Douglas A. Edmonds

Predicting delta avulsions: Implications for coastal wetland restoration

River deltas create new wetlands through a continuous cycle of delta lobe extension, avulsion, and abandonment, but the mechanics and timing of this cycle are poorly understood. Here we use physical experiments to quantitatively define one type of cycle for river-dominated deltas. The cycle begins as a distributary channel and its river mouth bar prograde basinward. Eventually the mouth bar reaches a critical size and stops prograding. The stagnated mouth bar triggers a wave of bed aggradation that moves upstream and increases overbank flows and bed shear stresses on the levees. An avulsion occurs as a time-dependent failure of the levee, where the largest average bed shear stress has been applied for the longest time (R 2 = 0.93). These results provide a guide for predicting the growth of intradelta lobes, which can be used to engineer the creation of new wetlands within the delta channel network and improve stratigraphic models of deltas.

The effects of sediment properties on deltaic processes and morphologies: A numerical modeling study

There is a pressing need to understand how different delta morphologies arise because morphology determines a deltas ecologic structure, resilience to relative sea-level rise, and stratigraphic architecture. We use numerical modeling (Delft3D) to explain how deltaic processes and morphology are controlled by the incoming sediment properties. We conducted 36 experiments of river-dominated delta formation varying the following sediment properties of the incoming grain-size distribution: the median, standard deviation, skewness, and percent cohesive sediment, which is a function of the first three properties. Changing standard deviation and skewness produces minimal morphological variation, whereas an increase in dominant grain size (D84) and decrease in percent cohesive sediment produce a transition from elongate deltas with few channels to semicircular deltas with many channels. This transition occurs because critical shear stresses for erosion and settling velocities of grains set the number of channel mouths and the dominant delta-building process. Together, the number of channel mouths and the dominant process—channel avulsion, mouth bar growth, or levee growth—set the delta morphology. Coarse-grained, noncohesive deltas have many channels dominated by avulsion, creating semicircular planforms with relatively smooth delta fronts. Intermediate-grained deltas have many channels dominated by mouth bar growth, creating semicircular planforms with rugose delta fronts. Fine-grained, cohesive deltas have a few channels, the majority of which are dominated by levee growth, creating elongate planforms with smooth delta fronts. The process-based model presented here provides a previously lacking mechanistic understanding of the effects of sediment properties on delta channel network and planform morphology.

Dynamics of River Mouth Deposits

Bars and subaqueous levees often form at river mouths due to high sediment availability. Once these deposits emerge and develop into islands, they become important elements of the coastal landscape, hosting rich ecosystems. Sea level rise and sediment starvation are jeopardizing these landforms, motivating a thorough analysis of the mechanisms responsible for their formation and evolution. Here we present recent studies on the dynamics of mouth bars and subaqueous levees. The review encompasses both hydrodynamic and morphological results. We first analyze the hydrodynamics of the water jet exiting a river mouth. We then show how this dynamics coupled to sediment transport leads to the formation of mouth bars and levees. Specifically, we discuss the role of sediment eddy diffusivity and potential vorticity on sediment redistribution and related deposits. The effect of waves, tides, sediment characteristics, and vegetation on river mouth deposits is included in our analysis, thus accounting for the inherent complexity of the coastal environment where these landforms are common. Based on the results presented herein, we discuss in detail how river mouth deposits can be used to build new land or restore deltaic shorelines threatened by erosion.

Is river avulsion style controlled by floodplain morphodynamics

River relocation, or avulsion, is a fundamental process that fills alluvial basins. However, it is difficult to predict avulsion patterns because the landscape conditions associated with different avulsion styles are currently unknown. Two end-member avulsion styles have been documented in modern rivers: progradational avulsions, during which significant floodplain deposition occurs as a new channel is built, and incisional avulsions, during which floodplain erosion captures flow from a parent channel. Here we propose that avulsion style is related to the tendency for overbank flows to erode or deposit sediment (i.e., floodplain morphodynamics). We present a scaling comparison of floodplain erosion and deposition rates, test it with morphodynamic modeling, and show field data from ancient deposits that are consistent with modeling results. These results demonstrate that floodplain morphodynamics may determine how rivers relocate and that assessing the relative influence of floodplain erosion and deposition may be useful for predicting sedimentation patterns in avulsive systems.

Predicting grain size in gravel-bedded rivers using digital elevation models: Application to three Maine watersheds

Riverbed grain size controls suitability of spawning habitat for threatened fish species. Motivated by this relationship, we developed a model that uses digital elevation models (DEMs) to predict bed grain size. We tested the accuracy of our model and two existing models with channel measurements from high-resolution airborne light detection and ranging (LiDAR) DEMs. All three models assume that bed grain size is a function of reach-average high-flow channel hydraulics (measured by shear stress or stream power). Our test data are field measurements of median grain size ( D 50 ) at 276 stations along four rivers in Maine. Pleistocene continental glaciation strongly influences the longitudinal profiles, which have alternating steep and gradual segments. We exploit the resulting variations in sediment supply to understand the controls on model success or failure in predicting bed grain size. Results show that all three models have ∼70% success in predicting D 50 within a factor of two overall, and better where the rivers are coarse gravel bedded (∼80% success where D 50 ≥ 16 mm). This similarity is unsurprising given that the models primarily rely on channel gradient (S) and drainage area as inputs. Measurements of S from LiDAR DEMs yield only a modest improvement in model success over those from topographic maps. We find that our model works best in sediment-starved steep reaches. Model failures fall into two broad categories: (1) relatively fine-grained ( D 50 D 50 . We argue that models based on airborne infrared LiDAR DEMs may reach a maximum around 80%–85% accuracy due to these sub-reach-scale factors, which cannot be easily measured from DEMs. The overall success of the models in predicting grain size indicates that the morphology of these channels has adjusted to the imposed S and sediment load during the ∼15 k.y. since deglaciation and through the period of anthropogenic channel change over the past three centuries.

