Dynamics of sedimentary systems
Physical processes at work on Earth’s surface sculpt our environment, present challenges for engineering designs, and create sedimentary records, which contain a useful, but aliased story of ancient environmental conditions. Therefore, there is significant merit in work that advances our understanding of surface processes, as our ability to predict consequences of ongoing climate change is firmly anchored to our ability to predict landscape responses and our understanding of ancient climate, as reconstructed from sedimentary records. To support these goals, my research has combined fieldwork in modern and ancient sedimentary systems with numerical and physical modeling to answer questions in the fields of geomorphology, sedimentology, and physical hydrology.
1 Sedimentary system dynamics and preservation
Prior work: Modern sedimentary systems cover most inhabited landscapes, and their rock record encodes a rich, but aliased history of past tectonic, climatic, and base-level changes (Jerolmack and Paola, 2010; Paola et al., 2018). Although reconstruction of ancient system is challenging, recently, Recent collaborative works have investigated an engineering challenge-why lobe orientation along the Yellow River Delta to wave climate is unable to explain why land loss engineered deltaic avulsions than naturally abandoned lobes (Carlson et al., 2021). Another engineering challenge exists at Sargent Beach, TX- as it is global hotspot for shoreline retreat. Recent collaborative work connects rapid post-storm rates of mud cliff erosion to storm-generated changes in cliff shape and sediment cover (Palermo et al., 2021). Other recent collaborative work has connected modern processes and simple numerical models to facilitate interpretation the sedimentary rock record. For example, preservation of allogenic signals in the aeolian sedimentary rock record (Cardenas et al., 2019a; Swanson et al., 2019a; Cardenas et al., 2019b). And recently, core and seismic interpretation paired with simple numerical modeling helped explain a late Holocene progradational event along the Central Texas Coast as a consequence of erosion and longshore transport of two major deltaic complexes (Odezulu et al., 2021).
Ongoing and future work: Ongoing work seeks to explore possible barrier island responses to future sea-level rise through the application of reduced complexity computer simulations of the expansive system of barrier islands along the Texas coast. Ongoing simulations forecasts the future response of Texas’ coastal barrier system may be like Holocene records of coastal barrier collapse during rapid sea-level rise (Ferguson et al., 2018): barrier islands within the central portion of the Texas coast may drown, forming subaqueous shoals. However, smaller transgressive barrier islands along the North and South, will continue to rapidly retreat landward due to frequent storm inundation and overwash (Swanson et al., 2017a, 2019b). work has been supported by the Institute for Coastal Plain Science 2021 REU program where single beam sonar bathymetric surveys and sediment samples were collected along the fluvial to tidal transition of the unregulated Ogeechee River, GA. Continuing work is set to explore how bedform morphology and bed material composition inherit signals from high frequency tides, and lower-frequency annual (winter) floods, in the broader context of backwater hydrodynamics.
2 Bedform dynamics and preservation
Prior work: Explaining dynamic bedform morphology in the context of physical processes, morphodynamic feedback operating between processes and environmental forcing, represent some of the basic challenges in geomorphology and sedimentary geology. For example, aeolian dune morphology and migration result from individual wind events that vary in direction, speed, and duration, but long-term dune orientation is unchanging where the overall environmental forcing remains the same. This fundamental dichotomy of aeolian systems is reconciled by way of understanding the scale dependence of dune motion and morphological change over individual wind events to an annual cycle of wind (Eastwood et al., 2012; Swanson et al., 2016). For the fluvial case, sinuously crested dunes often trail ephemeral helical vortices in turbulent wakes. When present, these vortices rapidly entrain and transfer significant quantities of sediment between dunes, causing rapid changes in dune morphology, that do not reflect mean flow conditions (Swanson et al., 2018). To explore morphological change over much larger spatial and temporal scales, I developed a numerical model to investigate aeolian dune pattern formation under different sets of allogenic boundary conditions (Swanson et al., 2017b). Recent work leveraged multibeam echosounder surveys collected along the lower 410 river kilometers of the Mississippi River to explore the lower dune size changes due to backwater hydrodynamics (Wu et al., 2020).
Ongoing and future work: Ongoing work funded by the American Chemical Society has centered on exploring the origin of sedimentary structures generated during the cessation of sand transport by turbidity currents. The transition from rippled cross-laminations to planar laminations in an idealized Bouma sequence has remained enigmatic. Ongoing undergraduate-mentees are constructing and applying micro-annular flumes to explore the role of syn-depositional dewatering (porewater expelled by consolidation) on bedload sediment transport and bed morphology. Concurrently, an NSF-supported project with undergraduate mentees is using digitized sedimentary exposures combined with numerical and flume experiments to explore anisotropy cross-stratification geometry- commonly used to reconstruct paleohydraulics. Future work is targeted to explore both modern, ancient, and synthetic sedimentary records created by bedforms within fluvial, marine, and aeolian systems forced by a variety of environmental conditions. This work will greatly advance our ability to invert the signals encoded in cross-stratified deposits to constrain ancient environmental conditions.
3 Applied Hydrology
Prior work: Fluid exchange between surface water and groundwater, termed hyporheic exchange, is an important vector for nutrients, dissolved solids, and pathogens. Hyporheic exchange and associated heat transport can be monitored using inexpensive geophysical field methods. For example, by logging time series of temperature change across the interface between surface water and groundwater, such as streambeds (Swanson and Cardenas, 2010), banks (Gerecht et al., 2011) and meander bends (Nowinski et al., 2012), the rate at which fluid moves across the sediment-water interface can be approximated using steady and transient vertical temperature profiles (Swanson and Cardenas, 2011). Before precipitation can infiltrate forested soils, it must first pass-through canopies. Therefore, the fate of precipitation is partially regulated by vegetation, and represents an important component of the hydrological cycle (e.g., Van Stan et al., 2021). In the context of coastal hammocks, storm-lain wrack deposits may strongly modify precipitation infiltration, as the mats of organic debris efficiently intercept incoming precipitation, which shallow reduces groundwater recharge (Van Stan et al., 2020). These co-authored works demonstrate my desire and ability to connect and collaborate with scholars of diverse interests within and beyond my discipline and department.
Ongoing and future work: Ongoing collaborative work lies at the intersection of ecohydrology and sedimentology and where I applied speciation modeling to re-evaluate the hypothesis that slightly acidic flow routed through canopies may infiltrate and dissolve carbonate substrates, forming dissolution cones. However, despite favorable conditions, these simulations fail to support this prevailing hypothesis (Van Stan et al., under review by Geomorphology).