Bankfull Discharge Research Articles

Dynamic of channel potholes on Precambrian geological sites of Chhota Nagpur plateau, Indian peninsula: applying fluvio-hydrological and geospatial techniques

Dynamic channel bed potholes or sculpted forms (circular, elliptical, oval, elongated, etc.) on riverbeds are generally formed due to the variation of lithology with structures and dominant fluvio-hydrological processes. Most of the channel bed potholes are confined at Bhakuyadi, Guridih and Ghatsila on Precambrian Chhota Nagpur granitic gneiss and tonalite gneiss of the river Subarnarekha and its tributaries. A total of 215 channel bed potholes are studied at three sites [Site 1 Bhakuyadi (n = 115), Site 2 Guridih (n = 65) and Site 3 Ghatsila (n = 35)]. The main objective is to find out the multi-dimensional growth on Precambrian bed rock channel and analyze the role of fluvio-hydrological factors toward their growth. Geospatial, fluvio-hydrological and statistical techniques have been applied to find out the potholes dynamic on lithological diversity of extended part of the Chhota Nagpur plateau of gneissic complex. Lineament features have been extracted using different SWIR and VNIR bands of two specific images like ETM and ASTER. Arc Hydrology tool (Hydro tool of ArcGIS) has been used to delineate stream networks and watershed of river Subarnarekha. Morphology of potholes like shape–form analysis has been done through morphometric index such as Surface Index, Bottom Index and Vertical Index. Downward and sideward potholes growth rate are calculated using mathematical indices, and depth–radius ratio has been assessed applying descriptive statistics. The results show that the multi-dimensional growth or pothole dynamics are immensely regulated by local condition such as rock types with structural imprints (fractures, joints, etc.). The absence of micro-geological structures on others lithological unit have failed to give birth to multi-dimensional growth of channel potholes along the flow path of river Subarnarekha and its tributaries within the Subarnarekha watershed. Chhota Nagpur granitic gneiss and tonalite gneiss with structural signatures help to form channel bed potholes within the flow path of river Subarnarekha and its tributaries. Statistical analysis reveals that there is a strong positive correlation between depth and radius of potholes. Subsequently, fluvio-hydrological factors (like corrasion, cavitation, hydraulic action, change of channel gradients, monsoonal bankfull discharge, etc.) clearly reflect the dynamic growth of channel bed potholes specifically during monsoonal months.

Meander geometry, hydraulics and sedimentary structures—a case study of the Usri River section, Jharkhand, India

The Usri River is a low sinuous ephemeral waterway of 90-km length, characterized by fluctuating stream discharge. During summer months (April–May), the river gets dried up almost completely exposing the point bars and the stream bed. It maintains an almost uniform gradient of 1:664 except in the lower 8–9-km reach, where the gradient changes to 1:150. The maximum bankfull width varies from 75 to 604 m, bankfull depth 1.30 to 3.02 m, stream bed slope 0.00125 to 0.00275, sinuosity 1.34 to 1.48, and bankfull discharge 210.40 to 351.91 m3/s. This study uses an enlarged data set comprising previous works and present study. The enlarged data set is used (1) to measure the different meander parameters of the Usri River; (2) to interrelate the meander parameters of the Usri according to the rules which have been empirically established in other parts of the world; (3) to derive a new equation from Chezy and Manning equations (Rouse 1961) and used in estimating the relative maximum discharge for two successive years, where the discharge hydrographs are not available; and (4) to establish an empirical relation between relative discharge and relative volume of sedimentation in the point bars. In the present study, two new linear equations have been established considering the ripple heights and cross-bedding thickness with water depth, one for small and large ripples when combined and the other for small ripples. Subsurface sedimentary structures in the point bars were studied by making trenches. It is observed that the frequency percentages of tabular type of cross-beddings increase downstream than the trough, lenticular type, possibly indicating the aggrading nature of the river with reducing effect of scouring downstream. The aggradation of sediments downstream is also reflected in the sigmoidal curve of the river profile, where the stream bed slopes of the studied four sites are considered.

