Unstructured-mesh modeling of the Congo river-to-sea continuum

With the second largest outflow in the world and one of the widest hydrological basins, the Congo River is of a major importance both locally and globally. However, relatively few studies have been conducted on its hydrology, as compared to other great rivers such as the Amazon, Nile, Yangtze, or Mississippi. The goal of this study is therefore to help fill this gap and provide the first high-resolution simulation of the Congo river-estuary-coastal sea continuum. To this end, we are using a discontinuous-Galerkin finite element marine model that solves the two-dimensional depth-averaged shallow water equations on an unstructured mesh. To ensure a smooth transition from river to coastal sea, we have considered a model that encompasses both hydrological and coastal ocean processes. An important difficulty in setting up this model was to find data to parameterize and validate it, as it is a rather remote and understudied area. Therefore, an important effort in this study has been to establish a methodology to take advantage of all the data sources available including nautical charts that had to be digitalized. The model surface elevation has then been validated with respect to an altimetric database. Model results suggest the existence of gyres in the vicinity of the river mouth that have never been documented before. The effect of those gyres on the Congo River dynamics has been further investigated by simulating the transport of Lagrangian particles and computing the water age.     

Development of a refinement criterion for adaptive mesh refinement in steam-assisted gravity drainage simulation

Steam-assisted gravity drainage (SAGD) is an enhanced oil recovery process for heavy oils and bitumens. Numerical simulations of this thermal process allow us to estimate the retrievable volume of oil and to evaluate the benefits of the project. As there exists a thin flow interface (compared to the reservoir dimensions), SAGD simulations are sensitive to the grid size. Thus, to obtain precise forecasts of oil production, very small-sized cells have to be used, which leads to prohibitive CPU times. To reduce these computation times, one can use an adaptive mesh refinement technique, which will only refine the grid in the interface area and use coarser cells outside. To this end, in this work, we introduce new refinement criteria, which are based on the work achieved in Kröner and Ohlberger (Math Comput 69(229):25–39, 2000) on a posteriori error estimators for finite volume schemes for hyperbolic equations. Through numerical experiments, we show that they enable us to decrease in a significant way the number of cells (and then CPU times) while maintaining a good accuracy in the results.     

Complex internal architecture of a debris‐flow deposit revealed using ground‐penetrating radar,Cass, New Zealand

Previous research on debris‐flow deposit structure typically reports little to no visually discernible stratigraphy. The preliminary findings presented here provide evidence for more complex internal deposit architecture with inverse grading and subunits thought to reflect individual flow surges. Ground‐penetrating radar surveys, geospatial data and field observations are used to describe 10 subunits traceable over the 14 lateral radargrams imaging the lower 38 m of the deposit. Additional subunits are depicted further upslope in a longitudinal transect. As well as demonstrating the need for continued investigation of deposit architecture using non‐traditional techniques, these results may help improve future interpretations of post‐event deposits.     

Cartography: visualization of spatial data (3rd ed.), by Menno-Jan Kraak and Ferjan Ormeling

Conventionally, a raster operation that needs to scan the entire image employs only one scanning order (i.e., single scanning order (SSO)), and the scan usually runs from upper left to lower right and row by row. We explore the idea of alternately applying multiple scanning orders (MSO) to raster operations that are based on the local direction, using the flow accumulation (FA) calculation as an example. We constructed several FA methods based on MSO, and compared them with those widely used methods. Our comparison includes experiments over digital elevation models (DEMs) of different landforms and DEMs of different resolutions. For each DEM, we calculated both single-direction FA (SD-FA) and multi-direction FA (MD-FA). In the theoretical aspect, we deducted the time complexity of an MSO sequential algorithm (MSOsq) for FA based on empirical equations in hydrology. Findings from the experiments include the following: (1) an MSO-based method is generally superior to its counterpart SSO-based method. (2) The advantage of MSO is more significant in the SD-FA calculation than in the MD-FA calculation. (3) For SD-FA, the best method among the compared methods is the one that combines the MSOsq and the depth-first algorithm. This method surpasses the commonly recommended dependency graph algorithm, in both speed and memory use. (4) The differences between the compared methods are not sensitive to specific landforms. (5) For SD-FA, the advantage of MSO-based methods is more obvious in a higher DEM resolution, but this does not apply to MD-FA.     

