SCOUR IN SEDIMENT MIXTURES UNDER SUBMERGED CIRCULAR VERTICAL JETS

Abstract

Hydrodynamic imbalance and instability in river system due to natural and anthropogenic causes has become a subject of concern. In rivers and streams, one of the most challenging problems is to minimize and to know the depth of scour due to changes in flow pattern around the hydraulic structures for its stability and economical consideration. The structures built in rivers and channels are subjected to scour around their foundations. If the depth of scour becomes significant, the stability of the foundations is endangered. Scour is a process of lowering of river bed due to removal of bed material in the vicinity of hydraulic structures by erosive action of the flowing water. In some of the structures like ski-jump, free overfall spillway, jet spillway, the water jet penetrates the sediment bed that reaches its deepest level and is deflected in upward directions resulting in a highly turbulent zone. Scour due to turbulent water jets impacts numerous hydraulic engineering projects. Practicing engineers and planners have been confronted with the task of finding a feasible and cost effective solution to protect scour around the structures. The investigator must seek ways to guide and control the process so as to minimize the risk of failure of the hydraulic structures. Therefore, estimation of maximum scour depth is required for the safe and economic design of hydraulic structures and their foundations. The scour of sand, gravels and other materials, which often occurs downstream of hydraulic structures, is of considerable importance, as excessive scouring process may endanger the stability of the hydraulic structures such as gates, weirs, culverts, spillways and grade-control structures, etc. There are two types of scour profile geometries, dynamic scour when the jet flow is in operating condition, and static scour which is the final bed scour hole produced, when the jet flow is stopped. Local scour due to jets is affected by variables such as jet velocity, jet height, nozzle size, type of structure and characteristics of the sediment. For given conditions of these variables, the scour depth also varies with time. Many researchers have performed laboratory experiments on scour processes due to jets. The experimental investigation on scour due to submerged vertical jet in cohesionless sediments was probably first conducted by Rouse (1940). Thereafter, a number of investigations have been carried out to study the response of submerged water jets in various sediment materials, jet velocities, jet heights and diameter of nozzles. ii Clark (1962), Sarma (1967), Westrich and Kobus (1973), Rajaratnam and Beltaos (1977), Rajaratnam (1982), Uyumaz (1988), (1988), Breusers and Raudkivi (1991), Aderibigbe and Rajaratam (1996), Aderibigbe and Rajaratam (1996), Donoghue et al. (2001), Mazurek et al. (2002), Ansari (1999), Ansari et al. (2003), Rajaratam and Mazurek (2003, 2005), Adduce and Mele (2004), Yeh et al. (2009), Mazurek et al. (2009), Dehghani et al. (2010), Chakravarti et al. (2013) studied scour due to jet in cohesionless sediment. Some studies in respect to scour and erosion around the hydraulic structures were studied by Li (1987 a&b), Srivastava and Contractor (1992), Chalov (1995), Patel and Ranga Raju (1996), Patel and Ranga Raju (1999), Ansari et al. (2002), Chalov et al. (2004), Sarma (2005), Pagliara et al. (2006), Pagliara (2007), Pagliara et al. (2008), Pagliara and Palermo (2008), Azamathulla et al. (2009), Fukuoka and Osada (2009), Goel (2010), Chahar (2011), Azamathulla and Zakaria (2011), Fukuoka et al. (2013), Dehghani et al. (2013). The process of scour due to jets in cohesionless sediment has been investigated at length in the literature. While adequate study has not been carried out in the cohesive soil which comes in the river system through surface erosion of upland areas along with cohesionless soil such as sand and gravel. Review of the literature reveals that only a few studies have been conducted related to scour in cohesive bed under submerged water jets (Raudkivi and Tan, 1982; Hanson, 1991; Hanson and Robinson, 1993; Ansari, 1999; Ansari et al. 2003; Mazurek et al. 2001; Mazurek and Hossain, 2007). The above studies were mainly focused on either clay or clay-sand mixtures. However, no study has been conducted so far on scour under submerged circular vertical jet in cohesive material consisting of clay-gravel and clay-sand-gravel mixtures to the best of our knowledge. Therefore, it is intended to study the effect of presence of cohesion i.e., clay on scour process in clay-gravel and clay-sand-gravel sediment mixtures under submerged jets; as such types of sediment occurs frequently in nature (Kothyari and Jain, 2008; and Jain and Kothyari, 2009). The main objective of the present study was to comprehend the scour process under submerged circular vertical water jets in cohesionless and cohesive sediment mixtures, and finding out the various scour parameters like maximum static and dynamic scour depths, temporal variation of scour depth and its various length scale parameters related to scour like radius of scour, dune height and volume of scour for safe and economical design of the hydraulic structures, and to ascertain its practical application. The experiments were carried out at the Hydraulics Laboratory of the Department of Civil Engineering, Indian Institute of Technology Roorkee, India, and were performed iii on a circular steel tank having diameter 1.25 m and depth 1.25 m, filled with the desired sediment up to a height of 0.80 m, while the water was filled in the remaining 0.45 m height of the tank. The impinging jet was produced by a nozzle fitted at the end of circular supply pipe of diameter 0.0254 m. Suitable arrangement