CHANNEL ADJUSTMENTS AND ISLAND DYNAMICS IN THE BRENTA RIVER (ITALY) OVER THE LAST 30 YEARS

Many gravel bed rivers in the European Alpine area suffered different ranges and types of human pressure that modified their morphology and altered their processes. This work presents the case of the middle portion of the Brenta River, historically impacted by human activities such as floodplain occupations, bank protection, gravel mining, hydropower schemes and water diversion. Dam operation and gravel mining have produced considerable modifications in the natural sediment regime generating important morphological channel responses (narrowing and incision). Large areas of the former active channel have been colonized by riparian vegetation, both as islands and as marginal woodlands. Overall, the river changed its morphological pattern from braided to wandering. The present study analyses the timing and extent of the planform morphological changes that occurred over the last 30 years along the middle portion of the river (20 km long) through the examination of aerial photos, repeated topographic measurements and hydrological data. A series of recent aerial photos (1981, 1990, 1994, 1999, 2003, 2006, 2008, 2010 and 2011) have been used to assess the medium and short‐term morphological changes of the floodplains and the active channel area. As to the medium‐term modification, the recent changes in in‐channel gravel mining have determined a new trend of active channel widening through erosion of vegetated areas. The analysis has also allowed to assess the morphological effect of single flood events. Only floods with recurrence interval higher than 8–10 years appear to be able to determine substantial erosion of floodplain and island margins. Copyright © 2013 John Wiley & Sons, Ltd.     

Rapid CHannel Widening Following Weir Removal due to Bed‐Material Wave Dispersion on the River Monnow,Wales

Kentchurch Weir, a low‐head weir on the river Monnow, Wales, was demolished in August 2011, releasing a sediment wave that had formed behind the structure for at least a century. We surveyed channel topography and bed‐material composition through a 1.5‐km long reach prior to weir removal and then periodically over a 2‐year period. The fill material was finer than the ambient bed material with all particles mobilized by bankfull flows. Rapid degradation of the 1460‐m³ sediment fill in the previously impounded reach occurred as bed material appeared to disperse downstream, consistent with other studies of sediment waves in gravel‐bed rivers. The riverbed profile was gradually smoothed through the study reach by degrading the elevated fill as a migrating knickpoint and aggrading the channel bed and bars immediately downstream of the former weir location. Extensive bank erosion was evident in the previously impounded reach with up to 10 m of widening following a single flow event, increasing channel width by more than 20%. Mitigation measures to enforce the riverbanks have been required as the gradual dispersion of the sediment wave continues to force flow diversion towards the riverbanks. The evolution of sediment stores behind flow obstructions follows that of sediment waves and theory available to describe wave evolution should do much to improve management efforts that seek to minimize channel widening following weir removal. Copyright © 2014 John Wiley & Sons, Ltd.     

黄河下游纵剖面自调整特性

本文根据丰富观测资料，对黄河下游纵剖面平行抬高趋向性、塑造均衡剖面趋向性、河口流路改道及其长期延伸地均衡剖面塑造的影响作了较深入的讨论。     

长江砂石料开采量初步分析

开采长江砂石料作建筑材料之用，已成为８０年代以来的新兴行业，砂石料开采用主要位于河床成型堆积体、弯道深槽和矶头冲刷坑下游堆积区、床沙输移区。砂石料的开采除了可能给防洪和航运安全带来不利影响外，对河道冲淤和河势也会带来重大影响，因此，应严格限制砂石开采区和开采量，长江上川江的卵石年开采最近期宜控制在２００万吨左右，中下游的砂石年开采量近期宜控制在５０００万吨以内。     

