HOW FAST DO SEDIMENT
GRAINS OF DIFFERENT SIZES SETTLE?
Molly F. Miller
Department of Geology
Nashville, TN 37235
Level: Junior high to senior high
Anticipated Learning Outcomes
- Students will define rate, turbidity, and settling rate
and explain the relation between settling rate and turbidity.
- Students will make semi-quantitative observations of
turbidity and make a graph showing change in turbidity against time.
- Students will explain how and why settling rate varies
with grain size.
- Students will interpret change in turbidity with time
as a function of settling rates of sediment of different grain sizes.
- Students will predict how the clarity of water in a stream
or in the ocean will differ during periods of flood vs. low water, or during
a storm vs. between storms, and explain the basis for this prediction.
Materials and Procedure
See student handout sheet. Results will vary depending
on sediment type used. Any relatively fine-grained, poorly sorted sand will
yield satisfactory results.
- The settling rate of a grain depends on its size, shape
and density as well as on the viscosity of the fluid through which it is
settling. Some generalities include:
- Smaller grains have a larger surface area to mass ratio
and thus their settling rates are slowed more by frictional drag than are
- Given two grains of equal size, the denser will settle
at a faster rate.
- Given two grains of the same mass, the flatter one will
settle at a slower rate because the flat shape will give a large surface
area on which frictional drag will act.
- Turbidity is cloudiness caused by sediment in suspension.
Sediment may be put into suspension by disturbance such as caused by waves
or currents, or, in this case by shaking. After quiet water conditions
are re-established, the sediment grains will settle, but not all at the
same rate. In general, large grains settle faster than smaller. This means
that the record of one event such as a storm will be a single graded bed
characterized by largest grains at the bottom (settled fastest) and smallest
on top (settled slowest).
- Turbidity can be measured by lowering an object into
the water and noting at what depth it disappears from sight as viewed from
the top. The more turbid the water, the shallower the depth at which it
- Turbidity in a stream generally increases with velocity.
As the rate of flow increases both the size of particles in suspension
increases and the amount of material in suspension increases. Stream velocity
is greatest during periods of high discharge which occur during floods.
Results and Discussion of Results
- Results of Tennessee River sand are shown below.
- Depth at which paper clip goes out of sight increases
with time. This is because fewer grains are in suspension as time goes
on, and more have settled to bottom. Those still in suspension are lower.
- The rate of settling is not constant. This is because
large grains settle very fast, resulting in rapid initial removal of the
grains from the water column, and rapid decrease in turbidity.
- Rate of change in turbidity becomes quite low. This is
because the small grains remaining in suspension have large surface areas
and settle slowly.
may not be be familiar with interpreting graphs showing rates. An exercise
on graphing and interpreting rates follows the student handout on the settling
rates experiment. This exercise can be done either before the settling experiment
or between observations while the experiment is in progress. Students might
also benefit from tabulating the data before constructing the graph.
Sediment Settling Worksheet
- Clear glass jar preferably 10 cm high, filled 1/3 full
with sediment and filled to the brim with water
- Paper clip bent as illustrated below
- Metric plastic ruler
Procedure: (see diagram above)
- With lid on tightly, shake jar vigorously. Let the jar
sit for 5 minutes. Note how much the water has cleared. Predict how long
it would take for the sediment to settle completely. Shake the jar again
- Quickly set jar down and open lid.
- Make observations as follows (two people are needed):
- One person lowers paper clip into water to level where
it disappears from view as observed from the top and holds it there. The
paper clip must be lowered next to the edge of the jar so it can be seen
through the glass.
- The other person measures the depth beneath the water
surface at which the paper clip disappears from view as observed from the
- Record the observation on a table with two columns on
it - one labeled "time after shaking", the other labeled "depth
of disappearance". Plot the point on the depth vs. time graph.
- Repeat every 5 minutes for the time your teacher tells
you; the optimal time of observation depends on the size distribution of
the sediment used.
- Explain what is meant by "rate", "turbidity",
and "rate of settling".
- Summarize your observation of how the rate of change
in turbidity changes with time after shaking.
- Explain the observed trend in rate of change in turbidity
as a result of different settling rates.
- Explain the relation of grain size to settling rate.
Then explain the change in turbidity as a result of settling of different
- How and why does turbidity of a stream change with water
- How long would it take for the paper clip to be visible
on the surface of the sediment?
- Does this time approach your predicted time?
Rate of Scoring Worksheet
Data from the sediment settling experiment will be plotted on a graph which
will show how the turbidity changed with time. From this graph you will
be able to determine the rate of change in turbidity: was it constant, or
did the turbidity change faster at the beginning of the experiment or at
the end of the experiment? Interpreting changes in rates from graphs takes
some experience as you will gain from this exercise.
The 3 tables on the attached data and graph sheet show a basketball team's
score through 3 different games (Games A, B, C). In the score column is
listed the team's score after so many minutes of play (listed in the column
"minutes") since the start of the game. For example, in Game A,
10 minutes after the game had started, the team had scored 10 points. By
the end of the game (40 minutes) they had scored 24 points. The scoring
rate is the number of points per unit of time. For example, in the first
10 minutes the scoring rate was 1 point per minute. (They scored 10 points
in 10 minutes.) In the last 26 minutes of the game (from 14 to 40 minutes
after the start) the team scored 9 points, increasing the score from 15
to 24. The scoring rate was 9 points per 26 minutes or 0.35 points per minute.
Plot the data from game A on the graph paper and connect the points to make
a line graph. Label the line "A". Repeat for games "B"
and "C". (Graph paper is on attached data and graph sheet).
Fill in the blank or circle the correct answer.
- In which game did the team score a lot of points early
in the game and few later?_______ In this game, the scoring rate was (high,
low) in the beginning of the game and (high, low) toward the end of the
game. The line on the graph at the beginning is (steep, flat); the line
representing the rate at the end of the game is (steep, flat).
- In which game did the team have a bad start, scoring
little at first, but coming on strong toward the end of the game?_______
In this game the scoring rate was (high, low) toward the beginning and
(high, low) toward the end of the game. Describe how the slope of the line
(that means its steepness or flatness) shows this change in rate.
- In which game did the team play consistently, scoring
evenly throughout the game? _______ Did the scoring rate change through
the game?_____ Does the line graph change its slope from steep to flat,
or flat to steep in this game? _________
- Compare the graphs of scoring in Games A,B, and C with
the graph showing turbidity vs. minutes after shaking from the sediment
experiment. Which game's scoring pattern does the experimental data most
closely resemble? ________ Explain what this means about the change in
turbidity and rate of settling of particles.
| Game A
|| Game B
|| Game C|