Average Surface Slope Research Articles

A Framework to Identify Rock Glaciers and Model Mountain Permafrost in the Jhelum Basin, Kashmir Himalaya, India

AbstractDeglaciation has led to the transformation of glaciers into rock glaciers in various mountainous regions worldwide. However, the science of permafrost and rock glaciers remains under‐researched in the Himalayan region. This study presents a detailed inventory, dynamics, and permafrost distribution map for the Jhelum basin in the Kashmir Himalaya. The Permafrost Zonation Map (PZM) is created using a Logistic Regression Model based on topographic and climatic variables. High‐resolution satellite images are used to identify rock glaciers as visual indicators. Active and relic rock glaciers are classified based on surface topography and geomorphology. Results reveal 207 rock glaciers in the Jhelum basin, covering ∼50 km2, with over 100 falling into the active category. The PZM aligns well with the global permafrost zonation index map. Slope, aspect, and elevation of rock glaciers are computed using the ASTER Digital Elevation Model. The average elevations range from 3,700 m to 4,550 m, and the average surface slope ranges from 12° to 26°, with maximum slopes from 25° to 65°. Most rock glaciers are oriented toward the south or southeast. Field investigations confirm that these rock glaciers occur in highly elevated regions with steep slopes. This study provides valuable information on the high areal abundance of permafrost in the Himalayan region and suggests increased risks of thawing permafrost due to climate warming in the future. The findings contribute to the understanding of permafrost and rock glaciers, filling knowledge gaps in the Himalayan context.

Monitoring and Analysis of Water Surface Slope of the Yarlung Zangbo River Based on Remote Sensing

Global warming has led to an increase in interest in river hydrology. Water surface slope is a basic parameter of river morphology and is critical in understanding riverine ecosystems. The Yarlung Zangbo River is an important international river. Scientific understanding of its hydrology has been severely limited by its harsh topography and the sparse distribution of measurement stations. This paper describes the first attempt to use ICESat-2 satellite altimetry data to calculate river water slope and introduces a new technique for studying river hydrology. Water level data for different stretches of the Yarlung Zangbo River were obtained from high spatial resolution ICESat-2 altimetry data. The data were analyzed to determine the water surface slope of the Yarlung Zangbo River, and variation in the water surface slope was examined for several river sections. The water surface slope of the Yarlung Zangbo River was greatest in summer and autumn and least in winter and spring. There may be seasonal cyclic changes in the water surface slope. The range for water surface slope was large and the difference in extremes is significant; the maximum average water surface slope was 1935.17 cm/km and the minimum water surface slope was 0.08 cm/km. The water surface slope of the Yarlung Zangbo River can be characterized as small in the upper and middle reaches and large in the lower reaches; it is related to dry and wet conditions determined by topography and climate. The upper and middle reaches of the Yarlung Zangbo River are semi-arid and the lower reaches of the Yarlung Zangbo River span three climate zones: semi-arid, semi-humid and humid. In addition, the water level information measured by ICESat-2 data is basically similar to the water level information measured by Hydroweb data, except that the water level information of individual virtual stations is quite different due to special reasons. It was found that ICESat-2 data had higher-accuracy water level data results and more river location information. Therefore, ICESat-2 satellite altimetry data can provide a baseline reference for the study of the Yarlung Zangbo River hydrology, and the technique used is applicable to other rivers.

Interplay of mechanics and chemistry governs wear of diamond-like carbon coatings interacting with ZDDP-additivated lubricants

Friction and wear reduction by diamond-like carbon (DLC) in automotive applications can be affected by zinc-dialkyldithiophosphate (ZDDP), which is widely used in engine oils. Our experiments show that DLC’s tribological behaviour in ZDDP-additivated oils can be optimised by tailoring its stiffness, surface nano-topography and hydrogen content. An optimal combination of ultralow friction and negligible wear is achieved using hydrogen-free tetrahedral amorphous carbon (ta-C) with moderate hardness. Softer coatings exhibit similarly low wear and thin ZDDP-derived patchy tribofilms but higher friction. Conversely, harder ta-Cs undergo severe wear and sub-surface sulphur contamination. Contact-mechanics and quantum-chemical simulations reveal that shear combined with the high local contact pressure caused by the contact stiffness and average surface slope of hard ta-Cs favour ZDDP fragmentation and sulphur release. In absence of hydrogen, this is followed by local surface cold welding and sub-surface mechanical mixing of sulphur resulting in a decrease of yield stress and wear.

