Low-flow Channel Research Articles

Urban River Design: A River Restoration Case Study

Human excessive requirements on functions of aquatic ecosystems cause that surface waters often suffer from significant morphological and hydrological alterations. Human induced hydromorphological changes of rivers belong among the most common ecological problems of the water bodies and they appear in all scales of river habitat. These changes have an impact in their ecological status and need to be solved in the near future. Successful sustainable river rehabilitation depends on the identification and understanding of key processes and anthropogenic stressors. These factors enable to estimate a proper longitudinal and lateral riverine system structure including stream habitat improvements at different scales. The article summarizes the basic principles of urban river restoration, that enables to approach to the state of dynamic stability and to increase the „absorbing capacity” of freshwater ecosystem. The authors also provide the background facts and principles of the restoration design of canalized reach of the River Trnavka, which flows through an urbanised part of the city Trnava. The Trnavka River has regulated flow regime affected by operation of Boleraz reservoir located 14 km in upstream direction. The river channel capacity equals to 50 m3.s−1 with return period 100 years and ecological flow is Q=0,150 l.s−1. The heavily modified nonprismatic channel has stone and concrete stream banks. The authors have proposed increasing sinuosity of low flow channel and decreasing capacity of the oversized channel by using gravel matraces with planting different types of macrophytes. The main goal of the proposed river restoration measures was to increase aesthetic value of complex restoration design of close public space (the Rose Park) and to implement technical and biotechnical measures that may improve ecological conditions and management operation of the Trnavka River.

Morphodynamic adjustments in the Yichang–Chenglingji Reach of the Middle Yangtze River since the operation of the Three Gorges Project

To investigate the evolution characteristics of the Yichang–Chenglingji Reach (YCR) of the Middle Yangtze River and develop simulation methods, the temporal and spatial morphodynamic adjustments of the YCR were analysed comprehensively on the basis of runoff, sediment load, and cross-sectional profiles data at 220 cross sections from 2002 to 2016. Results showed that significant channel degradation has occurred in the YCR, especially in the low-flow channels, due to a drastic reduction in sediment yield since the operation of the Three Gorges Project (TGP). The variations of upstream flow and sediment regimes would drive equilibrium channels to become increasingly narrow and deep, and the current bankfull width and depth were less than their equilibrium counterparts in the recent period. However, the recent channel evolution was mainly characterized by the prominent channel deepening via a one-way deformation due to the large-scale bank revetments within the YCR, where the reach-scale bankfull depth increased by 1.6 m in the span of 15 years. A semi-empirical method for determining the equilibrium values of channel degradation and aggradation volume at the YCR was proposed on the basis of a comprehensive analysis of the effects of flow, sediment concentration, sediment gradation, and water depth. Moreover, the equilibrium values of the channel width and depth were estimated with the Least Action Principle method. The calculation methods for the equilibrium width, depth, width/depth ratio, and aggradation/degradation volume were combined with the Delayed Response Model (DRM). Then, the methods for calculating the variations in channel geometry and aggradation/degradation volume were derived. Results showed that the proposed methods effectively simulated the channel geometry and cumulative erosion volume in the YCR from 2002 to 2016. In addition, the response of the channel evolution in the YCR was closely related to the previous four-year hydrological conditions, implying that channel evolution may lag behind the variations of flow and sediment discharges.

Aerial photo-based analysis of the hydromorphological changes of a mountain river over the last six decades: The Czarny Dunajec, Polish Carpathians

