The world’s deltas are increasingly at risk for disappearance due to a variety of factors including a rise in global sea levels, and compaction and downward shift in the wetland and delta floors due to natural and human-induced influences.
Deltas are distributary channel networks, meaning that as a river flows downstream, it forks into numerous, smaller channels to distribute water and sediment over a broad area. However, constructed features on deltas such as levees and control works can create superefficient channels that transport sediments into the sea, rather than allowing them to flow into the wetlands during floods. In fact, sediments that are deposited in the larger channels are often dredged in order to maintain the use of the channel for shipping. This means that the sediment that once allowed the wetlands to keep pace with sea level is no longer delivered to these areas, resulting in the increasing disappearance of wetlands underwater. Where possible, delta restoration efforts are needed to reverse land loss and restore wetlands by diverting sediment from the river onto the drowned wetlands. Equally important is a broader understanding of delta processes that can aid practitioners in managing human interaction with these complex systems.
Researchers at the St. Anthony Falls Laboratory (SAFL) are exploring the natural, self-sustaining processes of delta growth to better identify successful approaches for restoration and management efforts. Funded by the St. Anthony Falls Laboratory Industrial Consortium and the National Science Foundation, SAFL collaborators on deltaic systems research include Professors Chris Paola and Vaughan Voller, graduate students Man Liang, Antoinette Abeyta, Sarah Baumgardner and Dan Cazanacli, as well as post-doctoral researchers Nathanael Geleynse and Andy Petter. Using a combination of theoretical modeling and experimental studies, this group seeks to understand the growth dynamics of deltas to help predict rates of growth, the shapes and elevations that delta growth will produce, and the overarching impacts in the delta ecosystem.
Isolating Variables in Delta Growth Patterns
Given the complexity of deltaic systems, developing successful theoretical models that can predict the effects of various delta management inputs can be challenging. An important research initiative that has risen to the surface at SAFL is the development of a computational model that isolates and explores the most significant variables for large-scale pattern growth. This so-called Reduced-Complexity Model approach uses educated assumptions and process-based equations for fluid flow and sediment transport in delta channels. This establishes a framework for predicting delta evolution, including the effects of waves and tides.
The initial results produced from the SAFL model provide a reasonable reproduction of the features seen in existing deltas, and its dynamic processes such as abandonment of existing channels, formation of new channels, and bifurcation of a single channel. The model is also able to generate stratigraphy with coarse and fine sediment.
Next steps for this research project include rigorous comparison of the model against more detailed theoretical models, as well as against field and laboratory observation. Developing and testing this type of model can enhance understanding of how the processes in delta geomorphology interact with one another. The flexible structure and reduced complexity of these models can also better support hypothesis testing and education.
Understanding and Predicting Underwater Landslides on the Delta Front
Another significant aspect of delta research at SAFL explores how fine sediments create the morphology—form and structure—of deltas and how they are transported in freshwater delta systems, with special focus on the dynamics that create the front wall of the delta that slopes to the marine floor. This wall, called the delta front or foreset, builds up with sediment deposited from the river channel. Mass failures of the delta foreset, in which a body of sediment breaks away and avalanches down the slope, can cause damage to underwater infrastructure such as pipes, turbines and communication lines, and can generate huge waves or tsunamis. This research provides new insight on how these failures develop on the delta foreset, what the conditions are just prior to a failure, and what causes the large landslides down the delta front.
Current experimental studies at SAFL have taken a novel approach to explore these unique sediment failures by creating a system that is characteristic of marine delta fronts. One important strategy is the development of new sediment mixes that act more like the clay-based sediments found in physical deltas to introduce into experimental trials. As a result, researchers have produced natural, self-organizing flows that are characteristic of the flows and deposits found on the delta front. Using research facilities at SAFL with experiments over multiple time scales, the team reproduced a series of delta front build-up and landslide events.
Now that initial experimental studies are completed, researchers are reviewing the data captured to pinpoint the precise times at which landslides occurred and the frequency and size of the failures. The results generated from examination of these experimental studies can help researchers and delta restoration and management practitioners identify the characteristics that signal an impending failure. Knowing what to look for can support early prediction of where and when these failures are likely to occur.
Influence of Waves and Tides on Delta Systems
Experimental studies are also delving into the relative roles of the river, waves and tides on delta morphology, using basic wave- and tide-generation equipment in SAFL’s delta research facilities to produce small waves and rapid, shallow tidal cycles.
The team first builds a delta in ambient water over a period of approximately 550 hours, then introduces fluctuations in the water levels to simulate the movement of tides. Throughout the entire laboratory experiment, images capture the shape, size and changes of the delta. In addition, researchers scan the delta top with a SAFL-designed and built data acquisition carriage to show the networks and topography of the delta at the conclusion of the building phase, then again after the tidal phase is completed.
Although produced on a small scale, initial results from these experimental studies show the formation of tidal channel networks in deltaic systems. The research group first seeks to measure the shape and size of the delta at build out, and then examine and quantify any changes based on the introduction of tidal flows. Data gathered from these experiments about the relative impact of tides and waves on an experimental delta will help to identify the degree of influence each process has over the shape of deltas and coasts the world over. Combined with global wave and tide data and a database of river information, this research can help to predict which areas are most vulnerable to changes in wind and storm strength and direction resulting from global climate shifts.
Laboratory and Computational Research
As human influence on deltas grows worldwide, researchers, policymakers and practitioners will be charged with making significant choices regarding delta restoration and management. If researchers can predict delta growth, shapes and elevations, and the ecosystem effects, delta management organizations can better manage human interaction with these complicated systems. As SAFL researchers continue to develop new computational models and groundbreaking experimental studies, new information can be gathered to demystify the numerous processes interacting simultaneously across deltaic systems. “Deltas are fascinating landscapes - at once vulnerable and resilient, home to extremely productive wetlands and millions of people, and capable of recording their own history through sedimentation,” said Chris Paola, professor of Earth Sciences. “The better we understand them, the better we can live with them.”
Future studies include exploration of the morphologic features that catch and trap sediments as they move through a channel; determining the mechanisms by which rivers, waves, and tides interact to shape deltas and their channel networks; quantification of the shoreline and channel patterns; and prediction of how the channel patterns and shoreline evolve in time and respond dynamically to changes such as rising sea level.