ISSDOTv2 – A Method for Measuring Bed Load

Abstract

Bed-load transport is a complex process with wide ranging impacts on river morphology, sediment delivery, biological habitat suitability, and channel roughness, but limited measurements have been performed on some of the largest and most important river systems. Measuring bed-load transport is a challenging endeavor due to the large spatial and temporal heterogeneity in transport rates. Physical samplers have historically been utilized for smaller streams and creeks but become unreliable and logistically difficult to apply for large sand bed rivers like the Mississippi River. The Integrated, Section Surface Difference Over Time, version 2 (ISSDOTv2) method was developed to accurately determine the bed-load transport for these systems using sequential multi-beam bathymetry measurements of a translating dune field. Utilizing the high-resolution multi-beam bathymetry measurements, the ISSDOTv2 method can determine both the lateral variability in the bed-load transport, and, through integration, provide the total bed load for the river cross-section. The method has been incorporated into a numerical code with minimal user input, thereby removing sources of subjectivity and creating a consistent, repeatable process for determining the bed-load transport. The ISSDOTv2 code has been validated by flume measurements though a collaborative effort with the USDA-Agricultural Research Service National Sedimentation Laboratory in Oxford, MS (McAlpin et al., 2022). The flume tests provided a means of quantifying both the accuracy and uncertainty in the ISSDOTv2 numerical code. For real world applications, it is impossible to determine the accuracy as the true bed load is unknown, but the method used to quantify the uncertainty in the flume measurements can be extended to real world applications to determine a reasonable level of uncertainty in fluvial bed-load transport measurements. Quantification of the uncertainty in the results allows for more informed decision making with better consideration for the level of uncertainty and risks associated with the reported value. The ISSDOTv2 method has been utilized extensively with numerous measurements on the Mississippi and Missouri Rivers in particular. River managers have used these results for numerous purposes, including determination of suitable habitat for endangered mussels, creation of improved sediment budgets, validation of numerical model results, and geomorphic analysis. It is anticipated that additional applications will be identified as measurements become more common. One area of future research is evaluation of empirical/analytical bed-load transport formula using bed-load measurements on large sand-bed rivers. The ISSDOTv2 measurements of bed load provide an opportunity to quantify the accuracy of existing bed-load equations and to provide data that can be used to refine the relationships.