Preventing Landslides PDF

Preventing Landslides Grading. Although grading of slopes for development has increased the landslide hazard in many areas, carefully planned grading can be used to increase slope stability. In a single cut-and-fill operation, material from the upp...

Preventing Landslides Grading. Although grading of slopes for development has increased the landslide hazard in many areas, carefully planned grading can be used to increase slope stability. In a single cut-and-fill operation, material from the upper part of a slope is removed and placed near the base. The overall gradient is thus reduced, and material is removed from an area where it contributes to the driving forces and is placed at the toe of the slope, where it increases the resisting forces. Figure. Benching The upper-right quadrant shows a slope along the Pacific Ocean created However, this method is not practical on very steep, high slopes. As to reduce the overall steepness of the slope an alternative, the slope may be cut into a series of benches or steps. and provide for better drainage. (Edward A. Keller) The benches are designed with surface drains to divert runoff. The benches reduce the overall slope of the land and are good collection sites for falling rock and small slides (Figure). Slope Supports. Retaining walls constructed from concrete, stone-filled wire baskets, or piles (i.e., long concrete, steel, or wooden beams driven into the ground) are designed to provide support at the base of a slope (Figure LEFT). They should be anchored well below the base of the slope, backfilled with permeable gravel or crushed rock (Figure) and provided with drain holes to reduce the chances of water Figure. How to support a slope pressure building up in the slope (Figure BELOW). Some types of slope support: retaining walls, piles, and drains. Fig. Retaining wall This retaining wall, made of concrete cribbing with backfill, helps stabilize a road cut. (Edward A. Keller) Warning of Impending Landslides Landslide warning systems do not prevent landslides, but they can provide time to evacuate people and their possessions and to stop trains or reroute traffic. Surveillance provides the simplest type of warning. Hazardous areas can be visually inspected for apparent changes, and small rockfalls on roads and other areas can be noted for quick removal. Human monitoring of the hazard has the advantages of reliability and flexibility but becomes disadvantageous during adverse weather and in hazardous locations. Other warning methods include electrical systems, tilt meters, and geophones that pick up vibrations from moving rocks. Shallow wells can be monitored to signal when slopes contain a dangerous amount of water. These methods are part of real-time monitoring (Figure). In some regions, monitoring rainfall is useful for detecting when a threshold precipitation has been exceeded and shallow soil slips become more probable. Fig. Real-time monitoring of active landslides (a) Idealized diagram of how real-time landslide data are collected by sensors and transmitted to people. (b) Geologist measuring landslide movement. (Courtesy of Richard La Husen/USGS) Correcting Landslides After a slide has begun, the best way to stop it is to attack the process that started the slide. In most cases, the cause of the slide is an increase in water pressure, and, in such cases, an effective drainage program must be initiated. This may include installing surface drains at the head of the slide to keep additional surface water from infiltrating and subsurface drainpipes or wells to remove water and lower the water pressure. Draining tends to increase the resisting force of the slope material, thereby stabilizing the slope.  Snow Avalanches A snow avalanche is a rapid downslope movement of snow and ice, sometimes with the addition of rock, soil, and trees. As more people venture into avalanche-prone areas and more development occurs in these areas, the loss of life and property due to avalanches increases. The most damaging avalanches occur when a large slab of snow and ice, weighing millions of tons, fails due to the overloading of a slope with fresh snow or to development of zones of weakness within the snowpack. These slabs move rapidly downslope at velocities of up to 100 km per hour. Avalanches tend to move down tracks, called chutes, that have previously produced avalanches (Figure). As a result, maps delineating the hazard may be developed. Avoiding hazardous areas is obviously the preferred and least expensive adjustment to avalanches. Other adjustments include clearing excess snow with carefully placed explosives, constructing buildings and structures to divert or retard avalanches, or planting trees in avalanche prone areas to better anchor the snow on slopes. FIGURE 10.25 Avalanche hazard (a) Avalanche chute or track in the Swiss Alps. (Edward A. Keller) (b) Map of part of Juneau, Alaska, showing the avalanche hazard. (After Cupp, D. 1982. National Geographic 162:290– 305) Subsidence Interactions between geologic conditions and human activity have been factors in numerous incidents of