Local Channel Morphology Research Articles

Lateral meander migration of a medium-sized lowland river: Case study on the Rába river, Hungary

Engineering works greatly influence the lateral channel migration (LCM) of meandering rivers. We aimed to characterise the spatiotemporal characteristics of LCM during the last 174 years of the almost freely meandering Upper Rába (Hungary) and to identify units with distinctive LCM histories. The studied Rába's reach has been regulated just at some points. Due to the free meandering, its length varied between 119 and 133 km. The most intensive length increase (+291 m/y) took place between 2005 and 2008, and by the end of the process , the sinuosity had reached its historical maximum (SI 2018 = 1.93). The mean LCM was 3.3 m/y (1844-2018). The periods with intensive channel migration (max: 24-27 m/y) were followed by periods with low migration rates. Based on local channel morphology and LCM rates, 14 units were identified. The highest LCM rate was measured in the freely migrating units (R5: 5.8 m/y; R3: 4.4 m/y and R6 4.0 m/y).

Cryologic continuum of a steep watershed

Ice processes taking place in steep channels are sensitive to the thermal and hydrological regimes of upstream reaches and tributaries as well as to the local channel morphology. This work presents freezeup, mid-winter, and breakup data from four channels of increasing order located in a cold temperate watershed during the winter 2010–2011. From headwater channels to the main drainage system, water temperature, ice coverage, and ice processes are reported and related to weather conditions and to channel characteristics. Headwater channels only formed ephemeral ice features, and their water temperature reached as much as 4 °C in mid-winter. On the other hand, larger channels formed impressively large ice dams, some of them reaching 2 m in height. The development of a suspended ice cover partially insulated the channels; as a result, water temperatures remained above 0 °C even for air temperatures well below freezing. This work presents steep channels ice processes that have not been described in previous publications. The concept of a watershed cryologic continuum (WCC) is developed from the data collected at each channel order. This concept emphasizes the feedback loops that exist between morphology, hydrology, heat, and ice processes in a given watershed and can lead to a better understanding of ice processes taking place at any channel location within that watershed. The WCC can also contribute in improving our understanding of the impacts of climate change on the cryologic and thermal regimes of steep channels. Copyright © 2012 John Wiley & Sons, Ltd.

Numerical Study of Flow Affected by Bendway Weirs in Victoria Bendway, the Mississippi River

It has been observed that submerged weirs in bendways realign the flow and in general improve navigation conditions. This qualitative observation has been the basis for field design. This paper presents a study of hydrodynamics in the Victoria Bendway in the Mississippi River using three-dimensional numerical simulations. A numerical model, CCHE3D, was applied and computational results were compared to three-dimensional velocity data provided by the U.S. Army Corps of Engineers with reasonable agreement. The numerical simulation results were then used to analyze helical currents due to the channel curvature and the presence of submerged weirs. The simulated flow realignment near the free surface indicates that the flow conditions in the bendway were improved by the submerged weirs, however, the effectiveness of each weir depends on its alignment, local channel morphology, and flow conditions.

Biogeography, ecoregions, and geomorphology affect fish species composition in streams of eastern Oklahoma, USA

Stream fish assemblages are structured by biogeographical, physical and biological factors acting on different spatial scales. We determined how physical factors, geomorphology and stream habitat, influenced fish species composition (presence–absence) in eastern Oklahoma, USA relative to the ecoregion and biogeographic effects previously reported. We sampled fish assemblages and surveyed geomorphology and habitat at 107 stream sites in the Boston Mountains, Ouachita Mountains, and Ozark Highlands ecoregions in eastern Oklahoma. Partial canonical correspondence analyses (pCCAs) and variance partitioning showed that patterns of endemism related to drainage basins and ecoregions explained important variation in fish species composition in all streams, but stream size and local channel morphology explained more variation overall. Stream size effects were most important in explaining variability in fish species composition in both northeastern and southeastern Oklahoma streams. Local channel morphology and substrate characteristics were secondarily important. Variables typically considered important as fish habitat (aquatic vegetation, etc.) had little effect on fish species composition.

