Toolbox use 



In the application of the DMS to channel shoaling, some conditions often 

 require more detailed analyses of a channel's physical processes either to better 

 identify the shoaling mechanisms or to investigate possible mitigation methods. 

 Obviously, the type and complexity of the investigation will determine the 

 appropriate tool. For example, if the channel being investigated is located in a 

 well-sheltered inland waterway, then setting up a wave-refraction model serves 

 little purpose. 



The toolbox is presently in development. With continued application of the 

 DMS, the contents of the toolbox are expected to change as experience and needs 

 direct. Certain types of programs are expected to remain in the toolbox. These 

 include a hydrodynamic model, a wave model, and a sediment-transport 

 calculator. The DMS -Analytical Toolbox will contain recommendations for the 

 output format of these three tools. Development of the recommended formats is 

 ongoing. 



Because sediment movement creates shoals, the final form of the DMS- 

 Analytical Toolbox should contain a method for evaluating sediment-transport 

 potential under both quasi-steady currents and waves. As with all sediment- 

 transport investigations, interpreting results requires caution. For example, Yang 

 and Molinas (1982) compared seven sediment-transport (under steady currents) 

 formulae with 1,259 measurements from laboratory and field investigations. The 

 best performing formula predicted sediment-transport rates within a factor of two 

 of the measured rates in only 68 percent of the cases. 



Application of the sediment-transport calculator produces a spatially variable 

 scalar data set (a contour plot) of magnitude of sediment-transport rate. 

 Interpretation of contours of sediment-transport magnitude is often difficult. One 

 must remember that sediment-transport magnitude does not necessarily correlate 

 to erosion or deposition. Erosion and deposition align more closely to the 

 gradients of sediment-transport rate in the flow direction. If, for example, in a 

 specified control volume, the sediment transport entering the volume exceeds the 

 sediment transport leaving the volume, then most likely, the volume will contain 

 deposition. The appropriate analogy is that of filling a bucket with a hole in the 

 side. If water enters the bucket faster than it comes out the hole, the water level 

 will rise (accretion). If water enters at a slower rate, then the water level will fall 

 (erosion). The most prudent way to interpret these results is to conclude that 

 sediment originates in areas showing sediment transport and moves in the 

 direction of the velocity vectors. Figure 18 illustrates a contour plot of sediment 

 transport overlaid with the water- velocity vectors. 



The DMS-Analytical Toolbox can be the most difficult component of the 

 DMS to master. Obviously, implementing a hydrodynamic model or a wave- 

 refraction model requires an investment in learning. The purpose of the toolbox 

 in the analysis of shoaling problems will be to recommend a model choice, to 

 assist in the model setup by identifying the required data in the DMS-Data 

 Manager, and to make suggestions for output formats that best visualize the 

 physical processes that create the shoal. By following this procedure, the DMS 

 makes possible the creation of an experience base from which to compare (and 

 anticipate and avoid) future channel shoaling. 



Chapter 4 DMS-Analytical Toolbox 37 



