Geomorphological Origin Research Articles

Evaluación de respuesta sísmica usando la teoría de vibraciones aleatorias en tres perfiles de suelo de Lima, Perú

Seismic response analyses are performed using a minimum number of seismic records as input motions in order to achieve a statistically strong estimation. Unfortunately, the available information recorded from the current seismic networks is still scarce regarding events with considerable magnitude. In this context, the Random Vibration Theory (RVT) arises as an alternative tool for performing site response analyses without the need of seismic records, since it only requires adequate probabilistic seismic hazard assessment.
 In this study, RVT was applied to three shear-wave velocity profiles in Lima city with distinct geomorphological origin. These profiles are characteristic for gravelly, sandy and fine deposits so the influence of each soil type in their corresponding transfer function was taken into account. In that sense, the three RVT-based normalized response spectra show good agreement with the design spectra specified in the Peruvian code, despite some amplification in the short (below 0.10 s) and long (above 0.80 s) period ranges related to noise or far-field effects. Furthermore, RVT-based response spectra for La Punta and Villa el Salvador show good agreement with the time-series based analyses from a previous study.
 In addition, spectral acceleration values surpass those specified in the Peruvian code for a range beyond the corner period. This could suggest that the soil profile characterization based on the time-averaged shear wave velocity from the upper 30 m might be insufficient to evaluate the overall seismic behavior of a soil deposit. Therefore, additional parameters that account for the deeper soil substructure might be required.

The biogeochemical variability of Arctic thermokarst ponds is reflected by stochastic and niche-driven microbial community assembly processes.

SummaryShallow thermokarst ponds are a conspicuous landscape element of the Arctic Siberian tundra with high biogeochemical variability. Little is known about how microbes from the regional species pool assemble into local pond communities and how the resulting patterns affect functional properties such as dissolved organic carbon (DOC) remineralization and greenhouse gas (GHG) turnover. We analysed the pelagic microbiomes of 20 ponds in north‐eastern Siberia in the context of their physico‐chemical properties. Ponds were categorized as polygonal or trough according to their geomorphological origin. The diversity of bacteria and eukaryotic microbes was assessed by ribosomal gene tag sequencing. Null model analysis revealed an important role of stochastic assembly processes within ponds of identical origin, in particular for genotypes only occurring in few systems. Nevertheless, the two pond types clearly represented distinct niches for both the bacterial and eukaryotic microbial communities. Carbon dioxide concentration, indicative of heterotrophic microbial processes, varied greatly, especially in the trough ponds. Methane concentrations were lower in polygonal ponds and were correlated with the estimated abundance of methanotrophs. Thus, the overall functional variability of Arctic ponds reflects the stochastic assembly of their microbial communities. Distinct functional subcommunities can, nevertheless, be related to GHG concentrations.

Ratio LE para el ajuste de perfiles longitudinales en cursos fluviales de montaña. Aplicación a la cuenca del Río Almáchar (Málaga, España)

La ratio LE (Logarítmica exponencial) puede ser una herramienta geomorfológica eficaz para ajustar perfiles longitudinales de cuencas fluviales montuosas de pequeña extensión. Debe ser aplicada utilizando la altitud como variable independiente. Para el río Almáchar de la provincia de Málaga (España), con elevadas pendientes y una vocación agrícola fundamentalmente vitivinícola, se ha conseguido un ajuste final de R2=0,96. Estos cálculos permiten detectar y cuantificar en qué intervalos se han producido anomalías o rupturas (knickpoints) de origen geomorfológico y/o geológico. Para este caso, los tramos con mayores diferencias entre el perfil real y el ajustado fueron los intervalos de altitud comprendidos entre 200-300 m y mayores a 500 m.s.n.m. Los resultados muestran una correlación directa entre la disminución de las anomalías con la aparición de vegetación natural (0,86), y un aumento de ellas con el incremento de los cultivos de la vid y la construcción de caminos.

