David Mohrig

Hydroplaning of subaqueous debris flows

We report laboratory experiments that demonstrate that the fronts of subaqueous debris flows can hydroplane on thin layers of water. The hydroplaning dramatically reduces the bed drag, thus increasing head velocity. These high velocities promote sediment suspension and turbidity-current formation. Hydroplaning causes the fronts of debris flows to accelerate away from their bodies to the point of completely detaching from the bodies, producing surging. Instigation of hydroplaning is controlled by the balance of gravity and inertia forces at the debris front and is suitably characterized by the densimetric Froude number. The laboratory flows constrain hydroplaning to cases where the calculated densimetric Froude number is greater than 0.4. The presence of a basal lubricating layer of water underneath hydroplaning debris flows and slides offers a possible explanation for the long run-out distances of many subaqueous flows and slides on very low slopes.

Interpreting avulsion process from ancient alluvial sequences: Guadalope-Matarranya system (northern Spain) and Wasatch Formation (western Colorado)

Alluvial deposits of the Guadalope-Matarranya system (Oligocene, Ebro basin, Spain) and the Wasatch Formation (Eocene, western Colorado), provide time-integrated records of the process of river-channel avulsion. These sequences consist of isolated channel-belt sandstones incised into, and abruptly overlain by, flood-plain siltstones, indicating deposition by avulsive river systems. The geometry and distribution of channel incisions suggest that avulsion was not controlled by tectonics, climate, or base-level changes, but formed by autocyclic processes. Measurements from 221 channel fills in the Guadalope-Matarranya system and 38 from the Wasatch Formation allow us to statistically characterize channel geometries we infer to be associated with establishment and abandonment of individual river avulsions. Paleoflow depths in both systems average 1.4 to 1.6 m. Aggradation height (superelevation) of channel margin levees are, on average, 0.6 and 1.1 times paleoflow depth in the Guadalope-Matarranya and Wasatch systems, respectively. These results are consistent with values from recently avulsed modern rivers and suggest that (1) flow depth is the appropriate parameter against which to scale the critical superelevation necessary for channel avulsion; and (2) the increase in potential energy due to channel perching drives the lateral instability that is needed for avulsion to be successful. Numerous stacked channel fills indicate repeated reoccupation of the same site by avulsing channels. These reoccupation channels indicate that inherited flood-plain topography, here abandoned channel forms, was an important control on the arrival site of newly avulsed channels. Comparison of our results to others suggests two end-member types of avulsion can take place. Incisional avulsion, seen here, is characterized by an early incision phase followed by infilling by migrating bar forms. Aggradational avulsion begins with aggradation followed in time by stream integration into a single downcutting channel. We suggest that the type of avulsion is strongly influenced by whether or not the adjacent flood plain is well or poorly drained. In both cases subsequent aggradation and channel perching increase the chances that some triggering event will lead to avulsion.

Conditions for branching in depositional rivers

It is often taken for granted that rivers organize transport into a single active channel. In some net-depositional environments, however, flow of water and sediment is distributed in several stable channels. Such branching rivers may be confined in valleys (anabranching or anastomosed) or unconfined on deltas (distributaries), and their existence confronts us with the very basic question of what governs the spatial organization of channel patterns in sedimentary landscapes. Current models for equilibrium channel morphology cannot predict the occurrence of branching rivers because they do not consider dynamical processes such as avulsion, i.e., the rapid abandonment of a channel in favor of a new path at lower elevation. The requisite conditions for avulsion have been the subject of ongoing debate. Here we resolve the conditions leading to channel avulsion, and show that branching rivers occur when avulsion is the dominant mechanism of lateral channel motion. A compilation of field and laboratory data demonstrates that avulsion frequency scales with the time required for sedimentation on channel beds to produce a deposit equal to one channel depth. From the relative rates of bank erosion and channel sedimentation, we derive a dimensionless mobility number that accurately predicts the conditions under which anabranching and distributary channels occur. Results may be directly applied to modeling landscape evolution over human and geologic time scales, and for inverting formative environmental conditions from channel deposits on Earth and other planetary surfaces.