Towards an ecologically meaningful classification of the flow biotope for river inventory, rehabilitation, design and appraisal purposes

Recent research into the physical and ecological status of rivers has focused upon rapid field-based assessments of mesoscale habitat features in order to satisfy international regulatory requirements for habitat inventory and appraisal. Despite the low cost and time efficient nature of rapid field surveys, the robustness of such techniques remains uncertain. This paper uses data from over 4000 surveyed UK river reaches in the UK River Habitat Survey (RHS) database in order to seek linkages between surface flow conditions (flow biotopes), local channel morphology (physical biotopes) and biologically distinct vegetative and minerogenic habitat units (functional habitats). Attempts to identify one-to-one connections between surface flow types, units of channel morphology and functional habitats oversimplify a complex and dynamic hydraulic environment. Instead, a nested hierarchy of reach-scale physical and ecological habitat structures exists, characterised by transferable assemblages of habitat units. Five flow biotopes show strong correlations with functional habitats, and differing combinations of three of these account for over 60% of the distribution for all functional habitats. On this basis, a classification of environments for ecological purposes is proposed. Principal components analysis and agglomerative hierarchical clustering analysis are employed to objectively validate the proposed classification. Flow biotope assemblages may also represent reach-scale channel morphologies (step-pool, riffle-pool and glide-pool), although functional habitats exhibit differing ‘preferences’ for rougher or more tranquil environments within these. While the data and analysis are specific to the UK RHS, the methods and findings have wider application in situations, where rapid field appraisal methods and associated databases are deployed in water resource inventory and river rehabilitation design.

Dynamic model for constriction scour caused by large woody debris

AbstractLarge woody debris (LWD) can have a significant impact upon local channel morphology by creating scour pools and zones of reduced shear stress in which sediment is deposited. It is important to predict scour depths associated with LWD, as it is becoming increasingly common for debris to be added into river channels to improve sediment retention and create pools for aquatic habitat. Engineered log‐jams should therefore be designed using factor of safety engineering analysis, which includes estimates of associated scour and deposition rates. However, the rate and total depth of scour associated with LWD have not been modelled comprehensively, with authors resorting to the use of generic local and constriction scour models to predict scour depths. Also, constriction scour models presented, to date, do not calculate the rate of scour development. In this paper a model is presented for predicting the rate and total depth of scour associated with a channel constriction. The model is one‐dimensional and is based upon the sediment continuity equation, the calculation of specific head changes through the constricted reach and also allows for a variable free surface elevation above the bed at the constriction. This model could be applied to any channel constriction problem but here is used to determine scour rates and depths associated with deflector‐type LWD jams. Deflector jams are one category of jam type presented in a debris jam classification scheme, in which jam type is a function of the ratio of average riparian tree height to average channel width. Deflector jams, as the name implies, partially block the flow and therefore act as a channel constriction, which results in constriction scour. Copyright © 2002 John Wiley & Sons, Ltd.

Seasonal Changes in Bed Elevation in a Step-Pool Channel, Rocky Mountains, Colorado, U.S.A.

Scour and fill patterns at East St. Louis Creek, Colorado, were investigated via repeat, detailed surveys of the channel bed at 11 cross sections during the 1995 snowmelt season. Spatial variability was remarkably high, with significant differences in cross section scour and fill patterns over distances as short as 0.5 m. Most sites had small net changes in bed elevation, both daily and over the entire runoff season. The data and observations indicate the presence of small pulses of fine material that are temporarily deposited on top of the channel pavement in wider areas of the channel and near woody debris complexes. Scour and fill are primarily limited to the finer material of such pulses. ANOVA analysis indicates that although discharge was important in predicting changes in bed elevation, the relationship between discharge and bed mobility is complicated by the effects of local channel morphology and a slight hysteresis. Regression analysis shows that variations in channel width determine where finer sediments are deposited, and therefore the locations of greater change in bed elevation. The proximity of morainal ridges and boulders to the channel edge locally influence the channel width and also the distribution of woody debris complexes. Results of this study suggest that the channel morphology and sediment transport along some reaches of small, high-gradient streams in watersheds with a glacial history may not respond as substantially to changes in discharge characteristic as do other types of alluvial channels.

Seasonal Changes in Bed Elevation in a Step-Pool Channel, Rocky Mountains, Colorado, U.S.A.