Changes in hydromorphological conditions in the mountainous Czarny Dunajec River over the last six decades were investigated through the analysis of archival aerial photos and contemporary orthophotos. The aerial images from 7 dates between 1954 and 2009 were used to evaluate changes in selected hydromorphological parameters in six reaches delimited along the river course. As a result of channelization works and in-stream gravel mining, most of the analysed river course experienced dramatic hydromorphological alterations in the active river zone: change from the multi-thread to a single-thread channel, river narrowing associated with a reduced occurrence or elimination of channel bars and islands, transformation of the gravelly bed to a bedrock–alluvial or bedrock bed, stabilization of river banks with engineering structures, and disruption of longitudinal river continuity by transverse hydraulic structures. These changes largely reduced habitat heterogeneity in the river and disrupted or limited three-dimensional connectivity of the river ecosystem. However, in the reach that had generally avoided these human pressures, such negative changes did not occur and the number of low-flow channels and the proportion of islands in the active river zone increased during the study period. In all study reaches, positive changes occurred in the riparian and floodplain areas: the proportion of wooded channel banks and forest cover in the floodplain area increased over the last six decades. The degradation of hydromorphological conditions in the active river zone was reflected in a significant impoverishment of fish and benthic invertebrate communities as well as of ground beetles inhabiting low river benches. The study confirmed the usefulness of archival aerial photos in reconstructing temporal changes in river hydromorphology. The number of features that can be evaluated with this approach is smaller than in field-based assessments of contemporary rivers but larger than in studies based on historical maps or paleohydrological analysis.

Putah Creek hydrology affecting riparian cottonwood and willow tree survival.

Creating or recruiting new riparian forests to improve Lower Putah Creek (LPC) ecosystem functions is challenging under the modified stream flow regime developed after historic gravel mining and installation of Monticello Dam upstream. Hydrologic connectivity between riparian trees, shallow groundwater, and the low flow channel is essential towards maintaining these forests and related habitats through the annual summer and multi-year drought periods typical in this Sacramento Valley region of California. Despite increased average summer flows, significant mature cottonwood and willow tree mortality along the LPC riparian areas below the Putah Creek diversion dam in 2014 raised concerns over the soil and hydrologic factors affecting riparian vegetation survival. A forensic analysis was conducted combining annual canopy coverage fractions and tree ring studies with daily soil-water balances, low flow records, and available groundwater level Information from the past few decades to determine the key hydrologic factors affecting riparian tree survival along the LPC. The 2011-2016 drought was linked with greater than prior average soil-water deficits in 2012-2015 and lower initial soil-water storage on March 1 of 2012 and 2014 that would be expected to stress the trees. However, such stress was not apparent in decreased tree ring spacing during this period from mature (40-50years old), deceased, and living trees. Tree canopy coverage declined dramatically (by as much as 50% as compared to the previous decade average) only in the summer of the 2014 despite a ~ 35% increase in average summer flows from 2011 to 2014. However, the regional water table aquifer levels declined at an average rate of ~ 35mm/day in 2014 (as compared to ~ 17mm/day in previous decade) and by several meters overall between 2011 and 2016 suggesting that deceased trees lacked access to the water table aquifer or lateral stream seepage. The increased rates of water table decline and overall depth may be associated with a large increase in adjacent irrigated almond orchard areas in 2014-2016. Knowledge of the dynamic hydrologic factors controlling sustainability of riparian trees should better inform and guide future tree restoration efforts along the LPC.

Non-normality and classification of amplification mechanisms in stability and resolvent analysis

We seek to quantify non-normality of the most amplified resolvent modes and predict their features based on the characteristics of the base or mean velocity profile. A 2-by-2 model linear Navier-Stokes (LNS) operator illustrates how non-normality from mean shear distributes perturbation energy in different velocity components of the forcing and response modes. The inverse of their inner product, which is unity for a purely normal mechanism, is proposed as a measure to quantify non-normality. In flows where there is downstream spatial dependence of the base/mean, mean flow advection separates the spatial support of forcing and response modes which impacts the inner product. Success of mean stability analysis depends on the normality of amplification. If the amplification is normal, the resolvent operator written in its dyadic representation reveals that the adjoint and forward stability modes are proportional to the forcing and response resolvent modes. If the amplification is non-normal, then resolvent analysis is required to understand the origin of observed flow structures. Eigenspectra and pseudospectra are used to characterize these phenomena. Two test cases are studied: low Reynolds number cylinder flow and turbulent channel flow. The first deals mainly with normal mechanisms and quantification of non-normality using the inverse inner product of the leading forcing and response modes agrees well with the product of the resolvent norm and distance between the imaginary axis and least stable eigenvalue. In turbulent channel flow, structures result from both normal and non-normal mechanisms. Mean shear is exploited most efficiently by stationary disturbances while bounds on the pseudospectra illustrate how non-normality is responsible for the most amplified disturbances at spatial wavenumbers and temporal frequencies corresponding to well-known turbulent structures.

