Bedforms and Their Role in Influencing the Movement of Sediment and Flow of Water in Rivers - Term Paper Example

Sediment Transport Demonstration Channel Student Name ID number Course Tutor’s Name Date Abstract Bedforms are depositional features found in the bed of flowing water bodies such as rivers .They are formed as a result of movements of bed material due to water flow. The main aim of this experiment is to demonstrate the sequence bedforms which occur in an open channel with a movable, sand bed following an increase in the intensity of sediment transport. This experiment involves flow channel operated with sediment. Subsequently, the features appearing in the bed as the rate of the flow discharge and flume slope are increased gradually are keenly observed and analyzed in this report. Table of Content Nomenclature 3 Nomenclature The nomenclatures below are used in this experiment; Name Symbol Definition Reynolds Number Re U*D50/u Froude Number Fr FR=v/c Bed shear stress t t=pgRS Shear Velocity u* (gRS)0.5 Introduction Over the years, a number of studies have been conducted to investigate the origin of bedform in sand. Early studies focusing on sand movement were first undertaken by Brigadier Ralph Bagnold in the 1930’s. Prior to Bagnold’s study, no studies had been conducted to establish the origin of bedform in sand. Bagnold undertook his studies in the Saharan desert and mainly focused on understanding the movement of sand by wind (Welland 2011). In the 1960’s, Daryl Simons and Everret Richardson pioneered studies of bedform under flowing water rather than wind at the Colorado State University. These studies provided foundation for understanding bedforms in rivers and canals. This experiment replicates one of the studies conducted by Simons and Richardson on the sequence of bedforms that occur when the flow intensity and sediment transport are gradually increased over movable bedforms under high energy environments. The key purpose of this experiment is to demonstrate the sequence bedforms which occur in an open channel with a movable, sand bed following an increase in the intensity of sediment transport. This experiment will involve flow channel operated with sediment. Subsequently, the features appearing in the bed will be keenly observed as the rate of the flow discharge and flume slope are increased gradually. Relevant theory Bedforms are basically depositional features found in the bed of flowing water bodies such as rivers (Southard 1991). They are formed as a result of movements of bed material due to water flow. Small triangular bedforms are referred to as ripples. Most ripples are often few centimeter low and few millimeters high. They are also asymmetrical with an upstream slope referred to as the “stoss side” and steep downstream which is referred to as the “lee side”. The sediments that make up the ripple often move as bed load, slidding and rolling while in continuous contact with the bed. In the stoss side, there are grains entrained which roll upslope and fall rapidly below before resting in the trough within ripples. As a result, the ripple bedform gradually moves in the downstream direction. Instead of moving bodily, the bedform moves through the net total of all the grain movements. In this case, the speed of the moving bedform is slower than that of the grains or the water. As the velocities of the flow increases saltation takes place when grains are lifted above the bed by fluid commotion. The grains hop and bounce sporadically despite the fact that the main mode of motion is still through rolling. The paths used by saltating grains convey ballistic trajectories. This shows that they are not held by fluid forces in the course of saltation rather their submerged weight is supported by bouncing connection with the bed. This distinguishes them from grains which are suspended within the fluid as a result of support by anisotropic turbulence. Basically, the saltating grains are constituents of the bedload. Dunes are triangular bedforms that arise at high flow velocities. They are relatively larger than ripples. Naturally, dunes are based on the flows that produce them, this is what distinguishes them from ripples. Despite the fact that both dunes and ripples are triangular in shape, dunes are longer and flatter in shape than ripples. Moreover unlike ripples, dunes disrupt the free surface of the flow. In most cases water surface is slightly drawn over the dune crest and is embedded over the trough between dunes. Therefore, the waves in the water surface retract from the bed. This is particularly evident in the sub-critical or low regime flow. When the Froude number is calculated, it confirms that the flow above a dune bed is sub-critical. Boils linked to turbulence in the lee side of the dune are also embedded in the humped area of the water surface. These boils can be seen in rivers in the course of high flow stages. They indicate that a dune bedford is present though the bed cannot be directly observed. In most cases, dunes move downstream by grains saltating and rolling up the placid stoss slope and rapidly falling down the steep lee face. In the course of movement of mature dunes, separation of the bubbles at the lee face occurs when the flow is directed upstream. The impact is to continuously sweep grains back the avalanche face, sustaining this face at a slope a bit greater than the angle of inward friction for sand. Dune crests are often aligned across the flow width, however, even in a narrow flume they do not remain two dimensional. The crests rapidly become sinuous as deep scour holes develop next to wall based on the alternate sides of the channel. The scour holes and the patterns crests are linked with strong secondary currents that appear in the lee of the dunes as a result to skewing of the cross-stream. This impact is manifested at meander bends and contributes to a significant sediment sorting mechanism. As the intensity of the flow increases, the dunes which are washed out disappear and a plane bed with motion is developed. The water depth decreases whereas the longstream velocity increases. When observed along the flume length , the water surface appears to have a smooth glassy look. No undulations or boils are present due to the fact that large scales of turbulent structures are suppressed by high long stream velocities. As the intensity of the flow continues to increase, undulations referred to as standing waves are produced on the surface of water. Furthermore, waves begin to appear on the bed however, unlike dunes and ripples, the features are stationary and symmetrical. The waves present in the water surface become in phase with the ones in the