Lab 12 Coastal Geology I. Fluvial Systems Hydrologic Cycle Runoff that flows into rivers = precipitation (rain and snowmelt) [infiltration (loss to groundwater) + evaporation (loss to atmosphere) + transpiration (water taken up by plants)] The location where a stream starts is called the headwaters, or its source. The location where a stream ends is called its mouth. Gradient is the steepness of a stream, or the change in elevation over a certain distance. Gradient is generally higher near the source, and lower near the mouth of a stream. Where gradient is higher, the river exerts more erosive force, and therefore its channel may cut ravines and canyons. Where gradient is lower, a river course will meander, or move across low-lying valleys for reasons detailed below. Discharge is the volume of water flowing through a stream over a certain amount of time. It is usually expressed in cubic feet per second, and is measured by finding the velocity of flow through a certain stream channel area. Diagram of a Stream Meander Erosion Levee Levee Backswamp Marsh Point Bar Cut Bank Levee Erosion Levee As water flows down the part of a river with low gradient, its flow has a greater velocity on the outside of the meanders at the cutbank, so that is where erosion of the riverbank occurs. It has a slower velocity on the inside of the meanders, so deposition occurs there at the point bars. These processes cause the river channel to meander, or move and change course over time.
When the river overflows its banks in a flood, it will deposit sediment on the riverbank. The natural buildup over time of this sediment creates a levee, which is the highest point near a river. A river s natural tendency is to take the shortest route to its mouth. During flood causing events, such as spring snowmelt or during rainstorms, a river may break through its levee at an erosional point in its course, and flooding will ensue in the low, marshy backswamp area behind the point bar. If the flood is severe enough, the river will abandon its meander for the straighter course through the marsh, thus cutting off the meander and perhaps leaving an oxbow lake, like the one at Lake Providence, LA. Because the water velocity drops at the mouth of a river, all the sediment it carries will drop out and get deposited there. That deposits and accumulation of sediment at a river mouth is called a delta. II. Coastal processes and Landforms Coasts are places where land and water interact. Emergent Coasts are progradational, they are being elevated and building out into the water, which can happen in two ways: Uplift is the rising of the land. Regression is the sea level falling. Submergent Coasts are retrogradational, they are flooding and receding, which can happen in two ways: Subsidence is the lowering of the land. Transgression is the sea level rising. A single coastline may exhibit both emergent and submergent features according to locally variable conditions. Some big factors in determining coastal type: Sediment Supply coastlines near the mouths of major rivers are generally progradational because of the large amount of sediments that those rivers carry and deposit in the river delta (e.g. Mississippi River, Nile River). Tectonic Activity coastal areas near tectonically active plate margins may subside or uplift according to plate movements, and coastal features will reflect these effects (e.g. California). Sea Level Changes It has been determined that sea level has changed in the recent past according to the cycles of ice ages (low sea levels) and interglacials (high sea levels). The climatic and oceanic factors affecting this cycle are complex and are being studied, but it is thought that small perturbations in the overall environment (e.g. greenhouse gases) can have large effects on factors contributing to sea level changes. Currently the earth is in an interglacial interval. Waves are the manifestation of water motion due to winds blowing across the water, and are important for the erosion or deposition of sediments along coastlines.
Crest is the top of wave Trough is the bottom of wave Height distance from trough to top of crest Wavelength distance from crest to crest Wave Base depth below wave crest at which wave motion ceases, equal to about half the wavelength Refraction bending of waves seen in map view, caused by the slowing of waves as they approach shore. wave map view Longshore current flows along the coast in direction which it is being pushed by the waves. Surf zone is the area where waves break. When waves enter water shallower than the wave base, they change form until, in the surf zone, they break, and water motion becomes turbulent. Longshore drift is the process in which sediment in the surf zone is carried in the direction of prevailing (or average) wave motion. Sand and gravel are shuffled from the mouths of rivers along the lengths of the beaches. This process can either build up or erode coastal features depending on the sediment supply.
Storm Surge is a bulge of water pushed towards a shoreline by high winds or low atmospheric pressure associated with a storm or hurricane. Depending upon the storm intensity, they can cause sea level to rise 2 to 24 feet, causing coastal flooding and the destruction of ephemeral coastal features. Barrier Islands are long and narrow islands of built-up and reworked sediments lying offshore of the mainland. They insulate the mainland coast from erosive effects of longshore currents and storms. Topography and Flood Hazards Use the New Orleans and Vicinity Topographic map to answer the following questions. DO NOT WRITE ON or ABUSE THE MAP! 1. What is the scale and contour interval of this map? Scale: Contour Interval: 2. Examine the topographic map of New Orleans. In the event of a break in the levee and a flood, what is a good location for a home? Use nearest streets, etc. to indicate the area you have chosen. Explain why you chose this location. 3. What is the worst location for a home? Why did you choose this location? 4. What general area of New Orleans has the highest elevation? What feature (or features) does this high elevation correspond to? 5. On the next page, sketch an accurate topographic profile across the river, from the intersection of Carrollton and Banks, through Audubon Park to Westwego. FIRST construct and label the profile with a vertical scale and horizontal scale. Label which end of the profile is Westwego. THEN use the New Orleans map for land elevations and the bathymetric map at the front of the room to estimate the profile of the river bottom. Put the water surface in the river at +10 feet elevation.
