Single-thread Rivers Research Articles

Modelling shear stress distribution in ice-covered streams

Abstract Distribution of bed shear stress is the critical factor in regulating the meandering of single-thread rivers. However, the impact of ice cover on bed shear stress is largely unknown. In this study, we develop a theoretical model of cross-stream momentum balance to examine the distribution of bed shear stresses in ice-covered meandering rivers. To validate the theoretical model, field surveys were carried out in a river reach of the Red River in Fargo, North Dakota. Data monitoring was completed using an Acoustic Doppler Current Profiler to obtain time-averaged velocity profiles. Our theoretical model indicates that an ice covering develops high-shear zones near both the inner and outer banks, which might exacerbate sediment transport and enhance bank erosion. Velocity measurements confirm the results of the proposed model and demonstrate a clear impact of meandering river banks on velocity profiles and secondary flow patterns under ice cover. Based on our results, we hypothesize that ice cover increases turbulent stresses near banks, which in turn lead to the enhancement of the bed shear stress. Our work provides new insights into the impact of ice cover on bed shear stress distribution, which could play an important role in driving sediment-transport processes and the long-term morphodynamic evolution of meandering rivers seasonally covered by ice.

Effects of initial and boundary conditions on gravel-bed river morphology

Assuming that the equilibrium geometry of river channels does not depend on their initial state but solely on boundary conditions, several formulas have been derived that relate the channel depth and width to the river bankfull discharge and bed material. However, due to the existence of a threshold for sediment motion and the strong non-linearity between sediment transport and flow rate, this assumption might not be generally valid for gravel-bed rivers. This research clarifies the role of the initial conditions, more specifically the initial channel width, on the geometry of gravel-bed rivers considering a variety of boundary conditions. The approach includes laboratory experiments and two-dimensional modeling, reproducing the evolution of alluvial channels with different starting widths, discharge regimes and sediment input rates. The experiments represent the Arc River (France). Thus, the characteristics of this river were used in the numerical model to obtain a realistic virtual case complementing the experiments. Different boundary and starting conditions resulted in either braided or single-thread channels. We found that the initial width strongly influences the evolution process and leaves a footprint on the river braid-belt extension. The active width of braided systems and the width of single-thread channels do not depend on the starting condition. They depend on sediment input rather than on discharge variability. Different initial widths result in different final bed levels. This indicates that the initial channel width may affect the degree of channel incision or aggradation. The results of this study justify the use of equilibrium formulas for single-thread rivers.

Coupling threshold theory and satellite-derived channel width to estimate the formative discharge of Himalayan foreland rivers

Abstract. We propose an innovative methodology to estimate the formative discharge of alluvial rivers from remote sensing images. This procedure involves automatic extraction of the width of a channel from Landsat Thematic Mapper, Landsat 8, and Sentinel-1 satellite images. We translate the channel width extracted from satellite images to discharge using a width–discharge regime curve established previously by us for the Himalayan rivers. This regime curve is based on the threshold theory, a simple physical force balance that explains the first-order geometry of alluvial channels. Using this procedure, we estimate the formative discharge of six major rivers of the Himalayan foreland: the Brahmaputra, Chenab, Ganga, Indus, Kosi, and Teesta rivers. Except highly regulated rivers (Indus and Chenab), our estimates of the discharge from satellite images can be compared with the mean annual discharge obtained from historical records of gauging stations. We have shown that this procedure applies both to braided and single-thread rivers over a large territory. Furthermore, our methodology to estimate discharge from remote sensing images does not rely on continuous ground calibration.