Many-Objective Optimization of Sustainable Drainage Systems in Urban Areas with Different Surface Slopes

Sustainable urban drainage systems are multi-functional nature-based solutions that can facilitate flood management in urban catchments while improving stormwater runoff quality. Traditionally, the evaluation of the performance of sustainable drainage infrastructure has been limited to a narrow set of design objectives to simplify their implementation and decision-making process. In this study, the spatial design of sustainable urban drainage systems is optimized considering five objective functions, including minimization of flood volume, flood duration, average peak runoff, total suspended solids, and capital cost. This allows selecting an ensemble of admissible portfolios that best trade-off capital costs and the other important urban drainage services. The impact of the average surface slope of the urban catchment on the optimal design solutions is discussed in terms of spatial distribution of sustainable drainage types. Results show that different subcatchment slopes result in non-uniform distributional designs of sustainable urban drainage systems, with higher capital costs and larger surface areas of green assets associated with steeper slopes. This has two implications. First, urban areas with different surface slopes should not have a one-size-fits-all design policy. Second, spatial equality must be taken into account when applying optimization models to urban subcatchments with different surface slopes to avoid unequal distribution of environmental and human health co-benefits associated with green drainage infrastructure.

The Influence of Channel Deepening on Tides, River Discharge Effects, and Storm Surge

AbstractWe combine archival research, semi‐analytical models, and numerical simulations to address the following question: how do changes to channel geometry alter tidal properties and flood dynamics in a hyposynchronous, strongly frictional estuary with a landward decay in tidal amplitudes? Records in the Saint Johns River Estuary since the 1890s show that tidal range has doubled in Jacksonville, Florida. Near the estuary inlet, tidal discharge approximately doubled but tidal amplitudes increased only ∼6%. Modeling shows that increased shipping channel depths from ∼5 to ∼13m drove the observed changes, with other factors like channel shortening and width reduction producing comparatively minor effects. Tidal amplitude increases are spatially variable, with a maximum change 20–25 km from the estuary inlet; tidal theory suggests that increases in amplitude approximately follow , where x is the distance from the ocean and μ is a damping coefficient. Tidal changes are a predictor of altered surge dynamics: Numerical modeling of hurricane Irma under 1898 and 2017 bathymetric conditions confirms that both tidal and storm surge amplitudes are larger today, with a similar spatial pattern. Nonetheless, peak water levels are simulated to be larger under 1898 bathymetry. The cause is likely the record river discharge observed during the storm; as suggested by a subtidal water‐level model, channel deepening since 1898 appears to have reduced the average surface slope required to drain both mean river flow and storm flows toward the ocean. Nonetheless, results suggest an increased vulnerability to storms with less river flow, but larger storm surge.

Black-Faced Spoonbill Platalea minor Population Trends, Breeding Success, and Nest Characteristics in the Chilsan Archipelago, South Korea

