Philosophical Society of Washington

Minutes of the 2297th Meeting

Vice President Lloyd Mitchell called the 2,297th meeting to order at 8:23 pm February 24, 2012 in the Powell Auditorium of the Cosmos Club. Mr. Mitchell announced the order of business and introduced eight new members of the Society, including the speaker of the evening.

The minutes of the 2,296th meeting were read and approved.

Mr. Mitchell then introduced the speaker, Mr. Marwan Hassan of University of British Columbia. Mr. Hassan spoke on "Sand stories: fish, floods and folks in landscape evolution in river valleys throughout the world."

Mr. Hassan began by noting that soil erosion and desalinization played a role in the migrations and demise of several major ancient civilizations. The link between soil degradation and civilization suggests that understanding the processes of sediment transport remains a key issue even in modern times. Three important issues are sediment transport in gravel bed rivers, sediment mobilization by fish, and how land use and soil conservation methods impact sediment production, he said.

Mr. Hassan continued by describing how theoretical models that estimate the sediment transport in gravel bed rivers often predict values orders of magnitude higher than measured data. Rivers are very complex, with irregular boundaries and turbulent flow, and are difficult to fully model because they tend to develop surface structures that reduce sediment transport, he said. Models also do not include surface and subsurface composition differences that cause differing vertical sorting based on particle coarseness, he noted.

When rivers are starved of sediment by dams, they can be supplied sediment by humans but it is not clear how best to do so, Mr. Hassan explained. By comparing field evidence from two very different creeks and performing experimental work in a laboratory, a relationship was found between sediment feeding rates and transport rates of different grain sizes. This shift in transport curves from coarse to fine particles at the surface must be taken into account when supplying sediment, he said.

Mr. Hassan continued by asserting that during a flash flood, compared to snowmelt flow, there is insufficient time to sort sediment. Specifically, the amount of sediment sorting possible depends on the length of the falling region of the hydrograph describing the flow. Through this theoretical and experimental work, it was found that armoring surfaces develop during longer floods. However, sediment supply is still a primary control of the channel bed characteristics compared to hydrograph shape, he said.

Mr. Hassan then explained how fish change their environment to develop a suitable habitat, specifically noting the kind of bedforms salmonids produce during spawning season. Fish excavate material to lay eggs but it was unclear how much volume or impact this process had compared to floods, he said. Millions of sockeye salmon return to rivers in British Columbia every year and the known spawning period allows researchers to check sediment travel distance and burial depth by both floods and fish over several years of data collection.

It was found that fish mobilize sediment at values comparable to floods but provide vertical mixing superior to floods, Mr. Hassan said. Since weaker fish go further upstream where it is more difficult to dislodge the surface material, fish can actually mobilize two to three times more sediment than floods near the mouths of rivers. Further, since lakes regulate flood flows, fish must there dictate the movement of sediment below lakes.

Mr. Hassan then discussed how are land use and soil conservation methods impact landscapes around the world, noting that a graph of earth volume intentionally moved per capita shows a hockey stick shape over human history. The Yellow River basin has undergone a huge amount of erosion due to human activities, especially when terraces and checkdams became common during an agricultural reclamation, he said. Today, trees are being planted to help stop erosion and have managed to significantly reduce sediment yield but the amount of downstream sediment transport is still significant. Further examples included the Yangtze river, which has been dramatically changed due to the Three Gorges dam, and the Mississippi river, which is affected by the extensive dam system in the United States.

Mr. Hassan explained that soil conservation methods work best at the field and hillslope scales, but that floodplains and banks are the main sources of sediment. Dams and land conservation practices reduce sediment flux to the oceans, which must be carefully considered. Mr. Hassan concluded by noting that humans now transport approximately the same amount of rock and soil as rivers or volcanoes, implying a new informal geological age called the "Anthropocene."

After the question and answer period, Mr. Mitchell thanked the speaker, made the usual housekeeping announcements, and invited guests to apply for membership. At 10:02 pm, Vice President Lloyd Mitchell adjourned the 2,297th meeting to the social hour.

Attendance: 48
The weather: Mostly Cloudy
The temperature: 13°C

Respectfully submitted,

Justin Stimatze,
Recording secretary

Abstract & Speaker Biography
Semester Index - Home