Determination of predevelopment denudation rates of an agricultural watershed (Cayaguás River, Puerto Rico) using in-situ-produced Be in river-borne quartz

Accurate estimates of watershed denudation absent anthropogenic effects are required to develop strategies for mitigating accelerated physical erosion resulting from human activities, to model global geochemical cycles, and to examine interactions among climate, weathering, and uplift. We present a simple approach to estimate predevelopment denudation rates using in-situ-produced cosmogenic ¹⁰Be in fluvial sediments. Denudation processes in an agricultural watershed (Cayaguás River Basin, Puerto Rico) and a matched undisturbed watershed (Icacos River Basin) were compared using ¹⁰Be concentrations in quartz for various size fractions of bed material. The coarse fractions in both watersheds bear the imprint of long subsurface residence times. Fine material from old shallow soils contributes little, however, to the present-day sediment output of the Cayaguás. This confirms the recent and presumably anthropogenic origin of the modern high denudation rate in the Cayaguás Basin and suggests that pre-agricultural erosional conditions were comparable to those of the present-day Icacos.     

Alteration of bottom roughness by benthic organisms in a sandy coastal environment

This work deals with the potential influence of benthic communities on the sediment dynamics of the coastal zone, and specifically with the modifications to bottom roughness caused by communities and their effects on wave propagation across the coastal profile. Time-series of video observations of the sea bottom on the Ebro delta coast were analysed in order to estimate the bottom roughness associated with physical and biological morphological components and the sediment reworking rates caused by epifaunal organisms. Biological roughness was mainly caused by ophiuroids and tanatocenosis of bivalve and gastropod shells, which changed their abundance during the study period. The total biological roughness (K_bio) ranged between 0.27 and 0.81 cm and represented a significant part (<20%) of the total form drag roughness. Flattening of ripples caused by bioturbation was observed under low-energy conditions. Surface sediment perturbation and bioturbation rates were also estimated. Based on these observations a wave propagation model was applied in order to carry out a sensitivity analysis of the significance of biological roughness on wave dissipation in the study area under different wave conditions.     

Sewage treatment level and flow rates affect polychaete assemblages

The European Water Framework Directive (WFD) and the Urban Wastewater Treatment Directive (UWWTD) aim to improve water quality and reduce the environmental impact of wastewater, by requiring member states to introduce better treatment levels. This study was carried out in order to detect the most suitable treatment type or flow level for reducing benthic environmental impact. Therefore, soft-bottom polychaete assemblages affected by discharges of six treatment plants, with different treatment technologies and flow rates, were analysed over three consecutive years. Polychaete assemblages changed as a result of sewage discharge. These changes were related to treatment level and flow rates. Both disposal quality and quantity are important since changes in soft-bottom assemblages in stations affected by pre-treated sewage with medium and high flow rates are observed. However, the location with a low-flow pre-treated effluent has similar polychaete assemblages with regard to locations with biological treatment or controls.     

Very slow erosion rates and landscape preservation across the southwestern slope of the Ladakh Range,India

Erosion rates are key to quantifying the timescales over which different topographic and geomorphic domains develop in mountain landscapes. Geomorphic and terrestrial cosmogenic nuclide (TCN) methods were used to determine erosion rates of the arid, tectonically quiescent Ladakh Range, northern India. Five different geomorphic domains are identified and erosion rates are determined for three of the domains using TCN ¹⁰Be concentrations. Along the range divide between 5600 and 5700 m above sea level (asl), bedrock tors in the periglacial domain are eroding at 5.0 ± 0.5 to 13.1 ± 1.2 meters per million years (m/m.y.)., principally by frost shattering. At lower elevation in the unglaciated domain, erosion rates for tributary catchments vary between 0.8 ± 0.1 and 2.0 ± 0.3 m/m.y. Bedrock along interfluvial ridge crests between 3900 and 5100 m asl that separate these tributary catchments yield erosion rates <0.7 ± 0.1 m/m.y. and the dominant form of bedrock erosion is chemical weathering and grusification. Erosion rates are fastest where glaciers conditioned hillslopes above 5100 m asl by over‐steepening slopes and glacial debris is being evacuated by the fluvial network. For range divide tors, the long‐term duration of the erosion rate is considered to be 40–120 ky. By evaluating measured ¹⁰Be concentrations in tors along a model ¹⁰Be production curve, an average of ~24 cm is lost instantaneously every ~40 ky. Small (<4 km²) unglaciated tributary catchments and their interfluve bedrock have received very little precipitation since ~300 ka and the long‐term duration of their erosion rates is 300–750 ky and >850 ky, respectively. These results highlight the persistence of very slow erosion in different geomorphic domains across the southwestern slope of the Ladakh Range, which on the scale of the orogen records spatial changes in the locus of deformation and the development of an orogenic rain shadow north of the Greater Himalaya. Copyright © 2014 John Wiley & Sons, Ltd.     

