Adjustments in channel morphology due to land‐use changes and check dam installation in mountain torrents of Calabria (southern Italy)

In Mediterranean semi‐arid conditions, the availability of studies monitoring channel adjustments as a response to reforestation and check dams over representative observation periods, could help develop new management strategies. This investigation is an integrated approach assessing the adjustments of channel morphology in a typical torrent of southern Italy after land‐use changes and check dam construction across a period of about 60 years. A statistical analysis of historical rainfall records, an analysis of land‐use changes in the catchment area and a geomorphological mapping of channel adjustments were carried out and combined with field surveys of bed surface grain‐size over a 5‐km reach including 14 check dams. The analysis of the historical rainfall records showed a slight decrease in the amount and erosivity of precipitation. Mapping of land‐use changes highlighted a general increase of vegetal coverage on the slopes adjacent to the monitored reaches. Together with the check dam network installation, this increase could have induced a reduction in water and sediment supply. The different erosional and depositional forms and adjustments showed a general narrowing between consecutive check dams together with local modifications detected upstream (bed aggradation and cross‐section expansion together with low‐flow realignments) and downstream (local incision) of the installed check dams. Changes in the torrent bends were also detected as a response to erosional and depositional processes with different intensities. The study highlighted: the efficiency of check dams against the disrupting power of intense floods by stabilizing the active channel and the influence of reforestation in increasing hillslope protection from erosion and disconnectivity of water and sediment flows towards the active channel. Only slight management interventions (for instance, the conversion of the existing check dams into open structures) are suggested, in order to mobilize the residual sediment avoiding further generalized incision of the active channel and coast line erosion. Copyright © 2017 John Wiley & Sons, Ltd.     

How sediment composition and flow rate influence downstream channel morphodynamics upon dam removal

The process of dam removal establishes the channel morphology that is later adjusted by high-flow events. Generalities about process responses have been hypothesized, but broad applicability and details remain a research need. We completed laboratory experiments focused on understanding how processes occurring immediately after a sediment release upon dam removal or failure affect the downstream channel bed. Flume experiments tested three sediment mixtures at high and low flow rates. We measured changes in impounded sediment volume, downstream bed surface, and rates of deposition and erosion as the downstream bed adjusted. Results quantified the process responses and connected changes in downstream channel morphology to sediment composition, temporal variability in impounded sediment erosion, and spatial and temporal rates of bedload transport. Within gravel and sand sediments, the process response depended on sediment mobility. Dam removals at low flows created partial mobility with sands transporting as ripples over the gravel bed. In total, 37% of the reservoir eroded, and half the eroded sediment remained in the downstream reach. High flows generated full bed mobility, eroding sands and gravels into and through the downstream reach as 38% of the reservoir eroded. Although some sediment deposited, there was net erosion from the reach as a new, narrower channel eroded through the deposit. When silt was part of the sediment, the process response depended on how the flow rate influenced reservoir erosion rates. At low flows, reservoir erosion rates were initially low and the sediment partially exposed. The reduced sediment supply led to downstream bed erosion. Once reservoir erosion rates increased, sediment deposited downstream and a new channel eroded into the deposits. At high flows, eroded sediment temporarily deposited evenly over the downstream channel before eroding both the deposits and channel bed. At low flows, reservoir erosion was 17–18%, while at the high flow it was 31–41%.     

Modelling the long-term geomorphic response to check dam failures in an alpine channel with CAESAR-Lisflood

Globally, between 1950 and 2011 nearly 80,000 debris flow fatalities occurred in densely populated regions in mountainous terrain. Mitigation of these hazards includes the construction of check dams, which limit coarse sediment transport and in the European Alps number in the 100,000s. Check dam functionality depends on periodic, costly maintenance, but maintenance is not always possible and check dams often fail. As such, there is a need to quantify the long-term (10–100 years) geomorphic response of rivers to check dam failures. Here, for the first time, a landscape evolution model (CAESAR-Lisflood) driven by a weather generator is used to replicate check dam failures due to the lack of maintenance, check dam age, and flood occurrence. The model is applied to the Guerbe River, Switzerland, a pre-Alpine catchment containing 73 check dams that undergo simulated failure. Also presented is a novel method to calibrate CAESAR-Lisflood's hydrological component on this ungauged catchment. Using 100-year scenarios of check dam failure, the model indicates that check dam failures can produce 8 m of channel erosion and a 322% increase in sediment yield. The model suggests that after check dam failure, channel erosion is the remobilization of deposits accumulated behind check dams, and, after a single check dam failure channel equilibrium occurs in five years, but after many check dam failures channel equilibrium may not occur until 15 years. Overall, these findings support the continued maintenance of check dams.     

