Homework #8 2014, Earth Surface Processes, Humphrey
These questions are designed to
make you think about the meanings of some of the stuff we have done in
class. The calculations are not difficult,
but some of the questions require some understanding.
1. Reproduce the argument for Stoke’s Law for settling velocity, and therefore reproduce
the Stoke’s equation for low Reynolds number
particles (minus the value of the coefficient).
You might find it useful to follow the following line of logic:
a. Gravity force on particle
b. Find a scale for the viscous shear
stresses (hint, we used our work on debris flows to find the shear stress)
c. Total drag force on particle due to
surface shear stress
d. Balance gravity and viscous drag to get
Stoke’s Law (with an incalculable constant)
You
need to do more than just write the equations down; you need to label in words the important terms.
2 The
most common criteria for initiation of sediment motion (or loosely: sediment
transport competence) is the Shields plot. This uses the Shields criterion or
parameter: Q=
tb
/ ((
rsed
-rfluid
) g Dsed).
The Shields parameter predicts when the basal shear stress is just
enough that sediment of a particular size will be transported. There is a set of plots in the readings that
describe the behavior of the parameter. However if we stand back a bit we see
that the parameter is approx. 0.05 over a wide range of flows (Reynolds
numbers) typical in rivers and for a range of sediments also typical of rivers.
a) Use the
Shields criteria to estimate the maximum size of sediment in motion in the
Laramie river during a bankfull
flood. [Just to introduce some consistency, assume the depth of the Laramie river in minor flood is 1.5m and check the slope using
Google Earth or some other tool.] If you
can, comment on why the sediment size so calculated is probably somewhat too
large (hint: what is the real slope of the Laramie River)?
b) (a mini geomorph puzzle,
asking you to read a little on the web or in the library) Look at the Shield’s
plot (Google, or your text), the deviations from a flat line at high Reynolds
number are caused by changes in the type of boundary layers around a
particle. Why, at different Reynolds numbers,
can a boundary layer either increase or decrease drag, if the layer is laminar
or turbulent. If you don’t like that
vague a question, you can answer this instead: Why do golf balls have dimples?
3 In
class we discussed that the ratio of settling velocity (w) to shear velocity (v*) as a
discriminator of types of sediment transport.
Sediment with a ratio of less than about 0.2 is predicted to move in
suspension.
a)
Calculate the shear velocity (v*) for the Laramie river.
b)
Calculate the maximum size of particles that you would expect to find in
suspension in the Laramie River.
Fortunately you can get away with using the Stoke’s
fall velocity for the settling velocity since the particles will be small
enough to have a fairly low particle Reynolds number.
c) We also talked about the ratio of tb to tc as a discriminator for
types of sediment transport. Are these 2
ratios basically saying the same thing?
To be precise, the questions is: what is the difference between the 2
ratios for high Reynolds number particles?
(hint, try writing out the full equations for
the 2 ratios)
4 What is the order of
magnitude of the lift force on a 1cm diameter piece of gravel on a river bed,
with a 0.5m/s water flow over it and stagnant water under it. Use the Bernoulli equation we developed in
class, (it is also in the equation list on the web page). Is the force enough to lift the
particle? (to
get a scale for the lift force from the Bernoulli effect, you can assume the
water velocity difference is the full 0.5m/s) [hint:
the Bernoulli equation gives you a way to calculate the water pressure; that
water pressure can be turned into a force by multiplying by the cross-sectional
area of the particle]
5. (this weeks
geomorphic puzzle) Many lakes in cold regions of the world turn-over.
The phrase describes a phenomena that occurs once (or twice) a year; the water
in the lake moves so that in a short time the water at the top of the lake
sinks and flows under the water at the lake bottom, while the water that was at
the bottom circulates onto the top of the lake. This process is hugely
important for nutrient cycling. Why does it occur, where does the head
gradient come from? If you can, explain why the event is often sudden,
not gradual, and why it doesn’t occur in warmer regions. And as a bonus,
why doesn’t the ocean do the same thing. (hint:
think of temperature and density gradients)