Theories abound on how the river got those patterns


So, What were those circular patterns all up and down the Charles anyway?

My mail and telephone conversations might well be described as "Close Encounters of the Circular Kind" as theories have poured in in response to my photograph showing swirl patterns in the snow-covered ice on the Charles River. (MIT Tech Talk, Feb.10)

We asked what might have formed them, and you responded.

We're told, for example, that a note posted in an elevator in the Green Building (home to Earth, Atmospheric and Planetary Sciences) suggested that someone might like to give a talk about the swirls. So far, no talk. But a Green Building expedition led by Professor John C. Marshall did make a foray out to the river for a closer look.

"One of us ventured onto the ice and started prodding with a stick," he reported. "The dark, almost perfectly circular areas were iced-over but of a slushy consistency and less firm than the surroundings. Powdery snow was continually being blown onto the surface, sticking to the damp areas and melting."

"In the bitter cold we started speculating about what was happening. These speculations included: frozen-in eddies spawned by the sluggish flow of the Charles over the bottom; point sources of effluence issuing from submerged pipes; wind driven patterns akin the `crop-circle' phenomenon common in the summer in Wiltshire, England; signatures of traffic control helicopters hovering upside down over the Charles."

The last explanation was frivolous, of course, as we assume were the comments of several persons who attributed the circular patterns to the "same aliens" who presumably created patterns found in fields in England.

"Perhaps they were caused by the heat from their flying saucers," suggested one caller. Another surmised that the playful aliens had finished their work in England, became thirsty and had come to the Charles for water.

Although a few of the submitted theories were truly unto themselves, a number of observations fell into similar categories.

A popular one, advanced among others by Professor Jerome H. Milgram and colleagues in Ocean Engineering, graduate student Ken Olum in physics and Stu Nelson, formerly of the Sailing Pavilion, is that eddying winds in the river basin blew the snow into patterns.

Wrote Olum: "I think the swirl patterns...are frozen-in wind eddies. Any large ice surface is always cracking from thermal stresses, and sometimes the cracks are large enough that water can seep through onto the surface. If there is snow on the surface the water will be absorbed into the snow and then freezes again... If there is a strong wind it will make streaks on the surface... I've seen the linear form of this effect many times."

Nelson, in a similar vein, believes the patterns were a combination of uneven freezing (due to variations in depth and volume of water) and eddies of wind on the water as it freezes. The wind, he says poetically, meanders and snakes its way down the river, and since different parts of water skim (freeze) at different times, the wind eddies at freezing account for the patterns.

Graduate students Ron Hevey in physics and Bob Hyers of Materials Science and Engineering offer this, "based on the principle of faster-moving air creating lower pressure which causes more evaporation. The wind moving down the river creates vortices (swirls) when it hits objects (i.e. bridges, shoreline objects); these vortices have faster-moving air and the lower pressure therefore causes evaporation of the snow in those spaces where it's gone."

This seemed similar to a theory advanced by a husband-wife team, Ocean Engineering graduate student Eric Lamarre and his wife, Marie-Lyse Bergeron. They thought the circles might be the result of flow patterns around the pillars of the Harvard bridge, adding that "the dimension (of the circles) seems to match the separation between the bridge pillars."

Another popular theory focused on upwellings from the Charles itself. These included possible underwater springs; natural aeration of the river; MDC aeration of the Charles; something decomposing and bubbling to the surface; sewer outfall pipes; natural oxygen or gas bubbling to the surface; underground currents; or, warm water from industrial outflow pipes.

Retired Campus Police Chief James Olivieri recalled that a number of MDC aerators had been installed to stir up sediment and clean the river. However a call to MDC flood control manager Nick Winter yielded the news that, to his knowledge, "that compressor has not been turned on in at least a couple of years."

Geophysics graduate student Patrick McGovern said he was talking with Professor Bradford H. Hager of Earth, Atmospheric and Planetary Sciences when they were struck by how similar the circles were to some of those found on Venus, caused by the upwelling of magma. McGovern brought Magellan radar images of Venus to compare to the Charles River circles, and indeed there was some resemblance.

Karen Van Nederpelt, a fiscal officer in the Center for Transportation Studies, offered this observation: "On my way to work the other day, I noticed what looked like a swamp boat on pontoons on the ice. It was powered by huge fans, had a metal hull, and was going in circles."

Another first-hand observation came from David C. Ricks of Ocean Engineering: "I walk across the Harvard bridge twice a day. Here's what I saw over a period of a few weeks. First, a smooth sheet of ice formed over most of the surface of the river. Then, large holes of open water were formed by the ice moving around. Some of the holes came from the piers under the Harvard bridge on a very windy day that pushed the ice downstream. Other holes were created as the old smooth sheet separated into large pieces from being pushed around. Then, these areas of open water froze over. The ice forming in these holes included air and small broken plates of ice, so it has a rough texture and a different color. In the photograph, the dusting of wind-driven snow highlights the different textures of the ice.

"It's interesting how these holes froze to give circular patterns, even though the holes had irregular shapes when they were created. I saw that the ice grew into the holes from the outside toward the inside. Evidently, this made the holes more circular. And, as the holes were freezing over the course of a few days, the strength of the wind varied, and this caused different amounts of air and broken ice to be incorporated into the freezing ice at the edges of the holes. This gave the circular patterns of roughness and air content. It's also interesting how the swirls are now distributed around the river."

We'll leave the final words (for now) to Keith D. Alverson, a graduate student in Earth, Atmospheric and Planetary Sciences who was among Professor Marshall's team at the Charles and who provided this thoughtful explanation, with a proper MIT-type title" A Mechanism for the Formation of Patterns on the Charles:"

"The patterns in the snow cover...were roundish snow-free areas all of roughly the same size. Given that these patches formed in distinct locations on the stationary ice, it does not seem plausible that they were caused by either atmospheric or riverine disturbances such as `whirlwinds,' which would remain stationary in one spot over the ice. Instead, the patches must have been formed by a process inherent to the ice itself or related to the bottom of the river. Discarding the possibility of processes induced by irregularities in the river bottom, that leaves only ice processes.

"The process I propose relies firstly on the effects of local inhomogeneities in the snow cover. There were bound to be many small patches where the snow would melt. For instance, a crack in the ice would allow water to seep up and melt a small patch. Similarly a leaf in the snow, because of its dark color, will heat up and melt the snow around it. Instead of reflecting solar energy as snow does, a dark leaf absorbs solar energy and reradiates it at longer infrared wavelengths as heat...

"The next step is to compare the relative reflectivity and absorption of snow and ice. If the ice is dark, it, too, will maintain a higher temperature than the surrounding highly reflective snow cover. Hence, the snow will melt back around the edges of this patch, causing it to grow in diameter with time. The patch, being warm, will also become somewhat slushy... Thus any new snow that blows onto the patch will fall onto a wet surface, melt, and lose its reflective properties... The key element is the high reflectivity of snow compared to ice.

"The reason that patches such as those in the Charles...are not seen more often may be the relative infrequency of solid ice cover over the entire river followed by snowfall and a subsequent long period of temperatures very close to freezing such that the radiative processes rather than simple atmospheric temperature effects can dominate the melting of the snow cover."

A version of this
article appeared in the
February 24, 1993

issue of MIT Tech Talk (Volume
37, Number
23).


Topics: Oceanography and ocean engineering

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