The case of the braided river that meandered: Bar assemblages as a mechanism for meandering along the pervasively braided Missouri River, USA

Abstract This paper offers a mechanism for meandering in an otherwise braided river and then discusses its general implications for river processes and fluvial deposits. Braided rivers manage to meander without the paired point bars and single-thread channels that are instrumental in developing bends in other meandering rivers. The driving processes for meandering in these braided systems remain enigmatic. The unchannelized and prechannelized Missouri River is an example of a braided meandering river, and it provides an opportunity to gain insight into these processes. This study utilized historical maps, sequential air photos, and surficial geologic maps both to define the processes by which this braided river meanders, and to characterize the deposits produced by these processes. These data show that the Missouri River meanders by building point assemblages instead of point bars. Repeated accretion of midchannel and lateral bars to a common point on the bank forces development of a meander bend around a point assemblage comprising multiple amalgamated compound bars. This differs from single-thread systems, which expand and translate bends around a single compound point bar. Alternating development of point assemblages forces meandering over successions of meander bends. Braided meander loops grow by expansion and translation like single-thread rivers, but they also may contract to produce counterpoint assemblages. Contraction appears to be the more common means of loop abandonment compared to loop cutoff for the braided Missouri River. This differs from single-thread meandering rivers, where contraction is limited, and loop cutoff is consistently the dominant abandonment process. Deposits of the braided meandering Missouri River differ from deposits of single-thread rivers in the rarity of both meander scrolls and single-thread channel fills. Instead, point and counterpoint assemblages comprise fusiform bar elements bound by small filled remnants of anabranch channels. These assemblages are commonly bound by meander cutbank scars. Cutbank scars associated with contraction, however, tend to be composite rather than discrete erosional surfaces, and they do not tend to bind river-scale abandoned channel fills. The braided meandering Missouri River also differs from wandering rivers because wandering rivers meander by building compound bars instead of assemblages, are more gravelly, have less pervasive and much less mobile midchannel bars, and appear to reflect a transitional intermediate pattern instead of a stable hybrid pattern. Braiding and meandering both expend stream power, and both are mechanisms for achieving channel equilibrium. The Missouri River exhibits both of these processes in tandem; thus, meandering and braiding are not mutually exclusive processes. Braided meandering rivers like the Missouri River are less common than either straight-braided or single-thread-meandering rivers, but they are not unique. The long-held distinction of braided versus meandering patterns for rivers thus may be practical but is not definitive.

Wpływ cofki zbiornika zaporowego na koryto rzeki górskiej – perspektywy badań = The impact of dam-reservoir backwater on mountain river channel – research perspectives

Dam reservoir construction is one of the most important factors shaping river-valley morphology in the Anthropocene. While a large number (>58,000) of these constructions are in operation all over the world, we remain quite ignorant of what happens upstream of them (in so called backwater zone), especially for the case of gravel-bed rivers. Existing studies have shown that adjustments of the gravel-bed river in the backwater zone differ between the initial and long-term adjustments. The initial adjustments (occurring ≈ <20 years following dam construction) are controlled by large floods and in-channel deposition which trigger bi-directional bar↔bank interactions (bank erosion causing bar growth and vice versa) resulting in channel-widening. The long-term adjustments (≈ >20 years following dam construction) are characterized by river sinuosity increa sing and channel planform stabilization resulted from deposition of fine sediment and associated vegetation expansion. The long-term adjustments are controlled by the initial river morphology, which creates accommodation space for the deposition of fine sediment and for the associated expansion of vegetation on channel bars. The multi-thread river in backwater zone is significantly narrowed, its sinuosity increase (phase 1) and the planform is stabilized (phase 2). Whereas, in the case of initially single-thread river only planform stabilization occur (phase 2). This article summarizes recent findings on the backwater effects on gravel-bed channel morphodynamics, suggesting that backwater zones may be treated as hot-spots of human-induced changes in river geomorphology and biogeomorphology.

