Environmental Geology Chapter 11 COASTAL PROCESSES and RELATED HAZARDS Introduction >50% of world population concentrated in the coastal zones ~75% of U.S. population living in coastal states Coastal hazard problems compounded by increased human activities Tides - Tides are the oscillations in local sea level produced by the gravitational pull of the moon and the Sun (to a lesser extent) on the oceans. As the Earth spins, the overhead position of the moon sweeps across all latitudes. When the moon is overhead, gravity from it is a maximum, and the water is pulled upward to form high tide. This also happens on the opposite side of the earth at the same time as water is pulled away from the sides of the planet (where low tide is occurring). There are two high tides and two low tides in every 24- hour period. When the moon is closest to Earth, and the moon and sun are lined up together (either new or full moon), then tides are maximum (spring tides). When the sun and moon are at 90 degrees to one another with respect to the earth (first and last quarter moon), then tides are at a minimum (neap tides). Coastal Hazards Tropical cyclones (hurricanes in the Atlantic and typhoons in the Pacific) and other severe storms Marine floods and inland floods due to tropical storms Coastal erosion - MOST dynamic coastal environment: Convergent zone of continental and oceanic processes Tsunamis and tidal currents Rip currents Coastal Processes: Waves are formed by winds. The magnitude of waves is controlled by: Ø Speed of the wind Ø The duration of the wind Ø The fetch of the wind The area or distance blown by winds Tsunamis: Caused by earthquake or other seafloor vibration Waves - The height of a wave is the distance from the top (crest), or highest point of the wave to the bottom (trough) or lowest point. The distance between successive wave crests (or troughs) is called the wavelength. The wave height and wavelength are controlled by wind speed (wind blowing over water produces waves), length of time the wind has been blowing, and the area over which the wind is blowing over the water.
Wave and Water Movement Motion of water particles in waves is up and down in a circular (orbital) fashion. The wave/disturbance moves forward, but the water itself does not. Waves expend their energy along the coastline Wave translation: Decrease in wave length and velocity, but increase in wave height; deep water wave energy translating into wave breakers, pounding the shore. The long- term effect of wave translation: Coastal erosion and beach modification Wave refraction: The bending of waves and concentration of its energy on the protruding areas. The long- term effect of wave refraction: Straightening the irregular shoreline, erosion of headlands Plunging and Spilling Breakers - Beach erosion/wave erosion is produced by abrasion and by the impact pressure of waves breaking against the beach or headlands. Plunging breakers are the curled- over type of waves favored by surfers who want to ride the pipeline in Hawaii, and spilling breakers are the more low- key type that just break apart into white foam without the overturned lip. Coastal Processes: Currents - Horizontal movement of a large volume of seawater: Ø Due to oblique waves Ø Due to differences in water temperature Ø Due to differences in water salinity Ø Global currents or more local currents Longshore/Beach Drift = Littoral Transport - Longshore currents move sand down along the beaches. Eroding in one area and then redepositing in another. Beach drift and longshore drift combine to move sand along the coastlines (collectively known as littoral transport). Swash pushing sediment onshore in an oblique angle Backwash: Back flow of water and sediments perpendicular to shore by gravity Net effect: Zigzag longshore beach drift Changes of seasonal and weather conditions resulting in changes of beach faces and textures Beach Processes - The place where waves break is known as the surf zone, and is the only place where significant forward motion of water is actually occurring. Sand tends to be pushed onto the beaches by mild summer waves and be eroded into offshore bars during stronger winter storms. Offshore sand bars
can form and block bays (baymouth bars) or continue as spits beyond the end of headlands. When offshore sea stacks or breakwaters decrease wave action, sand bars can build up and connect these structures to land via a sand peninsula known as a tombolo. Rip Currents (aka rip tides or undertow) - Formation: A series of large waves surging to shore, then the rapid backflow of the piled up water in narrow zones. Up to 200 people killed and 20,000 rescued from rip currents per year in the U.S. It is important for people to recognize rip currents and to understand how to avoid or escape the hazard Swim parallel to the shore until outside the current.then head in to the beach. Coastal Erosion - Factors Ø Tropical cyclones, Nor easters Ø Tsunamis Ø Tidal actions Ø Long- term rise of sea level Ø Human activities Sea Levels and Climate Change - Sea level rises during warm climates when glaciers are melting away, and drops during ice ages as water is taken from the oceans and supplies the growing glaciers. Increased coastal erosion