Wide Shallow Channel Research Articles

At‐a‐station hydraulic geometry for reaches with bridge collapse events

AbstractThis study explores the at‐a‐station hydraulic geometry (AHG) for reaches with bridge collapse events. Eighteen reaches in Eastern United States, thirteen reaches in Appalachian Highland, and five reaches in Coastal Plain are examined. The methodology applied for retrieving AHG uses LiDAR (Light detection and ranging) data, and the study results are checked for both hydraulic and geomorphic consistency. The resulting data set is composed of five to thirty‐five measurements of water surface width, mean depth, and mean velocity at each of the 181 cross‐sections. The exponents of the AHG relationships vary considerably. Nonetheless, for most of the cross‐sections, width responds more rapidly to changing discharge, and velocity exponents are less than the width and depth exponents combined. Wide shallow channels with highly erodible beds and/or banks, the ability to transport large bed materials, and the ability to attain a super‐critical condition—are the common profile extracted for most of the cross‐sections across all sites. A definitive AHG configuration is found for the sites with the least human interference. Comparatively low variation of bank‐full geometry is also found for the sites with the least human interference. The prevalence of low flows and/or lower return periods of heavy‐tail flows are also exhibited for most of the sites. The study results suggest that the stream channel instability can be reasonably understood and predicted from AHG particularly if human interference is limited within the watershed. These findings have implications not only for the study of the risk of bridge collapse and bridge design but also to characterize instability in a more rigorous and practical way.

Taphonomy of drought afflicted tetrapods in the Early Triassic Karoo Basin, South Africa

The sedimentology and taphonomy of in-situ fossils from earliest Triassic strata (Induan) in the southern Karoo Basin of South Africa is presented as evidence for episodes of drought-induced mass death of the resident tetrapods. Abundant skeletons are preserved in a 2 m-thick tabular silty-sandstone capping a multi-storeyed low-sinuosity channel sandstone interpreted as a wide shallow channel that became progressively abandoned, with more ephemeral flow regime than in the underlying channels and subjected to intermittent flows of low-density sediment-laden floodwaters. Stratigraphic and planimetric distribution of 170 in-situ tetrapod fossils shows several clusters of up to eight closely-spaced articulated Lystrosaurus skeletons preserved in prone and spread-eagled body position. These are interpreted as drought-stricken carcasses that collapsed and died of starvation in and alongside dried-up water sources. Two of the specimens display an unusual micritic envelope with a distinctive pustular texture interpreted as permineralised mummified skin indicative of rapid desiccation after death. Bonebeds of disarticulated bones of multiple juvenile Lystrosaurus occur in shallow depressions within the rubified mudstones. Layering of different skeletal elements suggests some hydraulic sorting but the initial aggregation was likely a behavioural response to drought. Osteohistology of spread-eagled Lystrosaurus (L. declivis and L. murrayi species) skeletons show that they represent early juvenile stage which is in accordance with previous findings that throughout Pangaea Early Triassic Lystrosaurus died relatively young due to environmental stressors. Our results support the hyperthermal hypothesis that ∼252 Mya increased continental aridity, already a consequence of the coalescence of Pangaea, was critically intensified by volcanogenic greenhouse gasses from the Siberian traps. We propose that in the aftermath of the End-Permian mass extinction event, a succession of climatic drying episodes orchestrated a series of fully-functioning terrestrial ecosystems that were markedly different to those of the pre-extinction, and likely had a profound and lasting influence on the evolution of tetrapods.

Fluvial sedimentation and its reservoir potential at foreland basin margins: A case study of the Puig-reig anticline (South-eastern Pyrenees)

Fluvial fans represent one of the dominant sedimentary systems at the active margins of non-marine foreland basins. The Puig-reig anticline at the north-eastern margin of the Ebro Foreland Basin (SE Pyrenees, Spain) exposes continuous outcrops of Late Eocene-Early Oligocene fluvial deposits, from proximal to medial fluvial fan environments. The proximal deposits are found in the north limb of the anticline, especially in the northwest zone. These deposits are characterised by conglomerates with minor interbedded sandstones, with thick and wide sheet-like geometries with unscoured or variably scoured basal surfaces. These are interpreted to be the deposits of unconfined flash floods and wide-shallow channel streams. The medial deposits, covering the rest of the anticline, consist of interbedded conglomerates, sandstones and claystones. These are interpreted to have been deposited from braided to meandering channel streams and overbank areas. Distal deposits are found towards the south, beyond the anticline, and are characterised by sandstone and clay deposits of terminal lobes and lacustrine deltas. This study assesses the impact of the primary depositional characteristics, diagenesis and deformation of the most heterolithic portion of the system, with implications for increasing our understanding of folded fluvial reservoirs. Diagenetic processes, mainly mechanical compaction and calcite cementation, resulted in overall low intergranular porosity, with limited relatively high porosity developed in sandstone lithofacies in the medial deposits. Deformation associated with thrusting and fold growth resulted in the formation of abundant fractures, with relatively high fracture intensities observed in sandstone lithofacies in the anticline crest. This study shows that post-depositional processes can both improve and diminish the reservoir potential of basin proximal fluvial deposits, through the development of fracture networks and by compaction-cementation. The comparison of the Puig-reig anticline with other similar settings worldwide indicates that foreland basin margin locations may be potential areas for effective reservoirs, even in the case of low intergranular porosity.