RIVERBED DIGITAL ELEVATION MODELS AS A TOOL FOR HOLISTIC RIVER MANAGEMENT: MOTUEKA RIVER,NELSON, NEW ZEALAND

River management in New Zealand's laterally active gravelly rivers has permitted floodplain development and protection of agricultural resources and infrastructure. Management of these dynamic systems has been hailed as a success for the approaches adopted, namely straightening and confining the river using bank protection and managing riverbed levels by gravel extraction. However, this activity also impacts river morphological/habitat diversity and potential gravel resource, by replacing broad riparian corridors with narrower channels and reducing lateral connectivity with the floodplain. This paper quantifies river behaviour in three laterally confined reaches in the upper Motueka River over a 7‐year period, using annual high‐resolution ground surveys to address the nature of morphological change and associated sediment flux in these reaches with a view to informing management of the gravel resource. Surveys between 2004 and 2010 acquired data to construct digital elevation models (DEMs) of the active riverbed in three ~1‐km‐long reaches. Morphological budgeting based on differencing between successive DEM surfaces reveals complex spatial and temporal patterns of erosion and deposition, demonstrating complex reach dynamics. Overall, volumetric changes suggest these narrowed reaches have been net exporters of sediment, associated with continued channel degradation. This has left bar features, traditionally the focus of gravel extraction in the reaches, relatively isolated from all but extreme flows, limiting replenishment of the gravel resource. The paper demonstrates the utility of riverbed DEMs as a potential tool to frame river character and behaviour at the reach scale in gravel‐bed rivers, thereby providing an important contribution to holistic river management in these systems. Copyright © 2012 John Wiley & Sons, Ltd.     

Channel change and flood events since 1783 on the regulated river tay,Scotland: Implications for flood hazard management

An analysis of old maps and documentary sources reveals that major changes in river channel planform have occurred over the last 200 years on the River Tay system, Scotland, UK. Reaches showing natural river channel planform change, however, are relatively small and a stable planform is characteristic of many sections of the river. River planform instability appears to be controlled by channel bed slope, sediment load and the enhanced vulnerability of former river channel courses to erosion. Flood protection embankments built in the 19th and 20th centuries modified unstable multichannel wandering gravel bed river sections to narrower single-channel reaches, with limited lateral migration. On the River Tummel, 20th century impoundment has caused further geomorphological change in response to clearwater erosion close to the dam and aggradation processes within the regulated river downstream, but isolation of the effects of impoundment from those of channelization are problematic. An examination of the geomorphic effects of a high magnitude flood event in 1990 and historical accounts of earlier large floods reveal that the 1990 flood was the third largest since 1800 in the study area. Despite river regulation and bank protection the zones naturally characterized by instability are still susceptible to planform changes causing flood embankments to be breached, channel shifts and development of gravel bars.     

Island dynamics in a braided river from analysis of historical maps and air photographs

An analysis of island and active corridor dynamics is presented for a 16 km island‐braided reach of the gravel‐bed Tagliamento River (Italy) based upon information extracted, geocorrected and registered to a common base from three map (1803, 1833, 1927) and nine aerial photograph sources (1944/6, 1954, 1970, 1986, 1991, 1996, 1997, 1999, 2005). The active corridor width showed a general decline over the study period but with some recent widening. Adjustments in active corridor width were achieved through processes of floodplain avulsion, island attachment and progressive encroachment of the edge of the active corridor across gravel areas. These adjustments were accompanied by the preferential creation of dissection (floodplain avulsion) islands during periods of widening and the construction of mid islands within the corridor during periods of narrowing. Changes in island extent were achieved by rapid island turnover, which reached a maximum rate of over 50% per annum when corridor narrowing was most rapid between 1970 and 1991. Very few island surfaces were found to persist for more than 24 years. Despite this enormous dynamism and apparent cyclic behaviour, between 1944/6 and 2005 the ratio of island area to active corridor area remained relatively constant at around 0.08 and supported a consistently high bankfull shoreline to downstream length ratio of around 6 km · km^?1. These intrinsic properties of the dynamics of the study reach and other island‐braided channels need to be recognized and maintained by river managers because they represent a characteristic habitat dynamism that is crucial to the maintenance of ecological integrity. Copyright © 2008 John Wiley & Sons, Ltd.     