was provided to adjust the height of the jet above the sediment bed. The jet discharge was measured by calibrated Venturimeter fitted in the supply pipe. Locally available clay that was excavated from a depth of 1.0 m below the bed level of river bed was used. Various tests for determination of sediment properties were conducted as per Code of Practice. The clay material had a median size d50 = 0.014 mm, geometric standard deviation, σg = 2.1, sand had a median size, d50 = 0.24 mm and σg = 1.41, while gravel had a median size, d50 = 2.7 mm and σg = 1.21. The other engineering properties of clay material which were measured are liquid limit, WL = 43%, plastic limit, WPmax)(dγ = 22%, plasticity index, PI = 21%, maximum dry density = 16.75 kN/m3 The experiments were conducted with two sizes of nozzle of 12.5 and 8 mm diameter, and two jet heights of 0.15 and 0.30 m from sediment bed level. Two jet velocities of 7.19 and 5.12 m/s for 12.5 mm nozzle and 9.84 and 6.65 m/s for 8 mm nozzle were considered. In case of cohesionless sediment, three different types of sediment bed were prepared i.e., sand, sand-gravel mixture (equal proportion by weight) and gravel. Cohesive sediment mixture was prepared by mixing clay with gravel and sand-gravel. In all, three mixtures i.e., sand-gravel, clay-gravel, and clay-sand-gravel were prepared. In clay-gravel and clay-sand-gravel mixtures, the clay contents was varied in proportion varying from 10% to 60% by weight, while, in clay-sand-gravel mixture, equal proportion of sand and gravel were used. , optimum moisture content, OMC = 19% and relative density = 2.65. 1. Scour in Cohesionless Sediment and their Mixtures In all, 24 experimental runs were conducted in cohesionless sediment consisting of sand, sand-gravel mixture and gravel beds. The characteristics of scour under submerged circular vertical jets in cohesionless sediment were found different in each sediment mixture. Several shapes of scour hole geometries in sand, sand-gravel mixture and gravel beds were noticed. A close investigation of scour bed profiles revealed that the observed static and dynamic scour depths were high in sand bed compared to gravel and sand- iv gravel mixture. Also, the volume of scour hole, radius of scour hole and dune height were high in sand beds compared to gravel and sand-gravel mixture sediment beds. The size of scour hole was small in case of gravel beds while high in sand beds. In case of sand-gravel mixture, segregation of sand and gravel was noticed - fine material was deposited on the outer boundary of dune, while gravel was in the core of the scour hole. Temporal variation of the scour was measured and found that initially the rate of scour is high, however, it decreases with passage of time and attain an equilibrium stage – no significance scour takes place after attainment of equilibrium stage. Sediment size plays an important role in the process of scour in cohesionless sediment. The scour depth is inversely proportional to the size of cohesionless sediment. The experimental observations and analysis presented in this investigation established that the types of sediment have significant role on size of scour hole produced by water jets. The saturation time, Ts is defined as time required from start of the scour to achievement of 99% of the total scour. The variation of Ts Temporal variation of scour depth have been analyzed using the equations proposed by Lui et al., (1961), Sarma, (1967), Islam et al., (1986), Ansari et al., (2003). In order to estimate the temporal variation of scour depth, the value of exponent which appears in equation of temporal variation of scour depth is needed a priori. Analysis of data collected in the present study showed that the value of the exponent is a function of the jet velocity, diameter of nozzle, height of jet and the sediment size. is the function of the jet velocity, diameter of nozzle, height of jet and the sediment size. A relationship is proposed for the estimation of saturation time in cohesionless sediment. Various scour parameters like maximum static scour depth, maximum dynamic scour depth, radius of scour hole, height of dune and volume of scour hole have been analyzed using the data collected in the present study and that available in literature in case of cohesionless sediment. Maximum static scour depth was analyzed with erosion parameter in the case of cohesionless sediment using the present study data and data of previous investigators. It is found that the equation proposed by Adribigbe and Rajaratnam (1996) needs modification for better representation of the present and previous data. New modified equation has been proposed for estimation of maximum static scour depth. Variation of maximum static and dynamic scour depth is also studied with sediment size, nozzle diameter and jet velocity. It is found that variation of maximum static and v maximum dynamic scour depth can well be explained with other dimensionless parameters in place of erosion parameter (Ec Variation in radius of scour hole and dune height was analyzed with erosion parameter for data of present study as well as data of previous investigators. However, further analysis of data reveals that radius of scour hole can be accurately calculated using dimensionless parameters comprising jet velocity, diameter of nozzle, height of jet, sediment size in place of erosion parameter. The volume of scour hole was measured for each of the experimental run. It is found that the volume of scour hole is high in sand compared to the sand-grave and gravel. Higher jet velocity produces high scour volume. ). The time required for attainment of equilibrium state of scour i.e. saturation time was found to be low in sand beds as compared to sand-gravel mixture and gravel beds. Radius of scour hole, dune