A methodology for reconstructing wind direction, speed, and event duration from aeolian dune cross‐strata was developed from analysis of crescentic dunes at White Sands, New Mexico, during wind ...events. Dune lee faces were surveyed, lee‐face deposits mapped, deposition rates measured, grain size sampled by stratification type, and winds characterized from meteorological and field data. The spatial distribution of lee‐face stratification styles is a function of the incidence angle formed between the wind and the brinkline, with secondary controls by wind speed and dune sinuosity and height. Sets of wind‐ripple strata form at incidence angles of 25°–40°, grainfall/grainflow foresets over wind‐ripple bottomsets at 40°–70°, and grainflow/grainfall foresets at 70°–90°. Erosional reactivation surfaces form at incidence angles up to 15°; bypass surfaces up to 25°. The total sediment load is fractionated within lee‐face stratification types. Wind speed can be reconstructed from relationships between grain size, transport mode, shear velocity and grain‐settling velocity. Where the full range of grain transport modes occurs and grain size is limited by shear stress, the shear velocity and grain‐size range in each transport mode can be estimated by assuming the coarse fraction in grainflow strata traveled in creep, and the coarse fraction in grainfall traveled in saltation. The minimum duration of a wind event can be estimated using measures of shear velocity, dune height and dune forward migration. Method limitations arise with source‐area control on grain size, extremes in wind events, and severe truncation of sets of cross‐strata.
Key Points
A methodology is presented to reconstruct wind history from aeolian cross‐strata
The lower Colorado River (LCR) near Austin, Texas is heavily regulated for hydropower generation. Daily water releases from a dam located 23 km upstream of our study site in the LCR caused the stage ...to fluctuate by more than 1.5 m about a mean depth of 1.3 m. As a result, the river switches from gaining to losing over a dam storage‐release cycle, driving exchange between river water and groundwater. We assessed the hydrologic impacts of this by simultaneous temperature and head monitoring across a bed‐to‐bank transect. River‐groundwater exchange flux is largest close to the bank and decreases away from the bank. Correspondingly, both the depth of the hyporheic zone and the exchange time are largest close to the bank. Adjacent to the bank, the streambed head response is hysteretic, with the hysteresis disappearing with distance from the bank, indicating that transient bank storage affects the magnitude and direction of vertical exchange close to the bank. Pronounced changes in streambed temperature are observed down to a meter. When the river stage is high, which coincides with when the river is coldest, downward advection of heat from a previous cycles' warm‐water pulse warms the streambed. When the river is at its lowest stage but warmest temperature, upwelling groundwater cools the streambed. Future research should consider and focus on a more thorough understanding of the impacts of dam regulation on the hydrologic, thermal, biogeochemical, and ecologic dynamics of rivers and their hyporheic and riparian zones.
Hyporheic water is thought to infiltrate at the head of a riffle, which in turn is complemented by upwelling back to the stream at the tail of the riffle in a pool-riffle-pool (PRP) sequence. Heat ...tracing is a potentially useful method to characterize these hyporheic flow paths and quantify associated fluxes. Temperature was monitored within a PRP sequence for several days. Temperature in the hyporheic zone reflected the diel temperature change in the river but not uniformly. The observed thermal pattern exhibited deeper penetration of thermal oscillations below the head pool and shallower penetration below the tail pool. This pattern is consistent with the conceptual model of hyporheic exchange over a PRP sequence. One-dimensional analytical heat transport models were used at different points below the PRP sequence to estimate distributed vertical fluid fluxes. The calculated fluxes exhibit a trend that follows the expected distribution for a PRP sequence but modified for a losing stream (net fluid flux into the streambed). Deviation of both magnitude and distribution of fluxes from the conceptual "downwelling-to-upwelling" model is due partly to the dominantly losing conditions at the study site, but the trends are consistent with a losing stream undergoing hyporheic exchange. Violation of the assumptions in the analytical models most likely adds error to flux estimates. For this study, flux estimation methods using a temperature time-series amplitude analysis more closely matched field measurements than phase methods.
Temperature is a useful environmental tracer for quantifying movement and exchange of water and heat through and near sediment–water interfaces (SWI). Heat tracing involves analyzing temperature time ...series or profiles from temperature probes deployed in sediments. Ex-Stream is a MATLAB program that brings together two transient and two steady one-dimensional coupled heat and fluid flux analytical models. The program includes a graphical user interface, a detailed user manual, and postprocessing capabilities that enable users to extract fluid fluxes from time-series temperature observations. Program output is written to comma-separated values files, displayed within the MATLAB command window, and may be optionally plotted. The models that are integrated into Ex-Stream can be run collectively, allowing for direct comparison, or individually.
