Sediment Transport PDF

SEDIMENT TRANSPORT GEM 2105: SEDIMENTARY PETROLOGY AND STRATIGRAPHY BY JOSEPHINE MAXIMUS “WE RISE BY LIFTING OTHERS” - JOSEPHINE MAXIMUS  OUTLINE ▪Sediment Transport ▪Mass Movement ▪Triggering effect of mass movement ▪Troublesome materials that aid sediment transport ▪Types of Mass Moveme...

SEDIMENT TRANSPORT GEM 2105: SEDIMENTARY PETROLOGY AND STRATIGRAPHY BY JOSEPHINE MAXIMUS “WE RISE BY LIFTING OTHERS” - JOSEPHINE MAXIMUS  OUTLINE ▪Sediment Transport ▪Mass Movement ▪Triggering effect of mass movement ▪Troublesome materials that aid sediment transport ▪Types of Mass Movement ▪Causes of Mass movement ▪ Fluid Transport ▪Physical properties of fluids SEDIMENT TRANSPORT The movement of geological particles (sediment), from their weathered (in-situ) location to the environment where it is deposited. Typically due to a combination of mass wasting and/or the movement of the fluid in which the sediment is entrained. Erosion: the wearing away of the earth’s soil and rock material by ice, water, or wind. MASS MOVEMENT Processes of downslope movement of Mass movements are part of a continuum of surficial earth materials under the pull of erosional processes between weathering gravity are collectively called Mass Wasting”. and stream transport. Mass movement causes regolith and rock to move down- Mass Movement varies in size and speed. slope where sooner or later the loose Size: Varies from tiny mineral grains to particles will be picked up by another enormous volumes of rock and mineral transporting agent and eventually moved to material as great as thousands of km3. a site of deposition such as an ocean basin Speed: Mass movement processes are or lake bed. occurring continuously on all slopes; some act very slowly, and others occur very suddenly, often with disastrous results. Mass Wasting Event – Verapaz El Salvador, Nov. 2009 TRIGGERING EFFECT OF MASS MOVEMENT Gravity Angle of repose Troublesome earth materials Weak materials and structure Role of water Excessive steepened slope GRAVITY Gravity is the main force responsible for mass movements. Gravity is a force that acts everywhere on the Earth's surface, pulling everything in a direction toward the center of the Earth. On a flat surface, parallel to the Earth's surface, the force of gravity acts downward. So long as the material remains on the flat surface it will not move under the force of gravity. Of course, if the material forming the flat surface becomes weak or fails, then the unsupported support mass will move downward. On a slope, the force of gravity can be Another force resisting movement down the resolved into two components: a component slope is grouped under the term shear acting perpendicular to the slope, and a strength and includes frictional resistance and component acting parallel to the slope. cohesion among the particles that make up the object. The perpendicular component of gravity, gp, helps to hold the object in place on the slope. When the sheer stress becomes greater than The component of gravity acting parallel to the the combination of forces holding the object slope, gs, known as shear stress helps to on the slope, the object will move down-slope. move the object in the down-slope direction. THE ROLE OF GRAVITY AND SLOPE ANGLE Gravitational force acts to hold objects in place by pulling on them in a direction perpendicular to the surface. Shear stress is the downslope component of the total stress involved. ◦ Steepening a slope by erosion, jolting it by the effect of an earthquake, or shaking it by blasting, can cause an increase in shear stress. Normal stress is the perpendicular component. ANGLE OF REPOSE The angle of repose is the steepest angle at which a pile of unconsolidated grains remains stable, and is controlled by the frictional contact between the grains. In general, for dry materials the angle of repose increases with increasing grain size, but usually lies between about 30 and 45 o EFFECT OF WATER ON THE ANGLE OF REPOSE TROUBLESOME EARTH MATERIALS Expansive and Hydrocompacting Soils - Some water-saturated clays are stable so These are soils that contain a high long as they aren’t disturbed, but when proportion of a type of clay mineral called shaking occurs, just like sands, they can smectites or montmorillonites. Such clay turn into a runny fluid. These are referred to minerals expand when they become wet as as quick clays. water enters the crystal structure and increases the volume of the mineral. When such clays dry out, the loss of water causes the volume to decrease and the clays to shrink or compact (This process is referred to as hydrocompaction) e.g., peat WEAK MATERIALS AND STRUTURES Rocks often contain planar structures that If a weak rock or soil occurs between become slippage surfaces if weight is added or stronger rocks or soils, the weak layer will be support is removed. the most likely place for failure to occur, Bedding Planes - These are basically planar especially if the layer dips in a down-slope layers of rocks upon which original deposition direction as in the illustration above. occurred. Since they are planar and since they may have a dip down-slope, they can form surfaces upon which sliding occurs, particularly if water can enter along the bedding plane to reduce cohesion. ROLE OF WATER Although water is not always directly involved as the transporting medium in mass movement processes, it does play an important role. Addition of water from rainfall or snow melt adds weight