Nonlinear Propagation of Sound in Recently Settled Flocculated Sediments
Abstract
Cohesive sediments undergo changes in a whirlwind. Dumped out of the river and into the estuary, they get bathed in salty water and subject to turbulent motion. During this sequence of events, the clay particles form clay aggregates of larger size with higher settling rates than the clay particles. Once the flocs have settled, cohesive sediments may form a sediment deposit of mud. Our interest is in the factors that control the development of soundspeed within these muds. This paper addresses organic matter influences on floc aggregation and settling rates. In laboratory studies, organic matter type differed in mixtures with either bentonite or kaolinite clays. The organic matter types used were guar gum, a net positive biopolymer, and xanthan gum, a net negative biopolymer derived from bacterial exudates, similar to those commonly found in estuaries. These biopolymers were dissolved into low salinity water (0-10 ppt). The biopolymer mixture was degassed and during the degassing process, either bentonite or kaolinite clay was added to the vessel. Surprisingly, different settling rates occurred in the clay-biopolymer mixtures. The settling rates of the clay-guar mixtures was more rapid (1-2 days) than the settling rate for the clay-xanthan mixtures. While clay-guar consolidated further, clay-xanthan maintained consistency for more than 2 weeks with density slowly increasing during that period. Compressional soundspeed (Vp) measurements were made with depth through the vessel using 0.5 mHz piezoelectric transducers. It was found that Vp in water was similar to that of the clay-xanthan. Vp was the same in the upper 6 cm of mud as it was in the overlying water and Vp decreased to become slower with increasing depth. Compressional wave velocity (Vp) also changed slightly with the guar complexes below the sediment water interface to the depth of the vessel. Vp was slightly slower in the mud than in the water column. Vp of the water was 1480-1495 m/s whereas Vp within the clay-biopolymer was below the minimum Vp in the water column. This slight decrease in Vp with depth is consistent for that of naturally occurring surficial mud deposits. This work suggests that organic matter type can play a critical role in the rate of consolidation within a mud deposit, which has implications for mud strength development and transport potential.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMEP33A0978R
- Keywords:
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- 1815 Erosion;
- HYDROLOGYDE: 1862 Sediment transport;
- HYDROLOGY