Spatial grain size sorting in eolian ripples and estimation of wind conditions on planetary surfaces: Application to Meridiani Planum, Mars

The landscape seen by the Mars Exploration Rover (MER) Opportunity at Meridiani Planum is dominated by eolian (wind-blown) ripples with concentrated surface lags of hematitic spherules and fragments. These ripples exhibit profound spatial grain size sorting, with well-sorted coarse-grained crests and poorly sorted, generally finer-grained troughs. These ripples were the most common bed form encountered by Opportunity in its traverse from Eagle Crater to Endurance Crater. Field measurements from White Sands National Monument, New Mexico, show that such coarse-grained ripples form by the different transport modes of coarse- and fine-grain fractions. On the basis of our field study, and simple theoretical and experimental considerations, we show how surface deposits of coarse-grained ripples can be used to place tight constraints on formative wind conditions on planetary surfaces. Activation of Meridiani Planum coarse-grained ripples requires a wind velocity of 70 m/s (at a reference elevation of 1 m above the bed). From images by the Mars Orbiter Camera (MOC) of reversing dust streaks, we estimate that modern surface winds reach a velocity of at least 40 m/s and hence may occasionally activate these ripples. The presence of hematite at Meridiani Planum is ultimately related to formation of concretions during aqueous diagenesis in groundwater environments; however, the eolian concentration of these durable particles may have led to the recognition from orbit of this environmentally significant landing site.

Is It Feasible to Build New Land in the Mississippi River Delta

What if the Mississippi River levees were cut below New Orleans? What if much of the water and sediment were allowed to flow out and build new deltas? Could deltaic land loss be reversed, and indeed restored? Using a conservative sediment supply rate and a range of rates of sea level rise and subsidence, a physically based model of deltaic river sedimentation [Kim et al., 2009] predicts that approximately 700–1200 square kilometers of new land (exposed surface and in-channel freshwater habitat) could be built over a century (Figure 1).

CHANNEL DYNAMICS, SEDIMENT TRANSPORT, AND THE SLOPE OF ALLUVIAL FANS : EXPERIMENTAL STUDY

We present the results of an experimental study of alluvial fan sedimentation under conditions of constant inflow water discharge Qw, sediment supply Qso, and median grain size D. The study was designed to complement and test a recently formulated model of alluvial fan sedimentation and to emphasize the interactions between, and controls on, flow channelization and equilibrium fan slope. Flow channelization and fan sedimentation were studied under conditions of nearly steady, uniform aggradation. Steady conditions were achieved by imposing a steadily rising base level, just in balance with the average sediment aggradation rate. Experimental inflows covered a wide range of conditions, allowing examination of the effects of variations in Qw, Qso, and D in both bedload‐and suspension‐dominated environments. Experimental results were most consistent with an expanding‐flow channel model. Key experimental findings successfully and quantitatively predicted by the expanding‐flow theory include: (1) straight to slightly concave radial profiles of bedload‐dominated fans; (2) distinctly convexo‐concave profiles of suspension‐dominated fans; (3) a strong, inverse relationship between Qw and fan slope; (4) a strong, but secondary, relationship between Qso and fan slope; and (5) near‐independence of D and fan slope so long as transport stage is high and bedload transport dominant. However, potential scale effects in the experiments arose from reduced flow Reynolds numbers and incorrect geometric scaling of channel widths; no confident conclusions regarding the debate over the relative importance of “sheet‐floods” and braided channel flows can be drawn from the experimental data. Extrapolation to field scale is best accomplished through appropriate application of the theoretical model herein confirmed against experimental data.

Interactions between turbidity currents and topography in aggrading sinuous submarine channels: A laboratory study

We present results from a laboratory experiment documenting the evolution of a sinuous channel form via sedimentation from 24 turbidity currents having constant initial conditions. The initial channel had a sinuosity of 1.32, a wavelength of 1.95, an amplitude of 0.39 m, and three bends. All currents had a densimetric Froude number of 0.53 and an initial height equal to the channel relief at the start of the experiment. Large superelevation of currents was observed at bend apexes. This superelevation was 85%–142% greater than the value predicted by a balance of centrifugal and pressure-gradient forces. An additional contribution to the superelevation was the runup of the current onto the outer banks of bends. This runup height is described by a balance between kinetic and potential energy. Runup resulted in deposition of coarse particles on levee crests that were indistinguishable from those deposited on the channel bottom. Deposit thickness and composition showed a strong cross-channel asymmetry. Thicker, coarser, steeper levees grew on the outer banks relative to the inner banks of bends. Zones of flow separation were observed downstream from bend apexes along inner banks and affected sedimentation patterns. Sedimentation from currents caused the channel to aggrade with almost no change in planform. However, channel relief decreased throughout the experiment because deposition on the channel bottom always exceeded deposition at levee crests. The first bend served as a filter for the properties of the channelized current, bringing discharge at the channel entrance into agreement with the channel cross-sectional area. Excess discharge exited the channel at this filtering bend and was lost to the overbank surface.