Determinants of sediment properties and organic matter in beach and dune environments based on boosted regression trees

AbstractGrain size properties and the variation of organic matter in coastal beach and dune environments are assumed to be controlled by the intensity of aeolian processes, time and the sediment source. However, assumptions are based on relatively limited empirical studies. In this study, we examined which environmental variables are the main predictors of multiple topsoil properties. To achieve this, we analysed an extensive dataset systematically collected across all beach zones and a large geographical area at the Finnish Baltic Sea coast characterized by post‐glacial land uplift. We included a comprehensive set of predictors in the analysis and applied boosted regression trees, a modern modelling technique particularly suited for analysis without prior assumptions of the data model. The results suggest that mean grain size and sorting are mainly determined by northing and fetch. Northing, disturbance and fetch predicted the variation of soil organic matter while litter cover was strongly related to disturbance. Based on the analyses, we were able to identify the main drivers of multiple topsoil properties on land uplift beaches. Parent material is suggested to determine sediment textural properties, which largely masks the effects of transient processes. Mean grain size and sorting are highly interdependent: grains become finer and sorting improves with increasing shore exposure. The intensity of momentary geomorphic processes controls the accumulation of litter whereas the slower accumulation of organic matter in the soil is influenced also by the static exposure setting. Skewness and kurtosis of the grain size distribution are mainly influenced by unmeasured processes, potentially relating to the geomorphological origin of the sediment. Copyright © 2014 John Wiley & Sons, Ltd.

Application of elliptical Fourier analysis to watershed boundaries: a case study in Haiti

The shape of watersheds is linked to its geomorphological origin and its hydrological behavior. The present study uses elliptical Fourier analysis to describe shapes of watershed outlines. With two successive analyses, considering or not the outlet position, on a case study in Haiti, we identify on these shapes more than the classical shape descriptors such as elongation, asymmetry, regularity and curvature. This method completes traditional analysis of shape on watersheds and may ultimately help to identify a link between the shape and the behavior of watersheds.

The Landforms of the Făgărăș Mountains (The Argeș Mountain Catchment): Analysis of the Related Dynamic Processes

The present configuration of the Fagaras Mountains is a snapshot of the long-term evolution that brought about significant alterations of the landscape, and especially of the relief, which has acquired different features depending on the intensity of the relationship between the exogenous and endogenous agents. Relief shaping in the study area is controlled by the orographic and climatic features. However, the climate of the high and middle-height mountains is the main cause that determines the mechanism, the intensity and the spatial distribution of the shaping processes. The massiveness and the considerable height of the Fagaras Mountains, which exceed 2500 m altitude (Moldoveanu peak 2544 m, Negoiu peak 2535 m, Lespezi peak 2522 m, Vanatoarea lui Buteanu peak 2507 m, etc.), are responsible for the vertical zonation of climate and vegetation. The study area lies in the middle of the Southern Carpathians range, on the southern slope of the Fagaras Mountains (the Arges mountain catchment). The scale of investigated phenomena and the frequency of their occurrence allow us to distinguish two types of different phenomena on the basis of their geological or geomorphological origin. Each slope section was analyzed by using a series of parameters considered as being stable from the morphodynamic point of view.

Geomorphological origin of recession curves

We identify a previously undetected link between the river network morphology and key recession curves properties through a conceptual‐physical model of the drainage process of the riparian unconfined aquifer. We show that the power‐law exponent, α, of −dQ/dt vs. Q curves is related to the power‐law exponent of N(l) vs. G(l) curves (which we show to be connected to Hack's law), where l is the downstream distance from the channel heads, N(l) is the number of channel reaches exactly located at a distance l from their channel head, and G(l) is the total length of the network located at a distance greater or equal to l from channel heads. Using Digital Terrain Models and daily discharge observations from 67 US basins we find that geomorphologic α estimates match well the values obtained from recession curves analyses. Finally, we argue that the link between recession flows and network morphology points to an important role of low‐flow discharges in shaping the channel network.

About the presence of tropical steppe at Jebel Boudedma (southern Tunisia): a geomorphological origin?

En Afrique du nord, au cours des périodes humides du Quaternaire, les végétations tropicales étaient beaucoup plus étendues qu’aujourd’hui et atteignaient les marges septentrionales du Sahara. Lors des périodes arides, ces végétations se sont retirées vers les régions de plus basses latitudes. Et après le dernier optimum climatique humide de l’Holocène, le retour d’un climat plus aride a provoqué la disparition progressive des steppes tropicales des marges septentrionales du Sahara. C’est le cas du sud tunisien qui a perdu au cours des derniers millénaires toute sa steppe à Acacia raddiana à l’exception d’un dernier îlot qui a remarquablement subsisté au Jebel Bouhedma (Tunisie méridionale). La persistance de cette steppe sur le piémont sud de cette montagne et dans la plaine de Talah a longtemps été imputée à des causes édaphiques sans que jamais d’autres causes ne soient évoquées. Notre étude propose que le maintien de cette steppe dans la région du Jebel Bouhedma soit plutôt lié à l’existence d’un abri géomorphologique qui se serait mis en place pendant les phases arides du Quaternaire.