subsidence, the very slow to rapid sinking or settling of Earth materials. Most subsidence is caused either by: the withdrawal of fluids from subsurface reservoirs or by the collapse of surface and near-surface soil and rocks over subterranean voids. Withdrawal of Fluids The withdrawal of fluids—such as oil with associated gas and water, groundwater, and mixtures of steam and water for geothermal power—can cause subsidence. In all cases, the general principles are the same: Fluids in Earth materials below Earth’s surface have a high fluid pressure that tends to support the material above. If support or buoyancy is removed from Earth materials through pumping out of the fluid, the support is reduced, and surface subsidence may result. As the water is mined, the pore pressure was reduced and the grains were compacted; the effect at the surface was subsidence (Figure).  Sinkholes Subsidence is also caused by removal of subterranean Earth materials by natural processes. Voids—large open spaces such as caves—often form by chemical weathering within soluble rocks, such as limestone, dolomite, and evaporite rocks such as gypsum and salt. The resulting lack of support for overlying rock may cause it to collapse. The result is the formation of a sinkhole, a circular area of subsidence caused by the collapse of a near-surface subterranean void or room in a cavern. Sinkholes have caused considerable damage to highways, homes, sewage facilities, and other structures. Natural or artificial fluctuations in the water table are probably the trigger mechanism. High water table conditions enlarge the cavern closer to the surface of Earth by dissolving material, and the buoyancy of the water helps support the overburden. Lowering of the water table eliminates some of the buoyant support and facilitates collapse. FIGURE: Sinkhole swallows part of a town This Winter Park, Florida, sinkhole grew rapidly for 3 days, swallowing part of a community swimming pool as well as two businesses, a house, and several automobiles. (Leif Skoogfors/Woodfin Camp & Associates) Salt Deposits Serious subsidence events have been associated with salt mining. Salt is often mined by solution methods: Water is injected through wells into salt deposits, the salt dissolves, and water supersaturated with salt is pumped out. Because the removal of salt leaves a cavity in the rock and weakens support for the overlying rock, it may lead to large-scale subsidence. Coal Mining The subsidence is most common where underground mining is close to the surface of the land or where the rocks left as pillars after mining are weak or intensely fractured. Usually, only 50 percent of the coal is removed, leaving the remainder as pillars that support the roof, formed from the rocks overlying the mine. Over time, the pillars weather, weaken, and collapse, producing the surface subsidence. River and Flooding The floodplain, the flat surface adjacent to the river channel that is periodically inundated by floodwater, is, in fact, produced by the process of flooding. FIGURE: Floodplain (a) Diagram illustrating the location of a river’s floodplain. Streams and Rivers The hydrologic cycle involves the transport of water by evaporation from Earth’s surface, primarily from the oceans, to the atmosphere and, via surface and subsurface runoff from the land, back again to the oceans. Some of the water that falls on the land as rain or snow infiltrates soils and rocks; some evaporates; and the rest drains, or runs off, following a course determined by the local topography. This runoff finds its way to streams, which may merge to form a larger stream or a river. Streams and rivers differ only in size; that is, streams are small rivers. However, geologists commonly use the term stream for any body of water that flows in a channel. There are two basic types of rivers: the more common alluvial rivers where bed and banks of the river are sediment such as gravel and sand, or the less common bedrock rivers where bedrock is commonly exposed in the bed and banks. Bedrock streams and rivers are usually, but not always by any means, found in steep mountain areas. The region drained by a single river or river system is called a drainage basin, or watershed. A river’s slope, or gradient, is the vertical drop of the channel over some horizontal distance. In general, the slope is steepest at higher elevations in the drainage basin and levels off as the stream approaches its base level. The base level of a stream is the theoretical lowest level to which a river may erode. Most often, the base level is at sea level, although a river may have a temporary base level such as a lake. Rivers flow downhill to their base level, and a graph of elevation of a river against distance downstream is called the longitudinal profile. A river usually has a steeper-sided and deeper valley at high elevations near its headwaters than closer to its base level, where a wide floodplain may be present. At higher elevations, the steeper slope of the river causes deeper erosion of the valley. Increased erosion is due to the higher flow velocity of the river water