David Rosengarten & Robert Rothbaum, as rising sophomores from Great Neck High School, Long Island, spent 6 weeks at MIT in the summer of 2005 investigating sediment transport processes.

Research Project Advisor: Katharine Ruhl (now Huntington)

Collaborators: David Mohrig, Jim Buttles, Kyle Straub, Doug Jarolmack

Summer Research Program Advisor: Alan Schorn

Robert (left) and David (right), using Camsizer to measure sediment sample grain size distributions.

 

Abstract submitted to the Long Island Science and Engineering Fair

March, 2006

Sand Transport Across a Gravel Bed: An Experimental Flume Study of Spatial and Temporal Trends in Particle Size and Sediment Volume as a Function of Flow and Bed Conditions

Chemical analyses of sediment samples from modern river systems or ancient deposits are useful for estimating erosion rates over a wide range of spatial and temporal scales. However, such techniques generally depend on a series of assumptions regarding the erosion and transport history of the contributing area, or catchment. For example, that the grain sizes appropriate for analysis are well mixed by the time they reach the sampling site, that sediment moves rapidly through the system, and that a sample represents the contributing catchment in proportion to area . Understanding the grain-size dependence of mixing, storage, and transport of sediment in rivers also has numerous implications for the defense of aquatic ecosystems from pollution. This study attempted to test the validity of the assumptions that (1) all grain sizes are equally well mixed by the time they reach the outlet of a catchment and (2) all grain sizes arrive at the outlet of a catchment as the same function of time by measuring grain size distributions, bed topographies, and sand volumes with distance in a flume under varying flow and sediment input conditions. Sand was inputted through a flume with a gravel bed in each of four runs of the experiment. A laser was used to map the topography of the bed before and after each run. Flow velocities and depths at specific stations along the flume were measured, and between these stations the sand was separated from the gravel and analyzed in terms of grain size. Sediment grain size and volume distributions indicate possible grain size thresholds between the sand that was stored near the sediment input of the system and the sand that was well mixed and able to pass through the system. The thresholds as well as the volumes and mean sediment sizes in each bin varied with distance from the source similarly regardless of sediment input volume and flow velocity. The nature of such thresholds once calibrated to natural systems could be used to design erosion rate studies and used to design sediment traps to mitigate the effects of sediment pollution in fisheries.

 

Research Plan, July 2005

Motivation

Techniques based on detrital sediment samples taken from modern streams or ancient deposits often assume that the grain sizes you can sample and analyze are well mixed, not stored for significant amounts of time on the landscape, and represent the contributing catchment in proportion to area. This study attempts to test the validity of the assumptions that (1) all grain sizes are equally well mixed by the time they reach the outlet of a catchment and (2) all grain sizes arrive at the outlet of a catchment as the same function of time. Although these behaviors could not be known in detail for all rivers, the existence of possible grain size cutoffs between behaviors in space and time could have numerous implications for erosion rate studies from decadal to million year time scales and for the defense of aquatic ecosystems from pollution.

Testable Hypotheses

A critical cutoff of sediment grain size exists such that any sediment with grain size above this threshold will (1) be more poorly mixed by the time they reach the outlet of a catchment and (2) arrive at the outlet of a catchment more speedily, and any sediment with grain size below this threshold will (1) be more well mixed by the time they reach the outlet of a catchment and (2) arrive at the outlet of a catchment more slowly. Reasoning follows with a boulder trapped in the bed, hindering the bedload of the current, and so sediments of smaller grain size would get trapped between the cobble and gravel of the bed.

Experimental Design

The floor of a 9 meter long flume will be layered with gravel that will act as a bed and be controlled throughout the experiment. Water will be constantly flowing through the flume, and pulses of sand will be successively sent through the current. Each pulse of sand will be the same in terms of grain size, volume, and mass. The flow velocity will be varied with each successive run. A laser interfaced with the computer will map out the flume's bed geometry before and after the sediment is run through it as well as every time samples are taken in order to find out sediment volumes and channel features such as the interaction of sediment input with the bed. A trap will be built of chicken wire and placed above the recirculation vent of the flume so that no gravel can be carried through. An Acoustic Doppler Velocimeter (ADV) will be used to measure the flow velocities throughout the flume as the sediment is being carried through the flume by the current. Each run will be timed and photographed in intervals. The gravel will be partitioned after each run so that the deposited sand can be extracted using linoleum and a wet paper towel. The sand that is carried through by the current will be siphoned. These samples will then be analyzed using a laser scattering particle size analyzer for finer particles, the LPSA, and a digital camera based particle size analyzer for coarser particles, the Camsizer. The samples' masses and volumes will also be measured. The relative density between the sediment and the water can easily be determined thereafter. The software, Matlab, will be used to create a mathematical model using the dimensionless Rouse number, settling velocity divided by shear velocity times a constant, to analyze the results.

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