Braided Pollen Streams
June 11, 2009
I was walking to the office around the middle of April this year, a day after a heavy downpour had occurred. At each successive puddle, I noticed what appeared to be increasingly intricate pollen patterns on their surfaces. Some, as shown above in the top photo, reminded me of braided streams or river deltas seen from the air. Some puddles possessed meandering patterns surprisingly similar to those present in rivers as seen in the photo at bottom, which was taken on a flight over northern Florida. Since the pollen features were stationary over the timescale of the several minutes I was observing, the mechanisms producing them are probably quite different from their larger scale cousins. The area shown in the picture at top is about eighteen inches square (2,090 sq. cm), and the water was less than 1 inch in depth (1-2 cm). Some of the patterns were reminiscent of "slow" (or low Reynolds' number) flow in hydrodynamics. Some reminded me of the patterns reproduced in articles dealing with "chaos theory." It's possible that the combination of airflow over the surface of the puddle, coupled with ground frictional effects on any slow bulk flow of the water (on a slightly sloping or irregular road surface, for example) produced these patterns. I do remember politely asking a bicyclist to avoid the puddle I was trying to photograph!
By contrast, river meanders are caused by mechanisms that are fairly well understood. Meanders are even observed in ocean currents, such as the Gulf Stream, and in drainage channels on the surface of a glacier. In rivers, the processes of continuous erosion, transportation and deposition construct the ever-changing shape of rivers and their meanders. Imagine a slight bend in a river; as water flows around it, the outer concave bank will be "scoured." The water will then move downward at this bank (it has nowhere else to go) and then across the river bed towards the other bank. As it does this, friction with the bottom slows down the flow, allowing particles of silt and rock to be deposited there, though somewhat downstream due to the forward velocity of the river superimposed on this "crosswise" flow. The actual fluid "particles" will execute helical paths as a result of this, and the scoured bank will be steeper than the gentler slopes on the other side of the river. This will result over time in the meander becoming more and more pronounced until to continue would result in the river moving uphill: a no-no for rivers! Thus, the turning process begins again as gravity continues to pull the river water to a level of lower potential energy. Such meanders will also migrate downstream over time. Despite the considerable, even dramatic variations and size in bed conditions, all rivers have strikingly similar characteristics. Pollen photo taken at Old Dominion University in Virginia.