occurs during ice ages as water can flow down farther to reach the lowered sea level. Seacliff Erosion (e.g. landslide, mudflow, runoff) - Human activities promoting seacliff erosion through: Ø Urbanization Ø Added structures along the edge pools, and patios Ø Irrigation and other activities Hazard reduction: Reducing runoff (drainpipes), planed development and activities Rivers supply the sediment for beaches - So long as there is a constant supply of sediment from rivers, no net erosion has to occur. When sand resupply is blocked by groins, jetties, etc. (or is removed from a channel by dredging), enhanced beach erosion can occur downstream from these structures, while increased deposition occurs upstream of these features. Coastal Engineering Structures - Common structures: Seawalls, groins, breakwaters, and jetties Benefits Ø Improve navigation Ø Retard erosion, recreational beach expansion Problems
Ø Interference with longshore currents, causing unintended adjacent local erosion and deposition Building Beaches with Groins On the updrift side there is deposition & erosion on the downdrift side Beach Nourishment This is an Alternative to coastal engineering structures. Constructing a positive beach sand budget involves simply putting more (and hopefully slightly coarser) sand onto a beach to maintain a positive sand budget from year to year. A successful case: Ø Miami Beach, Florida, 1970s 1980s Ø 200 m wide beach, survived major hurricanes in 1979 and 1992 Remaining debate: Value of beach nourishment Barrier islands are long narrow islands (typically equivalent to big sand bars) separated from the main coastline by a body of water (e.g., a bay). The Outer Banks of North Carolina (of Wright Brothers at Kitty Hawk fame) are a prime example. Barrier islands are dynamic places, shifting back and forth, with inlets forming and reforming, under the influence of large storms. People occupy these naturally changing landscapes at their peril, as structures are not as naturally mobile as the sand bars on which they ve been built Human Activity and Coastal Erosion - Atlantic Coast: Barrier islands from FL to NY Example: Ocean City, Maryland Ø Summer resort city, high- rise developments Ø Inlet opened in 1933 hurricane, vulnerable for future hurricane or other severe storms, only matter of time Ø Local erosions due to development and removal of natural dunes Ø Rapid shoreline erosion in Assateague Island due to the starvation of sand supply blocked off by the Ocean City inlet jetties Human Activity and Coastal Erosion - Gulf Coast Last 100 years, coastal erosion along Texas coast accelerated by 30 40% Coastal erosions along the Gulf of Mexico Ø Due to land subsidence from the groundwater withdrawal and petroleum exploration Ø Reduction of sand supply from the damming of rivers Galveston Seawall - Constructed after 1900 Hurricane Disaster (~12k killed). All sand eroded away due to wave energy reflected off seawall
Human Activity and Coastal Erosion - The Great Lakes Periodic problems along the coasts of the Great Lakes Coastal erosion Ø Fluctuations of lake water level Ø Lack of natural frontal dunes Ø Erosion more severe along the Lake Michigan shoreline Tropical Cyclones - Warm ocean water transformed into rotating bands of thunderstorms- warm air around clear eye of subsiding air Ø Called Typhoons in Pacific Ø Called Hurricanes in Atlantic Ø Form where thunderstorms from over land move out over warm ocean waters (in tropical latitudes) and gather energy from the warmth Ø Counterclockwise rotation; low pressure system Tropical Cyclone - hazards Strong winds: Hundreds mph winds damaging structures, power lines, and trees Intense precipitation: Marine floods and inland floods Storm surge, if compounded with high tide: Great amount of flood and coastal erosion Annual average: Possible impact from 5 hurricanes along the Atlantic Coast The Saffir- Simpson Scale (from 1 to 5) used to describe the level or intensity of a cyclone Responses to Tropical Cyclones and Beach Erosion Ø Do nothing and bear the occasional losses Ø Evacuation, relocation, and change in coastal land use Ø Beach nourishment Ø Shoreline stabilization (groins, jetties, breakwaters) and raising buildings on stilts Perception and Adjustment - Perception: Largely affected by past experience, proximity to the coast, and probability of suffering damages Adjustment Ø Better protective structures Ø Better land use zoning Ø Better coastal mitigation planning and emergency management (preparation, evacuation and warning, post- storm management procedures)
Once humans become involved in altering the coastal zone, they must stay involved, or reap the consequences (down- current erosion, channel filling, structure damage and destruction) of the changes such involvement produces Managing Coastal Erosion - Five general principles Coastal erosion: A natural process, posing natural hazards as development approaching shore fronts Shoreline construction causes changes: Often better for some, worse for others Stabilization of the coastal zone: Protecting the interests of few or general public? Engineering structures: Design and consequences Structural vs. nonstructural alternatives to coastal erosion problems WSE 8/2012