Channel adjustments of the lower Trinity River,Texas, downstream of Livingston Dam

Channel cross‐sectional changes since construction of Livingston Dam and Lake Livingston in 1968 were studied in the lower Trinity River, Texas, to test theoretical models of channel adjustment, and to determine controls on the spatial extent of channel response. High and average flows were not significantly modified by the dam, but sediment transport is greatly reduced. The study is treated as an opportunistic experiment to examine the effects of a reduction in sediment supply when discharge regime is unchanged. Channel scour is evident for about 60 km downstream, and the general phenomena of incision, widening, coarsening of channel sediment and a decrease in channel slope are successfully predicted, in a qualitative sense, by standard models of channel response. However, there is no consistent channel response within this reach, as various qualitatively different combinations of increases, decreases or no change in width, depth, slope and roughness occur. These multiple modes of adjustment are predicted by the unstable hydraulic geometry model. Between about 60 km and the Trinity delta 175 km downstream of the dam, no morphological response to the dam is observed. Rather than a diminution of the dam's effects on fluvial processes, this is due to a fundamental change in controls of the fluvial system. The downstream end of the scour zone corresponds to the upstream extent of channel response to Holocene sea level rise. Beyond 60 km downstream, the Trinity River is characterized by extensive sediment storage and reduced conveyance capacity, so that even after dam construction sediment supply still exceeds transport capacity. The channel bed of much of this reach is near or below sea level, so that sea level rise and backwater effects from the estuary are more important controls on the fluvial system than upstream inputs. Copyright © 2005 John Wiley & Sons, Ltd.     

Morphological responses and sediment processes following a typhoon‐induced dam failure,Dahan River,Taiwan

The rates and styles of channel adjustments following an abrupt and voluminous sediment pulse are investigated in the context of site and valley characteristics and time‐varying sediment transport regimes. Approximately 10.5 x 10⁶ m³ of stored gravel and sand was exposed when Barlin Dam failed during Typhoon WeiPa in 2007. The dam was located on the Dahan River, Taiwan, a system characterized by steep river gradients, typhoon‐ and monsoon‐driven hydrology, high, episodic sediment supply, and highly variable hydraulic conditions. Topography, bulk sediment samples, aerial photos, and simulated hydraulic conditions are analyzed to investigate temporal and spatial patterns in morphology and likely sediment transport regimes. Results document the rapid response of the reservoir and downstream channel, which occurred primarily through incision and adjustment of channel gradient. Hydraulic simulations illustrate how the dominant sediment transport regime likely varies between study periods with sediment yield and caliber and with the frequency and duration of high flows. Collectively, results indicate that information on variability in sediment transport regime, valley configuration, and distance from the dam is needed to explain the rate and pattern of morphological changes across study periods. Copyright © 2013 John Wiley & Sons, Ltd.     

Bed stability variations after check dam construction in torrential channels (South‐East Spain)

The effects of check dams on the bed stability of torrential channels have been analysed in several tributary basins of the Segura and Guadalentín rivers (South‐East Spain). In order to illustrate the large variability in channel bed‐forms and bed sediment sizes along the stream, 52 reaches of 150 m in length were surveyed. This variability is due to the behaviour of check dams, which depends on bedrock control, bed slope, channel roughness, lateral sediment input and a highly variable sediment transport capacity. Though the purpose of check dams is to diminish the boundary shear stress, reducing the longitudinal slope, and to stabilize the channel bed, downstream they reduce the volume of channel‐stored material, favouring local scour processes, and upstream they can destabilize the sidewalls. The results enable us to evaluate the impact of every check dam on the bed morphology, distinguishing the structures installed in limy marl areas (e.g. catchment of the Cárcavo rambla, Cieza) and in schist and slate terrains (e.g. catchment of the Torrecilla rambla, close to Lorca). In the first type, bedrock and moderately thick granular beds predominate downstream from the check dams, so that the length of bedrock reaches and increase of roughness due to scour processes are the best indicators to verify its geomorphological effectiveness. On the other hand, the metamorphic areas drained by ramblas and gullies produce great quantities of gravel that are retained by check dams, creating more uniform and permeable beds, where the balance between sedimentation and scouring, and the ratio τ_c84/τ₀ (RBS), appear to be the parameters most frequently adopted to estimate the bed stability. Analysis of slope adjustments and the application of other indices to estimate the bed substrate stability (LRBS, SRI) and the structural influence of the dams (SIBS) corroborate the differences in bed stability found in the corrected reaches in each catchment. Copyright © 2007 John Wiley & Sons, Ltd.     