Toward a Better Understanding of Recurrence Intervals, Bankfull, and Their Importance

AbstractBankfull is a concept that is intimately tied to the annual flood series through the well‐accepted tenet that bankfull discharge occurs at approximately the 1.5‐year recurrence interval on the annual series. Thus, due to this association the annual series provides a useful diagnostic tool for helping to identify the bankfull elevation in the field. The partial‐duration series does not provide an equivalent tool because paired discharge and recurrence interval values from the flood frequency curve depend upon the minimum threshold selected for developing the partial‐duration series. However, the interpretation that bankfull discharge occurs on average once every 1.5 years, or two out of every three years from that bankfull discharge/recurrence interval relationship on the annual series is incorrect. Frequencies of small floods (those with recurrence intervals ≤10 years) should be obtained using the partial‐duration flood series because it contains a more accurate representation of the size and frequency of small events. We used discharge data from 11 streams in West Virginia watersheds that ranged from about 0.14 to 223 km2 to compare the two series and to illustrate the variability in small flood frequencies through time. Flooding to the bankfull stage was absent some years but occurred as many as four or five times during other years.

Assessment of river bed evolution with the aid of 2D hydrodynamic model with integrated sediment transport modeling capabilities

The aim of the study is to assess hydro-morphologic evolution of currently cut off meander for 3 river restoration scenarios during 10 days of bank-full discharge. To simulate hydro-morpho-dynamics...

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.

Development of Equations for Meander Movement Velocities by SCE Optimization Algorithm based on Experimental Study

Meandering rivers have a dynamic planform, which means that rivers move in radial direction with variable velocities. The movement velocities can be estimated from historical maps of specific river or using mathematical approach. There is not explicit method or suitable algebraic equation for estimation of river meander movement velocities. In this study, experiments were carried out in a laboratory flume, to develop novel equations to calculate meandering river movement velocities. We designated these empirical equations as the river resistance equations. The experiments were conducted with non-uniform, nonchosive sediment in a re-circulating flume with changeable slopes. The influence of bankfull discharge, bed slope and channel width on the meander movement velocities were discussed. Based on the experimental data, three novel empirical equations for prediction of Wx and Wexp and Wm were proposed. Moreover, the shuffled complex evolution (SCE) optimization algorithm was used to estimate the unknown parameters of the empirical equations. To validate SCE method, the result was compared with traditional multiple regression analysis. This comparison indicated that the SCE optimization algorithm outperformed the multiple regression analysis method in the empirical equations calibration.

Estimation of Bankfull Discharge in the Lower Yellow River

Bankfull discharge is highly correlated with bankfull elevation. The main channels and bankfull elevations in multi-thread sections are ambiguous and need additional indicators to define. Thus, two additional indicators (maximum width and effective width coefficient) were adopted to estimate the bankfull elevation of both multi- and single-thread cross-sections. Three geometric-criterion based definitions of bankfull elevation (minimum width-depth ratio, abruptly changing-point of wetted area to top channel width ratio, first maximum Riley bench index) were selected to estimate bankfull elevation. Typical cross-sections selected were the Huayuankou and Lijin cross-sections in the Lower Yellow River. More than 300 cross-sectional datasets of the Huayuankou and Lijin were collected to calibrate the value of effective width coefficient. Second, calibrated coefficients were verified by various applications, estimating the variation of sectional bankfull discharge in floods, estimating the variation of sectional bankfull discharge along a river reach, and estimating annual variation of reach-scale bankfull discharge. Third, sensitivities of physical parameters were further analyzed. Calculation and analysis showed that the effective width coefficient was necessary for estimating bankfull elevation in multi-thread cross-sections, and these three definitions could be adapted to estimate the bankfull elevation when coupled with these two indicators. Besides, the key point of this method was defining the maximum width and effective width coefficients, as other factors have limited effects on estimating bankfull discharge. Analyses can also provide information for field survey refering to the balance between time consuming and detailed measurements.

