Gaging.html

Stream Gaging

In order to understand how streams function, geologists have to be able to measure the characteristics of streams

How deep a stream is
How fast it is flowing
How much water is flowing in it.

This field trip will allow students to measure a local stream. This trip is accompanied by an exercise in interpreting stream data from the USGS.

On my field trip I was instructed by Dr. Tim Diehl of the USGS and helped by Judy Butler and Alex Rouse.

In looking for a stream, we wanted one which would be:

the proper size (large enough to be interesting, small enough to be safe)
accessible
clean enough for students to wade in
the location of one of the USGS stream gaging stations (so that we will have official data for comparison).

We selected the Harpeth River where it flows under Highway 100 in Bellevue.

Directions:

Drive west on Hwy. 100 (West End Avenue), to the intersection with Old Hickory Blvd. east Hwy. 245. (near Warner Park Nature Center)
Continue west on Hwy. 100 for 1.5 miles to the metal frame bridge over the Harpeth.
Just beyond the bridge, turn left and drive down to the parking lot beside the river.

When to take this field trip: In dry weather, the Harpeth River is ideal for gaging. In wet weather, however, it can become a raging torrent, completely unsafe to wade in.

Because middle Tennessee has a humid sub-tropical climate which is wet in the winter and dry in the summer, the river tends to rise and fall with the seasons. The USGS's chart of stream flow for the year 2001 shows this. Therefore, in general, the best time for gaging the Harpeth is summer or early fall.

Before you go, check the real time USGS data and go only if the depth of the river is not much more than 1 foot.

Stream Gaging Safety Rules
USGS safety rules require anyone working in a stream, beside a stream or above a stream to wear a life jacket.

Stream gaging involves three steps:

Measuring the width of the river
Measuring the depth of the river at regular intervals
Measure the velocity of the water at regular intervals.

Equipment for gaging a stream:

a rope at least 100 feet long (150 is better) marked of it 5 foot intervals
several large spikes (or 2'-4' lengths of rebar)
a heavy hammer
100-150 feet of cord (optional)
flagging tape (or plastic grocery bags)
a yard stick (or even better a good 5' ruler)
a measuring tape (at least 20' long)
several oranges

Finding an ideal site: When we arrived, Tim set off to find an ideal site. An ideal site minimizes water turbulence which can make measuring velocity difficult.

The stream should be flowing straight
The stream should be narrowing
There should be no major obstructions.

Tim found a good site about 50 yards downstream from the bridge. It is marked with flagging tape tied to a tree on the far side of the stream.

Before you start taking measurements, put one of your spikes in at the water's edge. You need to do this because the level of the Harpeth River can change suddenly due to release of water from the sewage treatment plant. If that happened while you were measuring, it would invalidate your measures.

Task 1: Measuring the width of the stream.
Take the high numbered end of the rope and tie it to a tree on the far side of the stream. One person should check that the stream is flowing at right angles to the rope while another person fastens the other end into the beach with one of the spikes.

Write down the measurement of the two edges of the stream. On the day we measured it, the near side of the stream was at 21' and the far side at 87', making it 66' wide.

Note that a small increase in the depth of the stream would make it very much wider.

Task 2: Measuring the depth of the stream.
Now take the yardstick and, at each 5' marker measure the depth of the stream. Call out the readings to your recorder on the shore. Now you have a profile of the stream.

We took our readings in inches, but because all our other readings are in feet, we will convert the readings to feet.

Depth 25' 30' 35' 40' 45' 50' 55' 60' 65' 70' 75' 80' 85'

inches
4

9

12

14

13

11

10

10

11

10

10

10

9

feet
.33

.75

1.00

1.17

1.08

.92

.83

.83

.92

.83

.83

.83

.75

Task 3: Measuring the velocity of the stream.
Take the measuring tape to one side of the stream and measure down 20'. Mark this point on the shore with flagging tape or a spike. 20' is a good distance on the Harpeth. The river runs slowly enough that it's easy to get an accurate measure.

Do the same on the other side. You can have your timer line these two points up from the shore or you can connect them with a string. Using a string makes measuring easier, and you can check the accuracy of your 20' a several points across the stream.

Set up a three person team. The first stands at the original line. Standing to one side, so that he/she will not interfere with the water flow, he/she releases the orange, shouting "mark' to let the timer start the stopwatch.

The second person stands at the finish line and shouts 'mark' when the orange crosses the finish line.

The third person holds the stopwatch and records the time.

Take three readings at each of the 5' marks.

A practical problem
In our first attempt at the 25' marker (in 4 inches of water), our orange got stuck on the bottom. Tim suggested that we take our reading using orange peel. This option worked for us because there wasn't much wind which might have blown the peel around, affecting the velocity reading.

Now you have all the data you need.

Before you leave, check your spike at the edge of the river to make sure the level hasn't changed. and be sure to leave the site as clean as you found it (if not cleaner).