New graphical methods for estimating aquifer hydraulic parameters using pumping tests with exponentially decreasing rates

Actual pumping tests may involve continuously decreasing rates over a certain period of time, and the hydraulic conductivity (K) and specific storage (S_s) of the tested confined aquifer cannot be interpreted from the classical constant‐rate test model. In this study, we revisit the aquifer drawdown characteristics of a pumping test with an exponentially decreasing rate using the dimensionless analytical solution for such a variable‐rate model. The drawdown may decrease with time for a short period of time at intermediate pumping times for such pumping tests. A larger ratio of initial to final pumping rate and a smaller radial distance of the observation well will enhance the decreasing feature. A larger decay constant results in an earlier decrease, but it weakens the extent of such a decrease. Based on the proposed dimensionless transformation, we have proposed two graphical methods for estimating K and S_s of the tested aquifer. The first is a new type curve method that does not employ the well function as commonly done in standard type curve analysis. Another is a new analytic method that takes advantage of the decreasing features of aquifer drawdown during the intermediate pumping stage. We have demonstrated the applicability and robustness of the two new graphical methods for aquifer characterization through a synthetic pumping test.     

Revisiting the morphological method in two-dimensions to quantify bed-material transport in braided rivers

Research in the 1990s showed that bed-material transport rates could be estimated at the reach scale in both one-dimension and, over small spatial scales (10s of m), in two-dimensions. The limit on the latter was the spatial scale over which it was possible to obtain distributed data on morphological change. Here, we revisit the morphological method given progress in both topographical data acquisition and hydraulic modelling. The bed-material transport needed to conserve mass is calculated in both one and two dimensions for a 1600 m × 300 m Alpine braided river “laboratory”. High-resolution topographical data were acquired by laser scanning to quantify Digital Elevation Models (DEMs), and morphological changes caused by the flushing of the water intake were derived from repeated surveys. Based on DEMs of differences, 1D bed-material transport rates were calculated using the morphological method. Then, a 2D hydraulic model was combined with a topographic correction to route sediment through the network of braided channels and to obtain a spatially variable estimate of transport in both downstream and cross-stream directions. Monte Carlo simulation was applied to the routing model parameters, allowing identification of the most probable parameter values needed to minimize negative transport. The results show that within-section spatial compensation of erosion and deposition using the 1D treatment leads to substantial local errors in transport rate estimates, to a degree related to braiding intensity. Even though the 2D application showed that a large proportion of the total transport was actually concentrated into one main channel during the studied low flow event, the proportion of transport in secondary anabranches is substantial when the river starts braiding. Investigations of the effects of DEM resolution, competent flow duration and survey frequency related to ‘travelling bedload’ and sequential erosion-deposition emphasized the critical importance of careful data collection in the application of the morphological method. © 2019 John Wiley & Sons, Ltd.     

Survival under chronic stress from sediment load: spatial patterns of hard coral communities in the southern islands of Singapore

Six reef sites were chosen along the west coasts of Singapore's southern islands, to: (1) quantitatively assess and compare coral community composition and structure, and recruitment rates, (2) assess the relationship between the aforementioned patterns and the environmental conditions, and (3) provide insights on potential processes that incorporate history at these study sites. Chronic exposure to high sediment load was the most obvious form of anthropogenic stress. Recruitment rates on ceramic tiles were low (1.4+/-1.0-20+/-14.7 recruits m(-2)year(-1)) but decreased towards the main island of Singapore as did hard coral cover and coral density. Coral fauna consisted of genera generally found in deeper waters (e.g., fungiids, foliose Oxypora, Leptoseris, and Echinopora) or those well adapted to turbid waters (e.g., Porites, Pectinia, Leptastrea, Montipora). Light extinction coefficient (K) and % live coral cover (%LCC) showed a strong and inverse curvilinear relationship (%LCC=13.60 *K(-3.40)). Similarly, the rate of sediment deposition (DFSPM) (Recruitment rate, RR=1.51-0.17 *DFSPM) and water clarity (RR=3.56-2.92 *K) exhibited strong and inverse relationships with recruitment rates. Although measured levels of the down-ward flux of suspended particulate matter and suspended solids were well within "normal" levels recorded in the literature, it was the proportion of benthic space, generic coral composition, and site history that offered compelling evidence of chronic exposure to increased sediment load. Clearly a reduction in both water clarity and live-coral cover has taken place since monitoring efforts began in the early 1970s, in fact coral cover has more than halved at all sites examined since the 1980s and benthic space was predominantly occupied by dead corals covered with sediment and filamentous algae.     