Spatial and temporal trends for water-flow velocity and bed-material sediment transport in the lower Mississippi River

Where rivers near the coastline, the receiving basin begins to influence flow, and gradually varied, nonuniform flow conditions arise. The section of the river affected by nonuniform flow is typically referred to as the backwater segment, and for large lowland rivers, this portion of the river can extend many hundreds of kilometers above the outlet. River morphology and kinematics vary in the backwater segment; however, these channel properties have not been explicitly related to properties of the flow and sediment-transport fields. This study examines the influence of spatially and temporally varying flow velocity and sediment transport on channel properties for the lower 800 km of the Mississippi River, a section of the river that includes the backwater segment. Survey transects (n = 2650) were used to constrain the cross-sectional area of water flow every ∼312 m along the Mississippi River channel for eight successive intervals of water discharge. Assuming conservation of water discharge, the local flow velocity was calculated at each transect by dividing water discharge by the local measurement of cross-sectional flow area. Calculated flow velocity was converted to total bed stress using a dimensionless friction coefficient that was determined by optimizing the match between a predicted and a measured water-surface profile. Estimates for the skin-friction component of the total bed stress were produced from the values for total shear stress using a form-drag correction. These skin-friction bed-stress values were then used to model bed-material transport. Results demonstrate that in the lower Mississippi River, cross-sectional flow area increases downstream during low- and moderate-water discharge. This generates a decrease in calculated water-flow velocity and bed-material transport. During high-water discharge, the trend is reversed: Cross-sectional flow area decreases downstream, producing an increase in calculated water-flow velocity and bed-material transport. An important contribution of this work is the identification of a downstream reversal in the trend for channel cross-sectional area due to variable water discharge. By accounting for the spatial divergences in sediment transport predicted over an average annual hydrograph, we demonstrate the tendency for channel-bed aggradation in much of the backwater reach of the Mississippi River (150–600 km above the outlet); however, a region of channel-bed erosion is calculated for the final 150 km. These results help to explain the spatial variability of channel morphology and kinematics for the lower Mississippi River, and they can be extended to other lowland river systems near the coastline.

Reconstructing relative flooding intensities responsible for hurricane-induced deposits from Laguna Playa Grande, Vieques, Puerto Rico

Extreme coastal flooding, primarily during hurricane strikes, has deposited sand-rich layers in Laguna Playa Grande, a back-barrier lagoon located on the island of Vieques, Puerto Rico. Silici-clastic grain-size distributions within these overwash deposits fine landward (away from the barrier and toward the mainland). A simple advective-settling model can explain this pattern of lateral sorting and is used to constrain the relative magnitude of past flooding events. A deposit associated with the A.D. 1928 San Felipe hurricane is used as a modern analogue to test the technique, which produces reasonable estimates for wave heights that exceed the barrier during the event. A 5000 yr reconstruction of local flooding intensity is developed that provides a measure of the competence for each overwash event to transport coarser-grained sediment a fixed distance into the lagoon. This reconstruction indicates that although the Laguna Playa Grande record exhibits large-scale changes in hurricane frequency on centennial to millennial time scales, the magnitude of these events has stayed relatively constant. Over the last 5000 yr, no evidence exists for an anomalously large hurricane or tsunami event with a competence for sediment transport greater than historical hurricane events.

Backwater and river plume controls on scour upstream of river mouths: Implications for fluvio‐deltaic morphodynamics

Sediment flux from rivers to oceans is the fundamental driver of fluvio-deltaic morphodynamics and continental margin sedimentation, yet sediment transport across the river-to-marine boundary is poorly understood. Coastal rivers typically are affected by backwater, a zone of spatially decelerating flow that is transitional between normal flow upstream and the offshore river plume. Flow deceleration in the backwater zone, as well as spreading of the offshore plume, should render rivers highly depositional near their mouths, leading to sedimentation and eventual elimination of the backwater zone at steady state. This reasoning is counter to observations of riverbed scour, erosional bed forms, and long-lived backwater zones near the mouths of some coastal rivers (e.g., Mississippi River, United States). To explain these observations, we present a quasi-2-D model of a coupled fluvial backwater and offshore river plume system and apply it to the Mississippi River. Results show that during high-discharge events the normal-flow depth can become larger than the water depth at the river mouth resulting in drawdown of the water surface, spatial acceleration of flow, and erosion of the riverbed. As proposed by Lane (1957), the transition to drawdown and erosion is ultimately forced by spreading of the offshore river plume. This points to the need to model coupled river and river plume systems with a dynamic backwater zone under a suite of discharges to accurately capture fluvio-deltaic morphodynamics and connectivity between fluvial sediment sources and marine depositional sinks.