Channel response to large-scale river training works: hunter river,Australia

Extensive river training works were undertaken on the upper Hunter River, Australia between 1956 and 1978 in response to rapid lateral migration and channel widening caused by a series of large floods between 1949 and 1955. River training involved artificial cutoffs, extensive realignment, structural bank protection works and tree planting. Channel length and hence sinuosity (ratio of channel length to valley length) decreased throughout each of six reaches located in the 78.9 km long study area as a direct result of the artificial cutoffs and realignment. Although the overall length reduction was less than 5%, one section experienced a 17% reduction. Localized channel straightening directly increased slope, decreased roughness and consequently increased flow velocity. The structural works have generally succeeded, converting an actively migrating stream into a laterally stable channel. Largely because of channel straightening and reduced sediment supply by floodplain reworking, the river has responded by eroding its bed by up to 1.1 m since the 1950s. Bed erosion preferentially removed the sand and fine gravel fraction producing an armour layer of coarse gravel. This armour layer halted further erosion by protecting the underlying fine material and is now only episodically mobilized by floods with return periods greater than 5.6 years on the annual maximum series. Unfortunately degradation was not predicted before the river training works were undertaken and it was only the fortuitous formation of the bed armour layer which prevented the degradation from becoming a major problem.     

Application of the Stream Evolution Model to a Volcanically Disturbed River: The North Fork Toutle River,Washington State,USA

In this study, a recently revised version of the channel evolution model, named the Stream Evolution Model (SEM), was applied to the upper North Fork Toutle River disrupted by the deposition of a 2.5‐km³ debris avalanche during the catastrophic eruption of Mount St. Helens in 1980. The results show that, in the first few years following the eruption, upstream channel reaches generally incised, evolving in SEM Stage 4 (i.e. degradation and widening), while downstream reaches aggraded, evolving in Stage 5 (i.e. aggradation and widening). However, starting in the late‐1980s, this simple pattern was disrupted by incision in the downstream reaches, which seemed to propagate upstream. Since the 1990s, lateral channel adjustments have become predominant as rates of vertical adjustment have slowed and river valley top widths relaxed to asymptotic values. Spatial and temporal sequences of channel evolution have tended to follow the sequences of stages expected according to the SEM, although these sequences have been frequently disrupted by renewed incision, secondary cycles of adjustment and the impacts of local geologic, geomorphic and hydraulic conditions. Within the quasi‐full SEM cycles, stages 4 and 5 were sometimes repeated, while stage 6 (quasi‐equilibrium) was sometimes omitted, and stage 8 (anabranching) only occurred in the downstream braided/anabranching reaches. According to the SEM, degradation, widening and lateral activity (stages 4 and 7) are forecast to continue until transverse valley profiles and channel planforms stabilize and floodplain and terrace surfaces are fully colonized by vegetation. Copyright © 2017 John Wiley & Sons, Ltd.     

分析潮汐河口稳定性的突变模型

本文针对潮汐河口的演变特点，依据冲积河流稳定性概念，提出了以河床冲淤强度表征的纵向稳定指标和以宽深比表征的河床横向稳定指标，并利用突变理论建立了由上述两个指标组成的河口稳定性的尖点突变模型，给出了河口稳定性的综合判据和河口演变图式。实际应用表明该模型能较好地解释河口演变过程和描述河口的稳定状态，可为充分合理地整治及开发河口提供依据。     

Estimating discharge in gravel‐bed river using non‐contact ground‐penetrating and surface‐velocity radars

Discharge measurement is a critical task for gravel‐bed channels. Under high‐flow conditions, the elevation of the riverbed changes significantly by intensive torrential flow. The stage–discharge relations commonly used for stream discharge estimation may no longer be adequate. The contact‐type velocity measuring is also subject to measurement errors and/or instrument failures by the high‐flow velocities, driftwood, stumps, and debris. This study developed a new real‐time method to estimate river discharge in gravel‐bed channels. A systematic measuring technology combining ground‐penetrating radar and surface‐velocity radar was employed. The rating curves representing the relations of water surface velocity to the channel cross‐sectional mean velocity and flow area were established. Stream discharge was then deduced from the resulting mean velocity and flow area. The proposed method was examined in a steep gravel‐bed reach of the Cho‐Shui River in central Taiwan. The estimated stream discharge during three flood events were compared to the prediction by using the stage–discharge relation and the index‐velocity method. The proposed method of this study is capable of computing reasonable values of discharge for an entire flood hydrograph, whereas the other two methods tend to produce large extrapolation errors. Moreover, when the computed discharge is used in 2D flood flow simulation, the proposed method demonstrates better performance than the commonly used stage–discharge and index‐velocity methods.     