The barrier river reach identification and classification in the Middle Yangtze River

Adjustments of upstream river regimes are one of the main factors affecting downstream fluvial processes. However, not all adjustments of river regimes will propagate downstream. There are some distinctive river reaches where upstream and downstream adjustments have no relevance. However, the irrelevance is neither caused by different river types nor by the different conditions of water and sediment; but rather, the channel boundaries and riverbed morphologies block the propagation effect. These are referred to here as the barrier river reach phenomena. The migration of the thalweg line is the essential reason for causing the propagation effect. Numerous influencing factors for thalweg migration exist, including 1) the average flow rate above the critical bankfull discharge, the average flow rate below the critical bankfull discharge, and their ratio, 2) the ratio of the duration of the aforementioned two periods, 3) the thalweg displacement at the entrance of the river reach, 4) the deflecting flow intensity of the node, 5) the ratio of the river width to water depth, 6) the relative width of the floodplain, and 7) the Shields number. In this study, the correlativity between the measured distances and the restricting indicators of thalweg migration in the Middle Yangtze River over the years was established. The barrier degree of 27 singlethread river reaches was subsequently assessed. These reaches included 4 barrier river reaches; 5 transitional reaches transforming from barrier to non-barrier; 10 transitional reaches transforming from non-barrier to barrier; and 8 non-barrier river reaches. Barrier river reaches were found to be important for maintaining the stability of the river regime and the transverse equilibrium of sediment transport in the downstream reaches. To some extent, the barrier river reaches may protect the natural dynamical properties from being destroyed by artificial river regulation works. Thus, they are of great significance for river management.

Model-Based Analysis of Macrophytes Role in the Flow Distribution in the Anastomosing River System

The impact of vegetation on the hydrology and geomorphology of aquatic ecosystems has been studied intensively in recent years. Numerous hydraulic models developed to date help to understand and quantitatively assess the influence of in-stream macrophytes on a channel’s hydraulic conditions. However, special focus is placed on single-thread rivers, leaving anastomosing rivers practically uninvestigated. To fill this gap, the objective of this study was to investigate the impact of vegetation on flow distribution in a complex anastomosing river system situated in northeastern Poland. The newly designed, one-dimensional, steady-flow model, dedicated for anastomosing rivers used in this study indicated high influence of vegetation on water flow distribution during the whole year in general, but—as expected—significantly higher in the summer season. Simulations of in-stream vegetation removal in selected channels reflected in Manning’s coefficient alterations caused relatively high discharge transitions during the growing season. This proved the significance of feedback between process of plants growth and distribution of flow in anabranches. The results are unique and relevant and could be successfully considered for the protection of semi-natural anabranching rivers.

Morphodynamic effects of stone and wooden groynes in a restored river reach

Restoration works on an 11 km long reach of the Spree River near Cottbus in Germany were accomplished in 2014. The overall objective was to improve the biodiversity, changing the morphological structure from a straight single-thread river to a braiding one with reshaped thalweg, and creating zones with varying flow velocities required for fish spawning. Several interventions were made, among which also two types of groynes were constructed along the reach: impermeable stone groynes, with an inclined crest so that only the tip is typically submerged during normal flow conditions, and wooden groynes made of several large trunks, which are mainly submerged at normal flow conditions. To compare local morphodynamic effects caused by the different types, flow velocities and bed topography were measured at distances of 12 m upstream and 16 m downstream of two selected groynes. The vertical profile was captured by measuring at five levels above the bed. Bed material was sampled at several locations around the groynes. The analysis of the measurements showed the effect of each groyne type on the flow velocity, bed topography and bed material. Furthermore, the differences in the effects of each groyne type on the flow, bed topography and bed material were highlighted.

Quantitative characterization of shales within tidally influenced fluvial valley fill deposits of the Ferron Sandstone, eastern Utah: Implications for hydrocarbon exploration