Buried channels provide keys to infer Quaternary stratigraphic and paleo-environmental changes: A case study from the west coast of India

High resolution shallow seismic data was acquired from inner continental shelf of Goa, west coast of India to map underlying stratigraphic and buried geomorphic features of shelf strata. Seismic data revealed characteristic channel incisions beneath 4–15 m thick sediment layer and corresponds to multi cycle incisions. Stratigraphic analysis of these incision signatures reveals three prominent subaerial unconformities S6, S7 and S9. These unconformities were exposed during the last glacial, penultimate glacial (MIS-6) and prior to penultimate glacial (MIS-8) periods. On the basis of interpreted age of subaerial unconformities and differences in their morphological features, observed channel incisions have been divided grossly into three phases of incision. Phase-1 incisions are older than ∼330 kyr BP, whereas, incisions of Phase-2 and Phase-3 correspond to ∼320–125 kyr BP and ∼115–10 kyr BP respectively. Plan form of these incisions varied from a straight channel type to ingrown meander and then to anastomosing channel types. These channels meet at the confluence of present-day Mandovi and Zuari rivers. The confluence point has varied in due course of time because of cyclic incision and burial with repeated sea level fluctuations. The preserved main channel width varies from ∼100 m to 1000 m, and maximum channel depth reaches up to ∼35 m. Comparison of quantitative and qualitative morphologic results of different phases of incisions suggest that Phase-2 channels had ∼33% more mean bank full discharge than that of the Phase-3 channels. Phase-2 incisions had been carved in higher hydraulic energy condition as compared to Phase-3 incisions implying that the Indian summer monsoon was better during formative stages of Phase-2 incisions.

Quantifying sediment storage on the floodplains outside levees along the lower Yellow River during the years 1580–1849

AbstractThe lower Yellow River channel was maintained by artificial levees between 1580 and 1849. During this period, 280 levee breaches occurred. To estimate sediment storage on the floodplains outside the levees, a regression model with a decadal time step was developed to calculate the outflow ratio for the years when levee breaching occurred. Uncertainty analysis was used to identify the likely outflow ratio. Key variables of the model include annual water discharge, a proxy for levee conditions, and potential bankfull discharge of the channel before flood season. Uncertainty analysis reveals an outflow ratio of 0.35–0.56. We estimate that during this period, 18.8–30.1% of the total ~312 Gt of sediment load was deposited on the floodplains outside the levees. Human‐accelerated erosion in the Loess Plateau caused a 4‐fold increase in sediment delivery to the lower Yellow River, which could not be accommodated by channel morphodynamic changes. As a result, 21.2–27.5% of the total sediment load was deposited within the levees, creating a super‐elevated channel bed that facilitated an uncommonly high breach outflow ratio. Hence, the factor of a large super‐elevation relative to the mean main channel depth should be considered when designing diversions to restore floodplains. © 2018 John Wiley & Sons, Ltd.

Palaeohydrology of lowland rivers in the Murray-Darling Basin, Australia

This study derives a new function describing the relationship of channel bankfull discharge (Qbf) to channel width in modern rivers of the Murray-Darling Basin (MDB) of southeastern Australia and applies this to dated palaeochannels of seven rivers to quantify late Quaternary discharge history in this important basin. All rivers show high MIS3 and MIS2 Qbf, declining in the Holocene. The Qbf of modern MDB rivers is correlated with total catchment precipitation but comparison with palaeochannel Qbf estimates shows that while enhanced runoff efficiency is necessary to account for much larger late Pleistocene palaeochannels, either lower or higher precipitation rates could have prevailed. A strong association between relative palaeo- Qbf enhancement and temperature suggests a temperature-mediated mechanism controlling river discharge, such as the fraction of precipitation stored as snow and thawing in spring, the enhancement of orographic rainfall, or CO2 feedbacks with vegetation cover. Significantly enhanced MIS3 Qbf requires an additional mechanism, such as increased rainfall. These findings are consistent with others that increased moisture availability was associated with past colder climates, although this was not necessarily the result of enhanced precipitation.

