Artificial Cutoffs Research Articles

Bed-level adjustments in the Arno River, central Italy

Two distinct phases of bed-level adjustment over the last 150 years are identified for the principal alluvial reaches of the Arno River (Upper Valdarno and Lower Valdarno). The planimetric configuration of the river in these reaches is the result of a series of hydraulic works (canalization, rectification, artificial cut-offs, etc.) carried out particularly between the 18th and the 19th centuries. Subsequently, a series of interventions at basin level (construction of weirs, variations in land use), intense instream gravel-mining after World War II, and the construction of two dams on the Arno River, caused widespread degradation of the streambed. Since about 1900, total lowering of the channel bed is typically between 2 and 4 m in the Upper Valdarno Reach and between 5 and 8 m in some areas of the Lower Valdarno Reach. Bed-level adjustments with time are analyzed for a large number of cross-sections and described by an exponential-decay function. This analysis identified the existence of two main phases of lowering: the first, triggered at the end of the past century; the second, triggered in the interval 1945–1960 and characterized by more intense degradation of the streambed. The first phase derived from changes in land-use and land-management practices. The second phase is the result of the superimposition of two factors: intense instream mining of gravel, and the construction of the Levane and La Penna dams.

Modelling inter-annual sea-ice variability off eastern Canada

Inter-annual sea-ice variability north of 55°N in eastern Canada was explored with a box model incorporating annually invariant, bathymetrically dependent ocean heat fluxes and near-surface currents. Using inputs of ice concentrations, regional surface temperatures and geostrophic winds at external model boundaries, ice composition was simulated in seven thickness categories at 10 day intervals during three annual ice seasons. Comparisons indicated good reproduction of observed inter-annual differences in regional ice volumes during critical January-March periods. Additional simulations of artificial cutoffs in southward ice fluxes showed that advective influence decreases with latitude and dominates the development of spring ice conditions in areas south of 60°N.

The response of the Lower Mississippi River to river engineering

An examination of the response of the Lower Mississippi River (LMR) to a variety of engineering activities is presented through the discussion of: (a) a brief history of engineering investigations and activities on the LMR; (b) the impact of artificial cutoffs on the channel geometry and water surface profiles of adjacent reaches; (c) the impact of channel alignment activities on channel morphology; and (d) the apparent impact of all of the LMR engineering activities on sediment dynamics in the channel. Investigations by many agencies reflect over 150 years of study of the hydraulics and hydrology of the LMR, which have contributed significantly to our understanding of large alluvial rivers. In an effort to provide for flood control and navigation on the largest river in North America, private landowners and the US Army Corps of Engineers have performed a wide range of river engineering activities, including construction of levees, floodways, artificial cutoffs, bank revetment, training dikes, dredging, channel alignment, and reservoirs on the major tributaries. This unprecedented program of river engineering activities on the river during the last 100 years has resulted in the evolution of a freely meandering alluvial river to a highly trained and confined meandering channel. The LMR has increased its overall gradient and average top-bank width and generally increased its channel depth. The immediate response of the river to increased gradient as a result of the construction of artificial cutoffs was dampened in some locations by local geological controls. Examination of the trends in sediment dynamics of the LMR reveals that the suspended load of the river has decreased during the 20th century. Conversely, a trend in the bed load transport in the channel for the years 1930 and 1989 cannot be determined with confidence because of the difficulty in acquiring representative samples. The highly trained river now responds to channel forming flows by attempting to build mid-channel bars rather than natural cutoffs of meanders. The LMR should maintain a relatively stable plan form in the intermediate future, barring a very large and unprecedented flood. The river will continue to adjust its channel geometry and its local gradients as a response to variations in significant discharges. Continued channel maintenance and occasional dredging will insure the present state of sediment and water transport efficiency.

SEDIMENT POLLUTANTS FROM A RIVER MEANDER REINSTATEMENT SITE : CONCENTRATIONS AND MANAGEMENT IMPLICATIONS

Since European settlement in the early 1800s, 66 artificial meander cut-offs have been created in the lower Latrobe River, Victoria. Contemporary waterway management practices include the reinstatement of a selection of these cut-off meanders to slow water flows and reduce river bed and bank instability. Infill sediments were collected from a range of depths from one of these meanders prior to its reinstatement to estimate the potential for remobilization of any sediment pollutants into the waterway. The characteristics of collected sediments were described and sediments were analysed for a range of pollutants including heavy metals, total petroleum hydrocarbons, monoaromatic hydrocarbons, organochlorine pesticides, polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs). Metal concentrations in the fine (<63 μm) sediment fraction increased with increasing sample depth; however, only increases in cadmium, chromium and mercury concentrations were statistically significant. Statistically significant increases in mercury concentrations with depth were also present in the entire sediment fraction. With the exception of mercury, concentrations of all metals assessed in meander sediments were similar to those reported as being at background concentrations. Mercury concentrations, although elevated, were below concentrations reported to adversely effect riverine biota. Concentrations of organochlorine pesticides, polychlorinated biphenyls, total petroleum hydrocarbons, monoaromatic hydrocarbons and polycyclic aromatic hydrocarbons were all below detection limits in the entire sediment fraction. As a consequence, reinstatement of the meander as part of a waterway management programme is unlikely to release significant quantities of sediment-partitioned pollutants into the mainstream waterway.

History and Effects of Channelisation on the Latrobe River, Victoria

Channel changes on the lower Latrobe River since the late 1800s include a 25 per cent reduction in channel length due to artificial meander cutoffs, increases in mean channel width, increased channel instability and incision of up to 1.05 m. Hydraulic changes at the Rosedale gauge include increases in bankfull flow velocity and a 67 per cent increase in channel capacity, resulting in a threefold reduction of over‐bank flow duration. The extensive channelisation works have reduced the frequency of minor flooding and the period offloodplain inundation, but these and other human impacts have severely degraded the stream environment.

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

AbstractExtensive 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.

Iterative broadening

Conventional blind search techniques generally assume that the goal nodes for a given problem are distributed randomly along the fringe of the search tree. We argue that this is often invalid in practice and suggest that a more reasonable assumption is that decisions made at each point in the search carry equal weight. We go on to show that a new search technique called iterative broadening leads to orders-of-magnitude savings in the time needed to search a space satisfying this assumption; the basic idea is to search the space using artificial breadth cutoffs that are gradually increased until a goal is found. Both theoretical and experimental results are presented.

Lifetimes of singly excited states in Cu^+ and Ag^+

The lifetimes of all the known singly excited states in Cu+ and Ag+ were calculated by using a realistic potential (Hartree–Slater) to represent the atomic (ionic) core. Since ab initio calculation of transition wavelengths still lags greatly behind experiment, we used the available experimental values. The results obtained are in good agreement with available sparse experimental data of lifetimes and are intended for guidance of future experimental investigations. The present numerical Coulomb-approximation approach is free from artificial integration cutoffs employed in the various widely used Coulomb approximations.

Mississippi River Cutoffs

Natural and artificial cutoffs on the Lower Mississippi River in the 50-mile stretch north of Vicksburg, Miss., are described with respect to their effect on river shortening and flood-stage loweri...