This study evaluates the proportion, length, and effective properties of thin (0.003 - 0.7 m [0.01 – 2.3 ft.]) shale beds and drapes in tidally-influenced channels within a compound valley fill with a focus on estimating geologically-based effective rock properties. The Cretaceous Ferron Sandstone is an outcrop analog for fluvial-tidal systems with primary reservoirs being deposited as tidally-influenced valley filling point bars. The study outcrops expose three valley systems in Neilson Wash of Utah. Light detection and ranging derived digital outcrop models have been used to characterize shale length, width, thickness and frequency of each valley fill succession. Long, uncommon, and anisotropic shales in Valley-1 (V1) were deposited in a braided setting with little tidal influence. In contrast, shales in Valley-2 (V2) were abundant, short, common, and equidimensional suggesting deposition by more tidally-influenced meandering rivers. Short, frequent, and equidimensional shales in Valley-3 (V3) were deposited in single-thread meandering rivers with less tidal influence. A sandstone-shale model was utilized to estimate the effects of shales on vertical to horizontal permeability ratio (kv/kh). The unique character of each depositional unit was reflected in resultant kv/kh distributions. The valley fill deposits, V1, V2, and V3 had an average kv/kh ratio of 0.11, 0.09 and 0.17, respectively. More tidally-influenced reservoirs such as the studied V2 had short, but frequent shales which resulted in low kv/kh estimates. Estimates of kv/kh for valleys that predominantly contained fluvial point bar deposits with lesser tidal influence (V1 and V3) were higher. The results of this study highlight the link between shale heterogeneity, reservoir architecture, and inferred flow parameters.

The mechanism of barrier river reaches in the middle and lower Yangtze River

Alluvial channel has always adjusted itself to the equilibrium state of sediment transport after it was artificially or naturally disturbed. How to maintain the equilibrium state of sediment transport and keep the river regime stable has always been the concerns of fluvial geomorphologists. The channel in the middle and lower reaches of the Yangtze River is characterized by the staggered distribution of the bifurcated river and the single-thread river. The change of river regime is more violently in the bifurcated river than in the single-thread river. Whether the adjustment of the river regime in the bifurcated river can pass through the single-thread river and propagate to the downstream reaches affects the stabilities of the overall river regime. Studies show that the barrier river reach can block the upstream channel adjustment from propagating to the downstream reaches; therefore, it plays a key role in stabilizing the river regime. This study investigates 34 single-thread river reaches in the middle and lower reaches of the Yangtze River. On the basis of the systematic summarization of the fluvial process of the middle and lower reaches of the Yangtze River, the control factors of barrier river reach are summarized and extracted: the planar morphology of single-thread and meandering; with no flow deflecting node distributed in the upper or middle part of the river reach; the hydraulic geometric coefficient is less than 4; the longitudinal gradient is greater than 12‰, the clay content of the concave bank is greater than 9.5%, and the median diameter of the bed sediment is greater than 0.158 mm. From the Navier-Stokes equation, the calculation formula of the bending radius of flow dynamic axis is deduced, and then the roles of these control factors on restricting the migration of the flow dynamic axis and the formation of the barrier river reach are analyzed. The barrier river reach is considered as such when the ratio of the migration force of the flow dynamic axis to the constraint force of the channel boundary is less than 1 under different flow levels. The mechanism of the barrier river reach is such that even when the upstream river regime adjusts, the channel boundary of this reach can always constrain the migration amplitude of the flow dynamic axis and centralize the planar position of the main stream line under different upstream river regime conditions, providing a relatively stable incoming flow conditions for the downstream reaches, thereby blocking the upstream river regime adjustment from propagating to the downstream reaches.

Phenomena and characteristics of barrier river reaches in the middle and lower Yangtze River, China

Alluvial river self-adjustment describes the mechanism whereby a river that was originally in an equilibrium state of sediment transport encounters some disturbance that destroys the balance and results in responses such as riverbed deformation. A systematic study of historical and recent aerial photographs and topographic maps in the Middle and Lower Reaches of the Yangtze River (MLYR) shows that river self-adjustment has the distinguishing feature of transferring from upstream to downstream, which may affect flood safety, waterway morphology, bank stability, and aquatic environmental safety over relatively long reaches downstream. As a result, it is necessary to take measures to control or block this transfer. Using the relationship of the occurrence time of channel adjustments between the upstream and downstream, 34 single-thread river reaches in the MLYR were classified into four types: corresponding, basically corresponding, basically not corresponding, not corresponding. The latter two types, because of their ability to prevent upstream channel adjustment from transferring downstream, are called barrier river reaches in this study. Statistics indicate that barrier river reaches are generally single thread and slightly curved, with a narrow and deep cross-sectional morphology, and without flow deflecting nodes in the upper and middle parts of reaches. Moreover, in the MLYR, barrier river reaches have a hydrogeometric coefficient of \({<}{4}\), a gradient \({>}\)1.2‱, a silty clay content of the concave bank \({>}{9.5}\)%, and a median diameter of the bed sediment \({>}{0.158}\) mm. The barrier river reach mechanism lies in that can effectively centralise the planimetric position of the main stream from different upstream directions, meaning that no matter how the upper channel adjusts, the main stream shows little change, providing relatively stable inflow conditions for the lower reaches. Regarding river regulation, it is necessary to optimise the benefits of barrier river reaches; long river reaches without barrier properties should be systematically planned and regulated; drastic bank collapse and sandbar shrinking should be urgently controlled to prevent the loss of barrier effects.