Long-term population monitoring is very important for the conservation of threatened and endangered species. The Black-faced Spoonbill Platalea minor is a globally endangered species and the majority of its population breed on uninhabited islands off the west coast of the Korean Peninsula. The Chilsan Archipelago in the southwest is the southernmost breeding site for this species in Korea and is expected to help buffer the potential dispersal of Black-faced Spoonbills from breeding populations currently concentrated in the Gyeonggi Bay area. As there has been a lack of information regarding the number of Black-faced Spoonbills, their characteristics, and their recent population trends, the present study investigated the population trends of Black-faced Spoonbills breeding in the Chilsan Archipelago. The study, from 2014 to 2018, examined breeding success in 2015 and 2018, and the size and location of nests in 2018. The number of nests ranged from 43 to 163 (2014, 43; 2015, 93; 2016, 84; 2017, 85; 2018, 163). The percentage of birds that attempted breeding between April and May was 84.9% in 2015 and 87.1% in 2018, while the breeding success rate was 69.9% in 2015 and 81.5% in 2018. The average nest size in 2018 was 55.06±6.09 cm long and 53.44±6.56 cm wide, while the average surface slope below the nest was 25.42±6.95°. In total, nine plant species were used as vegetation for nests. During the present study, the environmental conditions and the presence of anthropogenic disturbances seemed to affect breeding success. Therefore, strictly controlled access to breeding sites and effective management are needed to protect and conserve breeding populations of this endangered species.

РЕАЛИЗАЦИЯ СПОСОБА ОПРЕДЕЛЕНИЯ И ОБРАБОТКИ ДАННЫХ ПО ПАРАМЕТРАМ ПОДСТИЛАЮЩЕЙ ПОВЕРХНОСТИ АГРОЛАНДШАФТОВ СКЛОНОВЫХ ЗЕМЕЛЬ

The scientific article considers methods for studying the underlying surface of agro landscapes of slope lands - local and integral: contact and non-contact. Each group in turn includes a visual, optical and mechanical approach. For carrying out experimental studies on sloping lands, a method has been developed for determining the average slope, roughness and waviness of an elementary site in the field, and profilographs for its implementation. Using the moving average method, it is possible to break the data into components and isolate, on a general background, the average surface slope, randomly distributed irregularities, surface roughness and technological furrows with a corrugation of the surface. In order to automate the process of determining these parameters, a program has been developed that allows you to directly calculate and display information on the monitor screen. Field studies using a profiler with contactless profilograph were conducted on different soil fertility, including the examination of the field station, located on the slope of the complex after the pre-winter tillage BDM-3x4P in Morgaushskiy District of the Chuvash Republic. After data processing, the average slope of the elementary area for a certain point of the field was 0.06 or 3.440. The direction of the main soil cultivation was determined from the angle of deviation of the technological grooves from the direction of the slope, which amounted to 93.60. The data obtained are presented in the Excel spreadsheet as a relationship between two parameters: the angle of rotation and the height of the surface profile of the soil, in the form of a sweep. Further, using the moving average method for the field section with soil disking, the average slope of the surface was determined to be 3.440, the surface roughness was 3.54 mm, and the waviness (soreness) of the soil surface was 7.94 cm.

ANTARCTIC ICE SHEET SLOPE AND ASPECT BASED ON ICESAT’S REPEAT ORBIT MEASUREMENT

Abstract. Accurate information of ice sheet surface slope is essential for estimating elevation change by satellite altimetry measurement. A study is carried out to recover surface slope of Antarctic ice sheet from Ice, Cloud and land Elevation Satellite (ICESat) elevation measurements based on repeat orbits. ICESat provides repeat ground tracks within 200 meters in cross-track direction and 170 meters in along-track direction for most areas of Antarctic ice sheet. Both cross-track and along-track surface slopes could be obtained by adjacent repeat ground tracks. Combining those measurements yields a surface slope model with resolution of approximately 200 meters. An algorithm considering elevation change is developed to estimate the surface slope of Antarctic ice sheet. Three Antarctic Digital Elevation Models (DEMs) were used to calculate surface slopes. The surface slopes from DEMs are compared with estimates by using in situ GPS data in Dome A, the summit of Antarctic ice sheet. Our results reveal an average surface slope difference of 0.02 degree in Dome A. High resolution remote sensing images are also used in comparing the results derived from other DEMs and this paper. The comparison implies that our results have a slightly better coherence with GPS observation than results from DEMs, but our results provide more details and perform higher accuracy in coastal areas because of the higher resolution for ICESat measurements. Ice divides are estimated based on the aspect, and are weakly consistent with ice divides from other method in coastal regions.

