Geol 108 Lab #7 Coastal Studies Week of October 15-19, 2012

Hurricanes and Storm Surge : Before coming to lab, visit the following web site:

http://www.usatoday.com/graphics/weather/gra/gsurge/flash.htm

(with audio) “Hurricanes, Creation of dangerous storm surge”

This link takes you to an animation about hurricanes and storm surge. Be patient while the tutorial loads. Click through all seven frames of the lesson. Read the captions at the bottom of each frame. This tutorial will be helpful for understanding pages 4 and 5 of the lab exercise! Question 1. What two factors cause the bulge of water to develop?

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PART I. Coastal Erosion as a result of hurricanes INTRODUCTION

Hurricanes are tropical cyclones with winds that exceed 64 knots (74 mi/hr) and circulate counter-clockwise about their centers in the Northern Hemisphere (clockwise in the Southern Hemisphere.

Three types of damage: 1. wind damage 2. inland flooding due to heavy rain 3. storm surge at the coastline

This part of the lab exercise will examine coastal erosion resulting from two hurricanes – Ivan (Sept. 2004) and Katrina (Aug. 2005). Both of these storms hit the Gulf Coast of the U.S.

a. b.

Location Map 1. a = approx. landfall for Hurricane Ivan, September 2004 b = approx. landfall for Hurricane Katrina, August 2005

Figure 1. Storm surge – a dome of water that sweeps across the coast near the point where the eye makes landfall. The most devastating damage in the coastal zone is usually caused by storm surge.

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Figure 2. Path of Ivan and Katrina.

ABOUT THESE HURRICANES (from: http://www.nhc.noaa.gov) IVAN Ivan developed from a large tropical storm that moved off the west coast of Africa on August 31. It reached Category 5 strength on the Saffir-Simpson Hurricane Scale (see Appendix) three times before making landfall in Alabama as a Category 3 on Sept. 15-16. Widespread flooding resulted from Ivan’s rains, which fell on already saturated ground caused by Tropical Storm Bonnie and hurricane Frances that traversed much of the southeast in August and early September. An outbreak of 117 tornadoes developed over a 3-day period in the U.S. Storm surge of 10-15 feet occurred along the coasts from the Florida panhandle westward to Mobile Bay, Alabama. Portions of the Interstate 10 bridge system were severely damaged as a result of the severe wave action on top of the 10-15 ft. storm surge (see Color Plate 1). There was also a possible record observed wave height of 52.5 feet reported by the NOAA Buoy 42040 located in the north central Gulf of Mexico. KATRINA It was the costliest and one of the five deadliest hurricanes to ever strike the U.S. After reaching Category 5 intensity over the central Gulf of Mexico, Katrina weakened to Category 3 before making landfall on August 29. A precise measurement of the storm surge produced by Katrina along the northern Gulf coast is complicated by many factors, including the widespread failures of tide gauges. Additionally, in many locations, most of the buildings along the coast were completely destroyed, leaving few structures within which to identify still-water marks. An unofficial storm tide (actual level of sea water) observation of 28 feet at the Hancock, Mississippi Emergency Operations Center suggests that the storm surge produced by Katrina was as high as about 27 feet at that location. The surge appears to have penetrated at least six miles inland in many portions of coastal Mississippi and up to 12 miles inland along bays and rivers. The surge crossed Interstate 10 in many locations. Katrina produced a lesser but still very significant storm surge along the eastern Gulf coast of Mississippi and along the coast of Alabama. Observations suggest the storm surge was about 10 feet as far east as Mobile, Alabama where Katrina caused flooding several miles inland from the Gulf coast along Mobile Bay. The storm surge strained the levee system in the New Orleans area. Several of the levees and floodwalls were overtopped and/or breached at different times on the day of landfall.

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Precipitation amounts during the landfall along the northern Gulf coast were greatest along and just west of the track of the center. A large swath of 8-10 inches of rain fell across southern Louisiana and southwestern Mississippi, with a small area of 10-12 inches over eastern Louisiana. Katrina produced a total of 33 reported tornadoes. Questions A. Storm Surge 1. On the map below – (a) Draw arrows to indicate the motion (circulation) of the hurricane. (b) Explain the dashed water line (this is the water line during a hurricane).