Fracture distribution in a folded fluvial succession: The Puig-reig anticline (south-eastern Pyrenees)

Sedimentary rocks of foreland fold-and-thrust belts typically undergo intensive fracturing as folds grow. The resulting fracture networks can present significant variations depending on the distribution of sedimentary facies and the complex structural characteristics of fold-and-thrust belts. The Puig-reig anticline, located in the south-eastern Pyrenees, mainly exposes proximal fluvial deposits in the north limb and medial fluvial deposits in the rest of the anticline. Thus, this anticline constitutes an excellent case study to investigate the main controls on the distribution of fracture networks in folded fluvial deposits, in terms of structural position and lithofacies variations. Outcrops were selected to be representative of different structural positions, from the fold hinge to its limbs, and of a variety of the main lithofacies, from proximal to medial fluvial deposits. Fracture data were acquired using the linear scanline method. The results indicate that the anticline rocks have been affected by four sets of fractures. The north limb is dominated by thick conglomerate bodies with interlayered sandstones deposited from unconfined flash floods and wide-shallow channel streams in the proximal fluvial fan, and presents large fracture spacing and low fracture intensity but relatively large fracture length and aperture. The crest and the crest-limb transition zones are mainly characterised by interlayered conglomerates, sandstones, siltstones and clays, deposited from braided channel streams and overbanks in the medial fluvial fan, and present fractures with relatively high fracture intensity and variable fracture length and aperture. The south limb, composed of channel filling sandstone layers and stable overbank fine deposits in the medial fluvial fan, is characterised by low fracture intensity and small fracture length and aperture. Based on multiple linear regression analysis, fracture intensity is mainly controlled by the structural position, bedding thickness and lithological associations, with relatively more intense fracturing in thin sandstone layers with multiple interlayers of fine deposits in the anticline crest. The fracture length mainly depends on bedding thickness and is affected lithological associations. The fracture apertures are mainly controlled by lithofacies, with relatively higher apertures affecting conglomerate bodies. The results of this study are relevant for characterising similar systems in the subsurface, where data is scarce.

Spatiotemporal distributions and mixing dynamics of characteristic contaminants at a large asymmetric confluence in northern China

Previous studies of confluences have mainly concentrated on their hydrodynamic characteristics and morphological dynamics. However, the increasing environmental pollution in rivers has resulted in abrupt variations in water quality at confluences, thus affecting the ecosystems in river networks. Therefore, more attention should be paid to the pollutant distribution and mixing processes at confluences. In this study, several field experiments on pollutant concentrations were conducted at a large asymmetric confluence with a discordant bed and wide-shallow channel in northern China, where a heavily polluted tributary known as the Fen River joins the Yellow River. The results displayed that the heavily polluted tributary seriously affected the water quality at the confluence, especially in the downstream channel near the tributary. The contaminations at the confluence present a relatively strong temporal variability. The organic matter and dissolved oxygen show a similar seasonal variability with each other whereas ammonia nitrogen and total phosphorus are also similar to each other, but they have a different variability from the first group. These pollutants mostly demonstrated the same spatial distribution characteristics, with large horizontal concentration differences at the confluence. The standard deviation of the cross-sectional concentration gradually decreased with the downstream mixing of contaminants. Nonetheless, under conventional hydrological conditions, these contaminants were not completely mixed through a length of 27 river widths. Compared with a concordant bed, this discordant confluence exhibited a relatively fast mixing rate. The key factor affecting the mixing process at the confluence was the discharge ratio. Exceptionally, during the irrigation period (March–April), owing to the extremely low discharge of the tributary, the main stream flowed backward into the tributary, which enhanced the mixing motion to constitute an approximately full mixing regime at the mouth of the tributary. These results are of great importance in understanding the degree of contamination, spatiotemporal distribution and mixing dynamics of characteristic pollutants at complex river confluences.

