exercise 6 streams and flooding - central texas...

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Exercise 6 Streams and Flooding James S. Reichard

Georgia Southern University Student Name _________________ Section _______ In this lab you will:

learn to identify drainage divides and floodplains on a map in order to access the potential for flooding at a given location. You will also examine the historical discharge record of a stream and learn how to calculate the recurrence interval of flood events.

Background Reading and Needed Supplies

Prior to doing this exercise you should read Chapter 6 in the textbook. With respect to supplies, you will need a calculator, ruler, and colored pencils. Part I – Drainage Basins and Divides

As illustrated in Figure 6.1, a drainage basin is the land area that collects water for an individual stream or river. A drainage divide, shown in red, is an imaginary line that follows the crests in the landscape where surface water is forced to flow into different drainage basins. A large rain event, therefore, on one side of drainage divide may cause a flood in one basin and not another. Being able to locate drainage basins and divides on a map is important when evaluating flood hazards.

Figure 6.1

Ex 6 – Streams and Flooding


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1) Figure 6.2 is a digital relief map showing the topography of the area around the Big Thompson River in Colorado. This river drains the eastern portion of Rocky Mountain National Park (note that north is to the right).

a) Using a blue-colored pencil or marker, carefully outline the trace of the Big Thompson

River and its major tributaries. Note that part of the river has already been traced. b) Now that you have identified the stream channels, use a red-colored pencil and outline the

drainage divide that encompasses the Big Thompson drainage basin. As before, part of the divide has already been traced. Simply continue tracing the crests in the topography that separate those tributaries following into the Big Thompson from those that do not.

2) Based on the terrain and drainage network you outlined in Figure 6.2, what type of flooding would you expect to occur along the Big Thompson River, flash flooding or downstream flooding? Explain how you can tell.

3) On July 31, 1976, a large thunderstorm moved in from the east and pushed up against the

tall mountains making up Rocky Mountain National Park. The storm remained stationary for over three hours, dumping as much as a foot of rain on the bedrock surfaces of the Big Thompson drainage basin. The normally tranquil river turned into a raging torrent, cresting 20 to 30 feet (6 to 9 m) above normal and reached speeds of up to 50 miles per hour (80 km/hr). In a matter of two hours, the flood killed 145 residents and vacationers, and destroyed 418 homes and 152 businesses. a) If you had been living in the Big Thompson Canyon at the time of the 1976 flood, what was

the one thing you could have done that would have increased your chance of survival? b) Although in terms of human time the 1976 Big Thompson flood was a rare event, such

floods are common over geologic time. Since these floods may occur at any time, what steps could be taken that would help minimize the loss of life and property along the river?

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Figure 6.2. Digital relief map of the area around the Big Thompson River, located just west of Loveland Colorado. (Courtesy of NOAA)


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Part II – Floodplains Recall from the textbook that a floodplain (Figure 6.3) is the relatively flat area that lies

adjacent to a stream or river. During high discharge events, a stream will overflow its banks and inundate its floodplain. Also recall that in downstream areas where the gradient decreases and the channel approaches base level, rivers tend to meander more and produce wider floodplains. Floods here are referred to as downstream floods. In contrast to flash floods, downstream floods occur slowly and involve greater volumes of water. Of course the level to which any floodplain becomes inundated will depend on the amount of discharge. Figure 6.3

To minimize the risk of flooding, a building should be located as far above the elevation of the channel as is practical. Since placing a building on an active floodplain virtually guarantees damage from a flood, it is important to recognize the uppermost edge of a floodplain. As can be seen in Figure 6.3, the floodplain's edge is marked by an abrupt increase in slope. On a topographic map, this abrupt slope increase occurs where the contour lines become closely spaced. For example, the purple lines on the topographic map in Figure 6.4 coincide with the upper edge of the floodplain where the slope abruptly increases. Note that due to erosion, this change in slope is oftentimes more gradual and, therefore, not as obvious on a map. 4) Using a purple-colored pencil or marker, carefully trace the edge of the floodplain along the

north-south flowing stream on the eastern side of the map in Figure 6.4. Note that part of the floodplain has already been mapped for you.

5) Are there any buildings or roads within the floodplain? Explain why. 6) In Figure 6.4, part of the drainage divide on the south side of the tributary named Big Branch

has already been mapped for you. Continue tracing this drainage divide (in red) until you come to the southern most edge of the map.


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Figure 6.4 (USGS Glenville, GA, Quadrangle)


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7) The drainage divide you mapped in Figure 6.4 follows the high ground that is left behind when streams naturally etch, or cut, into the landscape. Describe the general relationship between drainage divides and human uses of the landscape that you see on the map (e.g., buildings and roads).

8) The western extension of the drainage divide you just mapped is shown in Figure 6.5. Using

a red pencil or marker, trace the other drainage divides that lead into the city of Glenville. 9) Notice how many of the divides that you mapped merge in the direction of Glenville.

a) In terms of landscape usage and flood hazard, explain why the city was originally built in an area where drainage divides converge.

b) The landscape throughout this part of Georgia's coastal plain is very similar to that of the

Glenville area. In terms of drainage divides, where do you suspect most towns and cities are located on the coastal plain? Why?

10) Find the circle labeled "Sewage Disposal" located to the southeast of Glenville. With

respect to the topography, give two reasons why the city's sewage treatment plant was located here.


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Figure 6.5 (USGS Glenville, GA, Quadrangle)



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11) The topographic maps in Figure 6.6 show some of the many housing developments that have been constructed in Stone Mountain, Georgia, which is a suburb of Atlanta.

a) Using a purple pencil or marker, continue tracing the edge of the floodplain in both maps. b) Circle those homes that are at a high risk of flooding with a black pencil or marker. c) List 3 reasons why a developer would build homes in an area with such a high potential for

flooding.