Channel evolution under changing hydrological regimes in anabranching reaches downstream of the Three Gorges Dam

Elucidating the influence of dams on fluvial processes can benefit river protection and basin management. Based on hydrological and topographical data, we analyzed channel evolution in anabranching reaches under changing hydrological regimes influenced by the Three Gorges Dam. The main conclusions are as follows: 1) the channels of specific anabranching reaches were defined as flood trend channels or low-flow trend channels according to the distribution of their flow characteristics. The anabranching reaches were classified as T1 or T2. The former is characterized by the correspondence between the flood trend and branch channels, and the latter is characterized by the correspondence between the flood trend and main channels; 2) on the basis of the new classification, the discrepant patterns of channel evolution seen in anabranching reaches were unified into a pattern that showed flood trend channels shrinking and low-flow trend channels expanding; 3) flood abatement and the increased duration of moderate flow discharges are the main factors that affect channel adjustments in anabranching reaches after dam construction; and 4) in the next few decades, the pattern of channel evolution will remain the same as that of the Three Gorges Dam operation. That is, the morphology will fully adapt to a flow with a low coefficient of variation. Our results are of interest in the management of the Yangtze River and other rivers influenced by dams.

Twentieth-century hydromorphological degradation of Polish Carpathian rivers

Case studies of the Soła, San and Dunajec rivers are used to demonstrate changes in planform geometry of the rivers draining the western and eastern parts of the flysch Polish Carpathians and those originating in the high-mountain Tatra massif since the second half of the nineteenth century. The number and width of low-flow channels and the length of river reaches are compared between the nineteenth-century and contemporary rivers, with analysis of other features of their planform geometry prevented by differences in the cartographic presentation of rivers on Austrian military maps and Polish topographic maps. In the second half of the nineteenth century, the rivers flowed in wide channels. The Soła supported a braided channel along its entire course, the Dunajec was mostly braided, with a single-thread channel within gorge sections, whereas for the San the braided pattern was limited to the reaches within intramontane and foreland basins. Over the twentieth century, the braided channel pattern was nearly completely eliminated and substituted by a single-thread pattern; multi-thread channel segments remained only in the upper course of the Dunajec River. Average low-flow-channel width significantly decreased, even by 66%, and this was accompanied by a reduction in the variability of low-flow channel width along the rivers. The change in planform channel geometry was accompanied by channel incision, which started earlier and has resulted in greater bed degradation in the foreland and foothill reaches of the rivers than in their mountain reaches. Twentieth-century river changes also encompassed a radical change in the facies pattern of channel sediments, accompanied by their coarsening and improved sorting and development of bed armour. Moreover, transformation of alluvial channel beds to bedrock ones took place in many reaches of Carpathian streams and rivers. The degradation of physical structure of Carpathian watercourses has resulted in adverse hydraulic and hydrological effects and a marked deterioration of habitat conditions for riverine biota.

Combined effects of multiple large-scale hydraulic engineering on water stages in the middle Yangtze River

