Stephanie Marchbanks

GEO 306: Dr. Schaetzl

December 6, 2000

Introduction

Human population increases in an area are often associated with land development, or urbanization. The extension of impervious surfaces (e.g., roads, driveways, parking lots, and rooftops) on a watershed can result in excess runoff into streams. This activity can increase stream volume, peak flow and peak flow duration, and decrease base flow.⁸ Figure 1 displays this concept with a hydrograph, which shows the discharge reading related to water height at the cross-section of the river.

The purpose of this study was to gather data of peak discharges of a stream in a recently urbanized watershed, where the population of the area in the watershed has significantly increased over the years. The data collected were analyzed to see if there was a relationship between changes in population and increased peak discharges over time.

Study Area

The first step was to identify a watershed that had both a high urban runoff potential (greater than 4% of the land area above 25% imperviousness) and a high population increase (greater than 7%) in a ten-year interval. The Environmental Protection Agency’s "Surf Your Watershed" website, http://www.epa.gov/surf/test.html, was utilized to find data on watershed vulnerability indicators, such as risk for high population change and imperviousness. Maps of population change for 1980-1990 and urban runoff potential for 1990 were analyzed to find watersheds that had the highest factors of both criteria. (Refer to Figures 2 and 3.) Finally the Upper Chattahoochee watershed, which encompasses Atlanta, Georgia, was selected. (See Figure 4.) This watershed had a 33.47 percent change in population between 1980 and 1990, and had 5.7104 percent of its land area above 25 percent imperviousness in 1990.^6,7

An Internet search was conducted to retrieve literature about the extent of urbanization over time with respect to population increases. The Atlanta, Georgia area was an excellent example of one of the fastest growing metropolitan areas in the United States.² (See Figure 5.) "The population of the metro area has more than tripled from the 1 million residents in 1950 to over 3 million today, with no slowdown in sight".⁹

The gaging station selected was at Sope Creek on S. Roswell Road in Cobb County, near Marietta, Georgia. (See figures 6 and 7). A gaging station is defined as the "…particular site on a stream…where systematic observations of hydrologic data are obtained."¹¹ The station’s drainage area was recorded at 29.2 square miles, the gage datum is 881.37 feet above sea level.¹¹ Additionally, the station is located in a region that has been urbanized since 1990, according to its relative location on the map in Figure 5. Note that the approximate location of the station has been marked on the map.

Methods

After finding a suitable watershed site that met both criteria, United States Geological Survey gaging station water data were obtained from their website: http://waterdata.usgs.gov/ . The station’s annual peak flow data for the water years (October to September) of 1963 to 1995 were downloaded in tabular format, as well as a graph of the historical stream flow daily values from 1985 to 1997.^10,11 These data are shown in Table 1 and Figure 8. The graph in figure 8 depicts the general trend of increasing discharge over time. Note that the base discharge is 900 cubic feet per second (cfs).¹¹

Next, an Internet search for demographic and growth information was conducted for the county in which the gaging station was located. "Cobb County was one of the first Atlanta Region counties to experience a suburban growth explosion. Population increased [approximately] 130 percent from 1970 to 1990. By 1990, Cobb was essentially an urban county."¹ Census data for the population of Cobb County was obtained from the Government Information Sharing Project website: http://govinfo.kerr.orst.edu/ .³ (See Table 2.)

Average values were then calculated for the annual peak discharge, grouped by decade. Finally, the rate of population increase was calculated for the Cobb County resident population, also grouped by decade. Tables 1 and 2 explain the mathematical methods of this analysis.

Results

Table 1 shows the data gathered for the annual peak discharges in cfs for Sope Creek, and the calculated average over four time periods for those data. The discharge increased significantly with time, as did the surrounding population.

Table 2 shows the data gathered for population and the analysis calculated to find the rate of population change, and percent population change for four time periods. Notice that the percent change in population occurred in the twenty-year interval between 1970 to 1990, where there was a 127.5 % change. A growing population over the years has obviously surrounded the Sope Creek.