Remote sensing of floodplain geomorphology as a surrogate for biodiversity in a tropical river system (Madre de Dios, Peru)

The complex floodplains of large rivers offer a striking example of how geomorphology, by dictating patterns in the frequency and duration of soil saturation and surface flooding, influences ecosystem structure and function. This study draws on multiple sources of data from remote sensing, together with ground observations and water sampling, to distinguish floodplain ecosystems in the Madre de Dios River, a tributary of the Amazon River in Perú. This remote tropical river meanders across sub-Andean alluvial deposits in a tectonically active region and creates floodplain surfaces of varying ages, including terraces that are above the reach of present-day river floods. Data from Landsat ETM+ (optical multispectral), JERS-1 (L-band radar), and the Shuttle Radar Topography Mission (C-band interferometric Digital Elevation Models) were integrated in an object-oriented image analysis approach to distinguish five classes of floodplain vegetation. Vegetation classes generally correspond with successional age and reflect the activity of the riverine meander belt. Stage data for the river show erratic fluctuations and an annual range exceeding 8 m, but the maximum depth of floodplain inundation varied from > 1 m close to the river to approximately 0.1 m on more elevated terraces. The major ion composition of standing waters on the floodplain during the dry season indicated the importance of emergence of local groundwater in maintaining saturated soils, particularly further from the river, where backswamp vegetation is distinct and includes palm swamps. Thus, a hydrological continuum exists from deep but sporadic river inundation near the river to constant soil saturation by groundwater emergence in distal backswamps, reflecting the geomorphological origin and age of the floodplain deposits. This hydrogeomorphic continuum results in fundamental ecological differences. The exceptionally rich biodiversity of the sub-Andean region may be ascribed in part to the enhanced biodiversity associated with fluvial geomorphological features, and, thus, conservation planning must account for the diverse landforms created by fluvial dynamics.

Scale effect on geomorphologic and kinematic dispersion

Geomorphologic dispersion relates to the fraction of the variance of residence time distributions that can be directly linked to the heterogeneity of the paths available to hydrologic runoff of a river basin in response to suitable rainfall pulses. It thus defines the dispersive characters of geomorphological origin. Hillslope transport processes and systematic variations of advective processes in the channel network define a kinematic component of dispersion. Here we suggest that channel‐borne kinematic effects are likely to be clouded by hillslope dispersion in a surprisingly wide spectrum of basin scales. We find that the signatures of hillslope transport on travel time distributions do not fade away rapidly with the size of the catchment (i.e., when the basin scale becomes much greater than the mean hillslope size), in particular because of their major impact on the skewness of the travel time distribution. We suggest that an appropriate description of hillslope‐channel transitions entails a combination of geomorphic and kinematic effects which plays a decisive role in shaping the main characters of the hydrologic response in cases of practical interest.

Benthic macroinvertebrate diversity in the middle Doce River: the beginning of the Brazilian Long-Term Ecological Research (LTER) Program

(2002). Benthic macroinvertebrate diversity in the middle Doce River: the beginning of the Brazilian Long-Term Ecological Research (LTER) Program. SIL Proceedings, 1922-2010: Vol. 28, No. 4, pp. 1827-1830.

Vegetational stability and rare species from Early Holocene Lewis

Summary Strata of complex geomorphological origin at Skigersta in the northeast of the Isle of Lewis include organic sediments which have provided a detailed pollen profile and two Early Holocene radiocarbon dates. These two sources of evidence suggest that a stable, pre-woodland, species-rich grass and sedge landscape may have existed for at least one thousand radiocarbon years (10,250 to 9250 BP). Included in the pollen taxa are Koenigia islandica and Polemonium caeruleum, neither of which is now native to Lewis. The former was present in the Middle Devensian (c. 30 kyr BP), perhaps hinting at survival in a local refugium during the Late Devensian glaciation.

