Velocity and Discharge of Soos Creek and the Green River

Transcription

1 IDS 101 Name Velocity and Discharge of Soos Creek and the Green River We will visit two streams on our field trip: Soos Creek (a small stream) and the Green River (a larger-sized stream). Our purpose will be to apply the information we have learned about rates and motion to the measurement of streams. The first stream characteristic we will examine is the velocity of the stream. Why study the velocity? The velocity and the turbulence (uneven flow) of the stream determine the size of sediment that can be transported by the stream. If a stream changes velocity, it ability to carry sediment will change. In some cases this means that the stream will change its character, such as going from one channel to many separate channels. This can create flooding and erosion of property. From our lab work, we know that to determine the velocity of a stream we must know the distance an object moves in a specific unit of time. To measure the surface velocity of the stream we will use marshmallows. We will establish a set distance along the bank of the stream. For each marshmallow we will determine the time required for the marshmallow to travel the established distance. Please answer the next two questions before the field trip--- Does it matter what distance we measure along the banks of the stream? Explain your ideas. How many marshmallows shall we measure? What determines whether we have lots of measurements or just a few? When we arrive at Soos Creek: Estimate the velocity of the stream in feet per second. Guesses are free today! Once we have determine the distance the marshmallows will travel and established some ground rules (such as, if a fish eats the marshmallow or it gets stuck in the vegetation along the banks, we will throw out that measurement) we will measure the surface velocity of the stream.

2 Enter your data in the space below. Be sure to include appropriate units! Compare your estimate of the velocity to what we measured. ***Another prediction if we could measure the velocity of the water below the stream surface, how will it change? Bob will wade into the stream if the water level is not too high and measure the velocity of the water at different depths. Record the data in the space below. stream surface strreambed

3 Clearly the velocity of the stream is lower as we go from the surface of the stream to the bottom. What might cause this difference in velocity? If we say that the velocity of Soos Creek is 3 feet per second and a river such as the Columbia is also 3 feet per second, it is apparent that velocity is not the only characteristic of the stream we want to measure. The major difference between Soos Creek and the Columbia River is their discharge. In a simple sense, the discharge is the flow of the water through a channel. The same type of concept applies to flow through a pipe. The easiest way to think of discharge is that it is the cross sectional area of the stream times the velocity of the stream. What velocity do we use? Is it the surface velocity or the average velocity? By convention, we use the average velocity in the profile view. If we measured the velocity profiles of many streams we would find that the average velocity is at 0.6 D where D is the depth of the stream. So, this means that if we measure the velocity at 6 tenths of the way from the surface to the stream bed, we will have the average velocity. We will divide the Soos Creek channel into units (or verticals) each two feet wide. We will measure the depth of the stream and we will determine the average velocity of the stream at 0.6D. If we multiply these three values (cell width, cell depth, and cell average velocity) we will get the discharge for that individual cell. To determine the total discharge for the stream, would we sum these cell discharges or average them? Explain. Imagine that the diagram below is a cross-section through a stream channel. Illustrate the process of collecting discharge data. water Stream channel Record the discharge data we collect in the table provided on the next page.

4 Measurements started from: Left bank facing downstream Right Bank facing downstream All velocities taken at 0.6 of the cell depth unless otherwise noted. Distance on tape Cell width (feet) Cell depth (feet) Velocity (ft/sec) Discharge for the cell (cfs) Distance on tape Cell width (feet) Cell depth (feet) Velocity (ft/sec) Discharge for the cell (cfs)

5 After our field trip we recommend visiting a web site called the Virtual River. We think that reviewing part of the web site will be sufficient, however if you want to go through the whole site, go to the second URL below: Recommended part of the Virtual River Web Site: To go through the entire Virtual River Web Site: Some recommendations to help you with this web site: If possible use a fast connection to the Internet, such as the computers in SMT 233 or in the library. Sometimes you may have problems if you try to access this site from a dial-up modem. We suggest using a hand calculator. There is one on the web site, but it faster to use your own. If you attempt to backup on the web site, you will lose the data for that channel. You will get a different channel with different conditions. If you have difficulty, please see Keith or Bob.

6 After you complete the web site, answer the following questions: The diagram below shows the cross section (profile) of a stream. The diagram is drawn to scale with one inch representing four feet. To get you started, the stream has been divided into four vertical cells. The average velocity of the water in each cell is shown in the table below the diagram. West Bank Surface of Stream East Bank Cell A Cell B Cell C Cell D 4 feet 4 feet Vertical Cell A B C D Average Speed 1 ft/s 3 ft/s 5 ft/s 4 ft/s Calculate the total discharge of the stream. Try to be organized and detailed and show your work! Make sure we can follow your calculations and/or reasoning. In cell C we are told that the average speed of the water is 5 ft/s. How far below the surface (in feet!) would you expect to measure the water speed and find a speed of 5 ft/s in cell C? Explain your answer. What are the units of velocity and discharge of a stream? How are velocity and discharge different?

7 Use the data we gathered while at Soos Creek to determine the discharge for Soos Creek. Show and example calculation below (just a number will not be sufficient). Go to the US Geological Survey web site below, and find the discharge of Soos Creek at the time we were visiting the stream. From this web site, the USGS prediction of the discharge for the time of day when we were at Soos Creek is. (remember to put the units down!)

8 Next we will go to the Green River. Again, estimate the velocity and the discharge of this stream before we measure the velocity! Record the data we collect for the Green River: Data for the Green River: (you must have the units) Distance Time Velocity In the map below, use an arrow to represent the velocity of the Green River in the various places we collect data. The higher the velocity, the longer the arrow and the lower the velocity, the shorter the arrow (we am looking for the relative patterns of velocity).

9 Is there a difference in the velocity of the marshmallows at Soos Creek and at the Green River? Which one is faster? From your observations about the two streams what are some possible reasons for the differences in velocity of the marshmallows between the two streams?