Evaluating methods to quantify sediment volumes trapped behind check dams,Salda(n)a badlands (Spain)

The determination of sediment yield in catchments based on the sediment trapped by check dams is becoming a subject of interest.In fact,several methods have been developed in recent years to estimate the sediment retained by check dams.The complexity,precision and accuracy of each method vary greatly.In this study,we evaluate the sediment trapped by check dams comparing the Sections method respect to the Prism,Pyramid,DTMs and Trapezoid methods.We analyzed a sample of 25 check dams(α-1=90%;ε=10%) in the Saldana badlands(Spain).The results showed that the Sections method offered a volume of retained sediment between the others,which gave an absolute variation from 22%to40%.The high variability of the check dam and sediment wedge sizes made necessary to compare methods in groups combining both characteristics.No significant differences in sediment volumes could be found between the methods for very small(height(h):1.8-2.3 m;trapped volume(V):6-102 m~3) or large check dams(h 2.3 m;V:165-387 m~3),while significant differences are found for small(h 1.5 m;V:1-229 m~3) or middle-sized check dams(h:2.2-3.2 m;V:65-235 m3).Nevertheless,volume differences between groups ranged up to + 25%.For these reason,the size of the check dam,the shape of the sediment wedge and the accuracy of the measure methods must considered when selecting an appropriate method to obtain the volume of retained sediment by check dams.A correct estimation of the sediment retention is needed to evaluate the role and efficiency of check dams in restoration projects or to estimate sediment yields.     

Recent channel adjustments in alluvial rivers of Tuscany,central Italy

Drastic channel adjustments have affected the main alluvial rivers of Tuscany (central Italy) during the 20th century. Bed‐level adjustments were identified both by comparing available topographic longitudinal profiles of different years and through field observations. Changes in channel width were investigated by comparing available aerial photographs (1954 and 1993–98). Bed incision represents the dominant type of vertical adjustment, and is generalized along all the fluvial systems investigated. The Arno River system is the most affected by bed‐level lowering (up to 9 m), whereas lower incision (generally less than 2 m) is observed along the rivers of the southern part of the region. Human disturbances appear to be the dominant factors of adjustments: the main phase of vertical change occurred during the period 1945–80, in concomitance with the phase of maximum sediment mining activity at the regional scale. The second dominant type of adjustment that involved most of the rivers in the region consists of a narrowing of the active channel. Based on measurements of channel width conducted on aerial photographs, 38% of the reaches analysed experienced a narrowing greater than 50% of the initial channel width. The largest values of channel narrowing were observed along initially braided or sinuous with alternate bars morphologies in the southern portion of the region. A regional scheme of channel adjustments is derived, based on initial channel morphology and on the amounts of incision and narrowing. Different styles of channel adjustments are described. Rivers that were originally sinuous with alternate bars to braided generally became adjusted by a moderate incision and a moderate to intense narrowing; in contrast, sinuous‐meandering channels mainly adjusted vertically, with a minor amount of narrowing. Copyright © 2003 John Wiley & Sons, Ltd.     

Erosion-control mechanism of sediment check dams on the Loess Plateau

Severe soil erosion occurs on the Loess Plateau in China, which makes the Yellow River the most sediment-laden river in the world. Construction of about 60,000 sediment check dams has remarkably controlled soil erosion on the Loess Plateau and reduced the sediment load of the middle and lower Yellow River. Nonetheless, little is known about the mechanism of erosion control and vegetation development of sediment check dams. The function of a single check dam mainly is trapping sediment, while the function of a train of check dams comprising dozens of or over hundreds of check dams in a gully encompasses controlling bed incision and reducing erosion energy. A formula was proposed to calculate the potential energy of bank failure and slope failure in a gully, which essentially constitutes the erosion energy. The erosion energy increases when gully incision occurs, which is induced by the incision of the Yellow River and its tributaries on the Loess Plateau. Sediment deposition in many gullies due to construction of check dams reduces the erosion energy to almost zero, which in turn greatly reduces soil erosion and sediment yield. Construction of check dams promotes vegetation development. The vegetation-erosion dynamics model was used to study the effect of check dams on vegetation development. Simulation results show that reforestation without check dam construction might result in an increase of vegetation cover in the first ten years and then a drop of vegetation cover to less than 10% in the later years. The check dams provide a foundation for vegetation development.     