Coastal cliff erosion is alongshore-variable and episodic, with retreat rates that depend upon sediment as either tools of abrasion or protective cover. However, the feedbacks between coastal cliff ...planform morphology, retreat rate, and sediment cover are poorly quantified. This study investigates Sargent Beach, Texas, USA, at the annual to interannual scale to explore (1) the relationship between temporal and spatial variability in cliff retreat rate, roughness, and sinuosity and (2) the response of retreat rate and roughness to changes in sand and shell hash cover of the underlying mud substrate as well as the impact of major storms using field measurements of sediment cover, erosion, and aerial images to measure shore platform morphology and retreat. A storm event in 2009 increased the
planform roughness and sinuosity of the coastal cliff at Sargent Beach.
Following the storm, aerial-image-derived shorelines with annual resolution
show a decrease in average alongshore erosion rates from 12 to 4 m yr−1, coincident with a decrease in shoreline roughness and sinuosity (smoothing). Like the previous storm, a storm event in 2017 increased the planform roughness and sinuosity of the cliff. Over shorter timescales, monthly retreat of the sea cliff occurred only when the platform was sparsely covered with sediment cover on the shore platform, indicating that the tools and cover effects can significantly affect short-term erosion rates. The timescale to return to a smooth shoreline following a storm or roughening event, given a steady-state erosion rate, is approximately 24 years, with the long-term rate suggesting a maximum of ∼107 years until Sargent Beach breaches, compromising the Gulf Intracoastal Waterway (GIWW) under current conditions and assuming no future storms or intervention. The observed retreat rate varies, both spatially and temporally, with cliff face morphology, demonstrating the importance of multi-scale measurements and analysis for interpretation of coastal processes and patterns of cliff retreat.
Fifteen transects of sediment cores located off the central Texas coast between Matagorda Peninsula and North Padre Island were investigated to examine the offshore record of Holocene evolution of ...the central Texas coast. The transects extend from near the modern shoreline to beyond the toe of the shoreface. Lithology, grain size and fossil content were used to identify upper shoreface, lower shoreface, ebb‐tidal delta and marine mud lithofacies. Interpretations of these core transects show a general stratigraphic pattern across the study area that indicates three major episodes of shoreface displacement. First, there was an episode of shoreface progradation that extended up to 5 km seaward. Second, an episode of landward shoreline displacement is indicated by 3–4 km of marine mud onlap. Third, the marine muds are overlain by shoreface sands, which indicates another episode of shoreface progradation of up to 5 km seaward. Radiocarbon ages constrain the onset of the first episode of progradation to ca 6.5 ka, ending at ca 5.0 ka when the rate of sea‐level rise slowed from an average rate of 1.6–0.5 mm/yr. Results from sediment budget calculations and sediment transport modelling based on reasonable estimates of an ancient shoreline shape and wave climate indicate that the first progradation was a result of sand supplied from erosion of the offshore Colorado and Rio Grande deltas. The transgressive phase occurred between ca 4.9 ka and ca 1.6 ka and coincided with a major expansion of the Texas Mud Blanket, which resulted in burial of offshore sand sources and the shoreface being inundated with mud. The second, more recent episode of shoreface progradation began ca 500 years ago with a maximum rate of ca 6 m/yr. This most recent change signals a healing phase of coastal evolution from the late Holocene transgressive event. Currently, the shoreline along the central Texas coast is retreating landward at an average rate of 0.30 m/yr, indicating that the second progradation event has ended.
Sediment cores from the central Texas coast were investigated to examine the offshore record of Holocene evolution of the central Texas coast. Two episodes of shoreface progradation of up to 5 km separated by 3–4 km of marine mud onlap were observed. Results from sediment budget calculations and sediment transport modelling indicate that the first progradation was as a result of increased sand supply from erosion of the offshore Colorado and Rio Grande deltas. The transgressive phase occurred between ca 4.9 ka and ca 1.6 ka and coincided with a major expansion of the Texas Mud Blanket.
► This study examines hyporheic processes in an experimental meander during flooding. ► Intra-meander flow is highly sensitive to gaining and losing stream conditions. ► Groundwater flux is invariant ...of stream stage except at onset and end of flooding. ► Vertical conduction within the meander cools water in the direction of flow. ► Flooding alters shallow but not deep groundwater temperatures.