12) Suppose you are in the market to buy a new home. Based on what you learned in this

exercise, describe how you might avoid purchasing a home in an area with a high risk of flooding.

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Figure 6.6 (USGS Stone Mountain, GA, Quadrangle, 1973 photo-revised edition) A) B)


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Part III – Recurrence Intervals As illustrated in Figure 6.7, a flood occurs whenever the discharge of a river increases to the

point where water leaves the channel and begins flowing onto the floodplain. Progressively larger discharge events naturally cause floodwaters to reach greater heights within the floodplain—hydrologists refer to the height of the water as stage. Of course streams are confined to their channels the vast majority of the time. In contrast, discharge events that are large enough to cause streams to overflow their banks are relatively rare.

Figure 6.7

In this section we will explore how scientists calculate recurrence intervals, which is a

statistical measure of the amount of time it takes for a discharge event of given size to reoccur. For example, a flood whose recurrence interval is 100 years means that 100 years should pass, on average, before the same discharge level occurs again. Note that because of the statistical nature of recurrence intervals, it could take more than 100 years for such a flood to repeat, but could just as easily take less than 100 years.

To calculate recurrence intervals for a given stream, hydrologists make use of historical discharge records. Here scientists normally tabulate the highest (maximum) discharge values that occurred during each year in the record. Because the focus is on the recurrence interval of very large discharge events (i.e., floods), it is best to have discharge records cover the longest span of time as possible. In general, the longer the historical record, the greater the probability that it will contain exceptionally large flood events.

In this portion of the exercise you will use the following equation to the compute recurrence

interval (RI) for individual discharge values in a historical record:

M1NRI +

=

where N = the number of records (in years), and M = the rank of a particular discharge. For example, if you had 25 years worth of annual discharge data (i.e., N=25), then the recurrence interval for the discharge that ranks 7th (i.e., M=7) would be calculated as follows:

years71.37

125RI =+

=


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13) Table 1 lists the annual maximum discharge for the Ogeechee River, located on the coastal plain of Georgia. Examine the data and then rank the annual discharge values in Table 2 from largest to smallest (1 being the largest). Next, calculate the recurrence interval in years for each of the discharge values. Round all calculations to 2 decimal places.

Table 1 Table 2

Year

Maximum Discharg

e (ft3/sec)

Rank (M)

Maximum Discharge (ft3/sec)

Year

Recurrence Interval

(RI) (years)

1974 9,000 1 1975 16,200 2 1976 7,720 3 1977 11,500 4 1978 17,300 5 1979 18,000 6 1980 27,900 7 1981 6,600 8 1982 8,320 9 1983 10,000 10 1984 6,560 11 1985 8,600 12 1986 8,200 13 1987 16,500 14 1988 2,700 15 1989 5,030 16 1990 5,620 17 1991 37,300 18 1992 6,560 19 1993 15,900 20 1994 6,850 21 1995 20,800 22 1996 8,550 23 1997 8,510 24 1998 28,200 25

14) On the graph paper provided in Figure 6.8, plot maximum discharge versus the recurrence

intervals you determined in Table 2. Then draw a best-fit line through the data points. Note that the y-axis on the graph paper is linear, whereas the x-axis is logarithmic. You should be able to figure out the log-scale based on the axis labels. If not, ask your instructor for help.

15) Based on your best-fit line, estimate what the discharge will be for a "20-year flood" on the Ogeechee River. To do this, draw a vertical line from the x-axis to the best-fit line, then draw a horizontal line over to the y-axis.



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16) Estimate the discharge for a 100-year flood on the Ogeechee. Here you will need to project (i.e., dash) your best-line out beyond the 26 years of the data set.

17) How reliable do you think your 100-year discharge estimate is? What would you need to

make it more reliable? 18) As a general rule of thumb, streams in the eastern United States flood, on average, every

2½ years. This means that a typical stream channel is able to contain any discharge event with a recurrence interval less than 2½ years. Using this generalization, show which discharge values on your graph were likely contained within the channel and which were likely floods.

19) Another useful measure of flood frequency is probability, which is the statistical chance or

likelihood that a particular discharge will occur in any given year. Probability is simply the reciprocal of recurrence interval (RI); percent probability is calculated as follows:

100RI1yprobabilit% ×⎟⎠⎞

⎜⎝⎛=

a) Calculate the percent probability for each of recurrence intervals listed in the table below.

Recurrence Interval (RI)

(years)

Percent probability

in a given year 1 2

10 20 50

100 1000

b) Describe a situation in which recurrence intervals would be more meaning when

evaluating flood frequency, then describe a situation where percent probability would be more appropriate.

Supplementary Information on the Flood Hazard in Your Area 1) Go to the Federal Emergency Management Agency's website http://www.fema.gov/ and click

on the "Flood Maps" link listed under "Flood Information". The direct URL to the Flood Map locator is listed below: http://msc.fema.gov/webapp/wcs/stores/servlet/FemaWelcomeView?storeId=10001&catalogId=10001&langId=-1

2) Another site with useful information is: http://realtyvan.com/flood.html

3) To examine where your home or business lies in relationship to the nearest stream channel,

you can also order a topographic map from the USGS. For ordering instructions, see the last page of Exercise 2.

http://www.fema.gov/

http://msc.fema.gov/webapp/wcs/stores/servlet/FemaWelcomeView?storeId=10001&catalogId=10001&langId=-1

http://realtyvan.com/flood.html


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