to the slope. Water can seep into the soil or rock and replace the air in the pore space or fractures. Since water is heavier than air, this increases the weight of the soil. If the material becomes saturated with water, vibrations could cause liquifaction to occur, just like often happens during earthquakes. SATURATION Destroys particle cohesion Effective Stress = Total Stress – Pore Water Pressure Water adds weight and Increases gravity (shear stress) EXCESSIVE STEEPENED SLOPE Excessively Steepened Slopes Unconsolidated granular particles assume a stable slope called the angle of repose Stable slope angle is different for various materials. Overly steepened slopes are unstable. Steepening is caused by: - Undercutting (by streams or human activity) - Addition of material to top of the slope (Natural deposition or Anthropogenic construction). REMOVAL OF ANCHORING VEGETATION Caused by: - Wildfires - Drought - Development, logging Plant roots provide a strong interlocking network to hold unconsolidated rocks and sediment. Vegetation removes moisture from the soil and may increase shear strength. GROUND VIBRATIONS Ground Vibrations From earthquakes: Seismic waves passing through rock may stress and fracture it Human activities Types of Mass Wasting 3 major types of slope failure: ◦ Falls. ◦ Slides. ◦ Flows. ROCK FALL AND DEBRIS FALL Rock Falls and Debris Falls - Rock falls occur when a piece of rock on a steep slope becomes dislodged and falls down the slope. Debris falls are similar, except they involve a mixture of soil, regolith, and rocks. Rockfall is the free falling of detached bodies of rock. It is common in precipitous mountainous terrain, where debris forms conspicuous deposits at the base of steep slopes. As a rock falls, its speed increases. ◦ V^2 = 2 gh, where: ◦ g = the acceleration due to gravity. ◦ h = the distance of fall. ◦ v = the velocity. ROCK SLIDE Masses of rock or sediment slide downslope along planar surfaces. Subdivision of slides: ◦ Rockslide and debris slide. ◦ Slump. ROCK SLIDE DEBRIS SLIDE Rock Slides and Debris Slides - Rock slides and debris slides result when rocks or debris slide down a pre-existing surface, such as a bedding plane or joint surface. Piles of talus are common at the base of a rock slide or debris slide. The rapid sliding of a tabular mass of bedrock. Movement occurs along an inclined surface of weakness DEBRIS SLIDE SLUMPS A slump is the downward rotation of rock or regolith that occurs along a curved surface. The upper surface of each slump block remains relatively undisturbed, as do the individual blocks. Slumps leave arcuate scars or depressions on the hill slope. Heavy rains or earthquakes usually trigger slumps. SEDIMENT FLOW Sediment flows are mass-wasting processes in which solid particles move in a flowing motion. Factors controlling flow: ◦ The relative proportion of solids, water, and air. ◦ The physical and chemical properties of the sediment. Water helps promote flow, but the pull of gravity on the solid particles remains the primary reason for their movement. There are two classes of sediment flows, based on sediment concentration: ◦ A slurry flow is a moving mass of water-saturated sediment. ◦ A granular flow is a mixture of sediment, air, and water (not saturated with water). SEDIMENT FLOW CREEP Very Slow downslope movements. Unnoticeable except to observations with long durations (days to decades). Evidence for creep is often seen in bent trees, offsets in roads and fences, and inclined utility poles. Curved tree trunks – a visible product of soil creep SLURRY FLOW The nonsorted or poorly sorted sediment mixture in slurry flows is often so dense that large boulders can be suspended in it. There are several key types of slurry flows. ◦ Solifluction: ◦ The very slow downslope movement of saturated soil and regolith. ◦ Rates of movement are less than about 30 cm/yr. ◦ Creates distinctive surface features: ◦ Lobes. ◦ Sheets of debris. ◦ Occurs on hill slopes in temperate and tropical latitudes, ◦ Regolith remains saturated with water for long intervals ROCK AVALANCHE Very rapid downslope movement Turbulent mass of broken-up bedrock. A rock avalanche can travel for several kilometres and reaches velocities of 40 m/s (> 140 km/h). Often leads to catastrophic flooding. CAUSES OF MASS MOVEMENT Shocks and vibrations Slope Modification - Modification of a slope either by humans or by natural causes can result in changing the slope angle. Undercutting - streams eroding their banks or surf action along a coast can undercut a slope making it unstable. Changes in Hydrologic Characteristics - heavy rains can saturate regolith reducing grain-to-grain contact and reducing the angle of repose, thus triggering a mass movement. Submarine slope failures on continental slopes and delta fronts can promote the formation of large submarine landslides SUMMARY FLUID TRANSPORT Mass wasting transports the weathered material to the valley floor Once there, fluid flow becomes the dominant means of transportation Water is the most important agent of erosion and transportation of geological materials WHAT IS FLUID The study of sediment transport requires some understanding of the principles of fluid flow Fluids are substances that change shape easily under their own weight. Sediment transport considers: 1. Air 2. water 3. Water containing various amounts of suspended sediment (slurry) PHYSICAL PROPERTIES OF FLUID The basic physical properties of these fluids are density and viscosity