Inputs of dissolved and particulate 226Ra to lakes and implications for 210Pb dating recent sediments

Gamma spectroscopy was used to measure radioisotope (²¹⁰Pb, ²²⁶Ra, ¹³⁷Cs) activities in sediment cores from 20 lakes and a wetland in Florida, USA. Nine profiles display relatively low (<5 dpm g^–1) and constant ²²⁶Ra activities, whereas 12 show high (>5 dpm g^–1) and variable ²²⁶Ra activities. In the latter group, most display up-core increases in activity. Upper sediments from two lakes (Round and Rowell) possess very high (>20 dpm g^–1) ²²⁶Ra activities that exceed total ²¹⁰Pb activities, clearly illustrating disequlibrium between ²²⁶Ra and supported ²¹⁰Pb. Supported ²¹⁰Pb activity is generally thought to come from in situ, ²²⁶Ra-containing detrital mineral particles, and is typically assumed to be in secular equilibrium with ²²⁶Ra activity. Since 1966, Round Lake has been augmented hydrologically with ²²⁶Ra-rich (6.2 dpm L^–1) groundwater pumped from the local deep aquifer. Adsorption of dissolved ²²⁶Ra to recent Round Lake sediments probably accounts for the high measured ²²⁶Ra activities and the pronounced disequilibrium between ²²⁶Ra and supported ²¹⁰Pb in topmost deposits. We suspect that many Florida waterbodies receive some ²²⁶Ra-rich runoff and seepage from groundwater pumped for irrigation, residential use, industrial applications, and mining. This may account for up-core increases in ²²⁶Ra activity measured in sediment cores from some Florida lakes. Significant groundwater pumping began within the last century, and there has been insufficient time for supported ²¹⁰Pb to come into equilibrium with adsorbed ²²⁶Ra in uppermost deposits. Input of ²²⁶Ra-rich groundwater to lakes may occur in any geographic region where local bedrock contains ²³⁸U and its daughters. When dissolved ²²⁶Ra adsorbs to recent sediments, it complicates accurate estimation of supported ²¹⁰Pb activity, and confounds calculation of unsupported ²¹⁰Pb activity that is used in dating models.     

Low-background gamma counting: applications for210Pb dating of sediments

Sediment cores from three lakes were dated with²¹⁰Pb using a constant rate of supply (CRS) model. We used low-background gamma counting to measure naturally occurring levels of²¹⁰Pb,²²⁶Ra, and¹³⁷Cs in sediment samples because sample preparation is simple and non-destructive,²²⁶Ra activity provides a direct measure of supported²¹⁰Pb activity for each sample analyzed, and¹³⁷Cs activity may provide an independent age marker for the 1962–1963 peak in atmospheric fallout of this radionuclide. In one core supported²¹⁰Pb activity was estimated equally well from²²⁶Ra activity of each sampling interval or from the mean total²¹⁰Pb activity of constant activity samples at depth. Supported²¹⁰Pb activity was constant with depth in this core. In a short freeze core, determining²²⁶Ra activity of every sample proved advantageous in estimating supported²¹⁰Pb activity because supported²¹⁰Pb activity could be estimated from²¹⁰Pb measurements only at the deepest sampling interval. Supported²¹⁰Pb activity estimated from²²⁶Ra activity also yielded more precise estimates of highly variable sedimentation rates. In the third core²²⁶Ra activity exceeded²¹⁰Pb activity at the top of the core and varied 20 fold with depth. This high input of²²⁶Ra in disequilibrium with²¹⁰Pb is attributed to recent erosion of radium-bearing materials in the drainage basin. These data invalidate the assumption that supported²¹⁰Pb activity is constant in sediment cores and can be estimated from the mean total²¹⁰Pb activity at depths where²¹⁰Pb activity is constant. We recommend using gamma counting or another independent assay of²²⁶Ra to validate the assumption of constant supported²¹⁰Pb activity in sediment cores if there is reason to expect that²²⁶Ra activity varies with depth.This is the fourth of a series of papers to be published by this journal following the 20 ^th anniversary of the first application of²¹⁰Pb dating of lake sediments. Dr. P.G. Appleby is guest editing this series.     