Sub-Bankfull Flow Frequency versus Magnitude of Flood Events in Outlining Effective Discharges. Case Study: Trotuș River (Romania)

Effective discharge, which represents the flow, or range of flows, that transport the most sediment over the long-term, was determined based on the mean daily flow discharge and mean daily suspended sediment discharge recorded between 1994 and 2014 at four gauging stations along the Trotuș River. This study proposes an efficient method for the estimation of effective discharge based on observed values of the suspended sediment load. By employing this method the suspended sediment load is no longer either under- or overestimated as in the cases when the assessment is based on sediment rating curves. The assessment on effective discharge was performed at two distinct levels: for the entire data series during the investigated time spans and, subsequently, for flows less than the bankfull discharge. The effectiveness curves of the suspended sediment transport characteristics revealed highly multimodal characteristics with many peaks, indicating ample ranges for the effective discharges. The main effective discharge corresponded to large flood events, which are typical for the upper end of the discharge range, whereas the secondary effective discharges corresponded to sub-bankfull flows, which are more frequent. The changes that occurred in the channel bed are reflected by the temporal variations in the effective discharge.

Gradual Transition From Net Erosional to Net Depositional Cyclic Steps Along the Submarine Distributary Channel Thalweg in the Rio Muni Basin: A Joint 3‐D Seismic and Numerical Approach

AbstractWe integrated measurements of cyclic steps and bankfull hydraulic geometry to estimate turbidity flow properties, yielding two main observations. First, three net erosional cyclic steps were recognized along a distributary channel thalweg based on (1) closed topographic depressions, (2) less asymmetrical cross‐sectional morphologies, (3) disrupted and discontinuous internal architecture revealed from seismic data, (4) erosional basal contacts, (5) stronger up‐current migration, and (6) relatively higher U1 (flow velocity just before each hydraulic jump), Q1 (bankfull discharge before hydraulic jump), and ΔEL (energy loss of hydraulic jump) of formative turbidity currents. In contrast, six net depositional cyclic steps were identified based on (1) open topographic depressions, (2) more asymmetrical morphologies, (3) well‐bedded and continuous internal structures, (4) gradual basal contacts, (5) stronger vertical accretion, and (6) relatively lower U1, Q1, and ΔEL values. These observations may aid in distinguishing between net erosional and net depositional cyclic steps. Second, cross‐sectional asymmetries (Ay) and width‐to‐depth ratios of the trough (W1/h1) of cyclic steps systematically vary with distance along the distributary channel thalweg from the point of overflow initiation (Di) and can be fit by a single relation, which, together with the fact that the two types of topographic depressions separate along these relations, demonstrate a gradual transition from net erosional to net depositional cyclic steps. U1, Q1, and ΔEL are proportional to the stoss‐side slope angle of cyclic steps (а) but are inversely proportional to Ay and W1/h1, indicating that a progressive downstream decrease in U1, Q1, and ΔEL (i.e., decrease in sediment transport capacity) probably caused the gradual transition from net erosional to net depositional cyclic steps.

Late Holocene channel pattern change from laterally stable to meandering – a palaeohydrological reconstruction

Abstract. River channel patterns may alter due to changes in hydrological regime related to changes in climate and/or land cover. Such changes are well documented for transitions between meandering and braiding rivers, whereas channel pattern changes between laterally stable and meandering rivers are poorly documented and understood. We hypothesize that many low-energy meandering rivers had relatively low peak discharges and were laterally stable during most of the Holocene, when climate was relatively stable and human impact was limited. Our objectives in this work are to identify a Late Holocene channel pattern change for the low-energy Overijsselse Vecht river, to develop and apply a novel methodology to reconstruct discharge as a function of time following a stochastic approach, and to relate this channel pattern change to reconstructed hydrological changes. We established that the Overijsselse Vecht was laterally virtually stable throughout the Holocene until the Late Middle Ages, after which large meanders formed at lateral migration rates of about 2 m yr−1. The lateral stability before the Late Middle Ages was constrained using a combination of coring information, ground-penetrating radar (GPR), radiocarbon (14C) dating, and optically stimulated luminescence (OSL) dating. We quantified bankfull palaeodischarge as a function of time based on channel dimensions that were reconstructed from the scroll bar sequence and channel cut-offs using coring information and GPR data, combined with chronological constraints from historical maps and OSL dating. We found that the bankfull discharge was significantly greater during the meandering phase compared to the laterally stable phase. Empirical channel and bar pattern models showed that this increase can explain the channel pattern change. The bankfull discharge increase likely reflects climate changes related to the Little Ice Age and/or land use changes in the catchment, in particular as a result of peat reclamation and exploitation.

