Small Vertical Variations Research Articles

A complete methodology to assess hydraulic risk in small ungauged catchments based on HEC-RAS 2D Rain-On-Grid simulations

This paper explores the use of the rain-on-grid (or direct rainfall) method for flood risk assessment at a basin scale. The method is particularly useful for rural catchments with small vertical variations and complex interactions with man-made obstacles and structures, which may be oversimplified by traditional hydrologically based estimations. The use of a hydrodynamic model solving mass and momentum conservation equations allows the simulation of runoff over the watershed at a basin scale. As a drawback, more detailed and spatially distributed data are needed, and the computational time is extended. On the other hand, a smaller number of parameters is needed compared to a hydrological model. Roughness and rainfall loss coefficients need to be calibrated only. The direct rainfall methodology was here implemented within the two-dimensional HEC-RAS model for the low-land rural, and ungauged, watershed of the Terdoppio River, Northern Italy. The resulting hydrographs at the closing section of the watershed were compared to synthetic design hydrographs evaluated through pure hydrological modelling, showing agreement on the peak discharge values for the low-probability scenarios, but not on the total volumes. The results in terms of water depth and flow velocity maps were used to create flood hazard maps using the Australian Institute for Disaster Resilience methodology. The Index of Proportional Risk model was then adopted to generate a basin-scale flood risk map, by combining flood hazard maps, damage functions for different building-use classes, and the value of reconstruction and content per unit area.

Unique current connecting Southern and Indian Oceans identified from radium distributions

We examined the spatial variations in 226Ra and 228Ra (activities) concentrations from the surface to a depth of 830 m in the Indian and Southern Oceans from December 2019 to January 2020. 226Ra concentrations at the surface increased sharply from 30° S to 60° S along a ~ 55° E transect (1.4–2.9 mBq/L), exhibiting small vertical variations, while 228Ra decreased southward and became depleted in the Southern Ocean. These distributions indicated the ocean-scale northward lateral transport of 226Ra-rich and 228Ra-depleted currents originating from the Antarctic Circumpolar Current (ACC). 226Ra concentrations indicated that the fractions of the ACC at depths of 0–800 m decreased from 0.95 to 0.14 between 60° S and 30° S. The ACC fractions in the subantarctic western Indian Ocean were higher than those previously reported in the eastern Indian region, indicating preferential transport of the ACC. The fractions obtained were approximately equivalent to those in the western Indian Ocean in the 1970s. This could be attributed to the minimal southward shift of the polar front due to global warming over the last 50 years.

Comparison of MISR and Meteosat-9 cloud-motion vectors

[1] Stereo motion vectors (SMVs) from the Multiangle Imaging SpectroRadiometer (MISR) were evaluated against Meteosat-9 cloud-motion vectors (CMVs) over a one-year period. In general, SMVs had weaker westerlies and southerlies than CMVs at all latitudes and levels. The E-W wind comparison showed small vertical variations with a mean difference of −0.4 m s−1, −1 m s−1, −0.7 m s−1 and corresponding rmsd of 2.4 m s−1, 3.8 m s−1, 3.5 m s−1for low-, mid-, and high-level clouds, respectively. The N-S wind discrepancies were larger and steadily increased with altitude, having a mean difference of −0.8 m s−1, −2.9 m s−1, −4.4 m s−1 and rmsd of 3.5 m s−1, 6.9 m s−1, 9.5 m s−1at low, mid, and high levels. The best overall agreement was found in marine stratocumulus off Namibia, while differences were larger in the Tropics and convective clouds. The SMVs were typically assigned to higher altitudes than CMVs. Attributing each observed height difference to MISR and/or Meteosat-9 retrieval biases will require further research; nevertheless, we already identified a few regions and cloud types where CMV height assignment seemed to be the one in error. In thin mid- and high-level clouds over Africa and Arabia as well as in broken marine boundary layer clouds the 10.8-μm brightness temperature-based heights were often biased low due to radiance contributions from the warm surface. Contrarily, low-level CMVs in the South Atlantic were frequently assigned to mid levels by the CO2-slicing method in multilayer situations. We also noticed an apparent cross-swath dependence in SMVs, whereby retrievals were less accurate on the eastern side of the MISR swath than on the western side. This artifact was traced back to sub-pixel MISR co-registration errors, which introduced cross-swath biases in E-W wind, N-S wind, and height of 0.6 m s−1, 2.6 m s−1, and 210 m.