bed. This is an evident sign of super-critical flow and thus standing waves are found in the upper region of bedforms. When the flow velocities move to the upper region, the amplitude of the surface waves significantly increases , the size of the sediment below also increases. Symmetrical waves referred to as antidunes whale-back the bedforms. The antidune move in an upstream direction against grain motion and the movement of water flow. When the flow intensity is increased further, strong waves appear obliquely out of the channel walls crossing the antidune crests. When flow velocity is very intense super critical flow is produced and intense turbulence linked with intense sediment transport and high stream power. Consequently, the flow adopts a surging pattern of alternate chutes and pools. Apparatus and Method This experiment will employ flow channel operated with sediment. The features appearing in the bed will be keenly observed as the rate of the flow discharge and flume slope are increased gradually. It will involve the use of apparatus such as; Point gauge Hook Armfield S8 MkII Clean sand of uniform size ranging between 0.1 and 0.3mm Procedure Sand of the same size between 0.1 and 0.33mm will be placed in the flume to a standard depth and subsequently leveled using the top of the overspill weir. The sand surface will be tamped down in order to ensure that it is packed lightly compacted or it is as light as possible. The collecting tank will be filled with clean water until the “full” mark. The flume will be set to a zero slope, the flow control valve will be closed and the pump set to its lowest level (I). Subsequently, the pump will turned on to allow water to enter into the flume. The slope of the flume will be increased gradually under uniform condition until further bed motion is determined. After awhile, the discharge setting will be increased to speed III. Subsequently, the flow control will be opened fully and the slope will be increased significantly. Results When the flume is set to a zero slope, the flow control valve is closed and the pump set to its lowest level , no sediment transport is observed and the bed is still immobile. In these conditions, there are no bedforms, transport and the bed is static. As the slope of the flume is increased gradually under uniform condition, small triangular bedforms begin to appear. These small triangular bedforms are ripples. They are asymmetrical, a millimeter high and two centimeters long. They also have a steep downstream base and an upstream slope. When the discharge setting is increased to speed III, the rate of the sediment movement and flow depth increases immediately. As the slope is increased incrementally grains begin to bounce and hop regularly. This mode is referred to as saltation. Increase in intensity contributes to the formation of larger triangular bedforms known as dunes. Nevertheless, they are flatter and longer topped and as a result they disrupt the free surface of the flow. As the slope is increased significantly, the amplitude is reduced and the dunes lengthen. This in turn results to the formation of long and low bedforms referred to as washed-out dunes (See appendix 1 for the sketches of observed bedforms and sediment motions). When the whole working length become occupied by the washed out dunes, measurement of water depth and slope are established as outlined in the table below; Flow rate Q (M3/s) Flume Slope S m/m Water Depth D (m) Reynolds Number Re (-) Froude Number Fr (-) Bed Shear T (N/M2) Shields Parameter Q (-) Shear Velocity U* (m/s) Stream Power Ω (W/m2) Bed Reynolds Number Re* Friction Factor F (-) Discussion The key aim of this experiment was to establish the sequence bedforms which occur in an open channel with a movable, sand bed following an increase in the intensity of sediment transport. Over time, engineers and scientists have established several approaches of predicting bedform linked with flow of a particular intensity over a sand bed. Some of the methods devised use simple indices like bed material grain size or mean velocity to represent the resisting and motivating forces that regulate sediment and water motion. This experiment involved flow channel operated with sediment. Afterward, the features appearing in the bed were keenly observed as the rate of the flow discharge and flume slope were increased gradually. The findings established through this experiment show that as the slope of the flume is increased gradually under uniform conditions, ripples begin to appear. The ripples formed are asymmetrical, triangular in shape, a millimeter high and two centimeters long. When the discharge setting is increased further, the rate of the sediment movement and flow depth increases significantly. Subsequently , saltation occurs as grains begin to bounce and hop regularly. As the intensity is increased further, dunes are formed. The dunes formed are also triangular like the ripples previously formed but they are relatively larger flatter and longer topped. Consequently, they disrupt the free surface of the flow. When the slope is increased, the length of the dune increases leading to the formation of long and low bedforms. Essentially, these findings confirm that bedforms play an important role in influencing the movement of sediment and flow of water in rivers or beaches (Armfield 2012). Conclusion Based on the findings established in this experiment, it is evident that, the flow of water over sand in either a beach or a river exerts a shear force on the bed. In a case where water flow is strong, sand grains are often lifted to roll and bounce alongside the bed. The shape of the bed in turn responds to the motions by changing into ripples. When the transport rate of the sand and energy of the flow increases, the bedforms transforms. Ripples are subsequently replaced by dunes which are larger in size. When the energy is increased these dunes are washed out to produce a flat bed. In extreme energy flow, anti dunes appear. Bedforms play a critical role in influencing the movement of sediment and flow of water in rivers or beaches (Armfield 2012). References Armfield 2012, Sediment Transport Demonstration Channel, S8MkII Issue 5, viewed July 9 2013 Southard J B 1991, “Experimental Determination of Bed-Form Stability”, Annual Review of Earth and Planetary Sciences 19:423. Welland M. 2011, Sediment Transport by Wind and Water: The Pioneering Work of Ralph Bagnold (pp. 399-429), In V. Badescu & Cathcart, R. Macro-enginering Seawater in Unique Environments: Arid Lowlands and Water Bodies Rehabilitation, Springer , London. Appendix 1: Observed bedforms and sediment motions http://www.fhwa.dot.gov Read More