2. Examine color Plate 1. Read the details in the map graphic. Where is the highest storm surge recorded and why would it be in this location? (hints - is this to the east or west of the hurricane eye, and how would that influence levels; what about local topography)

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3. According to the graphic on color Plate 1, the strongest winds passed over which area? B. Coastal Erosion along an island – Dauphin Island, Alabama Studying coastal erosion using laser technology - NASA's Airborne Topographic Mapper (ATM). The ATM was developed for climate change applications involving annual surveys of the Greenland ice sheet, but is also ideal for surveying the topography of beaches. As the aircraft flies along the coast, a laser altimeter scans a several hundred meter swath of the earth's surface acquiring an estimate of ground elevation every few square meters. Change is quantified by comparing pre-storm to post-storm surveys. Airborne scanning laser surveys are providing unprecedented data to investigate the magnitude and causes of coastal changes that occur during severe storms.

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4. Examine the poster of Dauphin Island, images A and B from Hurricane Ivan. Ivan made landfall 50 km to the east of this island (at Gulf Shores, AL). The “mounds” on the radar image are not sand dunes – they are houses. What is the elevation of the tallest house? (see map legend at lower corner of poster) __________ meters __________ feet 5. Explain in a few sentences what happened to the island as a result of Ivan, images A and B (there are two very noticeable changes). You might also look at the accretion/erosion image E (note green is for accretion). 6. Examine images C and D from Hurricane Katrina. Both images are after the storm. Katrina made landfall 150 km west of this island. What has happened to the houses? How many? How has the shape of the island changed?

(look also at image F illustrating accretion/erosion) C. Hurricanes, Natural Barriers, and New Orleans Read the poster “How the defenses break down” on the bulletin board. What are the natural barriers to storm waves, and how do they protect a coastal city?

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PART II. SANTA MONICA BEACH -- COASTAL ENGINEERING Reference: The Lab Book: Problem Solving in Geology, 2nd ed., 2000. Using the three photos provided by your TA: About 1905, a pier was built at the end of Colorado Avenue (photograph figure A). Almost immediately, it was obvious that the pilings of the pier were interfering with the longshore drift. The beach widened as sand built outward from the pier. Notice on each photograph a baseline (R-S-T-U) has been drawn. This represents the shoreline before the pier was built. Question 1. Examine figure A and determine how much the beach has grown outward around the pier. To do this, measure the distance from the baseline to the seaward edge of the sand at the edge of the pier point S. Use the scale provided with figure C, and make a paper ruler. How much has the beach grown at this point? Question 2. In 1934, the City of Santa Monica completed a breakwater seaward of the pier to provide safe anchorage for pleasure boats. The engineers thought that a breakwater that was not attached to the shore would permit sand to be transported along the beach while providing protection for small boats. Were they right – let’s find out. Using reference points R-S-T-U shown on the figures, determine how much the beach has grown at each point and record it in the following table. Make a paper ruler using the scale on the figure. Date Width of beach in feet at reference points R S T U Jan. 1928 Dec. 1937 Oct. 1949 Question 3. Describe and explain what has happened as a result of the construction of the breakwater. Question 4. Determine the rate of advancement of the shoreline at point T during the period from Dec. 1937 through Oct. 1949 (142 months). Your answer should be in ft/yr.

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Question 5. Assume that the rate of sand supply (from rivers) is constant, would you then expect the rate of advancement of the shoreline to be constant? Explain. Question 6. Engineers now understand that unconnected breakwaters built at a distance from the shore that is less than three times their length are likely to be joined to the shore by deposition in the wave shadow behind the breakwater. a. What is the length of the breakwater in feet? b. What is the distance from the breakwater to the 1928 shoreline? (hint – measure on figure B from the breakwater, along the pier, to the small “x” . This “x” represents the edge of sand in 1928) c. Given the rate of sand growth – how long (years) do you think it would be before the breakwater and the shoreline joined together? d. Would it have been possible to predict what would happen if the principle stated above had been understood at the time of construction of the Santa Monica breakwater? Question 7. Can you suggest the predominant direction of longshore current and drift – is it from the north, east, south or west?

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