THE EFFECTS OF LONGITUDINAL TRAINING WALLS (LTWS) ON TWO-DIMENSIONAL FLOW STRUCTURE IN THE GRAND RIVER, MICHIGAN (USA)

In recent years, longitudinal training walls (LTW) have received increasing attention in research and literature as an alternative engineering means for maintaining river navigation. Compared to the more commonly used groyne structures, LTWs are more environmentally friendly as it manages navigation in rivers with relatively limited impacts on aquatic habitat. LTWs were built in the Grand River (near Grand Rapids, Michigan) in the 1890’s for improving river navigation. Though their wooden structures have been degraded since construction, these structures are still having an impact on the dynamics of the river. This study explores the effects of the walls on spatial patterns of velocity, velocity changes in steady flow conditions and different discharge conditions along with sediment transport and long-term deposition. Bathymetric surveys of a ~38 km stretch of the river from Grand Rapids to the confluence with Bass River were conducted in 1906 along transects spaced roughly 90 meters apart. In addition to detailed depth measurements, the survey panels show the location of numerous wooden longitudinal training walls (LTWs) that were built in the river in the 1890’s as an effort to maintain navigation. The 1906 maps were georeferenced, and the bathymetric data were digitized, converted to bed elevation, and interpolated into an elevation model of the channel bed. Using the historical survey data and the accompanying analysis, this study investigates the two-dimensional flow structure in a section of the river using Telemac 2D software to simulate hydraulic conditions with and without the presence of LTWs. In the numerical model, drag coefficient and porosity values were changed in the domain to simulate the walls. This study is important for understanding the importance of spatial distribution of the walls from their simulated hydromorphic effects. Also, the findings will help to conceive river restoration management related to the decision-making of dam removal and dredging projects that have been proposed for the river.