6. What is the relief across your cross section area? What is the deepest point in the river (in feet) along your profile? Quaternary Geologic Map of the Lower Mississippi Valley 1. What is the depositional area at the mouth of a river called? 2. What is the channel type of the Mississippi River? What does this suggest about its gradient in Louisiana? 3. What type of coastline is southeast Louisiana? Name some features that lead you to this conclusion, and describe what processes are occurring in different areas. 4. The Quaternary period is subdivided into two epochs in the map legend. What are they? What is the age of the oldest and youngest units on the Quaternary Geologic Map? 5. On the Quaternary Geologic Map, note the different ages of deposits in areas around the mouth of the Mississippi River in southeastern Louisiana. On the generalized map provided with the worksheet, different areas of deposition are delineated. Number the general areas from 1=oldest to 4=youngest, in terms of the most extensive deposits exposed in each area. Put the name of those deposits from the Quaternary Geologic Map onto the generalized map.
Mississippi R. North Chandeleur Islands Gulf of Mexico Isles Dernieres Timbalier Islands 40 miles 6. What do the depositional areas you have labeled represent? What do they suggest has happened to the course of the Mississippi River in the past? Based on the ages given, what is the maximum amount of time (in years) it has taken for these different areas to be deposited? 7. What offshore landforms are shown in red on the Quaternary Geologic Map and are associated with the edges of the depositional areas shown on your generalized map? According to the description, what do these features consist of? Barrier Island Morphology and Coastal Restoration Barrier islands are sometimes all that protects the bays and wetlands of southwestern Louisiana from infiltration and erosion by the Gulf of Mexico. Since the natural flooding of the Mississippi River is curtailed by the Army Corps of Engineers, new sediments are not supplied to the barrier islands, and this may accelerate their erosion. During the end of 1997, restoration efforts were conducted on Grand Terre Island. Revegetation was done to stop erosion and a seawall was constructed on the bayside (northwest side) of the northeast part of the island to provide a protected area to dump sand dredged from offshore deposits. In 1998, Hurricane Georges blew over the area. How did the storm surge affect Grand Terre Island?
The first map (Figure 2) shows two images of Grand Terre Island, taken in 1997 and 2000. The images were divided into three categories, and the area of each category (in hectares) was calculated. You must calculate the total land for each image and the change in each category from 1997 to 2000. Put your answers in the table. Category 1997 image 2000 image Change Water 727.73 736.11 Bare Land 75.25 56.65 Vegetated Land 121.71 131.50 Total Land 1. Based on your calculations and observations, what effect did the storm surge of Hurricane Georges have on the total amount of land on Grand Terre? 2. Which single category appears to have lost the most area? Were revegetation efforts successful in protecting land area? 3. Did the seawall (seen in the 2000 image) provide protection for the dredge dump area behind it? The table below is the result of a matrix analysis in which the categories and areas from the two images are combined to get specific information about changes from 1997 to 2000. Class 1997 2000 1 Water Vegetated Land Type of Change Area Land Gain 11.70 2 Water Bare Land Land Gain 13.96 3 Vegetated Land Water Land Loss 14.77 4 Bare Land Water Land Loss 19.69
1. What are the totals for both land gain and land loss? How much land has been lost or gained on Grand Terre? 2. What is the rate of land gain or loss per year for the time period covered by the two images? 3. Given the total land area of Grand Terre, how many years it might take for the island to be completely built up or eroded away if the rate you calculated is a good estimate of the yearly average? Use the matrix image combining the 1997 and 2000 images of Grand Terre to answer the following questions: 1. Where are some of the largest areas of land gain located? What might they have resulted from (restoration efforts, coastal processes, hurricane effects etc.)? Be specific. 2. Where are some of the largest areas of land loss located? What might they have resulted from (restoration efforts, coastal processes, hurricane effects etc.)? Be specific.
3. Based on your assessment of the Hurricane Georges storm surge effects on Grand Terre Island, how do you think a rise in sea level might affect the island? How might a rise in sea level affect southwest Louisiana? Well Log Problem The United States Geological Survey is involved in a Louisiana coastal restoration project, which requires a large supply of sand to place on barrier islands. There is a large amount of sand below the seafloor in certain offshore areas. In order to obtain a large amount of offshore sand from one location, test cores were drilled to determine where the largest sand deposits are. Use the attached well log information from the test cores to answer the following questions. 1. How long (depth below seafloor) is each well core? Hindenberg: Logenberry: Huckelberry: 2. How thick is the sand unit in each well core? Hindenberg: Logenberry: Huckelberry: 3. How deep below sea level is the top of each sand unit? Hindenberg: Logenberry: Huckelberry: 4. How deep below sea level is the bottom of each sand unit? Hindenberg: Logenberry: Huckelberry: 5. If the sand dredge can only dig down to 120 feet below sea level, at which well site will the most (thickest) sand be obtained?
Well Log Data (depth below seafloor listed in feet) Hindenberg Core Seafloor is 35 feet below sea level Logenberry Core Seafloor is 50 feet below sea level Huckelberry Core Seafloor is 65 feet below sea level 0' Seafloor Seafloor Seafloor 0' 0' 10' 10' 9' 10' 20' 20' 20' 30' 32' 30' 30' 32' 40' 40' 45' 40' 50' 51' 50' 50' 60' 60' 60' 70' 70' 73' 70' 70' 80' 80' 80' 88' 90' Gravel Sand Silty Mud Muddy Silt