The effects of flooding on interactions between streams and shallow aquifers are poorly studied due to the unpredictable nature of flooding events. This study examines groundwater–surface water interactions in a constructed experimental meander over several simulated flood events using hydrologic and thermal monitoring. Detailed steady-state hydraulic head measurements were taken of the stream surface and in 59 piezometer nests during baseflow, bankfull flow, and overbank flooding conditions, allowing for three-dimensional hydrologic characterization. Additionally, a lithium tracer test was conducted, and pressure transducers and thermistors were deployed to capture transient behavior. Results demonstrate extensive coupling between the stream and adjacent alluvial aquifer under all discharge conditions. Water table elevation responds rapidly to changes in stream stage and re-equilibrates to order of magnitude increases in discharge within about 1h. Though flooding elevates the water table, steady-state hydraulic gradients within the meander are independent of stream stage. This results from the flow boundary condition imposed by the stream, which deepens in response to flooding but has a slope that is essentially independent of discharge. In addition to the stream boundary, flow within the meander is also controlled by loss of stream water to the subsurface, which directs hydraulic gradients towards the base of the meander. Finally, the temperature distribution within the meander during normal and bankfull flow mimics the water table distribution and results from advected warm stream water being progressively cooled by vertical conduction in the direction of groundwater flow. These findings suggest that areas of high reaction rates within meanders (hot spots) will be sensitive to whether a stream reach is gaining or losing water to the subsurface. Further, the location of these hot spots of is likely independent of stream stage, though other controls on reaction rates may be affected.
Aeolian dune motion is thought to be driven by an annual cycle of sediment‐transporting wind events. Each wind event drives uneven motion of dune crestlines, yet dune crestlines align as a trend to ...an annual cycle of wind. Understanding the variability in dune motion over such a cycle aids the interpretation of aeolian cross‐stratification, often available only in the limiting exposure of core and outcrop. Digital elevation models obtained by light detection and ranging are used to estimate dune brink motion and sediment flux along the sinuous crestlines of crescentic dunes at White Sands gypsum dune field (south‐central New Mexico, USA) over an annual cycle of wind. In tandem, meteorological observations over the same annual cycle are used to drive a kinematic model of dune crestline motion. Wind‐driven kinematic modelling does well to predict the mean and overall variation in sediment flux with compass direction. Digital elevation model‐based estimations of brink motion and sediment flux reveal that dune motion and sediment flux very nearly follow a circular normal distribution. Dunes at White Sands were found to achieve steady mean values of lee surface dip direction, brink motion and sediment flux within a sample window the size of approximately six dunes of average crestline length. Due to the symmetrical distribution of dune motion about the average lee surface dip direction, uneven motion of dune crestlines averages to become motion of dune crestlines normal to a trend, as predicted by wind‐driven kinematic models.
Over geologic timescales, forests have intercepted precipitation and thereby modified the intensity, duration, and spatial patterns of water fluxes to forest soils. Across a range of environmental ...conditions, persistent focused water flows can dissolve carbonate substrates, and form conical dissolution features—termed “dissolution cones.” These cones generally fill with soils, becoming localized soil (and water) reservoirs occupied by vegetation. A myriad of mechanisms are hypothesized to have formed dissolution cones. Prior work has sought to explain co-located palm trees and modern dissolution cones in tropical unconsolidated carbonates as the result of the chemical action of weakly acidic stemflow funneled by palm canopies down their stems, and into the substrate. Using a geochemical modeling program, PHREEQC, we find that for a range of environmental conditions and favorable assumptions, stemflow is unable to dissolve a benchmark volume of carbonate substrate that typifies tropical dissolution cones. Therefore, dissolution cone formation by abiotic dissolution from stemflow funneling is unlikely to be the chief geomorphic process. Further hypotheses to be tested are discussed.
For one-third of Earth’s land surface, precipitation passes through tree canopies (as throughfall or stemflow) before entering watersheds. Over a century of research has described fluxes of water and ...solutes along these “hydrologic highways”, yet little is known about their “traffic” –that is, the organisms and nonliving particulates frequently discarded from water samples after filtration in the lab. A comprehensive understanding of the composition of sub-canopy precipitation is necessary to estimate the total nutrient and pollutant inputs to watersheds for redistribution downstream, as well as to systematically investigate precipitation effects on organismal exchanges along the atmosphere–plant–soil continuum. Here, we review current concepts and research showing that the hydrologic highways from tree canopies to soil carry ecologically relevant quantities of biologic (viruses, microbes, microfauna, and meiofauna) and abiotic particulates. Their fate may have important consequences for the biogeochemistry and biodiversity of terrestrial systems.
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