Sediment transport by burrowing mammals,Marin County,California

Field observations suggest that burrowing activity is the primary mode of sediment transport currently active in a small grassland drainage basin in Marin County, California. Spatial concentrations of the 1150 gopher mounds surveyed vary from 0-16 mounds m^?2 on interfluves to 0.32 mounds m^?2 on sideslopes and in the topographic hollow, with localized concentrations of up to 2.88 mounds m^?2 on the margins of the colluvial deposit. Simple models of sediment transport by burrowing activity yield estimates of between 0.91 and 2.33 cm³ cm^?1 yr^?1 for the basin as a whole, with absolute minimum and maximum rates of 0.48 and 631 cm³ cm^?1 yr^?1. These values are similar to those previously estimated for this area (Lehre, 1982) and are nearly an order-of-magnitude less than average long-term sediment transport rates at the same site (Reneau, 1988).     

Effects of terrain smoothing on topographic shielding correction factors for cosmogenic nuclide‐derived estimates of basin‐averaged denudation rates

Estimation of spatially averaged denudation rates from cosmogenic nuclide concentrations in sediments depends on the surface production rates, the scaling methods of cosmic ray intensities, and the correction algorithms for skyline, snow and vegetation shielding used to calculate terrestrial cosmogenic nuclide production. While the calculation of surface nuclide production and application of latitude, altitude and palaeointensity scaling algorithms are subjects of active research, the importance of additional correction for shielding by topographic obstructions, snow and vegetation is the subject of ongoing debate. The derivation of an additional correction factor for skyline shielding for large areas is still problematic. One important issue that has yet to be addressed is the effect of the accuracy and resolution of terrain representation by a digital elevation model (DEM) on topographic shielding correction factors. Topographic metrics scale with the resolution of the elevation data, and terrain smoothing has a potentially large effect on the correction of terrestrial cosmogenic nuclide production rates for skyline shielding. For rough, high‐relief landscapes, the effect of terrain smoothing can easily exceed analytical errors, and should be taken into account. Here we demonstrate the effect of terrain smoothing on topographic shielding correction factors for various topographic settings, and introduce an empirical model for the estimation of topographic shielding factors based on landscape metrics. Copyright © 2008 John Wiley and Sons, Ltd.     

Complex multiple cosmogenic nuclide concentration and histories in the arid Rio Lluta catchment,northern Chile

Terrestrial cosmogenic nuclide (TCN) concentrations measured in river sediments can be used to estimate catchment‐wide denudation rates. By investigating multiple TCN the steadiness of sediment generation, transport and depositional processes can be tested. Measurements of ¹⁰Be, ²¹Ne and ²⁶Al from the hyper‐ to semi‐arid Rio Lluta catchment, northern Chile, yield average single denudation rates ranging from 12 to 75 m Myr^–1 throughout the catchment. Paired nuclide analysis reveals complex exposure histories for most of the samples and thus the single nuclide estimates do not exclusively represent catchment‐wide denudation rates. The lower range of single nuclide denudation rates (12–17 m Myr^–1), established with the noble gas ²¹Ne, is in accordance with palaeodenudation rates derived from ²¹Ne/¹⁰Be and ²⁶Al/¹⁰Be ratio analysis. Since this denudation rate range is measured throughout the system, it is suggested that a headwater signal is transported downstream but modulated by a complex admixture of sediment that has been stored and buried at proximal hillslope or terrace deposits, which are released during high discharge events. That is best evidenced by the stable nuclide ²¹Ne, which preserves the nuclide concentration even during storage intervals. The catchment‐wide single ²¹Ne denudation rates and the palaeodenuation rates contrast with previous TCN‐derived erosion rates from bedrock exposures at hillslope interfluves by being at least one order of magnitude higher, especially in the lower river course. These results support earlier studies that identified a coupling of erosional processes in the Western Cordillera contrasting with decoupled processes in the Western Escarpment and in the Coastal Cordillera. Copyright © 2008 John Wiley & Sons, Ltd.