Monitoring the channel process of a stream restoration project in an urbanizing watershed: a case study of Kelley Creek,Oregon, USA

Pacific Northwest (PNW) streams in the United States were impacted by the 20th century development, when removal of instream structure and channelization degraded an aquatic habitat. The lower Kelley Creek in southeast Portland, USA was channelized during the 1930's Works Progress Administration (WPA) projects. Stream restoration reintroduced pool‐riffle sequences and heterogeneous substrates to protect salmonids while mitigating impacts from flooding. We investigated whether the restored pool‐riffle morphology changed substantially following effective discharge events. We examined channel forms for four reaches representing three time periods—pre‐development (two reference reaches), development and restoration. We conducted thalweg profiles, cross‐sections and pebble counts along the reaches to examine how channel geometry, residual pool dimensions and particle size distribution changed following effective discharge events. The effective discharge flows altered the restoration reach more substantially than the reference reaches. The restoration reach decreased in median particle size, and its cross‐sectional geometry aggraded near its margins. However, the residual pool morphology remained in equilibrium. Richardson Creek's reference reach degraded at the substrate level, while Kelley Creek's reference reach remained in equilibrium. The restoration reach's aggradation may have resulted from sedimentation along the nearby Johnson Creek. In contrast, Richardson Creek's degradation occurred as upstream land use may have augmented flows. Stream channels with low gradient pool‐riffle morphologies are ideal for salmonid spawning and rearing and should be protected and restored within urban corridors. The findings of our study suggest that the connectivity of streams and the dynamic fluvial geomorphology of stream channels should be considered for stream restoration projects in humid temperate climates. Copyright © 2008 John Wiley & Sons, Ltd.     

MACROINVERTEBRATES IN ABANDONED CHANNELS: ASSEMBLAGE CHARACTERISTICS AND THEIR INDICATIONS FOR CHANNEL MANAGEMENT

Alluvial floodplains contain four types of abandoned channels: old river courses, oxbow lakes, oxtail lakes and riparian wetlands. These result from avulsions, meander cutoffs, ice‐jam floods and stem‐channel shifts, respectively. With the exception of old river courses, which belong to terrestrial ecosystems, abandoned‐channel types are important components of freshwater ecosystems. During 2006–2009, we conducted systematic investigations of macroinvertebrates in three types of freshwater abandoned channels and identified 93 taxa of macroinvertebrates belonging to 51 families and 88 genera. In the assemblage composition, most taxa were similar to those found in river‐isolated lakes and river mainstreams. Moreover, abandoned channels supported many unique benthic taxa, which are important complementary resources for the entire river system. In the abandoned channels that were covered with a layer of silt, standing crops were higher than that found in the river mainstream where the bed sediment consisted mainly of sand. Conversely, in abandoned channels that were covered with fine sand, standing crops were lower than in the gravel streambed. Some abandoned channels, such as the riparian wetlands of the East River that are freely connected with the mainstream, were characterized by the highest biodiversity and the greatest biomass. In regions less affected by human activities, abandoned channels need to be connected with mainstreams by flooding at least once every 3 years to maintain at least half of the maximum amount of macroinvertebrate resources. In regions more affected by human activities, abandoned channels need to be connected with mainstreams at least once every year. Copyright © 2011 John Wiley & Sons, Ltd.     

Geomorphological response of a river regulated for hydropower: River Fortun,Norway

