Relative importance of sediment abrasion in downstream fining of grains and production of fine sediment.

Project Description: 

Key Science Questions Involved:
1. What is the relative importance of abrasion versus selective transport in causing downstream fining of river sediments?
2. What is the fraction of fine material that is produced by abrasion of gravel?

The process of abrasion is defined by the production of fine sediments and sand that occurs by saltation of gravel, where particle-to-particle collisions supply the energy required to break apart grains (Figure 1). Although previous work has shown that lithology, grain shape, and energy of collision are contributing factors that control abrasion rates of river-bed material, little is known regarding the relationship between these factors and diminution rates. Without such knowledge, it is not possible to estimate the contribution of in-stream abrasion to determining grain sizes of river sediments, relative to the better-studied processes of chemical weathering and sorting by transport. In this project, we are investigating the controls on abrasion rates and the products of the abrasion process. The Luquillo Critical Zone Observatory (LCZO) in Puerto Rico is an ideal setting to study sediment abrasion because of the two different lithologies, volcanoclastic and quartz diorite, that comprise the channel sediments in the two otherwise similar study watersheds of the Rio Mamayes and Rio Icacos, respectively. The volcanoclastic watershed produces a wide range of river-bed grain sizes, whose distribution changes gradually downstream. Streams draining quartz diorite lithologies are rich in sand and boulders but are lacking in intermediate grain sizes. We hypothesize that in steep mountain sections of streams, sediment storage capacity is minimal while collision energies are higher; thus, abrasion should dominate over size-selective sorting for determining grain size fining downstream. Conversely, alluvial plain river sections with higher storage capacity should show strong size-selective sorting, while the importance of abrasion is reduced.
Pike et al. (2010) measured grain size at over 45 locations within the Mamayes watershed, with results indicating an overall fining of grains with distance from headwaters (Figure 2). In order to explain this downstream fining, we measure the shape of channel sediments in both watersheds to determine their downstream evolution. Results from experiments conducted by Durian et al. (2006) of abrasion of a two dimension square pebble in a rotating drum showed that areas of pebble that protruded, marked by high curvature values, tended to abrade faster. As the protruding corners of the square abraded away, the pebble became rounder causing abrasion rates to decrease (Durian et al., 2006). Since shape has been shown to control abrasion rates of grains, we are using this parameter as a proxy for observing the abrasion process. We assume that any rounding of the river sediments will be solely due to abrasion. We characterize grain shape with standardized shape parameters, as well as Fourier analysis, which allows us to track changes in grain roughness at a variety of length scales. Preliminary results of shape analysis of 36 locations within the Rio Mamayes watershed shows a distinct pattern of rounding in the last 10 km of the river (Figure 3). In order to further examine the mechanism behind abrasion, we will determine the mobility and collision energy of gravels that are transported by floods using results from the work Colin Phillips and Douglas Jerolmack are conducting on tracking the transport of grains using radio-tracer particles. Additionally, we can calculate the amount of fine sediments that should be produced by abrasion and compare them to hillslope sediments produced by chemical weathering. Using results from the sediment fingerprinting project being completed by Marcie Occhi and Jane Willenbring, we hope to determine the fraction of fines that are derived from the abrasion process. Laboratory experiments are being conducted at the University of Pennsylvania to tease apart the controls of lithology, collision energy and grain shape on the rates and styles of abrasion. Experiments will allow us to link the mechanics of granular collisions to parameters such as grain size and shape distributions that can be measured in the field. The results of this research will enable us to isolate the effects of in-stream abrasion on the downstream fining of grains in a river, and to understand the specific control that bedrock lithology exerts on this process.

Core Area(s) and/or Keywords: 

sediment transport, abrasion, lithology, downstream fining

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Person(s) Completing This Form: 
klitwin
Investigators: 
Doug Jerolmack, Kimberly Litwin
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