Investigation of water stages influenced by human projects provides better understanding of riverine geomorphological processes and river management. Based on hydrological data collected over ~60years, an extreme stage-extreme discharge analysis and a specific-gauge analysis were performed to research the individual and combined effects of multiple engineering projects on a long-term time series of water stages in the middle Yangtze River. Conclusions are as follows. (1) In accordance with the operation years of the Jingjiang cutoff (CF), the Gezhouba Dam (GD), and the Three Gorges Dam (TGD), the time series (1955–2012) was divided into periods of P1 (1955–1970), P2 (1971–1980), P3 (1981–2002), and P4 (2003−2012). Water stage changes during P1–P2, P2–P3, and P3–P4 are varied because of the differences in the types and scales of these projects. The stage decreased at Shashi and increased at Luoshan owing to the operation of the CF. Additionally, after the GD was constructed, the low-flow stage decreased in the upstream reach of Chenglingji and increased in its downstream reach, whereas the flood stage merely decreased at Yichang. Moreover, the TGD resulted in an overall decrease in low-flow stages and a limited increase in flood stages because of the differential adjustments of river geometry and resistance between the low-flow channel and flood channel. (2) Although differences existed in the scouring mechanisms between streamwise erosion associated with dams and headward erosion associated with cutoffs, particular bed textures in the gravel reach led to a similar adjustment that stage reduction at Shashi was the greatest of all stations, which caused the flow slope and sediment transport capacity to decrease in the sandy reach. (3) These engineering projects caused changes in average low-flow and flood stages that varied between Yichang (−1.58 and −0.08m respectively), Shashi (−3.54 and −0.12m), and Luoshan (1.15 and 0.97m) from P1 to P4. However, less influence was observed at Hankou owing to its remote location and the short impoundment time of the TGD. (4) Potentially detrimental decreases in low-flow stages and increases in flood stages should be monitored and managed in the future. Our results are of practical significance for river management and the evaluation of the influences of large-scale anthropogenic activities on the hydrological regimes of large rivers.

An Effect of Mass Transport and Pt Oxide Formation on High Current Density Performance in PEFC

Introduction High current density operation is essential to cost reduction of polymer electrolyte fuel cell (PEFC) system for automotive application. Although extensive numerical1,2) and experimental3,4) approaches have been conducted in order to understand and to predict a cell performance under high current density condition, there was no comprehensive analysis with microscopic transport resistance on Pt surface, catalyst activity change associate with Pt oxide formation, water and heat transport. In this study, polarization curves under high current density were validated with quantification of both proper catalyst activity and mass transport effects. Obtained results suggested that catalyst activity with oxide formation is important for deeper understanding of over-potential under high current density condition. Experimental and Numerical Modeling Polarization curves were evaluated under extremely low oxygen concentration (0.4%) and normal condition (10%) in order to validate the effect of heat, water and proton transport respectively. Supplied gases were controlled at 200 kPa (Abs.) with high enough flow rate into low pressure drop straight flow channel to minimize oxygen and reaction distribution along flow direction. To evaluate the initial state dependence of adsorbed species on Pt surface, scanning rate of potential was controlled very low (0.5 mV· sec-1) from low potential (0.3 V) to high potential (1.0 V) region. Pt loadings of test cell were 0.35 mg· cm-2 and 0.10 mg· cm-2to evaluate the effect of both mass transport resistance and activity change in catalyst associate with effective Pt surface area. In the numerical analysis model, continuous equations, momentum, conservation formula of chemical species, and potential (electronic and electrolyte) formulas were combined, and the source and sink term associated with electrochemical reaction was given by the Butler Volmer equation. Transport characteristic values were measured by the evaluation method described in previous reports 1,2) . Oxide formation on platinum surface was considered the active site change assuming Langmuir adsorption and adsorption energy change exerted by adsorbed species covering the site in the vicinity assuming Temkin adsorption3,4), coupled analysis of electrochemical reactions and three-dimensional mass transport corresponding to activity changes due to potential changes. In addition, the oxide coverage ratio was experimentally approximated by the CV test. The numerical model also made it possible to simplify to one-dimensional analysis for studying three-dimensional mass transport effect on polarization curves. Results and Discussions Fig.1 shows results of experiment (dashed line) and numerical validation (solid line) with/without considering effect of Pt oxide formation and three-dimensional mass transport. One-dimensional models analysis indicated that Pt oxide formation doesn’t have effects on limiting current density but has effects on over-potential nonlinearly from the region below 0.75 V. It was roughly in agreement that oxide species on catalyst surface almost all desorbed based on the CV test result in previous report3) and indicates that electrochemical reaction characteristics in low potential regions is also important for cell performance analysis. Furthermore, regarding the comparison of one-dimensional model and three-dimensional model considering Pt oxide, even if thin rib (0.2 mm) configuration is used under extremely low oxygen concentration operation, over-potential under high current density increased due to influences of three dimensional mass transport, electrochemical reaction distribution. Conclusion In this study, polarization curves under high current density condition were validated by the numerical analysis with both proper catalyst activity and three-dimensional mass transport effects. Validation results suggested over-potential under high current density operation is strongly affected by not only three-dimensional mass transport but also Pt oxide formation associated with potential change as well. Reference [1] Y. Tabuchi et al. / Electrochimica Acta., 56, 352 (2010) [2] Y. Fukuyama et al. / Electrochimica Acta., 117, 367 (2014) [3] S. Sugawara et al. / J. Eletrochemisty., 79, 404 (2011) [4] N.M. Marković et al. / J. Eletroanal. Chem., 467, 157 (1999) Figure 1