Channel response to a dam-removal sediment pulse captured at high-temporal resolution using routine gage data

In this study, we captured how a river channel responds to a sediment pulse originating from a dam removal using multiple lines of evidence derived from streamflow gages along the Patapsco River, Maryland, USA. Gages captured characteristics of the sediment pulse, including travel times of its leading edge (~7.8 km yr⁻¹) and peak (~2.6 km yr⁻¹) and suggest both translation and increasing dispersion. The pulse also changed local hydraulics and energy conditions, increasing flow velocities and Froude number, due to bed fining, homogenization and/or slope adjustment. Immediately downstream of the dam, recovery to pre-pulse conditions occurred within the year, but farther downstream recovery was slower, with the tail of the sediment pulse working through the lower river by the end of the study 7 years later. The patterns and timing of channel change associated with the sediment pulse were not driven by large flow or suspended sediment-transporting events, with change mostly occurring during lower flows. This suggests pulse mobility was controlled by process-factors largely independent of high flow. In contrast, persistent changes occurred to out-of-channel flooding dynamics. Stage associated with flooding increased during the arrival of the sediment pulse, 1 to 2 years after dam removal, suggesting persistent sediment deposition at the channel margins and nearby floodplain. This resulted in National Weather Service-indicated flood stages being attained by 3–43% smaller discharges compared to earlier in the study period. This study captured a two-signal response from the sediment pulse: (1) short- to medium-term (weeks to months) translation and dispersion within the channel, resulting in aggradation and recovery of bed elevations and changing local hydraulics; and (2) dispersion and persistent longer-term (years) effects of sediment deposition on overbank surfaces. This study further demonstrated the utility of US Geological Survey gage data to quantify geomorphic change, increase temporal resolution, and provide insights into trajectories of change over varying spatial and temporal scales.     

Morphodynamics of an erodible channel under varying discharge

Alluvial channels arise through the interaction between morphology, hydraulics, and sediment transport, known as the ‘fluvial trinity’. Over relatively short timescales where climate and geology are fixed but discharge and sediment supply may vary, this process facilitates adjustments towards steady state, where the system oscillates around a mean condition. The relationship between changes in conditions and geomorphic response may be highly complex and nonlinear, especially in systems with multiple modes of adjustment. This study examines the adjustment of an erodible channel with fixed banks and a widely graded sediment mixture to successive increases in discharge. With each increase in discharge, components of the fluvial trinity adjusted towards a steady state. Particularly at relatively low discharges, adjustments were controlled by intrinsic thresholds and highlighted important morphodynamic processes. Notably, there was a strong interplay between channel morphology and sediment transport, and an effect whereby larger-than-average grains controlled channel deformation. These two processes occurred at the bar scale and were highly spatialised, which has two important implications: (1) reach-averaged representations of process provide only partial insight into morphodynamics; and (2) models of rivers that suppress these process feedbacks and size-dependent transport may not replicate morphodynamics that typically occur in field conditions. The experiments provide quantitative evidence for conceptual models describing exponential approaches towards steady state and the potential for transiency if disturbance frequency exceeds the recovery time. They also highlight how in natural rivers, particularly those with greater degrees of freedom for adjustment (notably, lateral adjustment and meandering), continuous changes in discharge may lead to nonlinear rather than steady-state behaviour. In these settings, more holistic analytical frameworks that embrace different aspects of the system are critical in understanding the direction, magnitude and timing of channel adjustments.     

Experimental study on blocking and self-cleaning behaviors of beam dam in debris flow hazard mitigation

Blocking is one of the important features when a beam dam intercepts debris flow, while self-cleaning is another when managing suspended debris flow. Both features determine the debris flow control benefits of beam dam but the latter often is not considered in practical engineering design. In this paper, a series of specially designed flume experiments were done to simulate blocking and self-cleaning processes. The blocking ratio and deposition features were measured to contrast the blocking and self-cleaning performance before and after artificial self-cleaning. The experimental results reveal that the beam dam net opening, particle diameter of sediment, sediment concentration, and gradient of the channel are the main factors affecting blocking performance. A new criterion of blocking performance of beam dams that considers the interaction of multiple factors and can provide guidance to practical project design is proposed. For all three types of blocking, sediment deposited upstream of a beam dam can be effectively transported downstream by erosion from post-debris-flow floods, Self-cleaning performance is most efficient for temporary blocking, followed by partial-blocking, and total-blocking. The efficiency of self-cleaning largely depends on the change of the sediment deposit due to erosion. Finally, a discussion is given for the optimal design of an open-type check dam and the feasibility of synergistic effects of self-cleaning in combination with artificial cleaning. Some supporting artificial silt-cleaning should be implemented in practice. A beam dam will, thus, have more storage capacity with which to control the next debris flow event.     

