Thursday, December 21, 2023

Assabet River Flood 2023

Click on photos to enlarge

Powdermill dam (behind the
Subaru dealership on Rt, 62)
The Assabet River starts in the Westborough marshes that drain into the George H. Nichols flood control basin, then works its way first north, then east. For much of its length the Assabet drops five feet for every mile. Within Maynard, river elevation is 175 feet over the top of the Ben Smith Dam and 145 feet out the east side. All this downhillness means that water moves quickly through the Assabet watershed. The rainstorm of December 17-18 dropped about four inches of rain on earth that was already near-saturated by an earlier storm. Peak water height at the official Maynard recording site was 6.0 feet, reached Wednesday morning. Over 5.0 feet is considered a mild flood, over 6.0 feet a moderate flood. This recent peak was the highest since the flood of March 2010, which had reached 7.1 feet. The last significant flood before that was 7.17 feet recorded in 1987. The highest since record keeping began in 1942 was 8.94 feet, reached after Hurricane Diane, August 1955.
Powdermill dam, non-flood

Even though this latest was quote/unquote a “moderate” flood, there was next to no street flooding or property damage in Maynard. Rather, the water stays in its channel and just moves noisily faster. Only when the river exceeds eight feet does it get into buildings, including the mill complex.

There is a delay in time between the peak rainfall of storms and peak height of rivers. This is because water takes time to drain from the tributaries into the river. For the Assabet River, flood crest levels occur two to three days after the heavy rains began. Sometimes skies are clear and the sun shining while the water is still rising. Interestingly, although the Assabet and Sudbury rivers drain watersheds of approximately the same size, and thus reach similar flood volumes after heavy rainstorms, the Assabet crests much faster because of its steepness compared to the Sudbury. Henry David Thoreau observed that when the Assabet River in flood reached Egg Rock, Concord, where it merges with the Sudbury River to become the Concord River, the surge of water caused the Sudbury River to temporarily flow backwards for a distance of several miles. 

Danforth Brook dam,
December 2023 flood
Danforth Brook dam, in
drought condition
There is a history of severe floods on the Assabet River, especially before the two flood control dams, George H. Nichols Dam and Tyler Dam, were completed in the 1960’s. The impoundment area behind Nichols is kept partially full in order to be able to provide water to the Assabet in times of drought, but usually has a 500 million gallon flood hold-back capacity. Tyler’s impoundment is kept low between floods and has a hold-back capacity of 1,800 million gallons. The amounts sound huge, but the Assabet River’s days-long March 2010 peak of 2,400 cfs (cubic feet per second) converts to 1,500 million gallons per day, and the 1955 flood, courtesy of rains from the remants of Hurricane Diana, which predated the flood control dams, peaked in Maynard at 8.94 feet and 4,250 cfs which converts to 2,650 million gallons per day! Thus, the two flood control dams on the Assabet, plus flood control dams on some of the tributaries, are enough to mute the worst outcomes of these every 10 to 20 year floods, but not enough to prevent them completely.

It's interesting to  realize that Maynard draws less than 1,000,000 gallons per day for all its water needs, but there are no plans for Maynard nor any other community on the Assabet to create reservoirs that would provide a water supply. What we do hope for is that rain and snow melt recharge the aquifers that our wells depend upon.

Mill Street bridge, 2010 flood
Back before the flood control dams were in place, the November hurricane flood of 1927 is recorded as washing away both a dam and the Waltham Street bridge. The dam dated to when there was a papermill at what is now the site of the 7-11 convenience store.  A flood in March 1936 washed out the wooden bridge for Mill Street, replaced by stone arches. Hurricane Diane in August of 1955 brought the most rain recorded in any one month since a gauge was installed in 1942. The river crested at 8.94 feet. No bridges were lost. More recent floods of note occurred in March 1968, cresting at 8.15 feet, and January 1979, cresting at 8.11 feet. Retirees from Digital Equipment Corporation remember sandbagging the buildings in 1968 in an attempt to keep water out of the production facilities. Afterwards, DEC had the river retaining wall on its side built higher along the lowest stretch next to the mill buildings complex. The wall kept the river out in 1979.

An observation: for an undammed river, each flood moves tons and tons of rocks, dirt and organic debris such as trees, branches and leaves, downriver. This can raise the level of land under the river, especially when it reaches areas with less vertical drop per mile. However, all of these solids never reach the center of Maynard because those settle out in the miles-long body of water backed up by the Ben Smith dam. Only clear water overtops the dam, and then scours the river bottom through Maynard down to bedrock, hard clay and large loose rocks.  


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