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

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

What drives riparian vegetation encroachment in braided river channels at patch to reach scales? Insights from annual airborne surveys (Drôme River, SE France, 2005–2011)

AbstractUnderstanding riparian vegetation encroachment and its drivers is crucial for predicting alluvial river channel morphodynamics in different hydro‐climatic contexts. We investigated fine‐scale vegetation encroachment patterns and underlying physical controls along the Drôme River, France, following a 50‐year flood in 2003. Our study addressed the following questions: (a) What is the relative timing and the respective importance of vegetation patch recruitment versus patch expansion? (b) How well do regional/climatic factors explain the spatio‐temporal patterns of recruitment and expansion versus local and reach‐scale variation in geomorphic position and habitat? Along a 3‐km reach, we analysed a chronosequence of high‐resolution aerial images acquired yearly between 2005 and 2011. From each image series, we digitized patches of newly recruited vegetation and their lateral extent in following years to determine their expansion. This was complemented by elevation and plant structure data from airborne LiDAR data and field observations. The initial post‐flood vegetated area doubled within 6 years, with a pronounced recruitment peak in 2006–2007. Patch expansion showed a positive linear relationship to growing season temperature. Vegetation encroachment was greatest in a slightly aggrading sub‐reach compared to an incising sub‐reach. Initially, it occurred principally along low flow channel margins and on bar surfaces, later in intermittent channels. Our results confirmed that extensive recruitment can succeed floods with recurrence intervals smaller than 1‐in‐5 to 10 years, when the flood history resets the geomorphic framework. Recruitment extent depends in part on local hydrological connectivity, whereas higher temperatures can boost plant growth where there is a perennial water supply.

A continuum model of stresses in a vertical silo with a flow channel in the vicinity of the wall, using the principal stress cap surface approach for the bulk solids

Eurocode 1 (2006) gives design equations for eccentric stresses in silos, including flow channels adjacent to the wall. This has been modelled using the approach of Matchett et al. (2015, 2016).A three zone model was developed, consisting of:• The flow channel.• The transition zone.• The bulk of the solids.The flow channel and the transition zone were modelled by Janssen-type equations. The bulk was modelled by the principal stress cap approach.The transition zone is a complex region and has several purposes:1. To shelter the low stress flow channel from the high stresses around.2. To allow high principal stresses at the transition/bulk interface, within the yield locus.3. To form a transition between the dynamic flow channel and the static bulk.4. To allow transition from passive stress in the flow channel to active stress in the bulk.The model was calibrated against the data of Chen et al. (2007) for a full-scale silo, and described the data reasonably well, scaling axially and azimuthally.Large experimental data sets are required to calibrate a model. Unfortunate data points cannot be arbitrarily rejected.Further extensive, experimental data are needed to calibrate models.

Log transport and deposition in incised, channelized, and multithread reaches of a wide mountain river: Tracking experiment during a 20-year flood

