Lanie Rea, Kerri Zemko, Gabi Dziekiewicz, Craig Taylor
Natural Systems, Fall 1998
Dr. Hays Cummins
Our problem was to estimate the rate of erosion of the cut banks at the Pfeffer Park creek. We devised a sampling design that accurately measured the erosion of four particular cut banks. This data served as a basis that represented the erosion of the creek as a whole.
We had to consider many things: the total amount of rainfall in this season and in the area's history, the future rainfall (average annual), and finally estimate how it all affects the erosion of the banks. Our hypothesis predicted measurable differences in the rate of erosion after rainfalls (rainfall being a major factor in erosion). The erosion of the creek is particularly important because of the houses and buildings that reside near by. Through gathering and interpreting the data, we gained a better understanding of how the environment is shaped around us. This knowledge lead to a more in-depth understanding of how erosion has transformed a variety of landscapes, including such famous places as the Grand Canyon. In their article "Erosion and Suspended Sediment Transfer in River Catchments," Woodward and Foster cite, "Soil losses from water erosion, and the host of related problems associated with elevated suspended sediment…have intensified in recent decades, especially in those tropical and Mediterranean mountain environments where population growth and agricultural expansion have placed great strains on available land and water resources (Earth Summit, 1992; Prinz et al., 1994).
There are many reasons that our study of erosion is relevant. The study is of great importance after the August 1998 flooding of the Oxford area. With an estimate of the rate of erosion of Pfeffer creek we can predict when the city would have to intervene - supply sediment (dump sand or rock) or widen the creek to avoid unwanted consequences of nature. If an increase in annual rainfall is predicted, the necessary economic concerns can be met; the city can prepare for the effects of erosion. Our research is pertinent to the resource management of the Oxford community.
Another reason our study is important is because it gives a local example of erosion that occurs in every part of the world. It gives us a hands-on experience dealing with erosion, but also gives us an understanding of the process that relates to different forms of erosion in many parts of the planet.
Much of our literary research focuses on ways to prevent erosion in creeks, rivers, and oceans. Our physical research deals (directly) with the erosion of a sandy riverbed, an area in which there are few root structures to hold the soil in place. "Water erosion is currently a major threat to soil fertility and agricultural productivity in many parts of the world" (Woodward and Foster 356). After a farmer plows a field, the environment he creates is similar to the loose, sandy riverbed, and is therefore more susceptible to erosion. The same vulnerability applies to many riverbeds, such as the Mississippi and Ohio rivers, as well as areas where a river empties into the sea. Such areas are often important centers of commerce and their condition affects many people.
Flooding is another issue of substantial concern. By understanding processes of erosion, scientists can predict floods and implement the necessary precautions to protect thousands of people and businesses. These subjects, while they are not identical to our study, are still extremely important and relevant.
Materials and methods:
At four places in the creek (A, B, C, D) where there are visible turns and where the stream has definite cut banks, we set up rows of nails. We placed 5 nails in each of the 5 rows (a, b, c, d, e), in each section, and spray painted the ends of the nails. We measured the unpainted space (erosion) after every rainfall. We believe our method is statistically sound because the design provides an accurate representation of the erosion at different locations and heights of the creek’s cut banks.
In addition, we measured sediment content of the water at several random points in the creek after each rainfall, by taking water samples of equal volume and straining them through coffee filters, then measuring the mass of the filters and contents. Our first measurement was taken after a long dry spell, when very little disturbance of sediment had occurred. Therefore the first measurement acts as a control sampling and allows us to see the significance of later measurements, or the rainfall’s effect on sediment levels. After straining each sample we weighed them to determine the amount of sediment the water flow transported to new locations. As Woodward and Foster explain, "Accelerated soil and bedrock erosion on hillslopes commonly results in elevated suspended sediment concentrations and loads in rivers" (353). The rainfall causes more material to be washed into the stream, especially in the vicinity of the bluffs.
We also measured the velocity of the water to determine if there was a directly proportional relationship between the amount of rainfall, the amount of sediment within the water, and the rate of flow. We were careful to note any areas where sediment seemed to be building up (due to stream deposits) instead of eroding. "The imbalance between rates of soil development and potential rates of deterioration renders the soil a very fragile and precious resource (Kirkby, 1980)" (Woodward and Foster 356). In some parts of the creek, we found that soil at the cut banks decreased, while at some places it increased.
Time-line for project:
Phase One (Mid September)
We began formulating ideas for creating a student generated lab concerning Pfeffer Park creek. We created a sound sampling design to figure out the most efficient way of measuring the rate of erosion.
Phase Two (Late September)
We set up sampling design of 100 nails in the cutbanks at the creek. The nails were split up into four sections, each at a different location of the creek.
Phase Three (Late September--Late November)
We collected a total of 18 water samples on two different dates; once during dry weather, and once following rain.
We took a total of 400 measurements of the erosion on the nails on four separate dates following rain.
Phase Four (November--December)
We compiled our data and research to make graphs. We concentrated on different relationships of the data pertaining to the rows, columns, and stations.
Water: We acquired local rainfall statistics from John Klink. Using this data we calculated the amount of sediment that could have been displaced by rain over the time period sampled.
We started by gathering nine measurements of sediment after a heavy rainfall. The average of these measurements is 0.0067 grams of sediment per liter of water. The rainfall on that day was 0.375 centimeters. In order to set up the ratio, we summed the entire sixty-two day rainfall amount. From fifteen days of rain, the total was 11.275 centimeters.