Quantifying the signature of sediment composition on the topologic and dynamic complexity of river delta channel networks and inferences toward delta classification

Deltas contain complex self-organizing channel networks that nourish the surface with sediment and nutrients. Developing a quantitative understanding of how controlling physical mechanisms of delta formation relate to the channel networks they imprint on the landscape remains an open problem, hindering further progress on quantitative delta classification and understanding process from form. Here we isolate the effect of sediment composition on network structure by analyzing Delft3D river-dominated deltas within the recently introduced graph-theoretic framework for quantifying complexity of delta channel networks. We demonstrate that deltas with coarser incoming sediment tend to be more complex topologically (increased number of pathways) but simpler dynamically (reduced flux exchange between subnetworks) and that once a morphodynamic steady state is reached, complexity also achieves a steady state. By positioning simulated deltas on the so-called TopoDynamic complexity space and comparing with field deltas, we propose a quantitative framework for exploring complexity toward systematic inference and classification.

Avulsion flow-path selection on rivers in foreland basins

River avulsions help distribute sediment across the floodplain, and the frequency and flow path selected by each avulsion determines flooding and sedimentation patterns. However, rivers avulse infrequently, making it difficult to quantify flow-path behavior and test hypotheses posed by experimental and numerical studies. We used Google Earth Engine to create a novel data set of 55 avulsions that occurred from A.D. 1984 to 2014 in the Andean and Himalayan foreland basins. On each avulsion we measure hop length (the displacement of the avulsion across the floodplain orthogonal to the parent channel) and avulsion length (the length of the parent channel reach). Without controlling for external factors, such as climate or geology, we find that both hop length and avulsion length scale with parent channel-belt width. Avulsion channels tend to hop 2.5 channel-belt widths away from their parent channel and avulsion length is 13.4 parent channel-belt widths. We use these data to test between end-member scenarios where flow paths are randomly selected or steered by floodplain topography. Observed avulsions are inconsistent with flow-path predictions derived from a directed random walk model, and a scaling analysis of alluvial ridge size shows that hop lengths are set by alluvial ridge widths. These results suggest that avulsion flow-path selection in the Andean and Himalayan basins is driven by alluvial ridge topography, which promotes evenly spaced (or compensational) channel sand bodies over decadal time scales. These data and results are important for understanding controls on avulsion flow-path selection and how avulsion processes are represented in the stratigraphic record of foreland basins.

Testing Modified Dark Matter with Galaxy Clusters: Does Dark Matter know about the Cosmological Constant?

We discuss the possibility that the cold dark matter mass profiles contain information on the cosmological constant, and that such information constrains the nature of cold dark matter (CDM). We call this approach Modified Dark Matter (MDM). In particular, we examine the ability of MDM to explain the observed mass profiles of 13 galaxy clusters. Using general arguments from gravitational thermodynamics, we provide a theoretical justification for our MDM mass profile and successfully compare it to the NFW mass profiles both on cluster and galactic scales. Our results suggest that indeed the CDM mass profiles contain information about the cosmological constant in a non-trivial way.

Surface water‐groundwater connectivity in deltaic distributary channel networks

Delta distributary channel networks increase river water contact with sediments and provide the final opportunity to process nutrients and other solutes before river water discharges to the ocean. In order to understand surface water-groundwater interactions at the scale of the distributary channel network, we created three numerical deltas that ranged in composition from silt to sand using Delft3D, a morphodynamic flow and sediment transport model. We then linked models of mean annual river discharge to steady groundwater flow in MODFLOW. Under mean annual discharge, exchange rates through the numerical deltas are enhanced relative to a single-threaded river. We calculate that exchange rates across a <10 km2 network are equivalent to exchange through ~10–100 km of single-threaded river channel. Exchange rates are greatest in the coarse-grained delta due to its permeability and morphology. Groundwater residence times range from hours to centuries and have fractal tails. Deltas are vanishing due to relative sea level rise. River diversion projects aimed at creating new deltaic land should also aim to restore surface water-groundwater connectivity, which is critical for biogeochemical processing in wetlands. We recommend designing diversions to capture more sand and thus maximize surface water-groundwater connectivity.

Fluvio-deltaic avulsions during relative sea-level fall

Understanding river response to changes in relative sea level (RSL) is essential for predicting fluvial stratigraphy and source-to-sink dynamics. Recent theoretical work has suggested that rivers can remain aggradational during RSL fall, but field data are needed to verify this response and investigate sediment deposition processes. We show with field work and modeling that fluvio-deltaic systems can remain aggradational or at grade during RSL fall, leading to superelevation and continuation of delta lobe avulsions. The field site is the Goose River, Newfoundland-Labrador, Canada, which has experienced steady RSL fall of around 3–4 mm yr –1 in the past 5 k.y. from post-glacial isostatic rebound. Elevation analysis and optically stimulated luminescence dating suggest that the Goose River avulsed and deposited three delta lobes during RSL fall. Simulation results from Delft3D software show that if the characteristic fluvial response time is longer than the duration of RSL fall, then fluvial systems remain aggradational or at grade, and continue to avulse during RSL fall due to superelevation. Intriguingly, we find that avulsions become more frequent at faster rates of RSL fall, provided the system response time remains longer than the duration of RSL fall. This work suggests that RSL fall rate may influence the architecture of falling-stage or forced regression deposits by controlling the number of deposited delta lobes.