Distance of large wood transport during floods and conditions for wood deposition in wide mountain rivers are still insufficiently understood. Tracking of logs tagged with radio transmitters was used to investigate differences in depositional conditions and the length of log displacement during a 20-year flood between channel reaches of different morphology in the Czarny Dunajec River, Polish Carpathians. During the rising limb of the flood, logs were placed into the river at three locations: at the beginning of an incised reach, close to the beginning of a channelized reach, and 1km upstream from the beginning of a wide, multithread reach. The incised, channelized, and multithread reaches retained 12.5%, 33%, and 94% of tagged logs introduced to these reaches, respectively, and of the logs retained in the multithread reach, all were deposited in its upstream half. Significant differences in the length of displacement existed between the logs delivered to the river at the three locations, with logs placed into the river at the beginning of the incised reach moving the longest distance, and those delivered just upstream from the multithread reach moving the shortest distance before deposition. One-fourth of the logs were deposited in a low-flow channel or on a channel margin, one-fifth on the floodplain, and more than half on gravel bars. After the flood, river cross sections with deposited logs and a set of cross sections without any wood deposits were surveyed to collect data for one-dimensional modelling of hydraulic conditions at the flood peak. The cross sections with deposited logs were typified by significantly greater flow width and flow area and by significantly smaller mean flow depth, mean velocity, Froude number, mean bed shear stress, and unit stream power. Principal component analysis of the hydraulic parameters in the analysed cross sections grouped the two types of cross sections in distinct clusters, indicating that multithread cross sections differed in hydraulic parameters from all the other cross sections. The experiment confirmed findings from the previous wood inventory and numerical modelling of wood transport and deposition in the river, indicating that in a wide mountain river wood can be transported long distances in a narrow, single-thread channel. It is, however, preferentially deposited in a wide, multithread channel that functions as a natural wood trap preventing or considerably limiting further transfer of large wood to vulnerable sites. Therefore, it is crucial to avoid channelization of multithread reaches that would eliminate their functioning as natural wood traps. The hydraulic modelling provided a physical justification for the observed differences in wood behaviour between the distinct channel morphologies.

Medium and short term riparian vegetation, island and channel evolution in response to human pressure in a regulated gravel bed river (Piave River, Italy)

During the last decades, several Italian gravel-bed rivers suffered from different types and levels of human pressures that changed their morphological and vegetation patterns. This study aims to analyze the medium- and short-term evolution of vegetation cover, fluvial islands and main channel characteristics along a gravel-bed reach of the Piave River (North-eastern Italy), with special emphasis on the changes in the vegetation cover type and structure. The Piave River experienced different types of human disturbance such as gravel mining, hydropower schemes, and land use changes. A sequence of aerial photographs (1960, 1970, 1982, 1991, 1999, 2006, 2010 and 2012) have been analyzed to detect the medium-term (about 50yr) evolution of riparian vegetation along a reach about 30km-long in the middle course of the Piave River. In addition, LiDAR data (2003) and seven repeated topographic surveys (done between 2007 and 2011) have been used to quantify changes in three cross sections on a sub-reach about 2km-long, in order to analyze the channel evolution over a short period (1–8yr). The medium-term analysis revealed that changes in the river evolutionary trend depended on the variations in human activities both in the main channel and at basin scale. In fact, during the last five decades there has been a consistent and continuous increase of riparian vegetation within the river corridor, from around 50% up to 68%, which corresponds to a continuous decrease from around 46% to 29% in the area of exposed gravel and low flow channels. Considering the different vegetation cover types, there was a predominant increase of stable and tall vegetation from around 34% up to a maximum of around 67% (1999), then a slight decrease to around 62% (2010 and 2012). After a slight recovery phase subsequent to the cessation of gravel mining in the late 1990s and associated to flood events in the early 2000s, the Piave River appears to have been in an equilibrium phase in terms of bed elevation, planform morphology and areas of vegetation, which is becoming taller and more mature, as a result of the combination of flow regulation, reduced bedload input from upstream and lack of relevant flood events. These results suggest that in highly regulated — in terms of flow and sediment fluxes — rivers the cessation of gravel mining alone is not sufficient to revert a degradation trend and thus restore prior morphological patterns.