Relationships between discharge parameters and cross-sectional channel dimensions of rivers in an active margin influenced by tropical climate: The case of modern fluvial systems in the Indonesian islands

The present study investigates (1) the relationships between discharge parameters and bankfull channel width (Wb) based on statistical analyses of hydrological data obtained from 649 sites at rivers on the Indonesian islands of Java, Sumatra, and Kalimantan, and (2) the relationship between depth parameters and Wb using channel cross sections obtained from 86 sites at rivers on these three large islands. These analyses are used to describe geomorphological and hydrological features of fluvial systems developed in a convergent margin under the influence of a low-latitude tropical climate. The relationship between mean discharge (Qmean) and Wb shows distinct variation within the Indonesian rivers, and reflects regional variations in annual rainfall of a tropical climate. The relationship between bankfull discharge (Qb) and Wb shows only minor variations across the three islands, regardless of bed and bank material types. The Qb–Wb relationship in the Indonesian rivers has a pattern similar to that of fluvial systems in Europe and the North American continent, which are influenced by mid- and high-latitude climates. Minor regional variation in the Qb–Wb relationship, regardless of the tectonic and climatic settings, suggests that the development of fluvial morphology is controlled mainly by flood-related episodic discharge. In contrast, the empirical equations between maximum channel depth (db) and Wb and between mean channel depth (dm) and Wb show distinct regional variations not only within the Indonesian fluvial systems but also in fluvial systems of Europe and the North American continent. These regional variations are thought to reflect the fact that db–Wb and dm–Wb relationships are controlled by the tectonic and climatic settings of a drainage basin, and factors such as the interrelationships between precipitation, vegetation, and slope. Consequently, selection of an appropriate dm–Wb (or db–Wb) equation combined with an appropriate Qb–Wb equation permits us to reconstruct spatial and temporal variations in hydrological features in both modern and ancient fluvial systems more precisely than applying previously proposed equations to fluvial systems formed in active margin and low-latitude climatic settings.

Meander hydromorphology of ephemeral streams: Similarities and differences with perennial rivers

The geomorphology, hydrology and processes of ephemeral streams are poorly known and studies on the geomorphic characteristics of ephemeral meandering streams (EMS) are even less investigated. We collected geomorphic data (namely, channel width, sinuosity, wavelength and curvature radius) from 107 EMS reaches in different drylands of the world from GoogleEarth Pro® satellite images. Geomorphology features/processes, such as neck cutoffs, point scrolls and chute cutoffs, were also assessed. The main aim of this paper is to investigate similarities and differences between EMS and perennial counterparts of more humid areas (on which almost all hydrogeomorphic models are based) for the purpose of explaining why meandering rivers form in environments where the lack of vegetation, sporadic flows and a high sediment supply should favour the development of braided rivers. EMS showed both geomorphological similarities and differences with perennial rivers. Point bar chute cutoffs and point bar scrolls are the most common geomorphic features of EMS, suggesting a certain stability of meander behaviour, though within a continuous process of changes towards the most probable form, similarly to perennial streams. The bimodal distribution of curvature ratio values departs substantially from that of perennial rivers that are commonly in the 2–3 range. Data from the rivers in this study indicate that their channel pattern is well described by the same sine generated function used for perennial rivers. Wavelength to channel width ratio and bankfull discharge to streambed gradient of EMS unexpectedly plot in different areas of classical diagrams developed for perennial rivers. We interpret such differences in terms of excess energy expenditure for high sediment load transport. We also propose a new function to discriminate the meandering from the braided pattern of ephemeral streams, and present a conceptual model of meander formation in ephemerals streams based on an autogenic process of bank collapse and bar deposition.