Starting with the statistics from the day of heavy rainfall, and the rainfall sum for the period, we made conversions and predictions as follows.
One liter is equal to 0.00100003 cubic meters. We set up a ratio, with this value in the denominator on the left. In the numerator was 0.0067 grams per liter. We set this equal to X over one meter. Finding X would tell us how many grams per cubic meter of sediment were in the nine-meter length of creek sampled. X = 6.69 grams per cubic meter.
To find the volume of the nine-meter stretch in question, we had earlier observed and measured the creek when it was nearly dry. We saw only three or four shallow puddles, proving that the creek bed was fairly flat. Therefore, after it rained, we could be reasonably sure that our measurements would be accurate. We measured nine random spots for depth, the average of which was .35 meters. (.35 x .78 x 9 m = 2.457 cubic meters.) After 0.375 centimeters of rain, the volume of the section is 2.457 cubic meters, and, 16.44 grams of sediment are carried through the section (in one day).
After 11.275 centimeters of rain, which was the total for the sixty-two day period, the total sediment was 494.19 grams per 73.87 cubic meters. We know this based on the average of 6.69 grams per cubic meter of sediment per 0.375 centimeters of rain.
We know that 0.375 centimeters of rain corresponds to 2.457 cubic meters of water in the creek. We can use this in a ratio to find the volume for the 62-day period. The total volume for the period was 73.87 cubic meters of water.
For 1996 the total local rainfall was 134cm. We used this number, taken from a report by the National Atmospheric Deposition Program, to calculate our annual predictions. (We are assuming that the average rainfall for 1998 will be similar to that of 1996.) Hypothetically then, the total sediment estimation for 1998 is 5873.62 grams of sediment moving through the nine-meter stretch. In one mile of the creek, 1050555.67 grams of sediment move through each year. This amount of sediment is equal to 1.05056 metric TONS. Based on this knowledge, the necessary precautions could be taken to account for the sediment loss.
We observed significant erosion in sections A and B. However, C and D did not erode at all. They displayed sediment deposit instead, which we could not measure, so we threw out the data for C and D. Between sections A and B, there was not a significant difference in the overall amounts of erosion (Fig. 1). Comparing each row with A and B combined, there is a significant difference between rows (Fig. 2). Within station A, there is a significant difference between rows (Fig. 3). Within station B, there is not (Fig 4). Looking only at row a, there is a significant difference between A and B (Fig 5). Looking only at row b, there is a significant difference between A and B (Fig 6). For row c, there is no significant difference between A and B (Fig 7). For row d, there is no significant difference (Fig 8).
We are hesitant to draw any conclusions about the erosion at different heights (rows) or sections of the banks. Although there was an overall decrease of sediment for many test points in A and B, we observed a curious phenomenon. At a given nail, sediment would erode for a time, then build up, and even reverse again. This reversal of trends is visible in some of the time charts (Figures 10- 17).
Rainfall is a large factor in the addition of sediment to a body of water; however, rainfall also determines the volume and velocity of the body of water. In the case of the creek at Pfeffer Park, rainfall strips sediment from the steep slopes (and "the bluffs") alongside the creek, deposits this sediment in the creek, and carries it along to be deposited somewhere else. We have witnessed sediment both eroding and building up on the cutbanks. We have measured the erosion, but we did not account for the buildup when we designed our sampling method, so we could not measure the buildup.
Our rainfall and erosion data shows that our hypothesis was correct: rainfall does directly affect erosion rates. Additionally, we discovered that rainfall drastically affects the amount of sediment transported. (It puts sediment in the water, and by increasing the volume and velocity of the water, it helps move the sediment elsewhere.)
Some last-minute thoughts: If you read this whole thing, you deserve a good laugh.
So what does all this mean? Give us a minute, we have to figure that out...Okay, it be like dis. Ya gots da creek. Ya gots some rain and some stuff dat be floatin in da creek. An affer a lotta time, some a da stuff be buildin up at da sides a da creek. Dat be where we had dose Stations, See n Dee. All dat stuff builded up dere. But sometime, like how it be at Stations Aye n Bee, ya gits ta where da stuff be comin off da sides an a-goin in da creek. Dat be cuz a da rain, an dat be corrosion. Wedder it be buildin or goin away at a certain spot, well dat depend on da shape a da creek and da land around da creek. But we didn't done study dat. We done study just da corrosion. An we figgered out dat at da spots where it be corrodin, how much it be corrodin be dependin on how much it be rainin.
Yes, siree, an we even gots some numbers n grafs n such dat kin show ya Dose numbers, dey tell ya dat it be takin a long time 'fore a lot a dat creek be movin over through da park er whatnot,, but it be happenin. In fact, at dat Section Aye if da creek keep corrodin da same way it be doin now, it gonna corrode 22 ticks on da measurin pole in a year! Hell, in a few year, dat creek could be way da Sam Hill over n da nex county!
Waaaaail...uh...Momma's done figgerin to move da tomaters. We tol' her 22 ticks oughta git her a good nuther year. Paw, though, he so impressed wit dis here research he bout to move da whole family.
Klink, John. Rainfall data. Miami University. Sep-Nov, 1998.
National Atmospheric Deposition Program/National Trends Network. Sep 1997. http://nadp.sws.uiuc.edu/default.html
Woodward, James and Foster, Ian. Erosion and Suspended Sediment Transfer in River Catchments. Geology vol. 82, 1997: 353-376.
Return to the Topic Menu
IMPORTANT: For each Response, make sure the title of the response is different than previous titles shown above!