DESIGN OF SLIT DAMS FOR CONTROLLING STONY DEBRIS FLOWS

1 INTRODUCTION Stony debris flows are natural, highly concentrated water-sediment mixture, which forms wherever the simultaneous availability of water, debris material and an adequate slope, steeper than o10 are satisfied (Gregoretti, 2000). In mountainous regions of Taiwan, due to vast development and utilization of hills, stony debris flows are important from the point of disaster prevention, since they occur frequently and often bring about heavy loss of lives and properties. Therefo…     

Spatial variation of the vegetative roughness in Mediterranean torrential streams affected by check dams

This study focuses on the spatial variations in vegetative roughness associated with morphological channel adjustments due to the presence of check dams in Mediterranean torrential streams. Manning’s n values were estimated using methods established by previous studies of submerged and non-submerged vegetation in laboratory flume experiments and field work. The results showed a linear decrease in shrub density and rate of variation of the roughness coefficient versus degree of submergence with increasing distance upstream from the check dam, while downstream, the filling of the check dam and the bed incision had the most influence. A regression analysis was applied by fitting the data to different models: relationships between Manning’s n and the flow velocity were found to be of the power type for shrubs in all upstream sections, while relationships of flow velocity versus hydraulic radius in the sections closest to check dams showed a good fit to second-order polynomial equations.     

Silting patterns in the reservoirs of small- and medium-sized earthen check dams in humid subtropical monsoon regions

Silting of reservoirs is a ubiquitous process whenever water is impounded. Despite substantial work on the rates of silting, the spatial pattern of silting in reservoirs is not clearly understood. While it is anticipated that the variability of silting increases with decreasing reservoir size, not much is known about siltation in subtropical humid regions affected by monsoon rainfall. This paper presents the initial results of geomorphic analysis of six earthen check dams in the Shiwalik foothills of the Himalayas (India) in areas that are inhabited by high proportions of disempowered populations. These check dams include three small-sized dams (Dhamala-II, Rel Majra and Sukhomajri-II) and three medium-sized dams (Bunga-I, Parachh-II and Siswan). Field data were collected from each reservoir. The methods used included spatial interpolations of bed depth and silt thickness in each reservoir, Structure from Motion photogrammetry to extract multiple channel cross-sections from photographic scans along tributary mouths, texture analysis of bed and bank materials, and visual observations of hill slopes around the check dams. Based on this study, silt tended to concentrate either in the middle portion of the reservoir or near the dam; however, silt accumulation did not always occur along the dam or in the deepest portion of the reservoirs. Areas located downstream from these check dams are heavily used for various human activities and the channel network has almost been eliminated. While these earthen check dams may be cost-effective tools for water conservation in economically marginalized areas, if abandoned, these structures may pose a physical hazard to downstream communities. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.     

Hydraulic model tests for evaluating sediment control function with a grid-type Sabo dam in mountainous torrents

There are two kinds of Sabo dams in order to control sediment transport by debris flow and flash floods in mountainous area, which are closed and open-type＇s dams. In Japan, open-type＇s Sabo dams are constructed taking into account the continuity of sediment routing from upstream to downstream reach in a basin. A plan to construct a 20 m high grid-type Sabo dam which can capture a sediment volume of 400,000 m3 is proposed in the Amahata river basin in Japan. Hydraulic model tests are conducted to decide on the section for a dam （Section A, B） and the grid size such as clearance of vertical/horizontal bars for evaluating the plan. Several runs of flume tests are conducted and the sediment control function of the Sabo darn is discussed using several experimental data such as dimensionless sediment runoff rate from Sabo dam, temporal changes of bed profile and mean diameter and so on. It was found that sediment deposition in sediment storage area of Sabo dam was affected by curved channel, and that next the grid size of steel bars and thirdly the section of a dam was able to capture sediment in storage area of Sabo dam. Sediment was controlled well in the section B and in the grid size of 1.0×d95, and the problems related to sediment runoffafter sediment capturing in Sabo dam are pointed out.     

