Lecture 2: Mechanics of Sediment Transport

Why It Matters

Accurate sediment‑transport models underpin safe hydraulic infrastructure and enable reliable reconstruction of Earth and planetary surface histories.

Key Takeaways

• Sediment transport splits into bedload and suspended load, with overlapping zones.
• Measuring transport rates is challenging; suspended load easier than bedload.
• Saltation dominates aeolian transport, showing hysteresis and high spin rates.
• Density ratio dictates transport thresholds, varying from Earth to Mars.
• Bedforms evolve with flow, creating feedback that alters sediment dynamics.

Summary

The lecture surveys the mechanics of sediment transport in both fluvial and aeolian environments, outlining how particles move as bedload, suspended load, or wash load and describing the experimental foundations behind these concepts. It emphasizes the stages of motion—rolling, saltation, and suspension—while noting the difficulty of quantifying transport rates, especially for bedload, compared with more straightforward suspended‑load sampling. Key insights include Vito Vanoni’s microscopic flume experiments that revealed grain‑scale movements near the Shields curve, the semi‑empirical nature of transport‑rate formulas, and the pivotal role of density ratios (particle versus fluid) in setting aerodynamic thresholds. The speaker also highlights Ralph Bagnold’s pioneering wind‑tunnel work, the observable hysteresis in saltation onset and cessation, and the extreme spin rates of airborne grains. Illustrative examples range from the “magic screen” thought experiment for visualizing transport rates to real‑world wind‑tunnel strobe footage of saltating sand, and a comparison of threshold shear stresses on Earth, Mars, and Venus. The discussion of bedforms—ripples, dunes, antidunes—and their feedback on flow underscores the coupled forward and inverse problems faced by engineers and sedimentologists. Understanding these mechanisms is critical for hydraulic engineering design, river‑channel management, and predicting planetary surface processes, as accurate transport predictions inform infrastructure resilience and guide interpretations of sedimentary records.

Original Description

MIT RES.12-003 Fluid Motions, Sediment Transport, and Current-Generated Sedimentary Structures, Fall 2025

Instructor: John Southard

View the complete course: https://ocw.mit.edu/courses/res-12-003-fluid-motions-sediment-transport-and-current-generated-sedimentary-structures-fall-2025/

YouTube Playlist: https://www.youtube.com/playlist?list=PLUl4u3cNGP60q1Ib4pyz_FBs_lYUPr9MN

In this lecture, Prof. John Southard continues his exploration of sediment transport by focusing on how sediments move, how transport is measured, and how different flow conditions create distinct bedforms. He explains key concepts such as bedload and suspended load, transport rates, and the challenges of measuring sediment movement in natural systems. The lecture also introduces aeolian (wind-driven) transport, highlighting saltation and the role of density differences between air and water. A major portion is devoted to the formation and evolution of bed configurations—especially ripples and dunes—using both laboratory flume experiments and natural examples to illustrate how flow velocity, sediment size, and turbulence interact to shape the landscape. Southard emphasizes the dynamic feedback between flow and sediment, showing how bedforms both result from and influence fluid motion.

License: Creative Commons BY-NC-SA

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