produced by the steeper channel slopes. FIGURE: Drainage basin and river profile Idealized diagram showing (a) drainage basin, (b) longitudinal profile of the Fox River, (c) cross section of valley near headwater, and (d) cross section near base level. Sediment in Rivers The total quantity of sediment carried in a river, called its total load, includes the bed load, the suspended load, and the dissolved load. The bed load moves by the bouncing, rolling, or skipping of particles along the bottom of the channel. The bed load of most rivers, usually composed of sand and gravel, is a relatively small component, generally accounting for less than 10 percent of the total load. The suspended load, composed mainly of silt and clay, is carried above the streambed by the flowing water. The suspended load accounts for nearly 90 percent of the total load and makes rivers look muddy. The dissolved load is carried in chemical solution and is derived from chemical weathering of minerals in rock, sediment and soil in the drainage basin. The dissolved load may make stream water taste salty if it contains large amounts of sodium and chloride. It may also make the stream water “hard” if the dissolved load contains high concentrations of calcium and magnesium. River Velocity, Discharge, Erosion, and Sediment Deposition Rivers are the basic transportation system of the part of the rock cycle that involves erosion anddeposition of sediments. They are a primary erosion agent in the sculpting of our landscape. The velocity, or speed, of the water in a river varies along its course, affecting both erosion and deposition of sediment. Discharge (Q) is the volume of water moving by a particular location in a river per unit time. It is reported in cubic meters per second (cms) or cubic feet per second (cfs). Discharge is calculated as: Q = W x D x V where Q is discharge (cubic meters per second), W is the width of flow in meters, D is depth of flow in meters, and V is mean velocity of flow (meters per second). The equation is known as the continuity equation and is one of the most important relationships in understanding the flow of water in rivers. We assume that if there are no additions or deletions of flow along a given length of river, then discharge is constant. It follows that if the cross-sectional area of flow decreases, the velocity of the water must increase. This concept explains why a narrow river channel in a canyon has a higher velocity of flow. It is also the reason that rapids are common in narrow canyons. In general, a faster-flowing river has the ability to erode its banks more than a slower-moving one. Streams that flow from mountains onto plains may form fan-shaped deposits known as alluvial fans. Rivers flowing into the ocean or some other body of still water may deposit sediments that form a delta, a triangular or irregular-shaped landmass extending into the sea or a lake. FIGURE: Alluvial fan Alluvial fan FIGURE 9.7 Delta The delta of the Mississippi along the western foot of the Black River. In this false-color image, vegetation Mountains, Death Valley. Note the road appears red, and sediment-laden waters are along the base of the fan. The white white or light blue; deeper water with less materials are salt deposits in Death suspended sediment is a darker blue. Valley. (Michael Collier) The flood hazard associated with alluvial fans and deltas is different from hazards in a river valley and floodplain environment because rivers entering alluvial fan or delta environments often split into a system of distributary channels. That is, the river no longer has only one main channel but has several channels that carry floodwaters to different parts of the fan or delta. Furthermore, these channels characteristically may change position rapidly during floods, producing a flood hazard that is difficult to predict. Effects of Land Use Changes Streams and rivers are open systems that generally maintain a rough dynamic equilibrium, or steady state between the work done (i.e., the sediment transported by the stream) and the load imposed (i.e., the sediment delivered to the stream from tributaries and hill slopes). A stream tends to have a slope and cross-sectional shape that provides the velocity of flow necessary to do the work of moving the sediment load.6 An increase or a decrease in the amount of water or sediment received by the stream usually brings about changes in the channel’s slope or cross-sectional shape, effectively changing the velocity of the water. The change of velocity may, in turn, increase or decrease the amount of sediment carried in the system. Therefore, land-use changes that affect the stream’s volume of sediment or water volume may set into motion a series of events that results in a new dynamic equilibrium. Channel Patterns and Floodplain Formation The configuration of the channel as seen in an aerial view is called the channel pattern. Channel patterns can be braided or meandering, or both characteristics may be found in the same river. Braided channels are characterized by numerous gravel bars and islands that divide and reunite the channel. A