ANALYSIS OF THE EFFECT OF FLOW DIVERSION JETTIES AT PEARL RIVER AT STATE HIGHWAY 25 AT JACKSON, MISSISSIPPI: PROCEEDINGS OF THE 26TH MISSISSIPPI WATER RESOURCES CONFERENCE by P.C. Floyd Prepared by the U.S. Geological Survey 1996 TABLE OF CONTENTS Introduction Description of study area Data acquisition techniques Channel bathymetry and cross sections Comparison of pre- and post-jetty bathymetries Velocity vectors near Jetty "B" Conclusion FIGURES 1. Location of study area and cross sections surveyed using the Acoustic Doppler Current Profiler, Pearl River at State Highway 25 (Lakeland Drive) at Jackson, Mississippi 2. Cross sections at the downstream side of the downstream bridge showing pre- and post-jetty conditions, Pearl River at State Highway 25 (Lakeland Drive) at Jackson, Mississippi 3. Three-dimensional computer-generated image of the study area as viewed from downstream, Pearl River at State Highway 25 (Lakeland Drive) at Jackson, Mississippi 4. Plot showing the scour that has occurred since construction of the jetties, Pearl River at State Highway 25 (Lakeland Drive) at Jackson, Mississippi 5. Plot showing the fill that has occurred since construction of the jetties, Pearl River at State Highway 25 (Lakeland Drive) at Jackson, Mississippi 6. Shaded contour plot of velocities at Jetty "B" (section about 100 feet upstream of and parallel to upstream bridge), Pearl River at State Highway 25 (Lakeland Drive) at Jackson, Missisippi ABSTRACT On September 27, 1967, work was accepted by the Mississippi Department of Transportation (MDOT) for the completed construction of the two two-lane bridges at the Pearl River at State Highway 25 (Lakeland Drive) at Jackson, Mississippi (Figure 1). The crossing is located in a slight channel meander that bends toward the east (left) making the right bank a cutbank. A cutbank is the channel bank on the outsidse edge of the river bend at which flows are directed. A cutbank is defined as a steep stream bank, especially one formed on the center curve of a meander by erosion. At the time of bridge construction, the right bank was near MDOT Station 101+80. However, by 1991, due to the natural erosional processes of an actively meandering river, the right bank had eroded westward to near Station 99+80 (about 200 feet [ft]). The thalweg, or lowest point of flow in the channel cross section, had also moved westward from under the main bridge span near Station 103+40 to under the 40-ft-long approach spans near Station 100+60 (about 280 ft). Stationing, as used in this report, is a highway surveying term and is defined as the horizontal distance in hundreds of feet from an established origin with the pluses (+) designating intermediate point location in feet. For example, a Stationing of 99+80 is 9,980 ft from the origin of the survey. In an effort to prevent further bank erosion toward the west, the MDOT constructed two jetties upstream of the crossing near the right bank during the summer of 1993 (Figure 1). The purpose of these two jetties is to divert flows away from the eroding right bank toward an appropriate location under the highway bridges near Station 101+81, thereby protecting the right bank and causing the thalweg to move back (eastward) to an acceptable location under the 90-ft-long approach span. It was also anticipated that infilling of sediment would take place behind the jetties due to decreased flow velocities in that area. In August 1994, a 1-year cooperative project between the U.S. Geological Survey (USGS) and the MDOT began in which the USGS agreed to measure flow velocities at the jetties and survey the channel bathymetry (bottom mapping) in the vicinity of the crossing and jetties. The purpose of the project was to acquire new data to compare with existing data from prejetty surveys to determine if the jetties are functioning as designed. The majority of the data were to be acquired using a boat-mounted Acoustic Doppler Current Profiler (ADCP). The ADCP is capable of measuring flow velocities in three dimensions as well as its own direction and speed of travel as it traverses the river. Because the ADCP can keep track of its direction and speed, it can continuously compute its location (called bottom tracking). With these capabilities, the ADCP is a useful tool for the measurement of stream discharges and the acquisition of data for channel bottom-mapping. LizardTech's Djvu plug-in is needed to view these reports. CLICK HERE TO VIEW THE ENTIRE REPORT For questions or comments, contact K. Van Wilson.