professor Richard Vogel, a hydrologist at Tufts, estimates that the 100 year flow event on the Aberjona has increased from roughly 500 cubic feet per second based on flow records in the 40s and 50s to 1000 cfs based on flow records in the 60s and 70s to 1500 cfs based on flow records from the recent two decades ending in 2006. In other words, the 100 year flood has been steadily increasing and he believes that this is mostly due to land use changes during the period 1940 to 2006. He further estimates that if one looks at the trend line in flows (as opposed to the last the recent average), the 100 year flow may actually have be as high as 2,700 cfs. In other words, the current storm’s flow may be roughly only half as bad as the 100 year storm flow. And that estimate only includes the dramatic influence of land use changes but makes no assumptions about the effect of global climate change.
Question 4: Does River flooding relate to sewage backups? Yes, in a complex way. When there is a lot of rain, rain gets into the sewer system and raises water levels in the sewage system. This is what causes the system to overtop man holes and rise in peoples’ basements. High river water levels prevent the sewage system from overflowing into the rivers and force the backup higher.
Question 5: What should we be doing about the problem near term? The top actionable priority is to make sure we have clear flow in the Alewife and the Mystic. The Department of Conservation and Recreation cleared tons of debris from the Alewife Brook last summer. That clean up may have helped to avoid the disaster of a clog on the Alewife in this storm. And the 2003 cleanup by DCR of debris at the Craddock Bridge in Medford probably kept the 2006 Mother’s day storm from being more serious. Some people disagree about how much the Craddock Bridge by itself affects flow, but no one can disagree that, as currently built, it has a tendency to become occluded, with serious consequences. At 5:30 on Monday evening, I checked at the bridge and found that it was occluded by a huge dock structure that had broken loose and jammed broadside across the bridge. Water levels above the bridge were close to two feet higher than water levels below the bridge while the obstruction was in place. The blockage raised the peak flood levels in the Alewife by some undetermined amount, certainly less than two feet, but probably material. Crews were on site on very quickly after my calls, but darkness and swift currents prevented removal before Tuesday morning. The reconstruction of the bridge is in the design stage and all local and state officials are in support of the project. We will report an updated timeline on that project shortly. Another project in the works is the dredging of Blair Pond, which may help reduce the likelihood of local flooding at Belmont High School.
Question 6: What about sewage backups? There are several projects in the works that should reduce sewage backups. Arlington, Belmont and Cambridge have ongoing projects to reduce rainwater inflow and infiltration into their systems. Cambridge’s sewer separation project should specifically help in North Cambridge neighborhoods. Belmont’s project to separate Winn Brook sewers from Belmont Hill sewers, a project which has taken about 10 years of effort to bring into focus, is also underway. This will reduce the frequency with which Winn Brook sewers are overwhelmed. One additional issue that deserves study is just how the sewers were managed during the storm. There was some suggestion that MWRA could have lowered the water in the sewage system but did not open all available overflow mechanisms.
Question 7: What are medium term measures to be looked at? Increase the pumping capacity at the Amelia Earhart Dam. With the support of all local legislators from affected areas in Arlington, Cambridge, Belmont and Somerville, the legislature put an earmark into two successive budgets to study the addition of a fourth pump at the Earhart dam. A fourth pump would increase the rate at which flood waters can be thrown over the seawall, helping drive down flood levels. Engineers in DCR’s flood control group are consumed by the reconstruction of the failing dam between the Mystic lakes and Governor Patrick unilaterally cut the funding during the financial crisis in the Fall of 2008. Local legislators will continue to work together to restore this project.
Question 8: What should we be doing long term given that nothing we have on the drawing board right now will fully prevent the recurrence of recent events? Harder question. We need to look hard at ways to reduce the flow of waters into local rivers. That means a variety of detention and storage strategies. They have to be integrated into land use and development plans all over the watershed if they are to be meaningful. The amount of water involved in a big storm (flow equal to an acre-foot of storage every thirty seconds at the Aberjona in this storm) is such that local projects to increase storage will will not solve the problem. It may be more feasible to help local homeowners invest in measures to make their properties more storm resistant.
Please do share your ideas for flood control as comments to this post. Please share observations about this event and its effects in your neighborhood at this link about field observations.