The River Fortun is situated in south-western Norway and is typical of the fiord landscape catchments with large areas above 1000 m a.s.l, steep valley walls and an alluvial river plain at the valley floor. It was regulated for hydropower in 1963 and is Norway's 10th largest regulation scheme. The hydropower scheme has resulted in a large reduction of the frequency of large magnitude flood events. This paper describes the long-term geomorphological response of a gravel-bed river channel to this regulation. The study was initiated after complaints from local farmers of an increase in flooding of farmland after regulation. Forty-seven cross-profiles of the river repeatedly measured in 1973, 1989 and 1995 along a 1600 m reach have been used to quantify aggradation and degradation in the reach. The profiles show a reduction in channel size and capacity since 1973, with a net aggradation of 12000 m³. The upper part of the reach has been subject to the greatest amount of aggradation, with the river bed being raised by up to 1.5 m, while the lower part has degraded, with lateral erosion cutting banks back by up to 30 m. Aerial photographs from 1964 and 1984 show that former gravel-bars and flood channels have been subject to vegetation encroachment. The hydraulic model HEC-2 was used to compute the water level in 1973 and 1989. The water level has been raised by 0.6 m at low flows in the upper parts of the studied reach. This corresponds to an increase in discharge of between 20–30³/ s. Overbank flow therefore occurs at lower discharges today than prior to regulation. © 1997 John Wiley & Sons, Ltd.     

Reconstruction of the characteristics of a natural alluvial river–floodplain system and hydromorphological changes following human modifications: the Danube River (1812–1991)

Based on detailed historical surveys from 1812, the natural riverine landscape of a 10.25‐km‐long reach of the Danube River in the Austrian Machland region prior to channelization is analysed. Anthropogenically induced changes of fluvial dynamics, hydrological connectivity and aquatic habitat composition are discussed, comparing the situations following channelization (1925) and flow regulation (1991). In 1812 the alluvial river–floodplain system of the Danube River comprised a highly complex channel network, numerous gravel bars and extensive islands, with the main channel and side arms (eupotamon) representing about 97% of the entire water surface at low flow. The floodplain was characterized by relatively flat terrain and numerous natural trenches (former active channels) connected to the main channel. These hydromorphological conditions led to marked expansion/contraction of the water surface area at water level fluctuations below bankfull (‘flow pulse’). The high degree of hydrological connectivity enabled intensive exchange processes and favoured migrations of aquatic organisms between the river and floodplain habitats over a period of approximately 90 days per year. Overall in 1812, 57% of the active zone (active channels and floodplain) was inundated at bankfull water level. Channelization and construction of hydropower plants resulted in a truncated fluvial system. Consequently, eupotamal water bodies decreased by 65%, and gravel/sand bars and vegetated islands decreased by 94% and 97%, respectively, whereas the area of the various backwaters doubled. In 1991 the former ‘flow pulse’ was halved due to artificial levees and embankments, greatly diminishing hydrological connectivity and decoupling large areas of the floodplain from the main channel. Active overflow, formerly playing an important role, is now replaced by backwater flooding and seepage inflow in isolated water bodies. Copyright © 2003 John Wiley & Sons, Ltd.     

Downstream Regime Relations for Single‐Thread Channels

New empirically derived downstream hydraulic geometry relations are developed for single‐thread gravel/cobble‐bed and sand‐bed channels using the independent variables bankfull discharge, channel slope, and median bed grain size. Differences in channel response to the controlling variables are observed between gravel/cobble‐bed and sand‐bed channels. It is found that the inclusion of channel slope into regime relations is necessary for accurate channel geometry predictions and that bed grain size becomes an unnecessary variable. Accuracy of predictions is superior to many existing relations and comparable with those that already include slope as an independent variable. Variability of prediction errors is comparable with existing relations. Applications and implications of the new relations are discussed. Copyright © 2016 John Wiley & Sons, Ltd.     

三峡、丹江口水库运行前后坝下游不同河型稳定性对比分析

通过计算丹江口、三峡水库运用前后坝下游不同河型的河床稳定系数,分析了建库前后坝下游河床稳定性的变化情况及不同河型稳定性的差异,结果表明:坝下游不同河型的综合稳定系数表现为,分汊河型的稳定系数最大,蜿蜒河型次之,游荡河型综合稳定系数最小;同一河型在不同的河流甚至同一河流不同河段上的综合稳定系数存在差异;丹江口、三峡水库运用以后坝下游不同河型的综合稳定系数均呈增大趋势,河道总体呈现渐趋稳定的态势。     

Influences of valley form and land use on large river and floodplain habitats in Puget Sound