Linking Management and Riparian Physical Functions to Water Quality and Aquatic Habitat

Stream and wetland riparian areas are able to sustain a state of resiliency based on the ecosystem’s ability to attain the functions of its ecological potential. This resiliency allows an area to provide and produce desired and valued water quality and aquatic habitat ecosystem services. Maintaining healthy aquatic and riparian habitats depends on “management” allowing for, or facilitating natural recovery of riparian functions. Altering grazing management practices in Maggie Creek lead to changes in riparian functionality, water quality, and aquatic habitat. Maggie Creek basin, historically renowned for its fishery, is one of only a few watersheds in Nevada capable of supporting Lahontan Cutthroat Trout (LCT) (Oncorhynchus clarkia ssp. Henshawi) meta-populations. Prior to 1993, the majority of Maggie Creek was grazed by cattle throughout the growing season. Decades of intensive grazing, water development, and road construction degraded aquatic and riparian habitats. By the early 1990’s, a majority of the Maggie Creek watershed was rated as nonfunctional or functional-at-risk condition with unstable banks, channel incision, loss of riparian vegetation, wide shallow channels, excessive erosion and deposition, reduced stream flows, and increased water temperatures. As mitigation for their 1993 South Operations Area Project mine dewatering, Newmont Mining Company, in cooperation with the Elko District Bureau of Land Management (BLM) and the Elko Land and Livestock Company, developed the Maggie Creek Watershed Restoration Project to enhance LCT habitat. The project was developed to enhance 82 miles of stream, 2000 acres of riparian habitat and 40,000 acres of upland watershed primarily through prescriptive livestock management. Beginning in 1994, grazing systems were implemented for portions of the perennial/intermittent streams. This greatly reduced the frequency and duration of hot season grazing on Maggie Creek and its tributaries. The objective of this paper is to compare 1994 and 2006 stream and wetland riparian assessments using proper functioning condition (PFC) protocol and water quality data.

Instability of Laminar Flow in Wide Shallow Meander Channel

Yang, Y.; Liu, X., and Zhang, M., 2015. Instability of laminar flow in a wide shallow meander channel.The meander river has one of the most common types of natural river morphology, where the flow pattern influences the transformation and development of river morphology. The theory of the instability analysis of laminar flow in a wide-shallow meander channel with constant curvature is investigated in this study. The modified Orr-Summerfield equation, which is established in a curvilinear orthogonal coordinate for two-dimensional incompressible flow, is simplified for the case of wide-shallow channel. The characteristics of instability, such as the neutral curve of critical Reynolds number, contour of growth rate and characteristic spectrum of wave number are obtained by performing the numerical simulations. The computed results show that the characteristic spectrum of meander channel is similar to that of straight channel. It can thus be inferred that the flows in both channels have the same stability characteristics. Furthermore, the neutral curve moves leftward and the critical Reynolds number decreases with the increase of bank curvature. Secondly, the response to the disturbance wave number also increases, which results in the instability of the flow. Thus, this study explains the reasons for the laminar flow becoming unstable in meander channel and also establishes the fundamental theory to further study meandering rivers.

Arquitectura fluvial de las «Areniscas del río Arandilla». Triásico de Molina de Aragón (Guadalajara)

Los excelentes afloramientos de la parte superior del Buntsandstein (Triasico medio) en Molina de Aragon (Guadalajara), han permitido llevar a cabo un analisis sedimentologico detallado de las facies fluviales y de su arquitectura. Se han distinguido doce diferentes facies cuyas caracteristicas y asociaciones se describen en este trabajo. La evolucion fluvial indica la existencia de cuatro episodios deposicionales. Los dos episodios inferiores son caracteristicos de un sistema fluvial con canales de baja sinuosidad, inestables, de gran amplitud y poca profundidad. El drenaje de la cuenca estaba controlado por importantes descargas estacionales. El episodio intermedio es el resultado de la evolucion del sistema hacia facies mas distales. En esta etapa los canales son de menor tamano, la sinuosidad es mayor y existe un mayor porcentaje de depositos de granulometria fina relacionados con la llanura de inundacion. Por ultimo, durante el episodio final, se produce un rejuvenecimiento de la red fluvial, debido a la actividad tectonica que tuvo lugar en la zona. Se ha establecido una correlacion entre las causas de la evolucion de la arquitectura fluvial y las variaciones eustaticas globales segun la escala de Haq el al. (1987). De esta forma la disminucion del nivel del mar que tuvo lugar durante el Anisiense inferior podria corresponder al episodio final de la unidad «Areniscas del rio Arandilla».

