Beach Sediments Issues - Essay Example

Beach Sediments Beaches are dynamic forms of land that are constantly changed by the waves and the wind in a continuous cycle of formation and erosion. It is a gently sloping zone made up of unconsolidated sediments subject to wave action at the shore of an ocean ("The Columbia Encyclopedia" 4642). These beach sediments are a complex mixture of different minerals or organic particles such as gravel, sand and cobble fragments. Most of these sediments, about 80 to 90 percent, that make up a beach come from river sediments; the rest comes from the erosion of highlands near the coast. Some beaches are built to great widths by sediments washed to the sea by episodic floods, gradually eroding until the next major flood replenishes the sand ("Beach Formation and Types of Beaches and Sand"). Beach sediments are delivered per year in million cubic yards through longshore transport. Different beaches have different colors and textures because of the various sediments that make them up. There are beaches made up of eroded shale cliffs, multicolored agates ground and polished by the surf, feldspar minerals, ground quartz, and even iron minerals. The various make up of the sediments determine the origin of these sand beaches which help researchers know more information about a particular coast. Types of Beach Sediments Beach sediments can be classified into three main groups - mechanical, chemical, and organic sediments. Mechanical sediments, also known as clastic sediments, came from the erosion of oceanic rocks formed during the earlier times. These sediments are carried by streams or waves to the place where they are deposited. Ocean sediments, especially in the form of turbidites, are usually carried over and deposited at the bottom of continental slopes ("The Columbia Encyclopedia" 42907). Chemical reactions in seawater form chemical sediments which results in the precipitation of small mineral crystals that settle to the floor of the sea and finally form a chemically pure layer of sediment somehow. Organic sediments are formed from plant or animal actions. Sediments found in the deep parts of the ocean, known as pelagic ooze, are composed mainly of the remains of microscopic organisms. These organisms are mostly foraminifera and diatoms which are found from the overlying waters. There are also some traces of windblown volcanic and continental dusts found in organic sediments. Properties of Sediments Sediments have different properties which are being studied by researchers for different purposes such as coastal engineering. One property of sediments is its physical form. Sediments can be loose, fluid, hard or firm. Examples of loose sediments are sand and silt; mud is a form of fluid sediment. There are also sand forms that are firm and stiff as clay. Hard sediments are those rocks and coral pieces that can be found in some beaches. Sediments can also be classified in terms of their cohesiveness. Sediments can be cohesive, non-cohesive or mitigated. Sediments such as clay and firm sand are highly cohesive as each particle stick closely together. Mud and loose sands, as well as rocks and other loose particles, are non-cohesive. Mitigated sediments are mainly non-cohesive sediments with a little mix of clay. Non-cohesive sediment behavior in water is mainly controlled by mechanical forces. The hydrodynamics of a particle refer to its ability to remain still or become entrained if on the bed surface, or to remain in suspension or to cease movement if in motion ("Sedimentation Investigations of Rivers and Reservoirs" 7-1). The properties that crucially affect the non-cohesive particle's hydrodynamics are its size, shape and specific gravity. The behavior of cohesive sediments, on the other hand, is controlled by electrochemical forces and dependent on the particle size, sediment mineralogy, and water chemistry. Particle size is considered to be the most important property of non-cohesive sediments. The size of the particle can be defined by any of the four methods: a) Nominal diameter - with this method, the diameter of the particle is obtained by getting the diameter of a sphere with the same volume as the particle. b) Sieve diameter - this method measures the length of the edge of the smallest square opening where the particle can pass through. c) Sedimentation diameter - the diameter of a particle can be obtained by getting the diameter of a sphere with the same specific gravity and terminal settling velocity as the particle in the same liquid under the same conditions. d) Fall diameter - with this method, the diameter of the particle is the same as the diameter of the sphere with a specific gravity of 2.65 and has same terminal settling velocity as the particle in quiescent distilled water at 24C. Particle shape is another property important to non-cohesive sediments. This shape is defined by the particle's shape factor (SF), which is measured by the ratio of the shortest axis to the square root of the product of the measures of the longest and intermediate axes. These axes are perpendicular to each other. Natural non-cohesive sediment normally has 0.7 as the shape factor. This property affects the fall velocity, as well as sedimentation and fall diameters of the particle. Particle specific gravity is yet another property of natural sediments. Quartz, the most common mineral found in natural sediments, has a specific gravity of 2.65 due to its defiance to weathering and abrasion. A sediment mixture has an average specific gravity close to that of quartz. The fall velocity of a particle is the primary property governing the motion of a sediment particle in a fluid. Generally, it is described as how fast the particle falls or settles in a fluid. It is a function of different properties such as the shape, volume, and density of the particle and the density and viscosity of the fluid. The standard measure of the fall velocity is the average rate a particle would fall in quiescent distilled water at 24C. As for sediment deposits, there are also different properties that define them. These are porosity, specific weight and consolidation rate. The sediment deposit's porosity is defined as the ratio of the volume of voids to the total volume of the sample. Specific weight, on the other hand, is the deposit's weight per unit volume. Specific weight is expressed as the dry weight of the deposit. Consolidation rate is the measure of the deposit's process of compaction with time or imposing pressure. Sediment concentration is one property that defines water-sediment mixtures. It is defined by the weight of the dry sediment in the mixture per volume of the mixture and is expressed in terms of milligrams/liter. It can also be shown in terms of parts per million (ppm) which means the ratio of the mass of dry sediment in a water-sediment mixture to the mass of the mixture times 106 ("Sedimentation Investigations of Rivers and Reservoirs" 7-7). Another property of water-sediment mixtures is