Dam-induced base-level rise effects on the gravel-bed channel planform

Little is known on the planform evolution of gravel-bed rivers in base-level raised backwater zones upstream from dam reservoirs. The general model of river channel response to base-level rise predicts a decrease in river sinuosity. However, the observations of channel adjustments in the reservoir backwater document a narrower and more sinuous channel developed as a result of fine sediment deposition and vegetation expansion. Here, the long-term aerial photo-based observations (1963–2015) of two gravel-bed rivers of different initial channel pattern (the Dunajec and its tributary, the Smolnik) located in the base-level raised zone of the Rożnów Reservoir in the Polish Carpathians were analyzed. The results show that the initially multi-thread river in backwater was significantly narrowed and its sinuosity increased (phase 1), which was followed by the stabilization of channel planform (phase 2). However, in the initially single-thread river only phase 2 occurred. The rate of channel narrowing observed on the initially multi-thread river was positively related to the initial channel width (R2=0.90, p<0.0001) and bar width (R2=0.81, p<0.0001). The increase in sinuosity was higher in the previously wider channel section and did not occur in the valley-confined zones of the single-thread river. The results are conceptualized in a two-phase conceptual model, which hypothesizes that the trajectories of the long-term planform adjustments of gravel-bed river in backwater are controlled by the initial river morphology which creates accommodation space for fine sediment deposition and associated vegetation expansion.

Morphosedimentary features from a major flood on a small, lower-sinuosity, single-thread river: The unknown quantity of overbank deposition, historical-change context, and comparisons with a multichannel river

This study details overbank sedimentation on a single-thread, bedload stream, the Little Blue River (southeastern Nebraska and northeastern Kansas, USA), which experienced exceptional floods during May–June 2015. Downvalley elongate splays left deposits as thick as 1.2m and attained areas as great as 156ha. Splays consisted of fields of large, gravel-covered dunes 0.1 to 0.3m in height and extensive sand sheets with current ripples or current ripples atop low-relief, long-wavelength dunes. Erosion occurred chiefly in the heads of splays. Levee-floodplain channel complexes were more architecturally complicated, with channel–channel-fringing deposits deposited within ~110m of the banks of the channel. These complexes consisted of two amalgamated components: (1) sediment shadows (vegetation-induced sedimentary structures), as long as 6m and as high as 0.6m at their heads, which were produced by the interaction of direct overbank flow with rooted trees; and (2) indistinct, flat-topped bars (some of which were built around eroded remnants of sediment shadows) shaped chiefly by downvalley flood flow. Changes on in-channel bars, the erosion of bank crevasses as large as 70m in width, and the deposition of large woody debris also occurred during the floods.The earliest aerial photographs (1938) of the study area, taken after multiple low-flow years, show no evidence for recent overbank deposition. Increases in channel-belt area, the number of bars, and the sizes of bars in the Little Blue River took place after 1938, and most rapidly during 1938 to 1950, when annual discharge was increasing. Annual discharge has decreased since ca. 1990, and yet there has been a long-term increase in annual peak from since the 1930s. We surmise that increasing annual peak flows have led to larger floods and, since 1938, serial overbank deposition, as well a wider channel and numerous, large compound bars.