The meandering-braided river threshold: A reappraisal

The concept of a threshold discharge-slope (Q-s) combination that separates braided from meandering streams is critically re-examined, partly by review of previous data, and partly by analysis of new data from South Island, New Zealand. It is argue d that discriminant functions that use mean annual discharge provide a poor physical basis for interpretations based on natural processes, while bankfull discharge is also deficient because it is not a fully independent variable. The common use of channel (rather than valley) slope adds a further misleading component because channel slope is partly dependent on channel pattern sinuosity. Most previous studies have inadequately treated the role of bed particle size in pattern discrimination, failing to recognize that active gravel streams must plot higher on a Q-s chart than sand-bed channels — irrespective of pattern — because of the greater power requirements for bed material movement. Within any one size class of bed material there is no evidence to indicate a clear discrimination between braiding and meandering, only a weak statistical association between pattern and slope-discharge values. While this association may be explicable in terms of the high shear stress and stream power (that accompany high Qs-values) promoting braiding, the real prerequisite for braiding appears to be high loads of bed-calibre material (producing wide shallow channels), a factor that is only partly controlled by stream power. Accordingly, the search for a pattern threshold based on discharge and slope seems to be a futile exercise. An interpretation of existing pattern discriminant functions from the perspective of the threshold shear stress for particle movement is made. It is concluded that, in effect, these functions merely state that gravel-bed streams are more likely to be braided than channels in finer sediment. Statistically this appears to be valid, and reasons for that difference are offered based on the balance between imposed bed material load and channel capacity.

The impact of afforestation on stream bank erosion and channel form

AbstractModification of the land use of a small catchment through coniferous afforestation is shown to have influenced stream bank erosion and channel form. Field mapping and erosion pin measurements over a 19‐month period provides evidence of more active bank erosion along forested channel reaches than along non‐forested. Extrapolation of downstream increases in bankfull width, bankfull depth, and channel capacity with increasing basin area for the non‐forested catchment has demonstrated that afforestation of the lower part of the catchment has had a marked effect on channel form. Channel widths within the forest are up to three times greater than that predicted from the regression. These changes in bankfull width have led to stream bed aggradation and the development of wide shallow channels within the forest, and channel capacities within the forest are over two times that predicted from the basin area. The relationship between channel sinuosity and valley gradient for non‐forested reaches of the river also indicated decreased sinuosity resulting from afforestation. These changes in channel form result from active bank erosion within the forest with coarse material being deposited within the channel as point‐bars and mid‐channel bars. Active bank erosion is largely attributed to the suppression by the forest of a thick grass turf and its associated dense network of fine roots, and secondly to the river attempting to bypass log jams and debris dams in the stream channel.

Velocity distribution and the boundary layer at channel bends

An analysis of the accelerating forces available within a relatively wide shallow channel bend shows that the existence of higher velocities near the outside bank requires, in addition to the helical flow pattern commonly observed, that the outward flowing upper layers be unaffected by bed shear. Velocity measurements made in a model channel bend at a point unaffected by entry conditions have confirmed this requirement, indicating that the inward flowing bottom layers move within a boundary layer similar to those found near the surface of airfoils. The parameter (r/v) dv/dr, which should have the value −1 for free vortex flow and +1 for flow with constant angular velocity, exceeded +4 in these experiments and was found to be directly proportional to the friction factor. Theory developed indicates that this parameter is directly proportional to the width‐radius ratio also in those regions beyond the effect of entry conditions.

Origin of Inclosed Meanders on Streams of the Colorado Plateau

Meanders more or less narrowly inclosed by canyon walls characterize certain parts of many streams of the Colorado Plateau. On larger streams, the meanders appear to have been inherited from a preceding well-advanced erosion cycle; they are intrenched with inappreciable change of plan where hard rocks immediately underlay the former valley, but are greatly modified or obliterated where weak rocks have been encountered in deepening the valley. On smaller streams inclosed meanders are restricted to parts of the course carved in hard rocks, and their form indicates origin by downward-lateral corrasion, possibly or probably during the present erosion cycle. In soft rocks the tendency to cut horizontally so widens and clears the valley that the stream flows in a very wide shallow channel which is relatively straight. The origin of these features is mainly controlled by the strength of the rocks and relative loading of the streams.