FORAMINIFERAL ZONATIONS AS BASE LINES FOR QUATERNARY SEA-LEVEL FLUCTUATIONS IN SOUTH-SOUTHEAST BRAZILIAN MANGROVES AND MARSHES

Small vertical variations in occurrences of mangrove foraminifera and arcellaceans (thecamoebians) provide a zonation scheme that is applicable to the study of short-term sea-level change. Sampling along transects was carried out at three sites in southern Brazil and four sites along the Rio de Janeiro lagoon system. The Sai-Guacu fluvial-estuarine point bar that divides the states of Santa Catarina and Parana, and the Capinzal and Veiga islands at the ancient flood tidal delta of the Guaratuba Bay, were sampled during two summers for a total of 42 samples. In the state of Rio de Janeiro, the seaward borders of the Urussanga, Boqueirao and Fora lagoons, which comprise the incipient brackish mangroves of the Saquarema Lagoon system, were sampled in both summer and winter for a total of 17 samples. Three samples were collected during the summer at the freshwater marsh of Jacarepia Lake. These coastal sedimentary environments differ in latitude, climate, physio-chemical conditions, vegetation, and sediment type. Altitude, rate of sediment transport and salinity influence the distribution of three faunal zones: higher, middle, and lower. Statistics on the dead foraminiferal distribution and ecological indications given by live occurrences showed that Haplophragmoides sp. is the dominant species at the uppermost stations (higher high water—HHW, with a 6-cm vertical range) in both Guaratuba and Rio de Janeiro transects, whereas Trochammina inflata dominates the southernmost Sai-Guacu site. The Haplophragmoides - Trochammina association was also the most consistent in both dead and live HHW assemblages. Trochammina inflata and two forms of T. macrescens were found to be salinity-sensitive HHW assemblages, as were Polysaccammina hyperhalina and Miliammina fusca in the Guaratuba intertidal zone. In Saquarema, the intertidal zone is marked by T. macrescens, and Centropyxis aculeata predominates in Jacarepia. The subtidal zone in Parana contains Arenoparrella mexicana, Tiphotrocha comprimata, Trochammina macrescens f. macrescens, and Difflugia oblonga, whereas in Rio de Janeiro, T. macrescens f. polystoma predominates. Similar HHW assemblages are also observed in Oregon, British Columbia, Maritime Canada, and New England. Thus, the assemblages found in Brazil are potentially applicable to the study of sea-level variation as recorded in Quaternary deposits, like their counterparts in temperate marshes. These transects are the first well-constrained vertical transects of mangrove faunas in South America.

Euphotic zone variations in bulk and compound‐specific δ13C of suspended organic matter in the Subantarctic Ocean, south of Australia

The carbon isotopic compositions of suspended organic matter δ13CPOC collected from Subantarctic Zone surface waters south of Australia in November 1995 decrease southward from −20 to −26‰ and display strong correlations with aqueous carbon dioxide concentration ([CO2]aq), consistent with previous studies. In contrast, vertical profiles through the euphotic zone (top ∼100 m) of δ13CPOC at six stations display decreases with depth of up to 2.4‰. These decreases in δ13CPOC cannot be fully explained by the small vertical variations in [CO2]aq or its 13C content. Carbon 13 analyses of several individual sterols revealed similar isotopic changes with depth, suggesting that they derive from a fundamental depth control on primary production, rather than from algal community variations or remineralization processes. Growth rate μ appears to be the most likely source of the depth variations. The relationship between μ/[CO2]aq and δ13CPOC derived from surface water samples can explain the vertical variations of δ13CPOC within the mixed layer, provided integrated mixed layer growth rates are used. Below the mixed layer, differences between the observed δ13CPOC and the growth rate model can be explained by recent shallowing in mixed layer depth and the subsequent effect on growth rates. These results suggest that δ13CPOC determinations can be used to provide some information on the recent history of mixed layer processes and that interpretation of sedimentary δ13CPOC records should include consideration of possible growth rate and mixed layer depth variations.

LIBRARY LETTERS

LIBRARY LETTERS

Variations in REE and Sr-isotope chemistry of carbonate gangue, Castellanos Zn–Pb deposit, Cuba

Abundances of REEs and Sr, the magnitudes of the positive Eu anomalies (Eu/Eu *) and 87Sr/ 86Sr values in carbonate gangue of the Late Jurassic Castellanos SEDEX Zn–Pb deposit vary both laterally and, to a lesser extent, vertically. At the vent-distal site Sr, La and Eu/Eu *, determined by laser ablation microprobe analyses of individual grains, increase slightly from the base of the deposit upward. The La/Lu ratio only increases significantly in the upper part of the section. Both Sr/(Sr+Ca) and La/(La+Ca) in total carbonate, determined by ICP spectrometry, remain relatively constant or increase slightly from the base upward at the vent-distal location. Values of Sr/(Sr+Ca) correlate reasonably well with La/(La+Ca) throughout most of the vent-distal section. The Sr content and Eu/Eu * are higher and La much lower at the vent-proximal location and all three exhibit very large within-sample variations. La does not correlate with Sr at the vent-proximal site. The 87Sr/ 86Sr values at both distal and proximal sites lie between that for Jurassic (Oxfordian) seawater and that of a hydrothermal fluid with a 87Sr/Sr 86 value estimated from 87Sr/ 86Sr in the carbonate gangue of Cu mineralization in a nearby feeder channel beneath a small Zn–Pb deposit similar to Castellanos. Mixing of the REE- and Sr-enriched hydrothermal component with seawater in proportions calculated from the Sr-isotope data will explain the small vertical variations throughout most of the vent-distal section. However, the very low La abundances, the decoupling of the La from Sr and Eu/Eu * and the extreme micro-scale heterogeneity of REE distribution at the vent-proximal site suggest extensive leaching, redistribution and removal of REEs from early-stage carbonates by late-stage hydrothermal fluids at that location.