Spatial variation of erosion in a small,glaciated basin in the Teton Range,Wyoming, based on detrital apatite (U‐Th)/He thermochronology

Evolution of mountain landscapes is controlled by dynamic interactions between erosional processes that vary in efficiency over altitudinal domains. Evaluation of spatial and temporal variations of individual erosion processes can augment our understanding of factors controlling relief and geomorphic development of alpine settings. This study tests the application of detrital apatite (U‐Th)/He thermochronology (AHe) to evaluate variable erosion in small, geologically complex catchments. Detrital grains from glacial and fluvial sediment in a single basin were dated and compared with a bedrock derived age‐elevation relationship to estimate spatial variation in erosion over different climate conditions in the Teton Range, Wyoming. Controls and pitfalls related to apatite quality and yield were fully evaluated to assess this technique. Probability density functions comparing detrital age distributions identify variations in erosional patterns between glacial and fluvial systems and provide insight into how glacial, fluvial, and hillslope processes interact. Similar age distributions representing erosion patterns during glacial and interglacial times suggest the basin may be approaching steady‐state. This also implies that glaciers are limited and no longer act as buzzsaws or produce relief. However, subtle differences in erosional efficiency do exist. The high frequency of apatite cooling ages from high altitudes represents either rapid denudation of peaks and ridges by mass wasting or an artifact of sample quality. A gap in detrital ages near the mean age, or mid‐altitude, indicates the fluvial system is presently transport limited by overwhelming talus deposits. This study confirms that sediment sources can be traced in small basins with detrital AHe dating. It also demonstrates that careful consideration of mineral yield and quality is required, and uniform erosion assumptions needed to extract basin thermal history from detrital ages are not always valid.     

The use of sedimentary algal pigments to infer historic algal communities in Lake Apopka,Florida

The primary producer community of Lake Apopka, a large (125 km²), shallow (mean depth, 1.7 m), polymictic Florida lake, shifted from macrophyte dominance to phytoplankton dominance in the 1940s. Today, frequent wind resuspension of highly organic, unconsolidated sediments supports a meroplanktonic community that is predominantly diatoms, but during calm periods the algal community is dominated by planktonic cyanobacteria. Sedimentary algal pigments (chlorophyll derivatives and carotenoids) and chemical proxies for nutrient enrichment (polyphosphate, total phosphorus and biogenic silica) in three sediment cores were used to investigate historic changes in primary producers. Sediments were separated into three stratigraphic zones using multivariate statistical techniques. Stratigraphic zonation was established in each core although sediment deposition at one site was insufficient to adequately resolve temporal changes. These results show the importance of selecting suitable sites for paleolimnological studies. The oldest zone represents macrophyte-derived sediments, and the two overlying zones represent phytoplankton-derived sediments deposited since the 1940s. Algal pigments in the most recent sediment zone show little degradation, which might be due to the presence of viable meroplankton in the sediment. After the initial primary producer shift from macrophytes to phytoplankton, the lake experienced a short period of cyanobacterial dominance followed by a period of benthic diatom abundance before being replaced by the present algal community consisting of cyanobacteria and meroplanktonic diatoms. Chlorophyll derivatives and carotenoids were highly correlated with total phosphorus. Historic trends inferred from the data include algal and cyanobacterial productivity that increased with increased phosphorus loading. The study demonstrates that valid paleolimnological proxies for historic eutrophication are available in loosely consolidated sediments of shallow, subtropical lakes.