Mijanur R. Chowdhury, Ph. D.
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Project Focus
Cohesive sediment-laden underflows in coastal dredging and disposal

Background and Motivation
Each year large volumes of sediments are dredged from rivers, waterways, ports and harbors around the world primarily to maintain and enlarge their navigability. An estimated 230 million cubic-meters of sedimentary materials are dredged by the U.S. Army Corps of Engineers in the United States annually. Much of these sedimentary materials dredged (especially those from rivers, channels, lakes and estuaries) are cohesive fine-grained fluid mud that consists of water, cohesive sediment particles (clay and silt), and organic material.

Among the different dredging methods employed in removing fluid mud, the most common and economical maintenance dredging method is the hydraulic pipeline dredging method, in which the hydraulically dredged fluid mud is generally pumped and then transported through a suitable pipeline to dispose into a designated aquatic disposal area (Fig. 1). As soon as they are discharged, the fluid mud starts to descend in the water column and then flow away from the impingement point at the bottom in the form of an underflow due to the density difference of the fluid mud and the ambient water. In order to develop predictive models for the fate of the discharged fluid mud, it is of importance to understand the propagation dynamics of fluid mud underflows.

The main goal of this research, through a comprehensive experimental and theoretical investigation, is to understand the propagation dynamics of fluid mud underflows in relation to the density, rheological properties and release configurations of fluid mud.


Methods
  • The project wa carried out through laboratory experiments that involved discharging fluid mud at a constant volume flow rate from a submerged vertical pipe into a body of water (Fig. 1). 
  • The experiments involved a wide range of initial parameters (e.g., height of the discharge pipe from the bottom, flow rate, and concentration of the fluid mud at the time of discharge).
  • Before each experiment, the density and rheological properties of the prepared fluid mud mixture was measured. The fluid mud suspensions exhibited shear-thinning non-Newtonian behavior as the viscosity decreases with shear rate, except for the suspensions with low concentration values.



Fig. 1. Sediment-laden underflows , in the field (left figure) and experimental simulation (right)


Fig. 2. Different phases of undeflow propagation



Fig. 3. Sequential photographs illustrating the abrupt settling process in fluid mud underflows. (a) initially, the underflow propagates with the majority of the particles in suspension; (b) then, particles start to settle en masse and interstitial water starts to separate from the fluid mud, leading to (c) a turbid cloud propagation, as shown clearly in (d); photograph in (d) corresponds to the dashed rectangular area in (c)

Key Findings
The discharged fluid mud in the experiments descended as buoyant jets and formed axisymmetric underflows that flow away from the impingement
point.Analysis of the experiments revealed the followings propagation dynamics of the underflows:
  • Near the impingement point, underflows behaved as momentum-driven wall jets (Fig. 2). When the discharged dense fluids impinge on the bottom as energetic jets, the length of the horizontal wall jet was observed to scale with the momentum length scale and source bottom separation distance. For plume-like impingements, the length of the horizontal wall jets was found to be linearly proportional to the source-bottom separation distance.
  • After the momentum-driven wall jet phase, underflows propagated as inertial gravity currents which was modeled by a shallow water model.
  • The propagations of the underflows were greatly influenced by the non-Newtonian rheology of fluid mud.Few underflows in the experiments transitioned into the viscous propagation phase; these would not transition into the viscous phase if the underflow fluid were less viscous (e.g., saline solution), even for the same experimental conditions. A box model solution was developed for the viscous propagation of a non-Newtonian (powerlaw) axisymmetric gravity current.
  • Underflows in few experiments exhibited a phenomenon called abrupt settling in which the clay particles settled abruptly en masse and only a turbidity cloud propagated (Fig. 3). This process bears important physical significance in coastal dredge disposal operations in which mud mounds may form due to the abrupt settling process.

Publications

Peer-reviewed journal articles:
  • Chowdhury, M.R., and Testik, F.Y., (2015). “Axisymmetric underflows from impinging buoyant jets of dense cohesive particle-laden fluids”, Journal of Hydraulic Engineering, ASCE, Vol. 141, No. 3,04014079, 1-15. pdf link

Peer-reviewed conference proceeding:
  • Chowdhury, M.R., and Testik, F.Y., (2011). “Subaqueous sediment gravity flows from open water pipeline dredge disposal: laboratory experiments and mathematical modeling”, Conference on Coastal Engineering Practice: Engineering Sustainable Coastal Development, ASCE, 460-472. pdf link

Presentations:
  • Chowdhury, M. R.*, and Testik, F. Y., (2014). Propagation dynamics of mud slurry underflows, Pacific Institute for the Mathematical Sciences Workshop on Mathematicl Modeling of Particles in Fluid Flow, August 17-22, Banff International Reseach Station, Alberta, Canada. abstract link 
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