Characteristics of sedimentation and channel adjustment linked to the Three Gorges Reservoir

Construction of large dams is attractive because of their great benefits in flood control,hydropower generation,water resources utilization,navigation improvement,etc.However,dam construction may bring some negative impacts on sediment transport and channel dynamics adjustments.Due to the effects of recent water and soil conservation projects,sediment retention in the newly constructed large upstream reservoirs,and other factors,the sedimentation in the Three Gorges Reservoir(TGR)is quite different from the amount previously predicted in the demonstration stage.Consequently,based on the measured data,characteristics of sedimentation and the related channel deformation in the TGR were analyzed.The results imply that sediment transport tended to be reduced after the Three Gorges Project(TGP).Sedimentation slowed dramatically after 2013 and indicated obvious seasonal characteristics.Due to the rising water level in the TGR in the flood season,the yearly sediment export ratio(Eratio)was prone to decrease.The water level near the dam site should be reasonably regulated according to the flow discharge to improve the sediment delivery capacity and reduce sedimentation in the TGR,and to try to avoid situations where the flood retention time is close to 444 h.The depositional belt was discontinuous in the TGR and was mainly distributed in the broad reaches,and only slight erosion or deposition occurred in the gorge reaches.Sedimentation in the broad and gorge reaches accounted for 93.8% and 6.2% of the total sedimentation,respectively.The estuarine reach located in the fluctuating backwater area experienced alternate erosion-deposition,with a slight accumulative deposition in the curved reach.Sedimentation mainly occurred in the perennial backwater area.The insight gained in this study can be conducive to directly understanding of large reservoir sedimentation and mechanism of channel adjustment in the reservoir region in the main channel of large river.     

DEBRIS FLOW CONTROL BY USING SLIT DAMS

1 INTRODUTIONTaiwan is a hilly-mountainous island lying across the center of the tropic. The slopeland accounts fortwo thirds of the total area of Taiwan. Most mountains are consisted of geologically young rocks incategories of fissile slates and shales which are easily eroded by weather. TOrrents combined with fissileslates and shales form debris flows with a very strong destrUctive power. This kind of debris flow canpotentially occur for almost all torrents whose gradient is steeper th…     

Transport of suspended sediment under the dam‐break flow on an inclined plane bed of arbitrary slope

The problem of transport of suspended sediment after the break of a dam on an inclined bed is considered. To that end we use the shallow‐water approximation for arbitrary, constant slopes of the bottom, taking into consideration the effect of friction. The numerical technique and the frictional model are validated by comparison with available experimental data and asymptotic analytical solutions, with special attention to the numerical solution near the wetting front. The transport of suspended sediment down the inclined bed is obtained and discussed as a function of the slope of the bed for different values of the parameters characterizing the sediment and its transport properties. For sufficiently large times we always find the formation of roll waves near the water front, which affects the transport of sediments significantly. These strong oscillations are accurately computed with the numerical method used. The relative importance of the bed load (to the suspended load) sediment transport is also discussed as a function of the size of the sediment particles and the slope of the bed for different models on the initiation of sediment suspension from bed load. We also check the dilute sediment approach and characterize the conditions for its failure. Finally, the results of the present simplified model are intended to be used as tests of more complex numerical models. Copyright © 2007 John Wiley & Sons, Ltd.     

Network‐scale dynamics of grain‐size sorting: implications for downstream fining,stream‐profile concavity,and drainage basin morphology

We explore the link between channel‐bed texture and river basin concavity in equilibrium catchments using a numerical landscape evolution model. Theory from homogeneous sediment transport predicts that river basin concavity directly increases with bed sediment size. If the effective grain size on a river bed governs its concavity, then natural phenomena such as grain‐size sorting and channel armouring should be linked to concavity. We examine this hypothesis by allowing the bed sediment texture to evolve in a transport‐limited regime using a two grain‐size mixture of sand and gravel. Downstream ?ning through selective particle erosion is produced in equilibrium. As the channel‐bed texture adjusts downstream so does the local slope. Our model predicts that it is not the texture of the original sediment mixture that governs basin concavity. Rather, concavity is linked to the texture of the sorted surface layer. Two different textural regimes are produced in the experiments: a transitional regime where the mobility of sand and gravel changes with channel‐bed texture, and a sand‐dominated region where the mobility of sand and gravel is constant. The concavity of these regions varies depending on the median gravel‐ or sand‐grain size, erosion rate, and precipitation rate. The results highlight the importance of adjustments in both surface texture and slope in natural rivers in response to changes in ?uvial and sediment inputs throughout a drainage network. This adjustment can only be captured numerically using multiple grain sizes or empirical downstream ?ning rules. Copyright © 2004 John Wiley & Sons, Ltd.