Free instability of channel bifurcations and morphodynamic influence

Channel bifurcations are a fundamental element of a broad variety of flowing freshwater environments worldwide, such as braiding and anabranching rivers, river deltas and alluvial fans. River bifurcations often develop asymmetrical configurations with uneven discharge partition and a bed elevation gap between the downstream anabranches. This has been reproduced by one-dimensional (1-D) analytical theories which, however, rely on the empirical calibration of one or more parameters and cannot provide a clear and detailed physical explanation of the observed dynamics. We propose a novel two-dimensional (2-D) solution for the flow and bed topography in channel bifurcations based on an innovative application to a multi-thread channel configuration of the 2-D steady linear solution developed decades ago to study river bars and meandering in single thread river settings. The resonant value of the upstream channel aspect ratio, corresponding to the theoretical resonance condition of regular river meanders (Blondeaux &amp; Seminara,J. Fluid Mech., vol. 157, 1985, pp. 449–470) is the key parameter discriminating between symmetrical and asymmetrical bifurcations, in quantitative agreement with experimental observations and numerical simulations, and qualitatively matching field observations. Only when the aspect ratio of the upstream channel of the bifurcation exceeds resonance, is the bifurcation node able to trigger the upstream development of a steady alternate bar pattern, thus creating an unbalanced configuration. Ultimately, the work provides an analytical explanation of the intrinsic legacy between bifurcation asymmetry and the phenomenon of 2-D upstream morphodynamic influence discovered by Zolezzi &amp; Seminara (J. Fluid Mech., vol. 438, 2001, pp. 183–211).

Physical modelling of the combined effect of vegetation and wood on river morphology

The research reported in this paper employs flume experiments to investigate the potential effects of living vegetation and large wood on river morphology, specifically aiming to explore how different wood input and vegetation scenarios impact channel patterns and dynamics. We used a mobile bed laboratory flume, divided into three parallel channels (1.7m wide, 10m long) and filled with uniform sand to reproduce braided networks subject to a series of cycles of flooding, wood input, and vegetation growth. Temporal evolution of river configuration (in terms of the braiding index), vegetation establishment and erosion, and wood deposition amount and pattern was recorded in a series of vertical images. The experiments reproduced many forms and processes that have been observed in the field, from scattered logs in unvegetated, dynamic braided channels to large wood jams associated with river bars and bends in vegetated, stable, single-thread rivers. Results showed that the inclusion of vegetation in the experiments changes wood dynamics, in terms of the quantity that is stored and the depositional patterns that develop. Vegetated banks increased channel stability, reducing lateral erosion and the number of active channels. This promoted the formation of stable wood jams, where logs accumulated continuously at the same locations during subsequent floods, reinforcing their effect on river morphology. The feasibility of studying these processes in a controlled environment opens new possibilities for disentangling the complex linkages in the biogeomorphological evolution of the fluvial system and thus for promoting improved scientific understanding.

Modest Change In Fluvial Style With Varying Accommodation In Regressive Alluvial-To-Coastal-Plain Wedge: Upper Cretaceous Blackhawk Formation, Wasatch Plateau, Central Utah, U.S.A