steep slope and coarse sediment favor transport of bed load material important in the development of gravel bars that form the “islands” that divide and subdivide the flow. The formation of the braided channel pattern, as with many other river forms, results from the interaction of flowing water and moving sediment. If the river’s longitudinal profile is steep and there is an abundance of coarse bed load sediment, the channel pattern is likely to be braided. Braided channels tend to be wide and shallow compared with meandering rivers. They are often associated with steep rivers flowing through areas that are being rapidly uplifted by tectonic processes. They are also common in rivers receiving water from melting glaciers that provide a lot of coarse sediment. Some channels contain meanders, which are bends that migrate back and forth across the floodplain (Figure a). Although we know what meander bends look like and what the water and sediment do in the bends, we do not know for sure why rivers meander. On the outside of a bend, sometimes referred to as the cut bank, the water moves faster during high flow events, causing more bank erosion; on the inside of a curve, water moves more slowly, and sediment is deposited, forming point bars. As this differential erosion and sediment deposition continues, meanders migrate laterally by erosion on the cut banks and by deposition on point bars, a process that is prominent in constructing and maintaining some floodplains (Figure b). Overbank deposition, or deposition beyond the banks of a river, during floods causes layers of relatively fine sediments, such as sand and silt, to build upward; this accumulation is also important in the development of floodplains. Much of the sediment transported in rivers is periodically stored by deposition in the channel and on the adjacent floodplain. These areas, collectively called the riverine environment, are the natural domain of the river. FIGURE: Meandering river (a) Idealized diagram of a meandering stream and important forms and processes. Meander scrolls are low, curved ridges of sediment parallel to a meander bend. They form at the edge of a riverbank as sediment accumulates with plants. A series of scrolls marks the evolution of a meander bend. (b) Koyakuk River, Alaska, showing meander bend, point bar, and cut bank. The Oxbow lake formed as the river eroded laterally across the floodplain and “cut off” a meander bend, leaving the meander bend as a lake. (Accent Alaska/Alamy) Meandering channels often contain a series of regularly spaced pools and riffles (Figure). Pools are deep areas produced by scour, or erosion at high flow, and characterized at low flow by relatively deep, slow movement of water. Pools are places in which you might want to take a summer swim. Riffles are shallow areas produced by depositional processes at high flow and characterized by relatively shallow, fast-moving water at low flow. Pools and riffles have important environmental significance: The alternation of deep, slowmoving water with shallow, fast- moving water in pools and riffles produces a variable physical and hydrologic environment and increased biological diversity. For example, fish may feed in riffles and seek shelter in pools, and pools have different types of insects than are found in riffles. FIGURE: Pool and riffle Well-developed pool-riffle sequence in Sims Creek near Blowing Rock, North Carolina. A deep pool is apparent in the middle distance, and shallow riffles can be seen in the far distance and in the foreground. (Edward A. Keller) River Flooding The natural process of overbank flow is termed flooding. Most river flooding is a function of the total amount and distribution of precipitation in the drainage basin, the rate at which precipitation infiltrates the rock or soil, and the topography. Some floods, however, result from rapid melting of ice and snow in the spring or, on rare occasions, from the failure of a dam. The channel discharge at the point where water overflows the channel is called the flood discharge and is used as an indication of the magnitude of the flood. The height of the water in a river at any given time is called the stage. The term flood stage frequently connotes that the water surface has reached a highwater condition likely to cause damage to personal property  Flash Floods and Downstream Floods Flash floods occur in the upper parts of drainage basins and are generally produced by intense rainfall of short duration over a relatively small area. In general, flash floods may not cause flooding in the larger streams they join downstream, although they can be quite severe locally. Downstream floods cover a wide area and are usually produced by storms of long duration that saturate the soil and produce increased runoff. Flooding on small tributary basins is limited, but the contribution of increased runoff from thousands of tributary basins may cause a large flood downstream. A flood of this kind is characterized by the downstream movement of the floodwaters with a large rise and fall of discharge at a particular location. FIG: Flash floods