Typology of aquatic habitats in the great ouse, a small regulated lowland river

AbstractA study of aquatic habitat types was undertaken in August 1989 on the River Great Ouse. Habitat character at 44 sites was evaluated via twelve qualitative and quantitative environmental variables. Discriminant Analysis of the environmental data revealed a weak discrimination of sites corresponding with longitudinal changes in channel width. Distinction between the habitat types in the analysis was relatively weak. Compared with unregulated European lowland rivers of similar geomorphological origin, the Great Ouse possesses little transversal habitat diversity (abandoned channels, oxbows, etc.) and the ponding effect of water retention structures in downstream sections appears to have created a slight longitudinal gradient in channel width and depth. This comparatively low habitat heterogeneity corroborates a previous report of limited recruitment in the Great Ouse by 0 + fishes of both limnophilous and rheophilous reproductive guilds, with a predominance of generalist spawners.

Quelques réflexions sur les catastrophes naturelles en montagne

ABSTRACT. - A definition of natural castrophic phenomena is proposed, with a general review of their diversity and inter-relations (for exemple between earthquakes, rockslides...). A classification based on the genesis is suggested. First natural disasters from geological or/and geomorphological origin are described : tectonic movements and climatic changes which generate deseqilibriums in the landscapes (for example glaciations and déglaciations) have a fundamental importance. A classification of mountains according to the degree of risk is proposed. Climatic disasters, theirs characteristics and consequences, are numerous. Factors such as interfaces land/air, landforms, altitude, man impact environments... must not be under-estimated. From these observations, a geographical classification of natural disasters, on the basis of the magnitude and the surface concerned, is suggested in table 2 : ponctuai phenomena, «normal» catastrophe, meso, mega and giga catastrophes.

The habitat diversity and fish reproductive function of floodplain ecosystems

Fish reproduction in floodplain ecosystems, based on relative abundance and total biomass of 0+ juveniles, was studied using the synchronic approach to typological analysis in conjunction with Point Abundance Sampling by modified electrofishing. In 3 different flood plains of the Upper Rhone River, 1015 point samples yielding 4573 juveniles (0+) from 21 species were collected from 48 ecosystems of various geomorphological origin. The results demonstrate the lotic-to-lentic succession of floodplain ecosystems to be a series of non-sequential reproductive zones, with spawning conditions being reflected by the specific composition and guild structure of the YOY fish assemblages. The habitat diversity and the fish reproductive potential of floodplain ecosystems are strongly influenced by geomorphological origin and by past and present hydrological conditions. The YOY assemblages of autogenically driven ecosystems (usually of anastomose or meander origin) tend to differ both in composition and in quantity from those found in allogenically driven ecosystems (generally of braided origin). Ecosystems of intermediate character, and fish reproduction thereof, occur as the result of either ecosystem rejuvenation or senescence: autogenically driven ecosystems by allogenic mechanisms, or allogenically driven ecosystems by anthropic and/or autogenic mechanisms, respectively. Because of co-occurrence of ecosystems at similar and at different successional status, the flood plain as an entity is seen as ‘stable’ with respect to fish reproduction.

GUAYANA HIGHLANDS VERSUS GUAYANA LOWLANDS, A REAPPRAISAL

SummaryDuring the last 40 years, an impressive amount of biological, especially botanical, information concerning the Venezuelan Guayana region has been accumulated. Both the lowlands, as well as the distinctive highland mountains (‘tepuis’), evidence an outstandingly rich and diversified flora and vegetation. It has been postulated that this biological richness and the high degree of endemism are the result of long‐term and highly effective isolation suffered by the flat summit areas during extremely long geologic epochs. This static view is being challenged presently by the results of recent intensive fieldwork made in the Venezuelan Guayana. A detailed analysis of two highly evolved Guayana taxa, Stegolepis (Rapateaceae) and Bonnetia (Theaceae s.l.) shows a rather uniform distribution not only in summit habitats, but also in intermediate and lowland habitats. As a whole, both genera and several related taxa prove to have occupied successfully not only the ancient summit regions of the tepuis, but also the surrounding lowlands of much more recent geomorphological origin. Palaeoecological research also supports the hypothesis that the entire Guayana Shield area has been affected by climatic oscillations during the recent past as have the nearby Andes. This implies significant past fluctuations of the altitudinal vegetation belts in the tepui area and could well explain the wide altitudinal gradient of most of the Guayana Highland taxa. Also, the presence of convergent ‘paramoid’ vegetation types on the summits of the Chimantá Massif, indicates active speciation not only at the systematic level, but especially at the ecologic level. Thus, it is proposed that the Guayana highland and lowland biota have been and still are dynamically and genetically interdependent, not only within the Guayana region itself, but also with the adjacent complex of lowland and mountain biota.