MONITORING LATERAL MOVEMENT AND STABILITY OF CHANNEL BANKS ON THE PEARL RIVER IN MISSISSIPPI: PROCEEDINGS OF THE 22ND MISSISSIPPI WATER RESOURCES CONFERENCE by D. Phil Turnipseed and James A. Smith Prepared by the U.S. Geological Survey 1992 TABLE OF CONTENTS Introduction Description of the study area Pearl River at State Highway 35 near Carthage Pearl River at State Highway 25 (Lakeland Drive) at Jackson Pearl River at U.S. Highway 84 near Monticello Pearl River at U.S. Highway 98 near Columbia Approach Results Channel-geometry analysis Channel-bank stability analysis Channel-meander analysis Selected references FIGURES 1. Location of study sites on the Pearl River in Mississippi 2. Tops of channel banks and apparent channel shift caused by errors introduced during mapping and digitizing tops of channel banks for the Pearl River in the vicinity of U.S. Highway 84 near Monticello 3. Cross sections from soundings made during historical and recent discharge measurements at four crossings of the Pearl River 4. Cross sections showing probable critical failure surfaces for channel blanks at four highway crossings of the Pearl River. 5. Historical changes in tops of channel banks in the vicinity of four crossings of the Pearl River. 6. Change in sinousity with time on four reaches of the Pearl River ABSTRACT Rivers are subject to continuous morphological change. In alluvial river systems, it is the rule rather than the exception that banks will erode, sediments will be deposited, and flood plains and side channels will undergo modification with time (U.S. Department of Transporation, 1975). Changes in the meanders of rivers occur naturally, but can be accelerated when a channel is modified by engineering practices such as dredging, clearing, snagging, or straightening. Lateral erosion problems caused by river meander changes in many areas of the nation result in loss of arable land and endangerment of human life. Floods and droughts have been shown to accelerate these meandering and adjustment processes (Shen, 1971). Streams meander both naturally and as an adjustment process to accommodate increased channel gradients caused by channel modification. Many investigators have studied channel meandering and related changes in channel geometry. Linsley and others (1982) determined the median length of meandering streams to be about 1.5 times the valley length. Nanson and Hickin (1986) concluded the rate of channel migration (M) for streams in western Canada likely has the following relation:         M = f(w,W,D50,h,r) where: M is channel migration; w is stream power (product of discharge and slope per unit area of the bed); W is channel width; D50 is median size of the sediment in the base of the channel; h is bank height; and r is bend radius of curvature. They attributed about 70 percent of channel-bank movement of streams observed in western Canada to these few hydraulic and sedimentological variables. Osman and Thorne (1988) studied the relations between vertical and lateral erosion of channels and bank material properties, bank geometry, type of bed material, and flow characteristics. Meandering alluvial streams can cause localized channel-bank instability that can contribute to bridge failure. For example, in 1989, the State Highway 35 crossing of the Pearl River near Carthage was endanged by the near failure of a pile bent. Although the near-collapse of the bridge was caused by local scour around the pile bent, the problem began when the channel changed course near the crossing. In another area of the State, channel widening on the Homochitto River at State Highway 33 at Rosetta has resulted in channel widths 2 to 3 times the channel-bank widths that existed in about 1906 and necessitated replacement of the bridge in 1975. By 1974, the channel at a railroad crossing upstream of State Highway 33 had moved about 600 ft north of the main truss to an approach bridge span (Wilson, 1979). Concern about the lateral movement of river banks and the effects of bank erosion on bridges has emphasized the need for techniques to estimate future channel meandering. One possible approach to estimating future channel meandering is to study the historical patterns of channel migration by using remote sensing techniques and a Geographic Information System (GIS). The channel migration patterns, when used in combination with shear-strength properties of the soil and hydraulic flow information, can be used to esimtae future rates and direction of lateral erosion. The U.S. Geological Survey (USGS), in cooperatin with the Mississippi State Highway Department (MSHD), began an investigation in 1990 to develop appropriate methods for estimating channel meandering at selected sites on the Pearl River in Mississippi. Specific objectives of the study at these sites are to: (1) map and define past rates and direction of meandering; (2) measure planform change in channel dimensions and patterns; (3) digitally simulate the identified past channel meandering; and (4) relate patterns of change to available hydraulic and hydrologic data and shear-strength properties of the channel-bank material. The overall purpose was to develop methodology to assess past channel movement and current bank stability and correlate this information with recorded floods that probably initiated and provided the driving force behind much of the historical bank movement at these sites. The methodology may be used in future studies to estimate future study sites. The focus of this paper is to document historical changes in the channels and patterns of change at the sites. 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.