Density: commonly referred to as ρ (rho), is defined as mass per unit of fluid volume. Density affects: - The magnitude of forces that act within a fluid and on the bed as well as the rate at which particles fall or settle through a fluid (slower in denser fluids) Water can transport particles of much larger size than those transported by wind PHYSICAL PROPERTIES OF FLUID CONT’D Viscosity: A measure of the fluid’s resistance to flow. Low viscosity fluids flow readily and fluids with high viscosity flow sluggishly. The viscosity of water at 20°C is almost 55 times greater than that of air. Like density, viscosity increases with decreasing temperature of the fluid. Decreased turbulence reduces the ability of running water to erode and entrain sediment. TYPES OF FLOW Fluids in motion display two modes of flow depending upon: a) The flow velocity. b) Fluid viscosity. c) Roughness of the bed over which flow takes place. LAMINAR VS TURBULENT Laminar Flow: fluid will persist as a straight, coherent stream of nearly constant width Flow can be visualized as a series of parallel sheets referred to as streamlines The streamlines may curve over an object, but they never intertwine Laminar flow takes place only at very low fluid velocities over smooth beds TURBULENT FLOW Turbulent Flow: Streamlines become highly distorted and constantly changing Most flow of water and air under natural conditions is turbulent The upward motion of water particles in turbulent water masses slows the fall rate of settling particles (decreases their settling velocity) Turbulence tends to increase the effectiveness of fluid in eroding particles from a sediment bed Differences in laminar and turbulent flow arise from the ratio of inertial forces to viscous forces. Inertial forces, which are related to the scale and velocity of fluids in motion, tend to cause fluid turbulence. Viscous forces, which increase with increasing viscosity of a fluid, tend to suppress turbulence. Reynolds Number quantifies the relationship between these forces REYNOLDS NUMBER Thus, the Reynolds number can be used to predict whether flow will be laminar or turbulent and to derive some idea of the magnitude of turbulence. Low Reynolds number (below 500) = Laminar Flow High Reynolds number (above 2000) = Turbulent Flow SEDIMENT TRANSPORT THROUGH FLUID Sediment movement is a function of: Settling velocity of the particle. The magnitude of the current velocity. The turbulence of the transport fluid. RIVERS RIVERS The amount and size of sediment moving through a river channel are defined by three fundamental controls: Competence Capacity Sediment supply COMPETENCE OF A RIVER Competence: refers to the largest size (diameter) of sediment particle or grain that the flow is capable of moving. If a river is sluggish and moving very slowly it simply may not have the power to mobilize and transport sediment of a given size even though such sediment is available to transport. CAPACITY OF A RIVER Capacity: refers to the maximum amount of sediment of a given size that a stream can transport in traction as bed load. Capacity depends on: - Channel gradient - Channel velocity - Grain size (fines increase fluid density and increase capacity; large particles obstruct the flow and reduce capacity). SEDIMENT SUPPLY Sediment supply refers to the amount and size of sediment available for sediment transport. SEDIMENT LOAD TRANSPORT The sediment load of a river is transported in various ways: - Dissolved load - Suspended load - Wash load - Bed load DISSOLVED LOAD Dissolved Load: material that has gone into solution and is part of the fluid moving through the channel. It does not depend on forces of flow to keep it in the water column. The amount of material in the solution depends on: - Supply of a solute. - The saturation point for the fluid. - If dissolved load is very sensitive to water temperature. SUSPENDED LOAD Suspended Load: The clastic (particulate) material that moves through the channel in the water column. These materials, are kept in suspension by the upward flux of turbulence. The upward currents must equal or exceed the particle fall velocity for the suspended-sediment load to be sustained. Mainly silt and fine sand. WASH LOAD Wash Load: part of the suspended sediment Unlike most suspended-sediment loads, wash load does not rely on water turbulence to keep it in suspension. It is so fine (in the clay range) that it is kept in suspension by thermal molecular agitation (AKA Brownian motion) Wash load tends to be uniformly distributed throughout the water column. BED LOAD Bed Load: the clastic (particulate) material that moves through the channel fully supported by the channel bed itself. These materials are kept in motion (rolling and sliding) by the shear stress acting at the boundary. Mainly coarser sands and gravel. WIND/EOLIAN Very low density, low viscosity fluid Wind commonly transports particles of fine sand size and smaller. Sand-size particles move by traction (surface creep) and saltation and dust-size particles by suspension. Transport takes place at relatively high wind velocities, and the flow is commonly turbulent. GLACIER ICE The high viscosity of glacial ice causes it to flow very slowly. Glaciers are capable of moving huge volumes of sediment by scraping and plucking the underlying bedrock and adjacent valley. As the laminar flow of ice takes place within the glacier, the sediment of all sizes is carried along with the moving ice in contact with the bed. SUMMARY QUESTIONS