and downstream floods Idealized diagram comparing a flash flood (a) with a downstream flood (b). Flash floods generally cover relatively small areas and are caused by intense local storms with steep topography, often in a canyon. A distinct floodplain may not be present, whereas downstream floods cover wide areas of a floodplain and are caused by regional storms or spring runoff of a floodplain. Factors That Cause Flood Damage Factors that affect the damage caused by floods include: Land use on the floodplain Magnitude, or the depth and velocity of the water and frequency of flooding Rate of rise and duration of flooding Season (e.g., growing season on the floodplain) Sediment load deposited Effectiveness of forecasting, warning, and emergency systems Effects of Flooding The effects of flooding may be primary (i.e., directly caused by the flood) or secondary (i.e., caused by disruption and malfunction of services and systems due to the flood). Primary effects include injury, loss of life, and damage caused by swift currents, debris, and sediment to farms, homes, buildings, railroads, bridges, roads, and communication systems. Erosion and deposition of sediment in the rural and urban landscape may also involve a loss of considerable soil and vegetation. Secondary effects may include short-term pollution of rivers, hunger and disease, and displacement of persons who have lost their homes. In addition, fires may be caused by shorts in electrical circuits or gas mains broken by flooding and associated erosion Adjustments to Flood Hazards Humans responded to flooding by attempting to prevent the problem; that is, they modified the stream by creating physical barriers, such as dams or levees, or by straightening, widening, and deepening an entire stream so that it would drain the land more efficiently. The Structural Approach Physical Barriers. Measures to prevent flooding include construction of physical barriers, such as levees and floodwalls, which are usually constructed of concrete as opposed to earthen levees; reservoirs to store water for later release at safe rates; and on-site stormwater retention basins. Unfortunately, the potential benefits of physical barriers are often lost because of increased development on floodplains that are supposedly protected by these structures. The systems of levees, floodwalls, and structures to improve river navigation for barges transporting goods downriver control the smaller floods. For the largest floods, these same systems may constrain or retard flow FIG: Mississippi River levee (i.e., slow it down), and this results in higher levels of flood flow (stage) Channelization. Straightening, deepening, widening, clearing, or lining existing stream channels are all methods of channelization. Basically, it is an engineering technique with the objectives of controlling floods, draining wetlands, controlling erosion, and improving navigation. Opponents of channelizing natural streams emphasize that the practice is antithetical to the production of fish and wetland wildlife, and, furthermore, channelizing causes a stream to suffer from extensive aesthetic degradation. The argument is as follows: Drainage of wetlands adversely affects plants and animals by eliminating habitats necessary for the survival of certain species. Cutting trees along the stream eliminates shading and cover for fish and exposes the stream to the sun; the exposure results in damage to plant life and heat-sensitive aquatic organisms. Cutting hardwood trees on the floodplain eliminates the habitats of many animals and birds, while facilitating erosion and siltation of the stream. Straightening and modifying the streambed destroys both the diversity of flow patterns and the feeding and breeding areas for aquatic life while changing peak flow. Conversion of wetlands from a meandering stream to a straight, open ditch seriously degrades the aesthetic value of a natural area. FIGURE Natural versus channelized stream A natural stream compared with a channelized stream in terms of general characteristics and pool environments. (Modified after Corning, Virginia Wildlife, February 1975) Channel Restoration: Alternative to Channelization Many streams in urban areas scarcely resemble natural channels. The process of constructing roads, utilities, and buildings with the associated sediment production is sufficient to disrupt most small streams. Channel restoration uses various techniques: Cleaning urban waste from the channel, allowing the stream to flow freely, protecting the existing channel banks by not removing existing trees or, where necessary, planting additional native trees and other vegetation. Trees provide shade for a stream, and the root systems protect the banks from erosion. The objective is to create a more natural channel by allowing the stream to meander and, when possible, provide for variable, low-water flow conditions: fast and shallow flow on riffles alternating with slow and deep flow in pools. Where lateral bank erosion must be absolutely controlled, the outsides of bends may be defended with large stones known as riprap. FIGURE: Urban stream restoration (a) Channel-restoration design criteria for urban streams, using a variable channel shape to induce scour and deposition (pools and riffles) at desired locations.  