Rates and mechanisms of bedrock incision and strath terrace formation in a forested catchment, Cascade Range, Washington

Measurements of channel bed and bank incision into bedrock coupled with mapping and radiocarbon dating of strath terraces in the West Fork Teanaway River, Washington, provide insight into rates and mechanisms of river incision and strath terrace formation in a forested landscape. The West Fork drains 102 km 2 of the tectonically quiescent southeastern North Cascade Range, and it is rapidly eroding its bed and creating strath terraces in its lower reach. Minimum vertical incision, measured annually relative to nails embedded in the streambed, was greater in the seasonally exposed, weathering-dominated, high-flow channel (mean = 10.9 mm yr −1 ) than in the perennially wet, abrasion-dominated, low-flow channel (3.8 mm yr −1 ), documenting unsteady lowering of the channel margin. Ages of radiocarbon-dated materials from alluvium on strath terraces, 0.1 m to 5.4 m above the water surface, suggest three episodes of strath abandonment at maximum ages of ca. A.D. 830, A.D. 1560, and A.D. 1890, and average incision rates of 1.3 mm yr −1 , 1.4 mm yr −1 , and 7.4 mm yr −1 for the oldest to youngest surfaces, respectively. Weathering-promoted vertical incision in the high-flow channel provides a mechanism for “top-down” rapid lateral strath planation in which scour of alluvium on incipient strath terraces incorporates the surface into the high-flow channel, allowing rapid removal of bedrock weathered during wetting and drying cycles. Relationships among channel width, channel confinement by bedrock terrace risers, modeled bankfull shear stress, and alluvial bed cover suggest that rapid channel widening could also internally limit vertical incision by slowing incision as shear stresses decline and more alluvium is retained on the bed. The timing of the most recent (ca. A.D. 1890) strath abandonment corresponds with historic anthropogenic removal of fluvial wood, suggesting that the relative abundance of fluvial wood may influence episodes of vertical bedrock incision by affecting the retention of alluvium on streambeds.

Integrated River Restoration in a Mountainous City and Case Study

The river in a mountainous city is usually confronted with problems of short flood response time, water shortage in the dry season, artificialization of river channel, and shortage of hydrophilic spaces. Besides traditional requirements of flood control and drainage, the urban river also has functions of providing habitat, landscape, and recreation. We developed an integrated plan for river restoration in a mountainous city, based on the concept of safe, near natural, and convenient to enjoy water. We carried out case study of the Jiangshui River in Longkou City, Shandong Province, considering integrated aspects of flood control, water resources allocation, environmental protection, ecological restoration, and river landscape. Flood security was assessed by applying one-dimensional hydraulic model. Base flow was estimated, and it was maintained in the dry season by water saving in upstream irrigation areas and reuse of treated water. Water quality could be improved by increasing the collecting capacity of waste water. The low flow channel was meandering in the channel with water falls to increase habitat diversity and accessibility to water. The flood land was vegetated and constructed for citizens to enjoy water. The restoration project in the Jiangshui River was executed from 2013 to 2014. This study can help accumulate experiences of urban river restoration especially for the river in mountainous cities.

POINT BARS AS A SPECIAL FORMS OF THE CHANNEL TOPOGRAPHY INTERMEDIATE BETWEEN RIVER CHANNEL AND THE FLOOD PLAIN

The article provides rationalization for distinguishing special forms of the channel topography – point bars – representing an united aggregate of ridge-type macroforms of channel topography – parts of banks, bars, midstream sandbanks, which are drying during low water. They occupy an intermediate position between the lowflow channel and fl oodplain, both as a part of the channel and participating in its reformations and as initial element of the evolving fl oodplain. At high water levels they are in the subaqueous environment under the infl uence of the fl ow. With decreasing levels, they have a growing infl uence on the course of fl uvial processes, causing seasonal reorganization of the channel. At low water they represent the elements of land and being in subaerial conditions are exposed to wind and rain; they are also overgrown with vegetation. In the end, point bars over time become young fl ood plain. At the Upper Ob’ their stabilization due to the overgrowth takes from 12 to 25–30 years; on the Northern Dvina turning of the midstream sandbank into fl ood plain island with a height of 5 m takes about 100 years. Over the past few decades in almost all rivers of Russia an increased growth of riparian fl ats is witnessed, which is associated with fl ow regulation of rivers and their natural cutting-in.

