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Characterizing the sediment bed in terms of resistance to motion: towards improved modeling of saltation thresholds for aeolian transport
ABSTRACT: Models of aeolian transport thresholds generate a wide range of predictions, and error in threshold modeling leads to uncertainty in predicting aeolian events. This paper proposes a new characterization of the representative grain size for use in prediction of transport thresholds. This characterization is based on the distribution of resistance to motion in the sediment bed. The traditional grain size distribution uses a mean diameter to represent the sediment bed. However, the distribution of inertial forces resisting motion is not linearly proportional to the distribution of grain diameters, so that an arithmetic mean does not adequately represent the distribution of the resisting forces. A simple relation of shear stress to weight force is used to represent the threshold condition. Based on comparison with threshold observations drawn from the literature, the model provides reliable predictions of threshold stress and shear velocity for dry quartz grains over a wide range of grain sizes. Given that the dataset was drawn from studies employing a variety of experimental conditions and techniques, and that these studies spanned a range of nearly eight decades, the model is considered to provide a robust approximation of threshold conditions.
Aquatic vegetation mediates the relationship between hydrologic connectivity and water quality in a managed floodplain
ABSTRACT: We used conservative isotope tracers (deuterium and oxygen-18) and biologically relevant water quality measurements to assess connectivity of the Atchafalaya River to other waterways in its floodplain during the rising limb, peak, and falling limb of the 2011 flood pulse. We compared isotope tracers and water quality (dissolved oxygen and specific conductance) in biweekly samples at 83 sites in two areas that differed in their connectivity. We also compared tracers to an 8-year dataset of water quality measurements from the same sites. Although tracers clearly described differences in connectivity between the two floodplain areas and were correlated with concurrent measures of water quality, relationships were mediated by a strong temporal component and site-level variation in aquatic vegetation. Our results suggest a delay in floodplain water quality response to water inputs, and a strong influence of aquatic vegetation that locally overwhelms connectivity as a primary driver of local water quality.
Perirheic mixing and biogeochemical processing in flow-through and backwater floodplain wetlands
ABSTRACT: Inundation hydrology and associated processes control biogeochemical processing in floodplains. To better understand how hydrologic connectivity, residence time, and intrafloodplain mixing vary in floodplain wetlands, we examined how water quality of two contrasting areas in the floodplain of the Atchafalaya River—a flow-through and a backwater wetland—responded to an annual flood pulse. Large, synoptic sampling campaigns occurred in both wetlands during the rising limb, peak, and falling limb of the hydrograph. Using a combination of conservative and reactive tracers, we inferred three dominant processes that occurred over the course of the flood pulse: flushing (rising limb), advective transport (peak), and organic matter accumulation (falling limb). Biogeochemistry of the two wetlands was similar during the peak while the river overflowed into both. However, during the rising and falling limbs, flow in the backwater wetland experienced much greater residence time. This led to the accumulation of dissolved organic matter and dissolved phosphorus. There were also elevated ratios of dissolved organic carbon to nitrate in the backwater wetland, suggesting nitrogen removal was limited by nitrate transported into the floodplain there. Collectively, our results suggest inclusion of a temporal component into the perirheic concept more fully describes inundation hydrology and biogeochemistry in large river floodplain.
Floodplain biogeochemical processing of floodwaters in the Atchafalaya River Basin during the Mississippi River flood of 2011
ABSTRACT: The 2011 flood in the Lower Mississippi resulted in the second highest recorded river flow diverted into the Atchafalaya River Basin (ARB). The higher water levels during the flood peak resulted in high hydrologic connectivity between the Atchafalaya River and floodplain, with up to 50% of the Atchafalaya River water moving off channel. Water quality samples were collected throughout the ARB over the course of the flood event. Significant nitrate (NO3-) reduction (75%) occurred within the floodplain, resulting in a total NO3- reduction of 16.6% over the flood. The floodplain was a small but measurable source of dissolved reactive phosphorus and ammonium (NH4+). Collectively, these results from this large flood event suggest that enhancing river-floodplain connectivity through freshwater diversions will reduce NO3- loads to the Gulf of Mexico during large annual floods.
Geomorphic adjustment to hydrologic modifications along a meandering river: implications for surface flooding on a floodplain
ABSTRACT: Responses of large regulated rivers to contemporary changes in base level are not well understood. We used field measurements and historical analysis of air photos and topographic maps to identify geomorphic trends of the lower White River, Arkansas, USA, in the 70 years following base-level lowering at its confluence with the Mississippi River and concurrent with flood control by dams. Incision was identified below a knickpoint area upstream of St. Charles, AR, and increases over the lowermost ~90 km of the study site to ~2 m near the confluence with the Mississippi River. Mean bankfull width increased by 30 m (21%) from 1930 to 2010. Bank widening appears to be the result of flow regulation above the incision knickpoint and concomitant with incision below the knickpoint. Hydraulic modeling indicated that geomorphic adjustments likely reduced flooding by 58% during frequent floods in the incised, lowermost floodplain affected by backwater flooding from the Mississippi River and by 22% above the knickpoint area. Dominance of backwater flooding in the incised reach indicates that incision is more important than flood control on the lower White River in altering flooding and also suggests that the Mississippi River may be the dominant control in shaping the lower floodplain. Overall, results highlight the complex geomorphic adjustment in large river-floodplain systems in response to anthropogenic modifications and their implications, including reduced river-floodplain connectivity.
Sub-canopy evapotranspiration from floating vegetation and open water in a swamp forest
ABSTRACT: Among previous studies, there are large discrepancies in the difference between evapotranspiration from wetland vegetation and evaporation from open water. In this study, we investigate evapotranspiration differences between water and vegetation in a scenario that has otherwise not been extensively investigated: evapotranspiration from floodwaters in the sub-canopy environment. This study was conducted under a closed canopy baldcypress-ash-tupelo swamp forest in southeastern Louisiana. Water levels were measured in paired, partially-submerged evaporation pans, one with floating aquatic vegetation and the other without. Over the 5 month measurement period (June-November), average evapotranspiration rates from floating vegetation and open water were approximately 1.35?±?0.10 and 1.36?±?0.06 mm day-1, respectively. Open water evaporation was generally higher in summer, and evapotranspiration from the vegetated water surface was higher in fall, likely due to changes in the sub-canopy energy environment related to both regional climate and site canopy phenology.
Planform evolution of neck cutoffs on elongate meander loops, White River, Arkansas, USA
ABSTRACT: During the formation of a neck cutoff on a compound elongate loop, the upstream and downstream limbs can become oriented roughly subparallel with flow in opposite directions separated by a narrow meander neck. Immediately following cutoff of this thin neck, flow from the upstream limb is sharply redirected into the downstream limb over a short distance, leading to complex patterns of three-dimensional velocities that have implications for the evolution of the cutoff channel and the transformation of the abandoned bend into an oxbow lake. This paper investigates the process dynamics and planform evolution of neck cutoff and oxbow lake formation using measurements of flow velocities and time-series analysis of aerial photography for three neck cutoffs along the White River, Arkansas (USA) — each representing a different stage in the morphologic evolution from cutoff to oxbow lake. Results from this study suggest that the planform geometry of neck cutoff on an elongate meander loop can influence the spatial patterns of sediment erosion and deposition within the abandoned loop leading to increased hydrologic connectivity to the main channel, and contribute to the overall morphodynamics of highly sinuous meandering rivers.