Mega-meander paleochannels of the southeastern Atlantic Coastal Plain, USA

Paleodischarge estimates based on the slope-area method and channel boundaries determined from stratigraphic cross-sections indicate that large, terminal Pleistocene meandering paleochannels (“mega-meanders”) in river valleys of the southeastern Atlantic Coastal Plain of the United States represent bankfull flows that were at least double the magnitude of those on modern rivers. Correlation of radiocarbon- and luminescence-dated paleomeanders with previously reported pollen and eolian sedimentary records suggests that greater discharge was driven by seasonally wetter conditions resulting from dynamic changes to regional precipitation and runoff that occurred in association with global warming at the end of the Pleistocene. While reflecting larger channel-forming flows, the exceptionally large widths and radii of curvature of mega-meanders were nonetheless maintained by a relatively modest discharge magnitude that was between two and four times larger than modern bankfull flow and within the size range of the present two- to five-year flood. Despite their large planform, the paleochannels conveyed relatively modest bankfull discharges because a wide, shallow shape limited their cross-sectional area and hydraulic radius. Within the late Quaternary evolution of fluvial systems in the region, scrolled mega-meanders constitute a transitional meandering planform that remained influenced by large volumes of sandy bedload following the sand-bed braided channels of the late Wisconsin interval, circa 30–17 ka. In addition to greater discharge, the large planimetric dimensions of paleochannels reflect a lack of cohesive vertical accretion facies on paleomeander floodplains, a sediment regime that transported large quantities of bedload sand, and the influence of these factors on channel boundary composition, bank stability, and channel shape. Findings underscore the importance of reconstructing channel cross-sectional dimensions and slope when estimating discharge for infilled paleomeanders. This approach reduces uncertainties surrounding channel cross-sectional area, gradient, and boundary composition inherent to studies lacking subsurface data that rely upon meander geometry to retrodict discharge.

Combining remote sensing with physical flow laws to estimate river channel geometry

AbstractA reliable representation of river terrains is essential to river research. Field surveys of river channel geometry are time‐consuming, costly, and logistically constrained and thus would encounter difficulties to achieve sufficient spatial coverage, resolution, and frequency of resurvey. This paper aims to demonstrate an efficient approach to building a river terrain model (RTM), the emphasis being on how to combine bathymetry and topography derived from satellite images captured at different flow stages. A method for calculating the difference between high and low stages (DHLS) based on the uniform‐flow theory is proposed. Calculations are carried out for a 13‐km long meandering section of the gravel‐bed Goulais River in Canada, which features pools and riffles, alternating point bars, and midchannel bars. A RTM for this complex section has been successfully produced. It consists of three data components: bathymetry at low stage, topography at high stage, and DHLS. The results capture realistic characteristics, including thalweg shift, steep outer banks, and gradual inner banks. They also show realistic longitudinal and lateral locations of pools and riffles. To illustrate potential applications of RTM, this paper has computed extreme bed shear stresses at bankfull discharge through hydrodynamics simulations of depth‐averaged flow in the river section and further estimated bed‐sediment grain‐size distributions. The estimates compare well with field measurements. The DHLS can vary significantly along a river channel. The proposed method for determining it is not site‐specific, and hence applicable to other rivers. The novelty of the methodology discussed lies in combining remote sensing techniques with physical flow laws.

