A series of processes involving sediment delivery from channels, its dispersal onto the floodplain, and storage at their channel margins, often generates natural levees. When levees breach, they release water and sediment onto the floodplain, leading to crevasse splays or avulsions. Despite their importance in constructing floodplains and influencing channel mobility, levee growth is poorly understood. Presently, no model fully explains dynamic channel-levee evolution. A common assumption is that levee and channel bed aggradation rates are coupled. If they coevolve, levees should progressively accumulate along any aggrading channel belt, yet observations indicate otherwise. Using a one-dimensional numerical model, we investigate levee growth decoupled from channel bed aggradation across two flood scenarios wherein the flooded level: 1) exceeds the levee crest height (i.e., front loading); 2) is lower than the levee crest causing partial inundation of distal levee deposits (i.e., back loading). Initially, rapid levee aggradation confines the channel and increases bankfull depth, which mitigates flooding. As confinement slows levee growth, channel bed aggrades until bankfull depth is sufficiently reduced to trigger overflows. This releasing process increases flood likelihood and enhances overbank accumulation, promoting front loading and re-confining the channel. Our findings suggest aggradational channels may experience confined-release phases, characterized by episodic levee growth and fluctuations in bankfull depth. Rapid in-channel aggradation increases flood frequency and variability with more confined-release cycles. These results imply that river avulsions and associated floods might preferentially occur when the channel bed aggrades faster than adjacent levees, whereby the channel becomes shallower and destabilized.

Topographic roughness is a popular yet ambiguous metric used in surface process research for many applications that indicates something about the variation of topography over specified measurement intervals. In soil- and sediment-mantled settings topographic roughness may be framed as a competition between roughening and smoothing processes. In many cases, roughening processes may be specific eco-geo-hydromorphic events like shrub deaths, tree uprooting, river avulsions, or impact craters. The smoothing processes are all geomorphic processes that operate at smaller scales and tend to drive a diffusive evolution of the surface. In this article, we present a generalized theory that explains topographic roughness as an emergent property of geomorphic systems (semi-arid plains, forests, alluvial fans, heavily bombarded surfaces) that are periodically shocked by an addition of roughness which subsequently decays due to the action of all small scale, creep-like processes. We demonstrate theory for the examples listed above, but also illustrate that there is a continuum of topographic forms that the roughening process may take on so that the theory is broadly applicable. Furthermore, we demonstrate how our theory applies to any geomorphic feature that can be described as a pit or mound, pit-mound couplet, or mound-pit-mound complex.

Delta shoreline structure has long been hypothesized to encode information on the relative influence of fluvial, wave, and tidal processes on delta formation and evolution. We introduce here a novel multiscale characterization of shorelines by defining three process-informed morphological metrics. We show that this characterization yields self-emerging classes of morphologically similar deltas, i.e., delta morphotypes, and also predicts the dominant forcing of each morphotype. Then we show that the dominant forcings inferred from shoreline structure generally align with those estimated via relative sediment fluxes, while positing that misalignments arise from spatiotemporal heterogeneity in deltaic sediment fluxes not captured in their estimates. The proposed framework for shoreline characterization advances our quantitative understanding of how shoreline features reflect delta forcings, and may aid in deciphering paleoclimate from images of ancient deposits and projecting delta morphologic response to changes in sediment fluxes.

High-resolution topographic data reveal that meandering river floodplains can contain complex floodplain-channel networks. However, the influence of this topography on flood initiation and progression is unknown. In this study, we investigate how floodplain-channel networks influence flooding processes in low-gradient river systems. To accomplish this, we conducted a series of numerical modeling experiments using a two-dimensional (2-D) model built in Hydrologic Engineering Center’s River Analysis System (HEC-RAS). First, we simulated floods using 2-D HEC-RAS on the East Fork White River near Seymour, IN, USA, including the current, dense network of floodplain channels. Next, we simulated synthetic versions of this river system where we varied the connectivity among floodplain channels, and also between the floodplain channels and river channel. We found distinct differences in flooding patterns and flood hydraulics among model simulations. Numerical modeling experiments showed that increasing floodplain channel connectivity caused increased spatial variability of wet and dry surfaces across the floodplain, increased residence time of water on the floodplain, and increased floodwave attenuation. Results from this study indicate that topographic connectivity on the floodplain challenges the classic notions of flooding initiation, and potentially increases a floodplains ability to store matter.

River deltas are a compelling target for numerical simulation because they contain seemingly organized patterns and shapes at a variety of scales. For instance, most river-dominated deltas, regardless of size, have triangular to semi-circular planform shapes, channel networks, and channel bifurcations. The common presence of these features among most deltas in the world (Caldwell et al., 2019; Nienhuis et al., 2020) suggests there are consistent underlying physical processes controlling delta form and behavior. In this review, we discuss how numerical modeling, and more specifically a type of modeling focused on the morphodynamic feedback, has helped explore some of these key physical processes over the last 15 years.

Tree uprooting is an observable and consequential process that suddenly moves soil downslope, inverts the soil column, and roughens the surface with pit-mound topography. Quantifying fluxes due to tree throw is complicated by its stochastic nature and estimation requires averaging over a large area or long time. Here, we develop theory that leads to a dimensionless metric directly measurable from high resolution topographic data. The theory explains the flux and topographic roughness as a function of tree throw production and decay rate by creep-like processes. We then form a dimensionless variable that is the ratio of fluxes due to three throw versus creep-like processes. Applying the theory to hillslopes in Southern Indiana, we find that tree throw accounts for 10 to 20\% of the hillslope sediment flux. The theoretical and observational findings provide a framework and important constraints on quantifying Critical Zone function from topographic parameters such as roughness.