Introduction to Coastal Hazards Coastal areas are dynamic environments that vary in their topography, climate, and vegetation. Continental and oceanic processes converge along coasts to produce landscapes that are characteristically capable of rapid change. The impact of hazardous coastal processes is considerable because many populated areas are located near the coast. Hazards along the coasts may become compounded by the fact that global warming and the accompanying global rise in sea level are increasing the coastal erosion problem. The most serious coastal hazards are the following: Rip currents generated in the surf zone Coastal erosion, which continues to produce considerable property damage that requires human adjustment Tsunami, or seismic sea waves Tropical cyclones, called hurricanes in the Atlantic and typhoons in the Pacific, which claim many lives and cause enormous amounts of property damage every year Waves Waves that batter the coast are generated by offshore storms, sometimes thousands of kilometers from the shoreline where they will expend their energy. Wind blowing over the water produces friction along the air–water boundary. Since the air is moving much faster than the water, the moving air transfers some of its energy to the water, resulting in waves. The waves, in turn, eventually expend their energy at the shoreline. The size of the waves produced depends on the following: The velocity, or speed, of the wind. The greater the wind velocity, the larger the waves. The duration of the wind. Storms of longer duration have more time to impart energy to the water, producing larger waves. The distance that the wind blows across the surface, or fetch. The longer the fetch, the larger the waves. Within the area of the storm, the ocean waves have a variety of sizes and shapes, but, as they move away from their place of origin, they become sorted out into groups of similar waves. These groups of waves may travel for long distances across the ocean and arrive at distant shores with very little energy loss. FIGURE 11.3 Waves (a) Deep-water wave form (water depth is greater than 0.5 L, where L is wave length). The ratio of wave height to wave length is defined as wave steepness. If wave height exceeds about 10 percent (0.1) of the wave length, the wave becomes unstable and will break. Our drawing exaggerates wave height for illustrative purposes. The steepness of the waves in the drawing is about 1/3, or 0.33, which would be very unstable and would not exist long in nature. The wave groups generated by storms far out at sea are called swell. As the swell enters shallower and shallower water, transformations take place that eventually lead to the waves breaking on the shore. For deep-water conditions, there are equations to predict wave height, period, and velocity based on the fetch, wind velocity, and length of time that the wind blows over the water. This information has important environmental consequences: By predicting the velocity and height of the waves, we can estimate when waves with a particular erosive capability generated by a distant storm will strike the shoreline. Waves spend their energy when they reach the coastline. Wave energy is approximately proportional to the square of the wave height. When waves enter the coastal zone and shallow water, they impinge on the bottom and become steeper. Wave steepness is the ratio of wave height to wave length. Waves are unstable when the wave height is greater than about 10 percent (0.1) of the wave length. As waves move into shallow water, the wave period remains constant, but wave length and velocity decrease and wave height increases Waves change shape from rounded crests and troughs in deep water to peaked crests with relatively flat troughs in shallow water close to shore. Perhaps the most dramatic feature of waves entering shallow water is their rapid increase in height. The height of waves in shallow water, where they break, may be as much as twice their deep-water height (Figure 11.3c). Waves near the shoreline, just outside the surf zone, reach a wave steepness that is unstable. The instability causes the waves to break and expend their energy on the shoreline. FIGURE:Waves and beaches Motion of water particles in shallow water at a depth less than 0.25 L. Water at the beach moves up and back in the swash zone, the very shallow water on the beach face. Although wave heights offshore are relatively constant, the local wave height may increase or decrease when the wave front reaches the near-shore environment. This change can be attributed to irregularities in the offshore topography and the shape of the coastline. Figure in the left is an idealized diagram showing a rocky point, or headland, between two relatively straight reaches of coastline. The offshore topography is similar to that of the coastline. As a wave front approaches the coastline, the shape of the front changes and becomes more