Seepage Effects on Stream Power, Resistance, Incipient Motion and Regime of Sand Bed Channels including Its Design

Abstract

Common behavioral trends and characteristics of alluvial channels including rivers are extensively discussed in the literature. However, little is known about the hydrodynamic effects of seepage on alluvial channels. Factors like sand bed resistance, stream power of the channel, incipient motion of bed particles, and geometry of the channel cross section are significantly affected by seepage. This thesis presents the experimental investigations that are aimed to find out the quantitative effect of seepage, through a sand bed in downward (suction) direction, on the above mentioned factors. The problem in the sediment transport analysis is that the knowledge of complex interaction of several parameters with seepage cannot be fully obtained. In order to generalize the results, experiments are conducted in four rectangular smooth walled sand bed flumes under steady and fairly uniform flow conditions. Among the four, one is the Large Tilting Flume (LTF), which is 25 m long, 1.80 m wide and 1.00 m deep and with a seepage length of up to 20 m. This LTF is specially built at hydraulic laboratory, Indian Institute of Science exclusively for the present research work especially on alluvial channel regime. The experimental channels are designed to apply controlled amounts of uniform seepage flow in either direction in one flume (Flume-2), and only in downward direction to other three flumes (Flumes 1, 3 and 4). The application of seepage is perpendicular to the sand bed thickness over a sufficient length of the main channel. Appropriate instruments are used to accurately measure the basic experimental variables such as discharge in the main channel, seepage discharge, flow depth, water surface and bed slope, seepage gradients and cross sectional profiles. Experiments are carried out at different conditions (plane beds and curved shape channels) as explained below: Plane sediment beds Series – 1: Experiments to determine incipient motion of bed particles under no-seepage condition. Series – 2: Experiments on the non-transporting condition of the bed particles under both no-seepage and seepage condition. Series – 3: Experiments on the transporting condition of the bed particles under both no-seepage and seepage condition. Curved shape channels Series – A: Experiments to verify Lane’s (1953) geometric profile against higher discharge than prescribed by Lane (1953). Series – B: Experiments to verify the Lane’s (1953) geometric profile by allowing the discharge prescribed by Lane (1953). Series – C: Experiments to predict the final geometric profile by applying suction to Series – B experiments. A wide range of sediment particles are tested. Five different sized uniform sands (d50 = 1 mm, 0.56 mm, 0.65 mm, 1.00 mm and 1.77 mm) and gravel of size d50 = 8.00 mm are used for experimentation. Among the six sizes, three sizes (d50 = 0.56 mm, 0.65 mm, 2 mm) are used for seepage experimentation. The experimental data from the present experiments along with the available data from other sources on more sizes of sand are analyzed, thus covering a wide range of sand sizes. The following important results are obtained from the analysis. A new resistance equation has been developed for plane sediment beds (with little or no-transport) such that the average velocity in the channel depends on the shear velocity Reynolds number. A careful study has been done on incipient motion and concluded that incipient motion is better explained by critical stream power criterion for plane sediment beds. With the help of critical stream power criterion, a straightforward design procedure using design tables/design curves and analytical methods are presented to solve six possible design problems. For plane and non-transporting beds, in general, the stream power in the channel increases with suction and decreases with injection. The increase and decrease depend on the seepage power intensity parameter (NP), initial value of stream power (Ωbo), and critical stream power of the particles under no-seepage condition (Ωco). An expression relating all the influencing parameters is established to quantitatively estimate the stream power (Ωbs) variation with both the types of seepages, i.e., with suction and injection. It is found that the seepage has a significant influence in changing critical stream power for incipient motion of the bed material and the value is significantly different from the no-seepage critical value. An expression is established to quantitatively estimate the critical stream power with seepage (upward and downward) for a given critical stream power (Ωco) of the bed material under no-seepage conditions and initial stream power (Ωbo). It has been established that critical stream power curve used to define incipient motion is valid only for no-seepage condition of the bed and it cannot be used for sand beds under seepage condition, as seepage effects significantly alters the stream power. From the wide range of experimental data (including the observations from LTF) it is found that suction (downward seepage through the sand bed) enhances the transport or aids the incipient motion of bed particles which are initially at rest. Thus, suction reduces the stability and increases the erosion of bed particles when compared to no-seepage conditions. However, it is found that injection (upward seepage) affects in an opposite way, i.e., it can reduce the transport rate or even inhibit the incipient motion. Thus, injection increases the stability and reduces the erosion of bed particles when compared to no-seepage conditions. Therefore, it is concluded that suction increases the mobility of sand particles where as injection decreases their mobility. An expression to find the incipient motion with seepage (both suction and injection) is established in terms of stream power’s (Ωco, Ωbo and Ωbs) based on the present experimental data along with others' data. With the help of these expressions design procedure is developed for ten types of possible problems. A numerical model for spatially varied flow has been developed with the help of the seepage governing equations, developed in this thesis, to compute flow profiles along the channel length. A methodology of predicting the location of incipient motion section in sand bed channels affected by seepage is also presented. Channel geometry affected by seepage (suction) is established in the form of regression relationships for perimeter, flow depth and slope of the channel. Different combinations (bi-variate and tri-variate) of dimensional and non-dimensional regression relationships are developed. An approach to channel design has been developed based on the application of functional analysis of the salient variables that control the channel behavior. And also, it has been established that, Lane’s (1953) profile almost matches with experimental profile for no-seepage condition. The present investigation clearly shows the significance of seepage in altering the hydraulic and sediment transport behavior of sand bed channels. From the practicing engineer’s point of view it is hoped that present design procedures will be helpful in safe guarding the seepage affected channels.