Abstract Concept-driven sequence stratigraphic models of alluvial-to-coastal-plain successions suggest that fluvial architecture and style should transition from isolated, single-story, channelized sandbodies deposited by single-thread (typically meandering) rivers in mudstone-prone, high-accommodation intervals into densely stacked, amalgamated sandbodies deposited by multiple-thread (typically braided) rivers in low-accommodation intervals. Model predictions of changing fluvial style are tested by comparing the facies character, internal architecture, dimensions, and formative paleohydraulic conditions of representative, major fluvial sandbodies developed at different stratigraphic levels of an alluvial-to-coastal-plain succession developed under a progressively decreasing rate of accommodation creation (Late Cretaceous Blackhawk Formation, Wasatch Plateau, central Utah, USA). The major fluvial sandbodies have a similar facies composition, and consist mainly of cross-bedded, medium-grained sandstone with subordinate mudclast conglomerate, folded sandstone, and inclined sandstones and intercalated siltstones. Facies are arranged within a hierarchy of architectural components. Channel stories (mean width and depth of 90 m and 7.2 m, respectively) represent the migration of a paleochannel segment and adjacent bar, and they are amalgamated laterally into channel belts (mean width and depth of 400 m and 9.2 m, respectively) that in turn are stacked vertically into channel-belt complexes (mean width and depth of 400 m and 20 m, respectively). Channel stories and belts were deposited by a combination of three paleochannel types that occur together in most major sandbodies: (1) single-thread “cut-and-fill” channels; (2) single-thread, laterally accreting channels of low-to-moderate sinuosity; and (3) multiple-thread, wandering-to-braided channels. Estimated paleochannel slopes are uniformly low (< 0.04°), most sandbodies contain sparse, potential marine indicators (e.g., Teredolites-bored logs and tidally modulated? carbonaceous drapes along cross-bed foresets), and paleodischarge estimates imply that multiple-thread channels may have narrowed and branched downstream to form distributary networks. All of these features are consistent with a delta-plain setting. Channel story and belt stacking patterns within each major sandbody (channel-belt complex) are highly non-uniform, such that (1) there are no systematic trends shared by the sandbodies, (2) sandbodies do not result from systematic, short-term changes in accommodation, such as those associated with the incision and fill of coastal incised valleys, and (3) variability within each sandbody is more pronounced than variability between sandbodies. These results suggest that local variations in sediment flux and transport capacity, combined with local avulsion history, were the principal controls on the architecture and dimensions of the major sandbodies. The similarities in architecture between major fluvial sandbodies imply that these controls were not predominantly governed by proximity to the coeval shoreline (c. 40–100 km) or by long-term tectonic subsidence rate (c. 80–700 m/Myr), which controlled creation of accommodation.

Environmental impact of the 1.8 ka Taupo eruption, New Zealand: Landscape responses to a large-scale explosive rhyolite eruption

Large-scale ignimbrite eruptions from rhyolitic caldera volcanoes can trigger geologically instantaneous changes in sedimentary systems over huge areas by either burying existing environments or overloading them with vast quantities of unconsolidated particulate material. The post-eruption readjustment of the landscape to such perturbations is one of the most dramatic processes in physical sedimentology, exemplified here by the 1.8 ka Taupo eruption in the central North Island of New Zealand. This eruption generated voluminous fall deposits, then climaxed with emplacement of a c. 30 km 3 non-welded ignimbrite over a near-circular area of c. 20 000 km 2. Approximately 90% of the area, but < 50% of the ignimbrite volume, is represented by a landscape-mantling unit that covered the pre-eruption topography to a depth varying from c. 10 m in proximal areas to less than 15–30 cm distally. The remainder of the ignimbrite deposit is represented by landscape-modifying material that ponded in valley bottoms and depressions to thicknesses of up to 70 m, with no systematic variation in thickness with distance from source. The headwaters of many of the North Island's largest rivers were impacted by both the primary pyroclastic fall and flow material. Large-scale post-eruption remobilisation of this material, coupled with the re-establishment of fluvial systems, occurred in a distinct sequence as recorded by the evolution of sedimentary facies in different sub-environments. Following an initial period dominated by mass flows, re-establishment of fluvial systems began with the headward erosion of box canyons through the ponded ignimbrite deposits, a process often associated with the break-out of temporary lakes. Aggradational streams developed in these channels rapidly evolved from shallow, ephemeral, sediment-laden outbursts associated with flash flood events to deeper, permanent braided rivers, before declining sediment yields led to retrenchment of single thread rivers and a return to pre-eruption gradients and bedloads years to decades later. Typically the modern profile of many streams and rivers follow closely their pre-eruption profiles, and incision and erosion is overwhelmingly confined to the deposits of the eruption itself. Although the general remobilisation pattern is similar for all impacted river systems, detailed studies of the Waikato, Rangitaiki, Mohaka, Ngaruroro and Whanganui catchments show that the relative timing and scale of each eruption response phase differs between each catchment. These reflect differences in catchment physiography and hydrology, and the volume and type of pyroclastic material deposited in each. Ultimately, the landscape response reflects the relative spatial distributions of, and the volumetric ratios between, the volumes of pyroclastic debris, water, and accommodation space in the basin (cf. Kataoka and Manville, this volume).

