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21_hazards1_09_post - 21: Natural Hazards 1 Landslides...

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Unformatted text preview: 21: Natural Hazards 1 Landslides Landslides, Floods, Storm Surges Mississippi flooding New Orleans in May 1979. Photo: R. Peterson • Mass movement / mass wasting: Downhill movement of regolith and/or rock under the direct influence of gravity, i.e., without a transporting medium (water, wind, or ice). • More property damage is attributed to landslides than any other environmental factor. • In the U.S., landslides cause 25-50 deaths and $1-2 billion in property damage each year. Landslide induced by 1964 Alaska earthquake. Photo: S. MacCutcheon Beach erosion caused by Hurricane Hugo, South Carolina. Photo: D.M. Busch / O.H. Pilkey, Jr. Hazards 1: 1 Hazards 1: 2 Types of mass movement • Falls: very fast free-fall of rock and soil fragments through air, out of direct contact with slope. Types of mass movement • Avalanches: fast downslope flow of rock or debris mixed with air that rides on a cushion of compressed air. 2002: Rock avalanche triggered by earthquake along Denali fault partially covers Black Rapids glacier. Photo: D. Trabant /USGS. p. 573 Rock fall, Zion National Park, Utah. Photo: S. Allred Hazards 1: 3 Hazards 1: 4 p. 573 Types of mass movement Types of mass movement Fig. 16.15 • Avalanches: fast downslope flow of rock or debris mixed with air that rides on a cushion of compressed air. 1970: Debris avalanche triggered by earthquake in Peru traveled 17 km at 280 km/hr; consisted of up to 100 cubic meters of water, mud and rocks; and killed 66,700 persons in the towns of Yungay and Ranrahirca at the base of Mt. Huascarán. • Slides: moderately fast downslope movement of relatively intact blocks of rock or debris; sliding takes place on a specific surface (e.g., bedding plane) Photos: L. Cluff. Debris slide in valley in Kyrgystan. Fig. 16.1 Photo: M. Miller Hazards 1: 5 Hazards 1: 6 Types of mass movement Types of mass movement (Mud)flows: fast movements of rock, soil, and water as a fluid mixture p. 573 Lahar: volcanic Lahar: mudflow 15-m-high mudflow produced by seismic shaking of slopes weakened by heavy rain, Tadzhikistan, 1989. Photo: V. Shone Hazards 1: 7 Hazards 1: 8 Types of mass movement Types • Creep: very slow downhill movement of rock and soil, recognized by tilted gravestones & utility poles, curving tree trunks, etc. Photo: W.K. Hamblin Slump in soil, Sheridan, Wyoming. Photo: E.R. Degginger p. 572 • Slumps: intact blocks of rocks or unconsolidated debris that slide downward and tilt backward along concaveupward sliding surfaces Hazards 1: 9 Review Questions Fig. 16.2 Hazards 1: 10 Slope processes Angle of repose: maximum slope that unconsolidated material can sustain before slope failure occurs 21-1. Which of the following types of mass movement moves at the slowest rate? A. rock avalanche B. creep C. debris slide D. slump 21-2. Which of the following types of mass movement moves at the fastest rate? A. creep B. slump C. mudflow Q1. Which statement below is false? A. The angle of repose increases as the angularity of grains increases. B. The angle of repose increases as the size of the grains decreases. 21-3. Which of the following types of mass movement always travels down a curved surface? A. creep B. mudflow C. rock slide D. slump 21-4. Which of the following types of mass movement is likely to move most rapidly? A. creep B. rock fall C. rock slide D. slump 42°: Rounded pebbles 21-5. Which of the following types of mass movement is least coherent (most like a liquid)? A. creep B. mudflow C. rock slide D. slump 21-6. Which of the following types of mass movement involves relatively intact blocks of material? A. rock fall B. debris avalanche C. slump D. mudflow Fig. 16.14 Hazards 1: 11 Hazards 1: 12 Slope processes Angle of repose: maximum slope that unconsolidated material can sustain before slope failure occurs Q2. Moisture among sediment grains ____________. A. always increases the likelihood of mass wasting B. always decreases the likelihood of mass wasting C. can promote stability if present in small amounts, but promotes mass wasting if sediment is saturated D. can promote stability if the grains are saturated, but a small amount being present has little effect on the likelihood of mass wasting Slope physics • Gravity: force that acts straight down • Divided into force acting parallel to slope (Fd) & force acting perpendicular to slope (Fn) • Resistance force, FR=µFn, where µ is