parallel to the coastline. This change occurs because, as the waves enter shallow water, they slow down first where the water is shallowest, that is, off the rocky point. FIGURE 11.4 Convergence and divergence of wave Energy Idealized diagram of the process of wave The result is a bending, or refraction, of the wave front. refraction and concentration of wave energy at rocky points, or headlands. The refraction, or bending of the In Figure, the lines drawn perpendicular to the wave fronts, with wave fronts, causes the convergence of wave normals on arrows pointing toward the shoreline, are known as wave the rocky point and divergence at the bay. normals. Due to the bending of the wave fronts by refraction, there is a convergence of the wave normal at the headland, or rocky point, and a divergence of the wave normals at the beaches, or embayments. Where wave normals converge, wave height increases; as a result, wave energy expenditure at the shoreline also increases. The long-term effect of greater energy expenditure on protruding areas is that wave erosion tends to straighten the shoreline. The total energy from waves reaching a coastline during a particular time interval may be fairly constant, but there may be considerable local variability of energy expenditure when the waves break on the shoreline. In addition, breaking waves may peak up quickly and plunge or surge, or they may gently spill, depending on local conditions, such as the steepness of the shoreline and the height and length of waves arriving at the shoreline from a distant storm. Plunging breakers tend to be highly erosive at the shoreline, whereas spilling breakers are more gentle and may facilitate the deposition of sand on beaches. The large plunging breakers that occur during storms cause much of the coastal erosion we observe. Beach Form and Beach Processes A beach is a landform consisting of loose material, such as sand or gravel, that has accumulated by wave action at the shoreline. Beaches may be composed of a variety of loose material in the shore zone, the composition of which depends on the environment. The landward extension of the beach terminates at a natural topographic and morphologic change such as a sea cliff or a line of sand dunes. The berms are flat backshore areas on beaches formed by deposition of sediment as waves rush up and expend the last of their energy. Berms are where you will find people sunbathing. The beach face is the sloping portion of the beach below the berm, and the part of the beach face that is exposed by the uprush and backwash of waves is called the swash zone. Figure: Beach terms Basic terminology for landforms and wave action in the beach and near- The surf zone is that portion of the seashore environment where turbulent shore environment. translational waves move toward the shore after the incoming waves break. Breaker zone is the area where the incoming waves become unstable, peak, and break. The longshore trough and longshore bar are an elongated depression and adjacent ridge of sand produced by wave action. A particular beach, especially if it is wide and gently sloping, may have a series of longshore bars, longshore troughs, and breaker zones Transport of Sand The sand on beaches is not static; wave action keeps the sand moving along the beach in the surf and swash zones. A longshore current is produced by incoming waves striking the coast at an angle (Figure. Because the waves strike the coast at an angle, a component of wave energy is directed along the shore. If waves arrive at a beach perfectly parallel to the beach, then no longshore current is generated. The longshore current is a stream of water flowing parallel to the shore in the surf zone. This current can be surprisingly strong. The process that transports sand along the beach, called longshore sediment transport, has two components: (1) Sand is transported along the coast with the longshore current in the surf zone; and (2) The up-and-back movement of beach sand in the swash zone causes the sand to move along the beach in a zigzag path (Figure). Most of the sand is transported in the surf zone by the longshore current. Rip Currents When a series of large waves arrives at a coastline and breaks on the beach, the water tends to pile up on the shore. The water does not return as it came in, along the entire shoreline, but is concentrated in narrow zones known as rip currents Beachgoers and lifeguards call them riptides or undertow. They certainly are not tides, and they do not pull people under the water, but they can pull people offshore. Rip currents constitute a serious coastal hazard. Rip currents are usually relatively narrow, a few meters to a few tens of meters wide, and they dissipate outside the surf zone, within tens to hundreds of meters offshore. Rip currents can form quickly after the arrival of a set of large waves. They can be recognized as a relatively quiet area in the surf zone where fewer incoming waves break. FIGURE: Rip current Bird’s-eye view of the surf zone, showing a rip current, which is the return flow of water that forms as a result of incoming waves