Reading: Boggs Chapter 2
Particle Entrainment occurs when Fluid Forces > Resisting Forces
Applied Fluid Forces:1. Drag Force (τo / N) + Hydraulic Lift
τo = basal shear stress
= ρgd (sin theta) [where theta = angle of the channel
in degrees]
= ρgd (tan theta) [because tan = sin for very low
angles]
= ρgdS (for a stream channel with slope S) [where
S = slope expressed as a fraction, rise/run]
. . . We sketched and derived this, it's important for understanding how the weight and force of water is transmitted to sedimentary particles and causes them to be moved (transported) downstream.
2. Hydraulic Lift produced by Bernouli effect: conservation of energy along streamlines
Resisting Forces:
1. Gravity Force = (particle volume) (ρg-ρf) g
2. Frictional Resistance
3. "Other Factors" = grain shape, sorting, packing, bed roughness, molecular
cohesion, etc.
Extreme complexiy in the natural world makes it very difficult to accurately predict initiation of particle movement (entrainment). So this problem has been extensively studied through experiments and empirical observations.
Hjulstrum Diagram: shows the "critical" velocity required for initial movement as a function of grain size.
Once Transport is Initiated:Sediment is transported in bedload (rolling or saltation), in suspension, or wash load (always in suspension).
Bedform = any deviation from a flat bed, in sediment, produced by flowing fluid.
We sketched and discussed the formation and subsequent behavior of the essential asymmetric ripple bedform (your basic ripple). Can occur at many different scales and geometric variations (Fig. 2.8).
Note movies and animations of particle motion in Related Links at the course home page.
Bedforms include: ripples, dunes, sand waves, plane bed, antidunes (Table 4.2)
Formation of different types of bedforms depends on: (1) current velocity, (2) flow depth, and (3) sediment grain size. Figures 4.10, 4.11. Good examples of Process and Product.
Froude Number: Fr = U/(gd)1/2. Critical ratio of velocity:depth.
For Fr < 1, flow is said to be subcritical. For Fr = 1, flow is critical. For Fr > 1, flow is supercritical. There is a predictable evolution of bedforms that develop and are stable under conditions of subcritical, critical, and supercritical flow (Fig. 4.9).
Lower Flow Regime: Fr < 1. Upper Flow Regime: Fr = 1 or > 1.
Many experimental and field studies that show how different bedforms are stable under various combinations of velocity, depth and grain size. See Figures 4.10, 4.11, and USGS Bedform Sedimentology. We also watched some movies from Sediment Transport Movies by Paul Heller.
We discussed how to analyze and interpret cross-bedding in sedimentary rocks, what kinds of bedforms they represent, how to determine paleocurrent direction, and what we can or can't say about certain geometries and views of the structures (Fig. 4.20). Need full 3-D view of structures to be able to fully understand them.
Key distinction between straight-crested (2-D) and sinuous-crested (3-D) ripple forms (Fig. 4.20).
Here are a few examples shown in Field Photos