Ground beetle communities in a mountain river subjected to restoration: The Raba River, Polish Carpathians

Effects of passive restoration of mountain rivers on the organisms inhabiting exposed riverine sediments are considerably less understood than those concerning aquatic biota. Thus, the effects of a recovery of the Raba River after abandonment of maintenance of its channelization scheme on ground beetle (Coleoptera: Carabidae) communities were investigated by comparing 6 unmanaged cross-sections and 6 cross-sections from adjacent channelized reaches. In each cross-section, ground beetles were collected from 12 sampling sites in spring, summer, and autumn, and 8 habitat parameters characterizing the cross-sections and sampling sites were determined. Within a few years after abandonment of the Raba River channelization scheme, the width of this gravel-bed river increased up to three times and its multi-thread pattern became re-established. Consequently, unmanaged river cross-sections had significantly larger channel width and more low-flow channels and eroding cutbanks than channelized cross-sections. Moreover, sampling sites in the unmanaged cross-sections were typified by significantly steeper average surface slope and larger average distance from low-flow channels than the sites in channelized cross-sections. In total, 3992 individuals from 78 taxa were collected during the study. The ground beetle assemblages were significantly more abundant and richer in species in the unmanaged than in the channelized cross-sections but no significant differences in carabid diversity indices between the two cross-section types were recorded. Redundancy Analysis indicated active river zone width as the only variable explaining differences in abundance and species richness among the cross-sections. Multiple regression analysis indicated species diversity to predominantly depend on the degree of plant cover and substrate grain size. The study showed that increased availability of exposed sediments in the widened river reaches allowed ground beetles to increase their abundance and species richness within a few years after the onset of river restoration, but more time may be needed for development of more diverse carabid communities.

Wave Height Estimation from Shipborne X-Band Nautical Radar Images

A shadowing-analysis-based algorithm is modified to estimate significant wave height from shipborne X-band nautical radar images. Shadowed areas are first extracted from the image through edge detection. Smith’s function fit is then applied to illumination ratios to derive the root mean square (RMS) surface slope. From the RMS surface slope and the mean wave period, the significant wave height is estimated. A data quality control process is implemented to exclude rain-contaminated and low-backscatter images. A smoothing scheme is applied to the gray scale intensity histogram of edge pixels to improve the accuracy of the shadow threshold determination. Rather than a single full shadow image, a time sequence of shadow image subareas surrounding the upwind direction is used to calculate the average RMS surface slope. It has been found that the wave height retrieved from the modified algorithm is underestimated under rain and storm conditions and overestimated for cases with low wind speed. The modified method produces promising results by comparing radar-derived wave heights with buoy data, and the RMS difference is found be 0.59 m.

Forearc structure and morphology along the Sumatra‐Andaman subduction zone

AbstractThe Sunda subduction margin, the location of recent magnitude 8–9 megathrust earthquakes, is characterized by major changes in prism and forearc morphology and structure along its 5000 km length. On the Sumatra‐Andaman section, measurements of prism width and surface slope (α) indicate along‐strike segments, inclu1ding abrupt changes, of prism morphology: (1) a narrow and steep prism between Burma and the Andamans; (2) broad with an averaged gentle slope in the Andamans, Nicobars, and North Sumatra; (3) steep and narrow in Central Sumatra; and (4) wider and less steep offshore South Sumatra, decreasing in width to West Java. Prism width varies from ~90 to 180 km and average surface slope from ~1 to 3° with a strong inverse correlation between width and slope, also observed globally. The prism deviates from typical taper geometry in parts of the margin, notably offshore North Sumatra where it is characterized by a steep toe and broad plateau. Along‐strike changes in morphology are strongly linked to input sediment thickness. Sections of the prism toe represent key global examples of unusual landward vergent thrusting. These sections correspond to a thick sediment input and to a wide prism with shallow surface slope. A low basal shear stress or backstop mechanism may drive this style of faulting. Prism morphology and structure appear to be driven predominantly by input sediment thickness linked to oceanic basement topography, with sediment properties, plate smoothness, and orthogonal subduction rate and obliquity also contributing, and no clear role of plate age or dip.