Calculating Discharge

Looking at our findings. At each 5 foot mark, we took 3 readings (# of seconds to go 20 feet):

# Feet
25'

30'

35'

40'

45'

50'

55'

60'

65'

70'

75'

80'

85'

1st
62.62

25.32

25.51

24.94

20.62

19.23

19.79

20.22

24.00

37.77

47.46

56.59

53.27

2nd
58.72

32.16

28.95

20.10

26.11

17.03

14.27

21.74

33.54

38.39

45.55

68.47

59.72

3rd
54.78

38.01

25.19

22.49

24.63

19.46

16.55

24.19

28.70

36.88

46.41

55.09

68.37

We have a problem. We have three readings. How do we arrive at a single time. There are two ways of doing it:

Use the mean (average). Add the three readings and divide by three
Use the median. Pick the middle reading.

Tim Diehl recommends using the median. Occasionally your orange might get caught in an eddy which would lead to a reading which is way out of whack (called an outlier). You wouldn't want to average a reading like that into your final calculation.

Once we have picked our time, we need to calculate the velocity. Velocity is feet per second. Our orange floated 20 feet, so our velocity is 20/# seconds. Let's do it for the readings at 25 feet.

# of seconds = 58.72 (middle reading)
Velocity = 20/58.72 = 0.3 feet/second

# Feet

25'

30'

35'

40'

45'

50'

55'

60'

65'

70'

75'

80'

85'

Median

58.72

32.16

25.51

22.49

24.63

19.46

16.55

21.74

28.70

37.77

46.41

56.59

59.72

Velocity

0.3

0.6

0.8

0.9

0.8

1.0

1.2

0.9

0.7

0.5

0.4

0.3

Notice that the water moves faster in the center of the stream than on its edges. Why do you think that this is so?

While taking readings, Alex noticed that the bubbles on the surface of the water traveled faster than the orange, which was floating deeper in the water. Why do you think that this is so? Would the velocity be even slower, if we measured deeper in the stream?

We now have all of the measurements we need, but we still have a problem. The depth of the stream varies as we cross it, and so the the velocity.

We calculate the discharge by dividing the stream up into sections centered around our measurement points. The sections are five feet wide. For each sections we assume that

the depth is the depth at our center point
the velocity is the velocity at the center point

# Feet

25'

30'

35'

40'

45'

50'

55'

60'

65'

70'

75'

80'

85'

Width

4.5

Discharge

.57

2.33

3.92

5.19

4.40

4.77

5.04

3.8

3.2

2.2

1.8

1.5

1.1

Our total discharge is the sum of all of our totals = 39.8 feet^3/second

How does this compare to the official discharge. The USGS stream gaging station continually measures the Harpeth River that this bridge.

How the USGS Stream Gaging Station Works
The official readings are made this way. Every now and then, geologists measure discharge much as we just did. They measure discharge on different days, when the stage of the water differs. They then plot their findings on a graph called a rating curve. This curve shows the stage of the river as a function of discharge. For an example of a rating curve follow this link.
Once this graph is established for this point, all the USGS needs to do is to measure the stage (which can be done with the automated machinery on the bridge) and then calculate the discharge from the curve.

On June 22, from 9-10 am, the USGS measured the river level at 1.1 feet and the discharge at 24.0 cubic feet/second. USGS Data

Why did our reading come out so much higher?

Tim says that there are two reasons.

We measured velocity just below the surface. When USGS geologists are establishing their rating curve, they measure velocity slightly more than half way to the bottom (60% down). Water closer to be bottom moves slower than water near the surface so our velocities will be higher than theirs. Therefore, our discharge will be higher than that of the USGS. Tim says that if we had measured at 60% toward the bottom, our discharge reading would have been 80% of what we got. (39.8 * .8 = 31.8) This is still quite a lot higher than the official reading.

Tim thinks that the remaining difference may be caused by the fact that the USGS calculation may be wrong. Rivers are constantly changing, and therefore rating curves get out of data, and need to be recalibrated.

A Harpeth River Paradox

If people know anything about geology, they know the following two laws:

Sedimentary strata are deposited so that the oldest strata are on the bottom, and the youngest are on the top.
Rivers flow down hill.

Therefore, in a sedimentary landscape, you would expect that a river would flow from the youngest stratum toward the oldest.

Now look at a map of the Harpeth River.

The Harpeth River's sources are in the Stones River Group in South East Williamson County. The Stones River Group is the oldest stratum exposed in middle Tennessee. It then runs northwest, through the Nashville Group in northern Williamson County and across the southwest corner of Davidson County, through the Silurian and Devonian strata of southern Cheatham County, and finally through the Fort Payne Formation in which it flows into the Cumberland River.

The river flows from the oldest stratum exposed in middle Tennessee to the youngest.

Is this innocent looking river defying the law of gravity by flowing uphill?

Can you resolve this paradox?

Try to answer it yourself before you look at the solution.