Navigation Channel Sedimentation Solutions

This chapter describes navigation channel sedimentation, lists solutions to sedimentation problems in those channels, and recommends best practices for predicting navigation channel sedimentation responses – sediment deposition to and erosion of the channel bed plus surrounding bed and banks – resulting from those solutions. Sedimentation embodies the processes of erosion, entrainment, transportation, deposition, and the compaction of sediment. These are natural processes that have been active throughout geological times and have shaped the present topographic and bathymetry dimensions of our earth. Navigation is a cost-effective way to transport many goods and can be less environmentally intrusive when compared to other transportation methods such as rail and trucks. Sedimentation issues may impact environmental quality through increased vessel emissions (caused by longer transit times or additional trips required by light-loading vessels) and impacts to ecologically important habitats and communities. The chapter also presents some of the key concepts discussed in this book.

Reliable solutions to and accurate predictions of navigation channel sedimentation require a thorough knowledge of sediment transport processes, including the hydrologic and hydraulic forces driving those processes. This chapter provides an overview of significant waterway sedimentation processes. Broadly speaking, the principal external forces of interest are water, wind, and gravity. In riverine locations, gravity is the dominant external energy force, and the hydrodynamic equations need only to translate that force into flow velocity and boundary shear. The source, transport, and eventual sink of sediment represent a complex interplay of long- and short-term processes that must be understood to predict future behavior. Sedimentation processes occur at a variety of temporal and spatial scales. Cohesive sediment erosion, transport, aggregation, settling, deposition, and consolidation depend on the inherent physio-chemical characteristics of the sediment and their interaction with the water and its flow field.

Navigation channel design width, depth, and alignment depend on vessel size, traffic flow, and factors such as tides, water levels, winds, waves, and currents. Navigation channels can be categorized by depth: shallow-draft and deep-draft channels. Navigation channels may also be classified as inland or coastal channels. The US Army Corps of Engineers classifies the Gulf and Atlantic Intracoastal Waterways as inland navigation channels, but they are subject to many of the sedimentation processes of a coastal channel. Channel configuration and alignment - its geometric features - affect sedimentation processes in multiple, interacting ways. Some major aspects of those configurations are described in this chapter. Navigation channel sedimentation problems are most often deposition or excessive erosion. Off-channel sedimentation issues include erosion of and deposition to banklines, shallow bars, marshes, and in adjacent waterways influenced by navigation channels. Bankline and shoreline erosion can be a source of sediment supply to a channel, potentially increasing channel sedimentation.

Solutions to sedimentation problems may take many forms, ranging from nonstructural to structural to dredging. Keeping sediment in place is a basic step in preventing excess deposition and turbidity. The Coastal Zone Management Reauthorization Act of 1990 required states to develop plans for controlling nonpoint source pollution, including land erosion. Pascagoula Harbor, Mississippi, Navigation Project provides an example of keeping sediment out. The project includes two sediment traps, one on the east side of the Pascagoula Channel that runs from the Gulf of Mexico to Horn Island Pass and another trap on the east side of the channel through Horn Island Pass to the Gulf of Mexico. Keeping sediment navigable sounds like an oxymoron; however, Passive Nautical Depth is practiced worldwide in channels with fine-grained sediment through adoption of a nautical depth definition of the channel bottom based on a critical density of the sediment suspension.

Channel sedimentation responses to the solutions can be predicted by numerous methods grouped into four general approaches: field observations, desktop analyses, physical models, and numerical models. These approaches vary in terms of data requirements, complexity, reliability, applicability, cost, and time requirements. Field observations provide insight into the physical processes of sedimentation and data for desktop analyses, physical modeling, and numerical modeling. Bed material gradation data provide key input for sedimentation studies. Past dredging volumes, identified by time and channel reach, provide useful data for validating prediction methods and establishing rough relationships between deposition volumes and environmental events such as storms and floods. Desktop approaches – using equations and principles that can be solved without automated numerical methods – represent the simplest methods for estimating sedimentation rates and can be applied relatively efficiently. Physical models are designed by enforcing the scaling of one or a series of dimensionless relationships between the model and prototype.

Best practices for predicting channel sedimentation are expressed in this chapter in terms of models, with the emphasis on numerical models; however, these concepts can lead to best use of field data, desktop modeling, and physical modeling, as needed. Sedimentation problems are best solved through a methodical, step-by-step approach. The advent of easy-to-use numerical models has increased the temptation to begin modeling immediately without the necessary preparatory steps. The Conceptual Site Model can serve to summarize various lines of evidence and conceptualize the dominant processes relevant for sedimentation at the project site. Sedimentation model validation works best when it employs a weight of evidence approach that includes visual inspection of model results plotted with observed time series and spatial data and evaluation of model performance statistics. Deterministic future sedimentation quantitative predications must be expressed in probabilistic terms based on the joint probabilities of future conditions.

Navigation channel sedimentation problems are complex, the product of interacting weather, hydrology, hydraulics, sediment characteristics, and geomorphodynamics, as well as human activities. Projects successfully solving those problems require identifying sustainable solutions that may solve the problems and evaluating those solutions to ensure that they satisfy expectations. The first requirement for a successful solution is a project team with deep knowledge of channel sedimentation processes typical of the problem area. Next, identifying potential sedimentation solutions should be initially expansive, using a systematic evaluation of both the usual and unusual solutions. Finally, management of a navigation channel sedimentation study requires an organized approach just as systematic as the study's technical elements. Clear, written objectives, excellent client and team communications, and meticulous quality assurance/quality control are hallmarks of a well-managed study.