Climatic and morphometric controls on the altitudinal range of glaciers, British Columbia, Canada

To examine the relation between climate and glacier extent, we compared gridded, monthly temperature and precipitation data to the altitudinal range of glaciers in British Columbia, Canada. We related glacier relief to ablation season temperature (June—August), accumulation season precipitation (September— March), and morphometric variables that included slope, glacier order, and a shape index for 523 alpine glaciers that ranged from 5 to 15 km2 in surface area. A 1°C increase in mean June—August temperature equates to a 109 to 182 m decrease in glacier relief, and a 1 mm increase in mean monthly September— March precipitation equates to a 0.78 to 2.20 m increase in glacier relief. The most important morphometric controls on this glacier—climate relation include average surface slope, glacier order (analogous to stream order), and the ratio of upper accumulation area to lower ablation area width, all of which are positively related to glacier relief. We note strong relations between glacier relief and climate in all mountain regions of British Columbia, with glaciers of the interior ranges being most sensitive to spatial climatic variability. We show how our approach can be used to estimate past climatic conditions based on historical ice extents, such as at the ‘Little Ice Age’ maximum, and to predict potential future equilibrium glacier extents in a changed climate regime, such as those predicted by general and regional circulation models.

Effects of spatial grid resolution on river flow and surface inundation simulation by physically based distributed modelling approach

AbstractGrid‐based distributed hydrological models are considered to be a very effective flood modelling tool for basin‐wide flood risk analysis because of their capabilities of simulating river and surface inundations at high spatio‐temporal resolutions by taking advantages of grid‐based data from meteorological models, radar and satellite remote sensing. Selecting an appropriate grid size is critically important for any application of a grid‐based model, which requires proper understanding of effects of grid sizes on simulated outcomes. The paper presents the outcomes of a study conducted to analyse the effects of grid resolution on simulated river peak flows and surface inundation in two selected river basins using a process‐based distributed hydrological model.The outcomes show that grid resolution significantly affects the simulated river peak flows and surface inundation characteristics. In both cases, it has been found that the effects are mainly caused by changes of the topographic parameters as a result of changes of grid sizes. The reduction of average surface slope with the increase of grid size affects the simulated surface inundation extents and heights. There is a threshold resolution of digital elevation model (DEM) in the simulated flood inundation and beyond that the model outcomes become arbitrary. Averaged topographic values at coarse resolution beyond this threshold level do not represent any characteristics of locally elevated topographic features such as dykes, highways, etc. and their influence on flood inundation characteristics can be no more captured by the model. Copyright © 2008 John Wiley & Sons, Ltd.

Surface morphological evolution during annealing of epitaxial Cu(001) layers

Single crystal Cu(001) layers were grown on MgO(001) by ultrahigh vacuum magnetron sputtering at Ts=100 °C. Quantitative surface morphological analyses by in situ scanning tunneling microscopy show that the surfaces exhibit self-affine mound structures with a scaling exponent of 0.82±0.03 and a mound radius rc that increases from 31±8 to 39±6 nm for increasing layer thickness t=24–120 nm. In situ annealing at 200 and 300 °C leads to a thermodynamically driven mass transport that minimizes the surface step density, resulting in broader mounds and a smaller root mean square surface roughness σ. This effect is most pronounced for t=24 nm, for which rc increases from 31±8 to 70±20 nm and σ decreases from 1.3±0.1 to 0.74±0.08 nm, resulting in a decrease in the average surface slope from χ=7° to 2° and an increase in the average terrace width wT by more than a factor of 4. In contrast, wT increases by only 20% for t=120 nm. This remarkable difference between “thin” and “thick” layers is attributed to diverging surface morphological pathways during annealing: The strong smoothening for t=24 nm is due to a competitive coalescence process where some mounds grow laterally at the expense of their smaller neighbors, which die out. In contrast, the initially wider mounds of thicker layers (t=120 nm) combine to form a quasistable surface morphology that exhibits anisotropic mound structures, which limit mass transport and stabilize the surface step density.