height and volume of scour hole increase with increase of nozzle diameter, jet velocity while they decrease with increase of sediment size and jet height. The differences in maximum dynamic and static scour depths were higher in sand compared to gravel and sand-gravel mixtures. Relationships are proposed for the computation of various scour parameters like maximum static and maximum dynamic scour depths, radius of scour hole, dune height and volume of scour hole in sand, gravel and sand-gravel mixture using data available in the literature and collected in present study. The proposed relationships are able to predict the desired parameters within ± 20 percent error band. 2. Scour in Clay-gravel Cohesive Sediment Mixtures In all, 40 experimental runs were conducted in cohesive sediment beds consisting of clay-gravel mixtures. The geometrical characteristics of scour hole in case of clay-gravel mixture were found significantly different than that of cohesionless sediment. The scour geometries were different for various percentages of clay in the mixture. The scour profile for low percentage of clay i.e., 10% was similar to that of cohesionless sediment. A close investigation of the scour profile reveals that scour depth and size of scour hole decreases with the increase of clay percentage in the mixture. The slopes of scour hole were different with various percentage of clay in the mixture. The dynamic scour depth is much higher than the maximum static scour depth for low clay content, however, their difference decreases with increase of clay in the mixture. The dune height was low in case of higher clay percentage in the mixture. vi The temporal variation of scour depth in clay-gravel mixtures was studied with various clay percent in the mixture. The depth of scour reduces drastically with the increase of clay percent in gravel. It was also noticed in the experimentation that the rate of scour process also varies with clay percentages. The influence of cohesion was more apparent with clay percent more than 40% in the mixture. In such cases, the process of scour initiates after 20 to 40 minutes from start of the experiment. It is found that the saturation time (Ts) of scour is function of dimensionless parameters comprising jet velocity, diameter of nozzle, height of jet, sediment size and percentages of clay content (Pc For better representation of temporal variation of scour depth, the exponent, m). A new relationship is proposed for the estimation of saturation time in cohesive sediment consisting of clay-gravel mixtures. s of the equation describing the temporal variation of scour, is estimated for each experimental run of the present study. Analysis of computed value of ms For estimation of various scour parameters, the data collected in the present study have been used to formulate relationships for various scour parameters like maximum static scour depth, maximum dynamic scour depth, radius of scour hole; height of dune and volume of scour hole and it was found that dimensionless parameters comprising jet velocity, diameter of nozzle, height of jet, sediment size and percentage of clay content gives better results in place of erosion parameter. New equations have been proposed to estimate the above parameters within ± 20 percent error. revealed that it is a function of dimensionless parameters comprising jet velocity, diameter of nozzle, height of jet, sediment size and percentages of clay content. A new equation is proposed to compute the value of exponent for clay-gravel cohesive sediment mixtures. 3. Scour in Clay-Sand-Gravel Cohesive Sediment Mixtures In all, 44 experimental runs were conducted in clay-sand-gravel mixtures. The shapes of scour profiles developed in clay-sand-gravel mixture have irregular geometries with their side slopes ranging from 30o to 90o Analysis of temporal variation of scour depth in clay-sand-gravel mixtures reveals that the scour depth decreases with increase of clay percentage. It has been noted that the rate of scour process also varies with clay percentages. The influence of cohesion has . At lower percentage of clay i.e., up to 20%, large size of scour hole was observed with a significant dune height. Sand and clay were found on the sides of dune whereas gravel was found in the centre of scour hole. vii been found more significant with clay percent higher than 40% in the mixture. In such cases, the process of scour initiates after 30 to 50 minutes from start of the experiment. The value of saturation time of scour for each experimental run is estimated and correlated with jet velocity, diameter of nozzle, height of jet, sediment size and the percentages of clay content. A relationship is proposed for the estimation of saturation time in cohesive sediment consisting of clay-sand-gravel mixtures. In order to estimate the temporal variation of scour depth, the value of exponent in equation of temporal variation of scour depth is to be known a priori. Analysis of present study data revealed that exponent is function of the jet velocity, diameter of nozzle, height of jet, sediment size and the percentages of clay content. Accordingly, a new equation is proposed to compute the value of exponent for clay-sand-gravel cohesive sediment mixtures. For estimation of various scour parameters, the data collected in the present study have been used to formulate relationships for parameters like maximum static scour depth, maximum dynamic scour depth, radius of scour hole; height of dune and volume of scour hole and it was found that dimensionless parameters comprising jet velocity, diameter of nozzle, height of jet, sediment size and percentage of clay content give better results in place of erosion parameter. New equations have been proposed to estimate the above scour parameters within ± 20 percent error.