Brandon L. Edwards

Floodplain biogeochemical processing of floodwaters in the Atchafalaya River Basin during the Mississippi River flood of 2011

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.

Perirheic mixing and biogeochemical processing in flow‐through and backwater floodplain wetlands

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.

Comparison of Surface Moisture Measurements with Depth-Integrated Moisture Measurements on a Fine-Grained Beach

ABSTRACT Edwards, B.L.; Schmutz, P.P., and Namikas, S.L., 2013. Comparison of surface moisture measurements with depth-integrated moisture measurements on a fine-grained beach. Surface moisture measurement on beaches is an important component of beach groundwater and aeolian transport studies. There are several approaches to measuring beach surface moisture, but each has significant limitations. Several recent studies have used techniques that integrate moisture content over shallow depths, and this study aims to assess the utility of these measurements for characterizing moisture conditions at the sediment surface, and also to briefly comment on the usefulness of a handheld spectroradiometer for measuring beach surface moisture. Depth-integrated moisture measurements of the top 1.5 and 6 cm of sediment obtained with a time domain reflectometry probe were compared with collocated surface moisture measurements from the spectroradiometer. Results show that depth-integrated measurements overestimated actual surface moisture by an average of 2.5 and 4.4% moisture content for the 1.5- and 6-cm sampling depths, respectively. The maximum difference between surface moisture and depth-integrated moisture content was about 12% for the 6-cm depth and about 9% for the 1.5-cm depth. These results suggest that moisture measurements integrated over even shallow depths may not depict conditions at the surface accurately enough for some applications and may potentially provide a misleading description of moisture conditions at the surface. The spectroradiometer proved to be a useful method of measuring beach surface moisture (field calibration with an R2  =  0.99 and standard error of ±1.5% moisture content), but from a logistical standpoint, may not be well suited to measuring and mapping surface moisture over large areas.

Aquatic vegetation mediates the relationship between hydrologic connectivity and water quality in a managed floodplain

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.

Evaporation and the subcanopy energy environment in a flooded forest

Abstract The combination of tree canopy cover and a free water surface makes the subcanopy environment of flooded forested wetlands unlike other aquatic or terrestrial systems. Subcanopy vapour fluxes and energy budgets represent key controls on water level and understorey climate but are not well understood. In a permanently flooded forest in south‐eastern Louisiana, USA, an energy balance approach was used to address (a) whether evaporation from floodwater under a forest canopy is solely energy limited and (b) how energy availability was modulated by radiation and changes in floodwater heat storage. A 5‐month continuous measurement period (June–November) was used to sample across seasonal changes in canopy activity and temperature regimes. Over this period, the subcanopy airspace was humid, maintaining saturation vapour pressure for 28% of the total record. High humidity coupled with the thermal inertia of surface water altered both seasonal and diel energy exchanges, including atypical phenomena such as frequent day‐time vapour pressure gradients towards the water surface. Throughout the study period, nearly all available energy was partitioned to evaporation, with minimal sensible heat exchange. Monthly mean evaporation ranged from 0.7 to 1.7 mm/day, peaking in fall when canopy senescence allowed greater radiation transmission; contemporaneous seasonal temperature shifts and a net release of stored heat from the surface water resulted in energy availability exceeding net radiation by 30% in October and November. Relatively stable energy partitioning matches Priestley–Taylor assumptions for a general model of evaporation in this ecosystem.

Classification of Forested Wetland Degradation Using Ordination of Multitemporal Reflectance

Remote sensing classifications of wetland ecosystems can be difficult because of temporal variability of plant cover and hydrological conditions. We developed a multitemporal classification scheme to control for these factors in forested wetlands by classifying pixels within a principal-components ordination of multitemporal reflectance, using data from three Landsat bands from each of seven scenes across a 3-year period in each of two phenological conditions. The principal components of reflectance that defined the classification were statistically related to vegetative conditions of the Louisiana field sites in interpretable ways, so the resulting classification represents the desired ecological gradients more robustly than single-image classifications.

Changes in Shoreline Change Trends in Response to a Detached Breakwater Field at Grand Isle, Louisiana

Abstract This study examines changes in shoreline change trends at Grand Isle, Louisiana, in relation to the implementation of a detached breakwater field completed in 1999. Eight sets of aerial photographs and digital orthophoto quarter quadrangle mosaics ranging from 1973 to 2004 were analyzed to establish pre- and postconstruction shoreline trends. Preconstruction, the island experienced persistent growth at its downdrift end. Postconstruction, this accretion ceased, and the downdrift end of the island began to experience erosion. The area landward of the breakwater field was accretionary before construction and became stable, whereas accretion increased in the area immediately updrift of the breakwater field. Two key design parameters, breakwater length and distance from the shoreline, were used to assess the potential for the formation of salients. Results from an engineering model suggest that the breakwaters were constructed too far offshore for this typical response to occur. Nonetheless, results of the air photo analysis indicate a marked change in shoreline migration trends following construction of the breakwater field.

Discussion of: Delgado, I. and Lloyd, G., 2004. A Simple Low Cost Method for One-Person Beach Profiling. Journal of Coastal Research, 20(4), 1246–1252.

Abstract The accuracy of elevation measurements obtained with a recently described one-person beach profiling method is examined. A reanalysis of the available data indicates that the accuracy of this method is below the reported average of 99.76%, and actually ranges from about 91% to 99%. Of greater concern, the absolute error in elevation measurements (average = 15 cm, maximum = 35 cm) is shown to be large enough to potentially exceed temporal changes in profile elevation, effectively rendering those changes undetectable. It is suggested that the acceptable level of error for a given project be carefully considered prior to adoption of this method.

Planform evolution of neck cutoffs on elongate meander loops, White River, Arkansas, USA

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. A considerable amount of research has been aimed at improving our understanding of the evolution of meandering rivers (Brice, 1974; Ikeda et al., 1981; Parker and Andrews, 1986; Sun et al., 1996; Peakall et al., 2007; Parker et al., 2011), however our current understanding of the detailed processes that occur during and after cutoff events remains incomplete. Conceptual models of meander cutoff and oxbow lake formation have been primarily developed for chute cutoffs and relatively simple planform configurations. In these situations, the upstream and downstream limbs of the meander are connected by a relatively straight cutoff channel (Figure 1 A). However, during the formation of a neck cutoff on a compound elongate loop, the upstream and downstream limbs are roughly subparallel with flow in opposite directions, forcing flow redirection over a short distance (Figure 1B). These conditions of tight bend flow should become more pronounced as the ratio of radius of curvature to channel width become smaller, leading to complex patterns of three-dimensional velocities that could potentially influence the evolution of the cutoff channel and the transformation of the abandoned bend into an oxbow lake. In this paper, the formation and evolution of neck cutoffs and their associated oxbow lakes are