A study of straight stable channels and their interactions with bridge structures

Abstract

This thesis concerns a carefully controlled set of experiments, to study in detail the characteristics of straight stable channels and the alterations occurring in these channels due to the presence of hydraulic structures e. g. bridge piers, and abutments. A unique feature of the experiments is the mobility of the entire crosssection. The experimental data from present study are used to verify the characteristics of straight stable channels and the validity of the White et al (1981) theory and other published formulae for the prediction of the characteristics of these channels. The results show that the White et al (1981) theory is a useful tool for prediction of the hydraulic and geometric dimensions of stable straight channels. However, this theory underpredicts the water surface width, cross-sectional area, and sediment concentration of the laboratory channels for the range of applied discharges. It also overpredicts mean depth and water surface slopes. A bank profile investigation of the developed channels show that, for the prediction of bank profile, the fifthdegree polynomial of Diplas and Vigilar (1992) results in a better approximation than that obtained from the normal-depth method and the cosine profile. A hyperbolic function has been fitted to the bank profile data. This function approximates the bank profile even closer than the fifth-degree polynomial of Diplas and Vigilar (1992). The effect of the introduction of hydraulic structures e. g. bridge piers or abutments into the bed of developed channels and the instabilities produced was investigated. The interaction of piers and developed channels reveals that the main abrupt change in channel bed and bank is because of scour hole formation. An increase in channel width downstream of piers is observed. The rate of this increase is high initially and reduces thereafter. A mathematical model has been proposed to predict the temporal variation of scour depth in clear water and live bed scour conditions. This model, when compared with available data, predicts satisfactory results for most conditions. Scour topography due to the interaction of abutments and developed straight channels shows that the channel bank upstream of abutments was mainly scoured because of slowly circulating water ahead of the abutment. An equation is proposed to predict this scoured area.