Lateral movement and stability of channel banks near two highway crossings in the Pascagoula River basin in Mississippi

The meandering of alluvial streams has caused localized channel-bank instability that has endangered bridge structures in Mississippi.Knowledge of the movement of channel banks in the vicinity of bridge crossings is important in bridge design.The lateral movement of channel banks at the existing and proposed crossings of both State Highway 63 at the Chickasawhay River at Leakesville and U.S. Highway 98 at the Leaf River near McLain in the Pascagoula River Basin was studied by mapping meanders at various times and measuring channel geometry and soil strength at these sites.Historical aerial photography was used to determine cartesian coordinates of tops of channel banks, channel-bank widths and lengths, and land use in the stream drainage basins, as well as to map the lateral movement of channel banks at the two study sites.Photographic data were quantified and entered into a geographic information system.Historical channel meandering at the sites was studied to determine patterns of change.Also, change in sinuosity (river length/ valley length) was computed through time.General and site-specific soils data and channel geometry were collected at both study sites.Site-specific soils data, such as shear-strength properties of bank materials, in combination with hydraulic data, were used to compute existing channel-bank stability and to determine critical failure surfaces near the existing and proposed crossings of the Chickasawhay and Leaf Rivers.Although no recent significant lateral movement of channel banks was detected at the existing crossings, movement was detected upstream from both proposed alignments.Channel cross sections obtained at the existing State Highway 63 crossing of the Chickasawhay River between 1942 and 1990 indicated about 7 feet of scouring of the thalweg at the bridge.As much as 8 feet of scour of the channel bed was indicated by temporal changes in channel cross sections at the existing U.S. Highway 98 crossing of the Leaf River.Change in sinuosity was not significant for either study site.