friction coefficient. • Movement occurs when Fd>FR Q3. All other things being equal, increasing the friction coefficient (µ): A. decreases the resistance force and increases the likelihood of movement of the block. B. has no effect on the likelihood of movement of the block. C. increases the resistance force and decreases the likelihood of movement of the block. Fig. 16.13 Fig. 16.14 Hazards 1: 13 Slope physics Hazards 1: 14 Triggers of mass movement • Movement occurs when Fd>FR Fig. 16.15 Q4. All other things being equal, increasing the slope angle of possible landslide block ___. A. decreases the downslope force and decreases the likelihood of movement of the block. B. has no effect on the likelihood of movement of the block. C. increases the downslope force and increases the likelihood of movement of the block. Fig. 16.13 Hazards 1: 15 • Sliding is favored when plane of weakness is inclined ________ the slope or cut. Hazards 1: 16 Triggers of mass movement Triggers of mass movement • Removal of vegetation (e.g., forest fire): • Addition of water: 1) lubricates sliding surface, 2) weathers and weakens rocks (decreasing friction coefficient) 3) necessary ingredient for mudflows and debris flows • Earthquakes, explosions, volcanic eruptions (lahars) Fig. 16.17 • Slope angle: Enhanced erosion and mass movement following a forest fire in Yellowstone National Park, Wyoming. Photo: G. Meyer Hazards 1: 17 Hazards 1: 18 Triggers of mass movement Landslide recognition Fig. 16.22 • Undercutting of cliffs by wave action Hazards 1: 19 Fig. 16.19 Hazards 1: 20 Landslide mitigation Landslide mitigation Fig. 16.23 Artificially stabilize slope Fig. 16.23 + Divert excess water away from slope Revegetation: Revegetation: Redistribute materials to… to… Hazards 1: 21 Review Questions 21-7. Which of the following is responsible for all incidents of mass movement? A. blowing air B. flowing ice C. flowing water D. force of gravity E. friction 21-8. A. True / B. False: The downslope force increases and the normal force decreases as the slope angle decreases. 21-9. A. True / B. False: Mass movement occurs when the resistance force (normal force times the coefficient of friction) is less than the downslope force. 21-10. Which of the following factors decreases the risk of mass movement? A. nearby earthquakes B. excavation (removing soil, sediment and/or rock) into the base of a hill C. adding weight to the top of a hill D. adding vegetation to the side of a hill Hazards 1: 23 Construct safety structures Hazards 1: 22 Review Questions 21-11. A. True / B. False: Rockslides are more likely to occur in regions where weakness in the rocks (such as bedding surfaces) slope in the opposite direction as the land surface. 21-12. Which of the following increases the risk of mass movement? A. adding a small amount of moisture to loose, dry sediment B. waves breaking before they reach sea cliffs C. flooding the sediment with water to the point of saturation D. reducing the angle of the slope 21-13. A. True / B. False: Wildfires on slopes during the dry season are commonly followed by landslides during the rainy system because the burned vegetation is not as effective in binding the regolith as before the fires. 21-14. Which of the following is used to prevent landslides or minimize their damage? A. Increase the steepness of the slope. B. Remove excess vegetation. C. Divert excess water onto the slope. D. Artificially stabilize the slope. Hazards 1: 24 Floods Floods • Flood: an event during which the volume of water in a stream becomes so great that it covers the area outside the stream’s normal channel • Typically related to… • Floodplain: the flat land on either side of a stream that becomes covered with water during a flood • Floodstage: the height of the water in a stream necessary to cause flooding Fig. 17.32 Mississippi & Missouri rivers Photo: G.M. Ashley 1993 Hazards 1: 25 Hazards 1: 26 Floods & urbanization • Paved surfaces… surfaces… Fig. 17.41 Flood Prediction • Uses past record of discharge to predict frequency of future discharges • Measure discharge for 10-30+ years. • Determine maximum discharge for each year. • Rank each peak discharge from largest to smallest. • Urbanization has _________ discharge & _________ lag time between high rainfall events & floods. Hazards 1: 27 • Calculate recurrence interval, R= (n+1)/m, where n is number of years in record and m is the rank (for largest discharge, m=1; next largest, m=2, etc. Hazards 1: 28 Fig. 17.39 Flood Prediction Flood Mitigation • Construct artificial levees, but