In this paper, we use a system‐wide census of large river and floodplain habitat features to evaluate influences of valley form and land use on salmon habitats along 2,237 km of river in the Puget Sound region of Washington State, USA. We classified the study area by geomorphic process domains to examine differences in natural potential to form floodplain habitats among valley types, and by dominant land cover to examine land use influences on habitat abundance and complexity. We evaluated differences in aquatic habitat among strata in terms of metrics that quantify the length of main channels, side channels, braid channels, and area of wood jams. Among geomorphic process domains, habitat metrics standardized by main channel length were lowest in canyons where there is limited channel migration and less potential to create side channels or braids, and highest in post‐glacial and mountain valleys where island‐braided channels tend to form. Habitat complexity was lower in glacial valleys (generally meandering channels) than in post‐glacial valleys. Habitat abundance and complexity decreased with increasing degree of human influence, with all metrics being highest in areas classified as forested and lowest in areas classified as developed. Using multiple‐year aerial photography, we assessed the ability of our methods to measure habitat changes through time in the Cedar and Elwha Rivers, both of which have recent habitat restoration activity. We were able to parse out sources of habitat improvement or degradation through time, including natural processes, restoration, or development. Our investigation indicates that aerial photography can be an effective and practical method for regional monitoring of status and trends in numerous habitats.     

River bed evolution due to channel expansion: general behaviour and application to a case study (Kugart River,Kyrgyz Republic)

We study the effect of spatial variations of river width on bed aggradation and degradation processes, making use of a one‐dimensional numerical model of channel morphodynamics. We refer to a peculiar case, the downstream reach of the Kugart River (Kyrgyzstan). The river has been partly channelized in the recent past with the aim of reducing the flooding risk for the surrounding villages; the consequent reduction of channel width in some reaches was also expected to improve channel conveyance with respect to the high sediment load produced in the upper river basin. The resulting longitudinal sequence of relatively sharp channel expansions and contractions has, however, triggered rapid siltation rates, especially in the narrowest reaches. This motivated the application of a 1‐D numerical model of river morphodynamics. Abrupt channel expansions are found to be the main driving forces for aggrading processes, which may extend for long distances from where they are generated. In order to obtain a thorough understanding of the morphodynamics of channel expansions, we first apply the model to simple test cases. This allows us to characterize the basic features of the problem and the dependence of bed evolution on the upstream Froude number Fr and on the expansion ratio r_b, which are the most relevant controlling parameters. We invariably find that deposition occurs in expansion regions with bed aggradation observed both upstream and downstream. The deposition prism progressively increases its height and lengthens both in the upstream and downstream directions. The deposition process is particularly intense, in terms of deposition prism height, in super‐critical conditions. Moreover, it is found that higher values of Fr strongly reduce the time scale of morphological processes and faster deposition rates are further facilitated by abrupt expansions. The present outcomes are relevant for assessing the expected altimetric response of river bed to the implementation of localized channelization works and to local river widening, a practise which is increasingly being employed within river restoration projects, with the aim of enhancing habitat diversity. Copyright © 2008 John Wiley & Sons, Ltd.     

Change detection of gravel mining on riverbeds from the multi‐temporal and high‐spatial‐resolution formosat‐2 imagery

Gravel mining from river channels is conducted in many countries around the world, yet ground‐based monitoring of these activities requires considerable manpower and is not very effective. Therefore, innovative and effective approaches to monitoring gravel mining are urgently required. Deployed as a high spatial resolution sensor in a daily revisit orbit, Formosat‐2 has proved to be an ideal satellite for site surveillance. Using one known event of gravel mining in the Tseng‐Wen River, Taiwan, between March 2004 and March 2005, we analysed nine Formosat‐2 images taken during this period to summarize three characteristics of gravel mining in the images. Based on these characteristics, a standard procedure for processing Formosat‐2 imagery to detect gravel mining is proposed. This procedure is validated against ground truthing collected by an unmanned helicopter flying at low altitude. The evolution of this gravel‐mining event in the Tseng‐Wen River is described by processing all historical Formosat‐2 imagery using the proposed procedure. This standard procedure has been successfully incorporated into the Formosat‐2 automatic image processing system and has been used to monitor gravel mining on a daily‐basis. Copyright © 2008 John Wiley & Sons, Ltd.