the sediment discharge. This is the amount of sediments per unit of time that passes a cross section which is expressed in terms of tons/day or cubic feet per second (cfs). While sediment discharge indicates the rate of transport of the sediment, sediment load defines the type of material that is being transported. Sediment load is generally based on the mode of transport, by the availability in the streambed, or by method of measurement. According to the discussion in "Sedimentation Investigations of Rivers and Reservoirs": Based on the mode of transport, sediment load can be divided into bed load and suspended load. Bed load is the sediment load transported close to the bed where particles move intermittently by rolling, sliding, or jumping. Turbulence supports suspended load throughout the water column, and sediment is swept along at about the local flow velocity. Based on its availability in the streambed, sediment load can be divided into bed-material load and wash load. Wash load consists of the finest particles in the suspended load that are continuously maintained in suspension by the flow of turbulence and, thus, significant quantities are not found in the bed. Particles that move as suspended load or bed load and periodically exchange with the bed are part of the bed-material load. [] Based on measurement technique, sediment load is described as either measured or unmeasured. [] The unmeasured load includes some of the suspended and usually all of the bed load. (7-8) Other compositional properties of sediments are mineral composition, density, strength, and grain shape and abrasion. As mentioned above, quartz is the most common mineral found in sediments because of its defiance to physical and chemical changes. That and feldspar form almost 90 percent of the material found in temperate latitude beaches. Though feldspars are more commonly found on earth's surface, they are more prone to chemical weathering converting them to clay minerals and solutions. Quartz sand and clay commonly found on shores far from mountains. Carbonate sands are formed from organisms that precipitate calcium carbonate. These sands form up to 100 percent of the beach material found in places like tropical island beaches. They are composed of shell fragments, coral and algal fragments and oolites. Other minerals that contribute a small percentage to the beach sand are usually termed as heavy minerals due to their specific gravity being normally higher than 2.87. They may come in strong colors such as black and red which may sufficiently color the entire beach. They can be used to trace sediment pathways from the parent rocks. Density, another compositional property of sediments, is defined as the mass of the sediment per unit volume. Density of sediments is a function of its composition. The strength of a particle is defined as the maximum stress a particle can resist without failing, for a certain loading type. Quartz sand grain is a very strong material. Its difference from the composite sandstone is due to weak intergranular cement and to flaws such as grain boundaries, bedding planes, cleavage, and joints that have a higher probability of being present in larger pieces ("Coastal Engineering Manual: Part III" III-1-17). Grain shape is a function of the grain's composition, size, original shape, and weathering history. Early attempts on classifying sediments through its shape developed three size scales. These scales are the overall shape or sphericity of the particle, the roundness or the amount of abrasion of the corners of the particle, and the very fine scale roughness or microtexture. Recent approaches eliminated these divisions with one procedure that utilizes fractal geometry types of analysis. Measuring and Collecting Beach Sediments In beaches, sediment distribution change in time and space. Beach sediments are normally coarser and more badly sorted during winter seasons than in summer. There are also more variations of sediment materials in the foreshore and trough regions than in the nearshore and dunes. A single sample may be enough to characterize sediments at a particular location, though getting a set of samples may even be better. Different samples from different parts of the beach can reduce the variability in spatial grain size distributions on beaches (qtd. in "Coastal Engineering Manual: Part III" III-1-11). Composite samples can be formed by physically combining different samples before sieving or by mathematically combining individual sample weights. Several techniques can be done to analyze the size of beach materials. Pebbles and coarser sediments can be directly measured using calipers. For materials up to 75mm, coarse sieves can also be used. For tiny particles such as sand, sieves are used to analyze them. A stack of sieves of square-mesh woven-wire cloth is used, with each sieve different by having a decreasing nominal opening from top to bottom. A container is placed under the bottom sieve. The sediment sample is then poured over the top sieve then covered. The stack is then placed on a shaker for approximately 15 minutes to let the sediments settle. Different sizes of sediments will settle on a sieve where it cannot pass through anymore. After which, the quantity of sediments on each sieve is weighed. Sediment size fractions of silts and clays are determined through the use of either pipette or hygrometer method. These methods determine the amount of time that different size fractions of these materials remain suspended. Coulter counters are also occasionally being used. Collecting different samples for quantifying or measuring can be done using different sampling equipments which are all standardized to produce valid and comparable measurements. Depth-integrating sampler is an instrument used to get water-sediment sample by lowering it to the streambed and rising to the surface at the same rate. The nozzle, which comes in different diameter sizes, is always open. The particles that this instrument can collect range from clays through sands. Point-integrating samplers can be used to collect a sample at any point in the water column. Sampling is controlled by a rotary valve, electrically operated. Sampling is done by placing the sampler at the streambed before opening the valve, then opening the valve to start collecting while rising up the surface. Other samplers available are auxiliary or automatic sampling equipment, bed samplers and bed-load samplers; each has its own unique purpose. Each of these samplers also uses its own technique in measuring or quantifying the sediments. Works Cited "Beach Formation and Types of Beaches and Sand." Beach California. 28 Mar. 2006 . "Coastal Engineering Manual: Part III." Engineer Manuals. 30 Apr. 2002. US Army Corps of Engineers. 28 Mar. 2006 . The Columbia Encyclopedia. New York: Columbia University Press, 2004. "Sedimentation Investigations of Rivers and Reservoirs." Engineer Manuals. 31 Oct. 1995. US Army Corps of Engineers. 28 Mar. 2006 . Read More