Turbulent flow over three‐dimensional dunes: 2. Fluid and bed stresses

Dunes formed in response to fluid flow exert a total boundary stress on the fluid that is made up of form drag and skin friction, the latter of which is generally considered important for predicting sediment transport and dune evolution. Previous research has used various methods to estimate the total stress and its subcomponents, with recent work suggesting the application of a spatial average to the equations of motion. A complete, three‐dimensional (3‐D) form of this analysis is derived and applied to recent measurements of turbulent open‐channel flow over 3‐D dunes. Spatial averages of Reynolds shear stresses over 3‐D dunes cannot be used to predict total boundary shear stress. However, estimations of total boundary shear stress from the spatially averaged equations of motion agree with not only the sum of form drag and skin friction but also the direct measurements of average free surface slope. Form‐induced stresses from secondary circulations augment the relatively low Reynolds shear stresses over the 3‐D dunes. These low turbulence levels have ramifications for sediment transport predictions, in that use of the total boundary shear stress in prediction of sediment transport over 3‐D dunes must account for these low turbulence levels so as not to overpredict transport rates.

Effect of Lateral Water Surface Profile on Side Weir Discharge

The average water surface slope in the lateral direction is taken into account as a local parameter ψ to study flow over a side weir. It was later shown that for smaller side weir lengths and side weir portions with no entrance and exit transition effects, ψ can be obtained from a numerical integral and also from the measurements of water surface elevations in the lateral direction. The effect of elemental weir length was further determined. Dividing the weir length into smaller computational segments has the effect of lowering the water surface to approach the measured profile, the downstream ends being coincident. The model was verified using experimental data.

Prediction of concentrated flow width in ephemeral gully channels

AbstractEmpirical prediction equations of the form W = aQb have been reported for rills and rivers, but not for ephemeral gullies. In this study six experimental data sets are used to establish a relationship between channel width (W, m) and flow discharge (Q, m3 s−1) for ephemeral gullies formed on cropland. The resulting regression equation (W = 2·51 Q0·412; R2 = 0·72; n = 67) predicts observed channel width reasonably well. Owing to logistic limitations related to the respective experimental set ups, only relatively small runoff discharges (i.e. Q < 0·02 m3s−1) were covered. Using field data, where measured ephemeral gully channel width was attributed to a calculated peak runoff discharge on sealed cropland, the application field of the regression equation was extended towards larger discharges (i.e. 5 × 10−4m3s−1 < Q < 0·1 m3s−1).Comparing W–Q relationships for concentrated flow channels revealed that the discharge exponent (b) varies from 0·3 for rills over 0·4 for gullies to 0·5 for rivers. This shift in b may be the result of: (i) differences in flow shear stress distribution over the wetted perimeter between rills, gullies and rivers, (ii) a decrease in probability of a channel formed in soil material with uniform erosion resistance from rills over gullies to rivers and (iii) a decrease in average surface slope from rills over gullies to rivers.The proposed W–Q equation for ephemeral gullies is valid for (sealed) cropland with no significant change in erosion resistance with depth. Two examples illustrate limitations of the W–Q approach. In a first example, vertical erosion is hindered by a frozen subsoil. The second example relates to a typical summer situation where the soil moisture profile of an agricultural field makes the top 0·02 m five times more erodible than the underlying soil material. For both cases observed W values are larger than those predicted by the established channel width equation for concentrated flow on cropland. For the frozen soils the equation W = 3·17 Q0·368 (R2 = 0·78; n = 617) was established, but for the summer soils no equation could be established. Copyright © 2002 John Wiley & Sons, Ltd.