Table 3 clearly summarizes the relationship between population increases and peak discharge over time. Between 1970 and 1995, the Cobb County population increased by 167 percent. The 1970 to 1980 interval of averaged annual peak discharges was 2,800 cfs, which rose to 3,138.6 cfs during the 1980-1990 interval, and then elevated to 4,581.7 cfs between 1990 and 1995.

Conclusions

The increased height of flood peaks, or peak discharge, during heavy storms occurs when precipitation infiltration into the soil is reduced due to impervious surfaces. Overland flow, or the amount of water that runs into the stream channel without being infiltrated, increases and there is a reduction of recharge to the groundwater body, which decreases the base flow contribution to stream channels.⁵ Assuming that major population increases result in urbanization of an area and an increased amount of impervious surfaces, it is expected to see higher events of peak discharge events in streams in the affected area, as represented by the data gathered and analyzed for Sope Creek. 

References

1. Atlanta Regional Commission. "Cobb County Population and Employment". Accessed November 2000; available from http://www.atlantaregional.com/datasheets99/COBB_DATA_SHEET99.gif

2. Atlanta Regional Commission. "Regionalism and Urban Sprawl"; GMA annual meeting, June 24, 2000 PowerPoint presentation. Accessed November 2000; available from http://www.atlantaregional.com

3. Government Information Sharing Project. "USA Counties-Cobb County Georgia: Population, Total and Selected Characteristics". Accessed November 2000; available from http://www.govinfo.kerr.orst.edu/cgi-bin/usaco-list?30-067.gac

4. "Latitude/Longitude: 33.9264/-84.4440". Topographic map. Accessed December 2000; available from http://www.mapquest.com, powered by TopoZone.com

5. Strahler, Alan H. and Arthur N. Strahler, Modern Physical Geography, 4^th ed. (New York: John Wiley & Sons, Inc., 1992), p. 306.

6. United States Environmental Protection Agency. "Population Change: 1980-1990", rev. March 1999. Accessed November 2000; available from http://www.epa.gov/iwi/1999april/iii13_usmap.html

7. United States Environmental Protection Agency. "Urban Runoff Potential–1990", rev. March 1999. Accessed Nov. 2000; available from http://www.epa.gov/iwi/1999april/iii13_usmap.html

8. United States Environmental Protection Agency, Office of Water. "Urbanization and Streams: Studies of Hydrologic Impacts". Rev. March 1998. Accessed Nov. 2000; available from http://www.epa.gov/OWOW/NPS/urbanize/report.html.

9. United States Geological Survey, Water Division, "The Effects of Urbanization on Water Quality: Population Growth", rev. February 2000. Accessed November 2000; available from http://fs1dgadrv.er.usgs.gov/edu/urbanpop.html

10. United States Geological Survey. "Historical Streamflow Daily Values for Sope Creek (S Roswell Rd) Nr Marietta, GA (02335870)"; Retrieved from WATSTORE. Accessed November 2000; available from http://waterdata.usgs.gov/nwis-w/GA/data.components/hist.cgi?statnum=02335870.

11. United States Geological Survey. "Peak Flow Data"; Retrieved from the National Water Data Storage and Retrieval System (WATSTORE). Accessed November 2000; available from http://www. waterdata.usgs.gov/nwis-w/GA/data.components/peak.cgi?stratnum=02335870

TABLE 1

TABLE 2

TABLE 3

Figure 1. Stream Hydrograph ⁸

Figure 2. Population Change in Watersheds, 1980-1990 ⁶

Figure 3. Urban Runoff Potential for Watersheds – 1990

Figure 6: Map of region surrounding Sope Creek (S Roswell Rd) Near Marietta, Ga.

Figure 8. Historical Daily Stream Values for Sope Creek.  

*Note that base discharge is 900 CFS. Higher amounts of discharge above 900 CFS occurred more frequently as time progressed.