Scour and fill patterns at East St. Louis Creek, Colorado, were investigated via repeat, detailed surveys of the channel bed at 11 cross sections during the 1995 snowmelt season. Spatial variability was remarkably high, with significant differences in cross section scour and fill patterns over distances as short as 0.5 m. Most sites had small net changes in bed elevation, both daily and over the entire runoff season. The data and observations indicate the presence of small pulses of fine material that are temporarily deposited on top of the channel pavement in wider areas of the channel and near woody debris complexes. Scour and fill are primarily limited to the finer material of such pulses. ANOVA analysis indicates that although discharge was important in predicting changes in bed elevation, the relationship between discharge and bed mobility is complicated by the effects of local channel morphology and a slight hysteresis. Regression analysis shows that variations in channel width determine where finer sediments are deposited, and therefore the locations of greater change in bed elevation. The proximity of morainal ridges and boulders to the channel edge locally influence the channel width and also the distribution of woody debris complexes. Results of this study suggest that the channel morphology and sediment transport along some reaches of small, high-gradient streams in watersheds with a glacial history may not respond as substantially to changes in discharge characteristic as do other types of alluvial channels.

Channel Morphology, Water Temperature, and Assemblage Structure of Stream Insects

Temperature is known to be an important mechanism affecting the growth and distri- bution of stream insects. However, little information exists that describes how variable temperatures are among streams of similar size, especially in physically heterogeneous landscapes. We measured summer daytime temperature and the structure of riffle benthic insect assemblages from 45 montane streams in California. Summer stream temperature was nearly randomly distributed across large-scale geographic gradients of latitude (6?) and elevation (2000 m). The lack of geographic trends in summertime stream temperature appeared to be caused by the strong relationship between local channel morphology and summer water temperature. Mean daytime water temperature was most strongly related to the % of the channel present as pools, which did not vary systematically with either latitude or elevation. We used multiple multivariate regression analysis, non-metric multidimensional scaling (NMDS), and graphical techniques to both quantify differences in insect assemblage structure among streams and to determine the degree to which assemblage structure was related to temperature. NMDS anal- yses were conducted on 3 similarity matrices based on: 1) presence and absence of all aquatic insect taxa encountered during the study, 2) densities of the 16 most numerically abundant taxa, and 3) population biomasses of the 16 most common taxa. All 3 analyses showed that variation in assem- blage structure among streams was significantly related to temperature, although assemblage struc- ture was most strongly related to sampling date-a consequence of sampling over a 98-d period. Temperature probably influenced assemblage structure in 2 ways: 1) by influencing developmental rates of individual taxa and overall assemblage phenology, thus affecting the relative abundances of taxa found on a specific sampling date, and 2) by excluding taxa unable to tolerate certain temperature ranges. Because of the strong dependency of assemblage structure on temperature and the lack of strong geographic trends in temperature among these streams, much of the measured variation in assemblage structure appeared to be unrelated to latitude or elevation. These results have important implications for both our understanding of natural biogeographic patterns of lotic organisms and our ability to detect and model the effects of climate change and other thermal alterations on stream ecosystems.

COARSE WOODY DEBRIS AND CHANNEL MORPHOLOGY: A FLUME STUDY1

ABSTRACT: In recent years, logs and other structures have been added to streams for the purposes of altering channel morphology to improve fish habitat. This flume study was conducted to evaluate the effects of coarse woody debris on local channel morphology. Wooden dowels were used to simulate the effects of individual logs in a stream, and scour depth and surface area were determined at the end of each test run. The maximum scour depth was significantly correlated (90 percent confidence level) with both the vertical orientation of the dowels and the channel opening ratio; the scour surface area was significantly correlated (90 percent confidence level) with both the flow depth and the vertical orientation. Upstream‐oriented dowels caused relatively large streambed scour and also deflected flows toward the streambank. Downstream‐oriented dowels generally caused less bed scour and appeared to provide better bank protection because flow was generally deflected from the bank. In conjunction with data from field studies, these results provide information on the effects of orientation, hydraulic function, and relative stability of coarse woody debris in streams.

Effects of valley and local channel morphology on the distribution of gold in stream sediments from Harris Creek, British Columbia, Canada

Abstract Large (60 kg, minus 2 mm) sediment samples collected from a 5-km reach of a mature highland stream were used to investigate the effects of bar morphology and channel slope on the downstream dispersion of Au. Results are compared with those for an abundant heavy mineral (magnetite) and are discussed in terms of differential entrapment of light and heavy minerals by gravels and differences in supply of magnetite and Au to the study reach. Gold is selectively trapped in gravels; therefore, highest Au concentrations, a long dispersion train and lowest between-sampling-location variability were obtained with heavy-mineral concentrates of the minus 105+74 μm fraction from sandy gravels. In contrast, sands in bar-tail eddy pools gave the least reliable Au concentrations, due to random (Poisson) sampling errors and local post-depositional winnowing, and show rapid downstream anomaly decay.