The development of crystalline basement aquifers in a tropical environment

Abstract Chemical weathering is the dominant process in the development of a weathering profile on crystalline basement rocks. In the saturated lower part of the weathering profile, groundwater is the principal chemical reagent and the rate of groundwater flow determines the rate of weathering reactions. The search for successful borehole sites in this environment also depends upon finding locations where the rate of groundwater flow is at an optimum. This search could therefore be facilitated by an appreciation of the part which groundwater flow plays in the chemical weathering process. The principal factors which control the quantity of groundwater flow through a unit cross sectional area of the weathering system are the availability of recharge, the permeability of the weathered material and the hydraulic gradient between the recharge and discharge areas. Of these three, the importance of recharge is clear as this determines the input to the groundwater system. The permeability of the weathered material is a function of the extent of chemical weathering and is therefore intrinsically linked to the history of groundwater flow through the system. The remaining independent variable is the hydraulic gradient. This is normally developed in what is generally a relatively thin weathered layer which closely follows the surface morphology. The hydraulic gradient is therefore a function of the average surface slope over the groundwater flow path and therefore related to the surface morphology. A plot of recharge against surface morphology therefore becomes a valid projection upon which the various groundwater provinces can be plotted and provides a useful means of classifying the various occurrences of groundwater within the weathered profile.

A study of geometric and true levelling in electroplating using stylus methods

Under suitable conditions the process of electroplating improves the surface characteristics of the workpiece. This effect is called levelling and arises from two fundamental mechanisms: (i) geometric levelling and (ii) true levelling. Assessment of percentage levelling using surface profilometry ( R a ) is well established. However, the present work suggests that the use of more complex surface parameters obtained by digital surface metrology gives a better understanding of the mechanisms involved. Since the techniques are both quick and reliable they should form a major tool in the development of plating solutions and the establishment of the optimum plating conditions. The relations between the original surface finish R a , the average surface slope and the levelling achieved for varying plating thicknesses with geometric levelling and combinations of geometric and true levelling are discussed. Finally some preliminary results are reported for computer simulations of geometric and true levelling of actual surfaces.

Bed forms and stratification types of modern gravel meander lobes, Nueces River, Texas

ABSTRACTAll major streams draining the southwestern flank of the Edwards Plateau in south‐central Texas transport large volumes of gravel and sandy muddy gravel and are developing meander lobe sequences consisting predominantly of coarse gravel. The largest of these streams, the Nueces River, has a sinuosity index of 1.3 and an average stream surface slope of 1.8 m/km in the study area. Stream discharge is variable and has ranged from no flow to more than 17,000 m3/s.Mean clast b‐axis length for the ten largest clasts at thirteen sample sites ranged from 2.5 to 10.8 cm. Velocities of 2.7‐4.4 m/s 1 m above the stream bed are required to transport these clasts. Stream velocities of these magnitudes occur about once in 8 years when discharge of the Nueces River exceeds 3300 m3/s. Mean grain size of Nueces River alluvium ranges from 1.2 to 3.4 cm. At a flow depth of 1 m, sediment of this size has a critical erosion velocity of 1.8‐3 m/s. Velocities of this magnitude occur about once in two years when discharge exceeds 340 m3/s. Under these conditions flow is subcritical, with critical shear stresses on depositional surfaces ranging from 6.4 to 12.7 kg/m2.Gravel clasts are imbricated and channel bed forms are predominantly transverse gravel bars with slip faces ranging up to 2 m high and wavelengths in excess of 100 m. Stratification includes graded planar crossbeds and horizontal beds. Lower lateral accretion face sediments are also predominantly transverse bars; upper lateral accretion face deposits occur as longitudinal gravel ridges deposited in the lee of vegetation and, less commonly, as chute bars. Near the upper limit of meander lobes where vegetation is heavy, mud and muddy sand occur as overbank deposits; in these deposits sedimentary structures other than desiccation cracks are rare.Sedimentary sequences in gravel meander lobe systems deposited by low sinuosity streams are graded or non‐graded horizontal beds and planar cross‐beds overlain by mud and muddy sand interbedded with horizontally bedded gravels. Sequences may be several metres thick, but probably do not exceed 8‐10 m in thickness. These deposits in turn are overlain by overbank deposits of mud and muddy sand. Similar sedimentary sequences occur in the extensive Quaternary terraces that parallel the Nueces River.