Showing papers on "River engineering published in 1992"

Alluvial cutoffs as indicators of former channel conditions

Abstract: Although alluvial cutoffs record accurately the geometry, bedforms, and bed material of the channel when last active, few attempts have been made to use cutoffs in studies of channel changes. A detailed record of historical channel changes on the lower Hunter River in Southeastern Australia has shown that this channel responds to naturally alternating periods of high and low flood activity, called flood- and drought-dominated regimes respectively. Sinuosity decreased from 3.84 in 1870, to 2-66 in 1893 and to 1-38 in 1970 through the development of eight cutoffs. The channel also aggraded with medium sand burying the former bed material of mixed mud, coarse sand, and gravel. Channel straightening was a response to increased flood frequencies during the flood-dominated regimes of the late 19th and 20th centuries, combined with localized river engineering works and increased sand load. Detailed stratigraphic studies were carried out on three neck cutoffs and one chute cutoff which were abandoned in 1890, 1950, 1952, and 1956. A comparison of former and present bed elevations and bed material size showed similar trends to those determined by the historical record, confirming the reliability of cutoffs as indicators of former channel conditions. The sedimentary infills of the cutoffs are not uniformly fine grained as recorded previously in the literature. Relatively thin, fine-grained fills were deposited during the drought-dominated regime of the first half of this century but thick, coarser-grained fills were deposited after 1949 during the flood-dominated regime. All fills fine upwards. Cutoff infills provide a record of changing flood activity and sediment loads.

Particle Size Distributions of Bed Sediments Along the Thalweg of the Mississippi River, Cairo, Illinois, to Head of Passes, September 1989

Abstract: : Changes in Mississippi River bed material gradations between Cairo, IL, and head of Passes, LA, between 1932 and 1989 were determined. In September 1989, bed material samples were collected from the thalweg of the river along the 955-mile reach. In all, 504 samples were collected at 417 locations. Results were compared to a similar sampling program conducted in 1932. In general, the 1989 bed contained less coarse sand and gravel and less very fine sand than the 1932 lied. Upstream of the Old River Structure near river mile 300 the bed was generally finer in 1989 than in 1932. Downstream from river mile 300 the median grain size was about the same, but the distribution was more uniform, with less very fine sand.

Dilution Method for Measurements of Unsteady Discharges in Mountain Streams

Abstract: In river engineering problems information on discharges and water levels is essential for many reasons. For irrigation and drinking water purposes it is very important to know how much water can be extracted from a river. Managers of waste water stations need to know how much they can release without exceeding the norms, and information on flood waves in the past help to predict the probability of flood waves in the future. Discharge measurements are therefore necessary. Different methods to measure a river discharge exist, such as the velocity area method, the moving boat method and methods using structures like flumes and weirs. Another method is the dilution method, based on the dilution of a soluble, non-disintegrating substance, to be released in the river. For steady flows, the principles of this method are rather simple, and recommendations on how to perform the measurements already exist (ISO Handbook). However, for unsteady flows, this method is much more complicated, and even its applicability is questionable. Studies on this topic, in which rivers were schematised as prismatic channels have already been carried out. In the present study the practice is the central issue. The applicability of the dilution method is investigated for mountain rivers with irregular cross-sections, big rocks and unsteady discharges. In chapter 2 the principles of the dilution method are explained, and the studies made thus far are outlined, as well as the approach in the present study. In chapter 3 the basic equations for the motion of water and transport processes, necessary for a transport model that can deal with the typical problems of an irregularly shaped mountain river, are formulated. A numerical scheme is derived in chapter 4, resulting into a flow-and-transport-simulating computer program (FATS). Dilution discharge measurements are simulated in imaginary rivers under flood wave conditions . The river discharge is then a time dependent upstream boundary condition, and a few measurements (water levels, concentrations) are generated. In chapter 5 a numerical procedure is developed (FINDQ) to compute the upstream river discharge, which uses only these measurements without any further information on characteristic river parameters. Determination of these parameters is an identification problem, which is solved using the DUD procedure. The quality of the discharge determination can be estimated by comparing the result with the original upstream boundary condition in FATS, supposed to be 'true'. Finally, in chapter 6, attention is paid to the field work and equipment required to obtain real measurements for the discharge determination. Using the FINDQ-DUD algorithm, the river discharge as a function of time can then be computed to a certain degree of accuracy, which is the eventual aim of this study.

Vegetated Streams in the Context of Flood Damage

Abstract: Human impact on the aquatic environment, in order to exploit available water potential (quantity, quality, position in gravity field), involves changing characteristics of the water system. Human impact on river systems can be divided into two categories: impacts due to pollution of water and sediments, impacts due to river engineering works.