Effect of urbanization on stream hydraulics

AbstractUrbanization results in major changes to stream morphology and hydrology with the latter often cited as a primary stressor of urban stream ecosystems. These modifications unequivocally alter stream hydraulics, but little is known about such impacts. Hydraulic changes due to urbanization were demonstrated using two‐dimensional hydrodynamic model simulations, comparing urban and non‐urban stream reaches. We investigated three ecologically relevant hydraulic characteristics: bed mobilization, retentive habitat, and floodplain inundation, using hydraulic metrics bed shear stress, shallow slow‐water habitat (SSWH) area, and floodplain inundation area. We hypothesized that urbanization would substantially increase bed mobilization, decrease retentive habitat, and due to increased channel size would decrease floodplain inundation. Relative percent area of bed disturbance was 4 times higher, compared with that of the non‐urban stream at bankfull discharge. SSWH availability rapidly diminished in the urban stream as discharge increased, with SSWH area and patch size 2 times smaller than the non‐urban stream for a frequently occurring flow 0.7 times bankfull discharge. Floodplain inundation decreased in frequency and duration. These results demonstrate changes in hydraulics due to urbanization that may impact on physical habitat in streams. New “water sensitive” approaches to stormwater management could be enhanced by specification of hydraulic regimes capable of supporting healthy stream habitats. We propose that a complete management approach should include the goals of restoration and protection of natural hydraulic processes, particularly those that support ecological and geomorphic functioning of streams.

Application of numerical model in evaluating the effects of a planned reservoir on reach-scale bankfull discharge of the lower Wei River

Predicting the responses of an alluvial channel to changes in flow and sediment supply is essential for engineering design. Many methods have been developed in the last few decades to describe sectional bankfull characteristics (elevation and discharge); however, studies on long-term reach-scale bankfull discharge are still limited. In this study, a hydraulic model is built to calculate the reach-scale bankfull discharge, and the effects of reservoir building on downstream bankfull discharges are discussed. The studied river reach is located at the lower Wei River (WR), where the planned Dongzhuang Reservoir would be built on its largest tributary, the Jing River. A quasi-two-dimensional numerical model coupled with a bankfull discharge estimating method is put forward to calculate the reach-scale bankfull discharge. The soundness of the model is verified. Results show that the temporal variation of reach-scale bankfull discharge of the lower reach of the WR would be highly influenced by the planned reservoir, especially during the first 20 years of operation. The effect of the planned reservoir on bankfull discharge may reach its maximum when the total trapped sediment load reaches approximately 75% of the reservoir capacity. Our results show that after the first 17 years of operation, the effect of the planned reservoir on bankfull discharge of the river reach may decrease gradually. The soundness and predictive capability of the coupled model have also been calibrated by comparing with existing reservoirs. All analyses indicate that the numerical model can be used to predict the changed reach-scale bankfull discharge of the lower WR.

Bankfull discharge as a key control on submarine channel morphology and architecture: Case study from the Rio Muni Basin, West Africa

Bankfull discharges play a pivotal but underappreciated role in determining submarine channel morphometrics and architecture. This hypothesis is suggested by the power law relationship between channel morphometrics and discharge and the logarithmic relationship between channel architecture and discharge. We apply this relationship to the Rio Muni deep-water channels of Equatorial Guinea in three ways. Firstly, relatively dilute turbidity currents with a relatively low discharge of ca.1.1 × 104 to 6.9 × 104 m3/s tended to enhance channel confinement and thalweg deposition, but to discourage lateral spreading of the flow. This, in turn, reduced depth and sinuosity (SI) of the channel form as the fill aggraded, as previously reported for younger channels on this slope. These vertically aggraded channel belts [represented by relative angles of channel trajectories (Tc) ranging from 47.2° to 81.0°] are narrower (averaging 634 m), thinner (averaging 23 m), and straighter (mean value of SI = 1.17) than their laterally migrated counterparts. Secondly, turbidity flows with a relatively high discharge of ca.4.1 × 104 to 15.8 × 104 m3/s appear to have reduced channel confinement, but to promote thalweg erosion and lateral spreading of the flow. This, in turn, increased depth and sinuosity of the channel form as it laterally migrated, forming laterally migrated channel belts (represented by a relatively low Tc of 21.8° to 49.0°) that are wider (1.5×), thicker (2×), and more sinuous (1.2×) than their vertically aggraded counterparts. Thirdly, a gradual decrease of discharge through time likely drove an architectural transition from lateral migration to vertical aggradation and associated migrating-to-aggrading channel trajectories.