STABILIZING EFFECT OF WWTP DISCHARGE ON WATER QUALITY AND FISH ASSEMBLAGE STRUCTURE. A CASE STUDY

We examined the effluent from a municipal (Ny regyh za, Hungary) wastewater treatment plant (WWTP) on hydrophysicochemical properties and on diversity, community structure, and stability of fish assemblages at the recipient low flow channel system during a two-year period. The WWTP outflow increased significantly the nutrient concentrations (e.g. NO2 and NO3 concentrations increased to 4x and 8x respectively), and the regime (with the permanent ~0.23m3/s load) at the recipient channel sections. The wastewater outflow not only altered, but stabilized the physico-chemical variables measured, and the water regime in the recipient channels. Thus the natural, periodic fluctuation of the environmental variables was diminished in the study period. The WWTP outflow caused significant changes in the fish fauna as well. High abundances and taxa richness were found in the stocks inhabiting the charged watercourse sections. At the same time, species composition and relative abundances of fish stocks proved to be more constant at the impaired sites. Our results show that the WWTP outflow caused altered, but significantly more stable environmental conditions. These alterations were favourable for the emergence of a more diverse and more stable fish community on the recipient channel sections. Hence, the dynamic variability in fish assemblage structure that is characteristic of natural lowland stream was not apparent in these perturbed, semi natural habitats.

洪水営力による樹木管理後の幼木林動態を踏まえた樹林化抑制手法の考察

Gravel-bed rivers in Japan are now facing a multitude of problems relating to fluvial environment and flood control. These problems include the expansion of river beds with less disturbance by flooding and the growth of the riverine trees and the decrease of high-quality gravel-bed bars and the immobilization of low-flow channel. In particular, the gravel bed river that dominated by vigorous growth of Willows and Robinia pseudo-acacia, sometimes increase the riverine trees by vegetative propagation under the flood disturbance. In addition, expansion of the overgrowth density is likely to occur as the young riverine trees that easily fallen by flood disturbance. In this study, to target the gravel bed of Watarase river that Robinia pseudo-acacia, etc. grows in the river, evaluated the dynamics of young riverine trees after felling, and proposed the excavation shape of gravel-bed bar for forestation suppressing by using flood disturbance.

Environment-friendly reduction of flood risk and infrastructure damage in a mountain river: Case study of the Czarny Dunajec

Migration of a mountain river channel may cause erosional risk to infrastructure or settlements on the valley floor. Following a flood of 2010, a cutbank in one of the bends of the main channel of the Czarny Dunajec, Polish Carpathians, approached a local road by 50m. To arrest the erosion of the laterally migrating channel, water authorities planned construction of a ditch cutting the forested neck of the bend, reinforcement of the ditch banks, and damming the main channel with a boulder groyne. In order to avoid channelization of the highly valued, multithread river reach that would deteriorate its ecological status and cause increased flood risk to downstream reaches, an alternative approach to prevent bank erosion was proposed. The new scheme, applied in 2011, included opening of the inlets to inactive side braids located by the neck of the bend of the main channel. This solution reestablished the flow in the steeper low-flow channels, allowing us to expect a cutoff and abandonment of the main channel during subsequent floods. Gravelly deflectors were constructed directly below the inlets to the reactivated side channels to divert the flow into the channels and prevent the water from entering the main channel. Hydraulic measurements performed before and after the implementation of the scheme confirmed that it enabled shifting the main water current, with the highest average velocity and bed shear stress, from the braid closest to the road to the most distant braid. Similar surveys of fish and benthic macroinvertebrate communities indicated that flow reactivation in the side channels was beneficial for these groups of river biota, increasing their abundance and taxonomic richness in the reach. Not only was the implemented solution significantly less expensive, but it also enhanced ecological functions of the multithread channel and the variability of physical habitat conditions and maintained the role of the reach as a wood debris trap. However, avulsion of the main channel in the river bend immediately upstream during the flood in May 2014 again caused erosional risk to the road, although at another location. This indicates that with the highly unstable, multithread channel pattern of the Czarny Dunajec, the best practice of river maintenance in a relatively unmanaged valley reach would be allowing free channel migration within the floodplain area and reinforcing, where necessary, the boundary between the erodible river corridor and the managed terrace.