Question 4: Does River flooding relate to sewage backups? Yes, in a complex way. And that estimate only includes the dramatic influence of land use changes but makes no assumptions about the effect of global climate change. In other words, the current storm’s flow may be roughly only half as bad as the 100 year storm flow. 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 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. 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. High peak river flows drive up flood waters. They send water straight through the storm drain system into the rivers, increasing the peak flow in the rivers. The replacement of absorbent soils with impermeable roofs and roads makes a big difference. Question 3: How often can we expect storms with this local flooding impact? Hard to say, but much more frequently than every 100 years. 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. When the Mystic floods, the Alewife Brook tributary also floods. High river flow could be explained by locally heavy rainfall or by saturated soil that absorbed little of the rainfall. 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. The highest day of the Mother’s day storm was only 1,110 cfs. 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 river flow coming down from Winchester on the Mystic was 140 times greater than its calm day level of 10 cubic feet per second. Question 2: Was the flood itself off-the-page by the numbers? Apparently so. (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.) 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. However, it was not quite as dramatic for the whole regional system. So, this storm was a little more spreadout in time but did approach a 100 year event locally. 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. A 100 year storm is a storm with severity that has a one percent chance of recurring in any given year. Local rain gauges suggested that Belmont got 8.5 inches of rain from the 3 day storm. The rain started falling in Belmont on Saturday morning and continued through Monday evening - in other words, roughly a 60 hour storm. Question 1: Was it an off-the-page storm overall? Not a 100 year storm, but definitely big. Please do share your thoughts and observations on this event. While we have made progress over the last few years, it is clear that we have to keep at our efforts. We had both severe sewer backups and severe storm-water flooding in many neighborhoods in Arlington, Belmont and Cambridge. Most of us were disappointed by how our regional infrastructure performed during the most recent flooding event.
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