Reduced‐complexity flow routing models for sinuous single‐thread channels: intercomparison with a physically‐based shallow‐water equation model

AbstractReduced‐complexity models of fluvial processes use simple rules that neglect much of the underlying governing physics. This approach is justified by the potential to use these models to investigate long‐term and/or fundamental river behaviour. However, little attention has been given to the validity or realism of reduced‐complexity process parameterizations, despite the fact that the assumptions inherent in these approaches may limit the potential for elucidating the behaviour of natural rivers. This study presents two new reduced‐complexity flow routing schemes developed specifically for application in single‐thread rivers. Output from both schemes is compared with that from a more sophisticated model that solves the depth‐averaged shallow water equations. This comparison provides the first demonstration of the potential for deriving realistic predictions of in‐channel flow depth, unit discharge, energy slope and unit stream power using simple flow routing schemes. It also highlights the inadequacy of modelling unit stream power, shear stress or sediment transport capacity as a function of local bed slope, as has been common practice in a number of previous reduced‐complexity models. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Morphological response to river engineering and management in alluvial channels in Italy

In response to various types of human disturbance, most Italian rivers have experienced considerable channel adjustment during the last centuries and in particular in the last decades. This paper reviews all existing published studies and available data, and aims to reconstruct a general outline of the main channel adjustments that have occurred in Italian rivers during the past 100 years. Two main types of channel adjustment have been recognized: (a) incision, which is commonly on the order of 3–4 m, but in some cases is even more than 10 m; (b) narrowing, with channel width reduction up to 50% or more. In some reaches, these adjustments have led to changes in channel pattern in particular from braided to wandering. Such channel adjustments are due to several types of human intervention, particularly sediment extraction, dams and channelization. A strong temporal relationship (specifically, short reaction times) between human disturbance and channel adjustment can be inferred, but trends of adjustment are available for only a few rivers (e.g. the Po, the Arno and the Piave Rivers). These trends show that incision and/or narrowing are more intense immediately after the disturbance and then slow and become asymptotic; the same trends also suggest that larger rivers could have longer relaxation times. The results of this study are synthesised in a general classification scheme that summarises the main styles of adjustment observed in Italian rivers. According to the scheme, braided rivers adjust through prevalent narrowing with varying rates of incision, whereas single-thread rivers adjust mainly through a more pronounced incision accompanied by various amounts of narrowing. The scheme, representing initial and final (present) morphologies and not including intermediate stages of channel adjustment, will need to be tested on the basis of more detailed data to have a wider application both to the Italian context and to fluvial systems elsewhere, affected by similar types of human disturbance causing a reduction of sediment supply.

Experimental Steep, Braided Flow: Application to Flooding Risk on Fans

Flooding processes occurring on alluvial fans are considerably different from those occurring along single thread rivers with well defined floodplains. Active erosion, rapid sedimentation, and the uncertainty in flow path make the prediction of flood evolution and extent difficult. Based on a large scale experiment, this study investigates the long term evolution of the flow on a steep, noncohesive sediment surface resembling a complex of merged alluvial fans. The results are pertinent to the assessment of flooding hazard on alluvial fans. At any given time, the average flow occupancy was 21% of the surface. However, the flow was characterized by active channel switching and overflow processes. The percentage of the surface remaining dry was found to decay harmonically with time. A reworking time was defined as the time at which half of the surface that was initially dry remained dry, whereas the other half was inundated at least once. An empirical expression was developed in which reworking time is proportional to the average cross sectional area of flow and inversely proportional to the sediment supply.