it is too expensive to build levees able to withstand all floods • Plot recurrence interval (R) vs. discharge (flood(floodfrequency graph) graph) • Designate floodways adjacent to stream channels where… where… • Data typically plot on a straight line, which can be used to predict the discharge associated with a “50-year” or “100-year” flood. Q5. What is the annual maximum discharge associated with a "100-yearflood"? A. ~550 ft3/s B. ~600 ft3/s C. ~650 ft3/s D. ~700 ft3/s Fig. 17.36 Q6. What is the probability that a discharge of 400 ft3/s will occur in any given year? Probability is [1/(recurrence interval in years)]x100%. A. ~1% B. ~5% C. ~10% D. ~20% E. ~100% Hazards 1: 29 Hazards 1: 30 Flood hazard maps Coastal hazards • Use floodfrequency data to determine river height (stage) for 100stage) yr and 500-yr floods. • Storm surge: abnormally high water level produced by lowpressure of hurricane (causing water to rise upward) and strong landward-blowing winds (causing water to pile up); causes extensive coastal erosion • Use topography (elevation) data to map how much of area around channel will be flooded. Fig. 17.37 Example: 2-yr flood stage: 10 ft; 100-yr flood stage: 50 ft Example: ft; Hazards 1: 31 Hazards 1: 32 Katrina storm surge Rising sea level Fig. 18.39 Shoreline after 100ft rise in sea-level Shoreline after 20-ft rise in sea-level Present-day shoreline Hazards 1: 33 Photos: US Army Corps of Engineers Topographic maps Gentle slopes: contour lines are… are… Steep slopes: contour lines are… are… Hazards 1: 35 Contour lines connect points of equal elevation Hazards 1: 34 ep Ste Topographic maps e slop p ee St pe slo Contour lines connect points of equal elevation Q7. The contour interval (difference in elevation between two adjacent contour lines) is: A. 5 ft. B. 10 ft. C. 20 ft. D. 50 ft. Q8. The maximum elevation on the map is approximately: A. 230 ft B. 240 ft. C. 260 ft. D. 280 ft. Q9. The gentlest slope is located at: A, B, C, or D. Hazards 1: 36 280 260 240 220 180 160 140 120 Contour lines connect points of equal elevation Human activity Fig. 18.41 • Groins: barriers built at high angle to beach that cause upcurrent deposition and downcurrent erosion; one part of the beach is protected at the expense of another • Jetties function like groins to protect a harbor inlet. • Breakwaters: designed to dissipate waves and to protect a harbor or beach, but result in a scalloped beach. Hazards 1: 37 Review Questions 21-15. A. True / B. False: Urbanization has increased the lag time between a precipitation event and runoff. 21-16. A. True / B. False: A 100-year flood is less catastrophic than a 50-year flood because it happens less frequently. 21-17. If a 50-year flood occurs on the Mississippi River in 2010, then ___. A. another flood of that magnitude will not occur until 2060 B. another flood of that magnitude can occur before 2060, but the likelihood is initially small and increases thereafter C. another flood of that magnitude can occur before 2060, but the likelihood is initially large and decreases thereafter 21-18. A. True / B. False: Artificial levees, if built well enough, can prevent all floods from occurring in a protected part of the floodplain. 21-19. A. True / B. False: Seawalls provide, at best, short-term protection from coastal erosion. Human activity • Seawall: barrier built to protect the coastline, but the abrupt discontinuity causes all surf energy to be dissipated on an unnaturally narrow beach, causing beach erosion and undermining of the seawall Fig. 18.42 Best approach: Hazards 1: 38 Review Questions 21-21. Which of the following is not a contributing factor to the destructiveness of a storm surge? A. arrival of storm surge at or near the time of natural high tide B. landward-blowing winds associated with storms C. high air pressure associated with storms that forces water toward shore. 21-22. Sand groins ___. A. have solved the problem of beach drift B. increase erosion rates in the region upcurrent from the groin C. increase erosion rates in the region downcurrent from the groin D. increase depositional rates in the region downcurrent from the groin 21-23. Based on the pattern of erosion and deposition associated with the groins in the photo, the longshore current was flowing ___. A. east to west B. north to south C. south to north D. west to east 21-20. A. True / B. False: If all the continental ice sheets were to melt, New Brunswick, NJ, would be submerged below sea level. Hazards 1: 39 Hazards 1: 40 N ...
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