Classification of North American Coastlines

GEOMORPHOLOGY

FINAL REVIEW

COASTS

Moving seawater can: a. ERODE b. TRANSPORT c: DEPOSIT sediment 1. Waves: Waves are wind-generated. Therefore, any stretch of water that has wind blowing across it can develop waves. Waves reaching the coast are wind-generated oscillatory waves. Water particles undergo an oscillation or orbit, which decreases with depth, becoming negligible at half a wavelength depth. When the wave approaches the coast the wavelength becomes smaller, while the wave height becomes larger. This steepens the wave until eventually it "breaks", sending a mass of water forward. This is how the energy of the waveform is transferred to the energy of moving water.

Wave Erosion The effect of the wave energy depends largely on the slope of the coast. 1.Where the coast is steep -> cliffs -> formation of a wave-cut platform (the cliff is undergoing parallel retreat and the wave-cut platform is the marine equivalent to a pediment); Differential erosion can form arches and erosional remnants called stacks. 2. Where the slope is gentle, the wave energy is dissipated over a wider intertidal zone, creating lower energy conditions and allowing sediment to accumulate, forming a wide beach. There is no cliff to erode, but sediment transport occurs in the form of longshore (or littoral) drift. Depositional features = spits, beaches, baymouth bars, tombolos, deltas, barrier islands.

Classification of North American Coastlines

A. TECTONIC SETTING: 1. Convergent coastlines: West Coast. Characteristics = erosion>deposition; relatively straight; mountainous; sea cliffs; rocky coasts; beaches in sheltered bays; marine terraces; narrow continental shelf; active volcanism; earthquakes. 2. Passive margin coasts: East Coast/Gulf Coast. Characteristics = deposition>erosion; low-lying; wide coastal plain + continental shelf; often submerged; common spits, barrier islands, beaches.

B. LOCAL PROCESSES: Local effects are superimposed onto the larger scale tectonic features. These processes are subdivided into PRIMARY COASTS - dominated by subaerial processes; and SECONDARY COASTS - dominated by marine processes.

1. Primary Coasts: a). Stream erosion coasts: areas dominated by well-developed stream networks which have been partly submerged by rising sea-level. b). Stream deposition coasts: at the mouths of large rivers, deltas may dominate the coastline e.g. Mississippi Delta c). Glacially eroded coasts: due to their ruggedness and great relief, submerged glacially-eroded valleys usually dominate coastlines, whether in the east or west. Fjords tend to be straight and very deep (glacially scoured).

2. Secondary Coasts: a). Wave eroded coasts: characterized by cliffs, stacks, arches, wave-cut platforms. b). Marine deposition coasts: characterized by beaches, barriers, marshes: in areas of wider coastal plain drowned by post-glacial sea-level rise.

Hurricane Impacts:

A hurricane is a tropical cyclone: a cyclonic storm that originates in the tropics. Tropical cyclones typically form over large bodies of relatively warm water – tropical seas. They derive their energy through the evaporation of water from the ocean surface, which ultimately condenses into clouds and rain when moist air rises and cools. Hurricanes are called typhoons in southeast Asia and cyclones in Australia.A cyclonic storm has low pressure at the center and counter-clockwise rotating winds. Typical hurricanes are about 300 miles wide. In the northern hemisphere, there are easterly winds in the tropics, so hurricanes move from east to west and generally curve to the north, striking the gulf and east coast of the U.S. A hurricane making landfall generates a storm surge that is greater on its right-front quadrant than its left-front quadrant, due to the counter-clockwise cyclonic circulation of winds.Category 3-5 hurricanes (on the Saffir-Simpson hurricane scale – wind speeds of 111 to 155+ mph) generate storm surges typically of 3-5 m in height.Hurricanes can erode, transport and deposit sediment because of the combination of storm surge and large waves driven by strong winds. Erosion removes sand from beaches and dunes. Pre and Post storm surveys can be used to assess erosion. Surveys can be conducted with LIDAR (Light Detection and Ranging).

Storm surge deposition: storm surges can reach many tens of km inland and transport sediment into nearshore environments, including coastal marshes. Sediment deposition causes marsh aggradation which counters sea-level rise and forestalls submergence.

Glacial Landscapes

The Quaternary Period is divided into 2 epochs - the Pleistocene (ca. 2 mybp – 10,000 ybp) and the Holocene (10,000 ybp - present). The Pleistocene was a time of colder climates that resulted in a number of extensive glaciations or ICE AGES. ALPINE GLACIATION, characterized by valley glaciers, in mountainous areas of the western U.S.; and CONTINENTAL GLACIATION, characterized by large unconfined ice sheets, over much of Canada and the northern states.

Landforms of Alpine Glaciation. EROSION is the dominant process in glaciated mountainous regions. The glaciers are confined to pre-existing steep valleys and tend to erode vertically. Wide U-shaped glacial troughs, hanging valleys, horns, cirques, truncated spurs, tarns and aretes are the result.

Landforms of Continental Glaciation. DEPOSITION is dominant in lowland areas at the margins of continental glaciers - the northern states in North America. This is where debris carried in the ice is released as the ice melts. Further back from the ice front, (mainly in Canada) erosion can occur by SCOURING of the surface by debris (rocks) carried at the base of the ice. General scouring of the surface removed much topsoil from these areas, which is slow to recover due to the cold climates. Scouring also creates small grooves or striations or lager troughs aligned with the direction of ice flow; many larger troughs filled with water to become lakes. Much of the landscape of the northern mid-west is dominated by depositional landforms. Deposition can occur directly from the ice - erratics, ground moraine, drumlins and end moraines - or from meltwater (mainly gravel, sand, mud, clay) in the form of outwash deposits and eskers.

APPLIED GEOMORPHOLOGY

(Note: the abstracts of the studies are provided below – you should read the article for a full review).

ASSESSING THE IMPACT OF WEIR CONSTRUCTION ON RECENT SEDIMENTATION USING CESIUM-137

Ten cores were obtained from a marsh developed along Mad Island Slough, Texas, upstream of a weir constructed in 1948. The cores were analyzed for cesium-137 to identify time-stratigraphic marker horizons and calculate recent sedimentation rates. The cesium-137 analysis provided a 1954 marker horizon in 9 of the 10 cores. A second marker horizon, present in all 10 cores, consisted of an abrupt downcore change in lithology from dark organic-rich muds to grey organic-poor sands. This transition was tentatively identified as coinciding with 1948 and the beginning of marsh sedimentation. Resulting sedimentation rates show that surprisingly little sedimentation has occurred behind the weir, averaging only 27 cm in almost 50 years. Sedimentation rates in the marsh declined from an average of 2.4 cm/yr in 1948-1954 to 0.32 cm/yr in 1954-1994. A similar trend of declining sedimentation has been documented for adjoining Mad Island Lake, suggesting that land use changes in the lake's watershed have reduced the sediment supply in recent decades. The results also suggest that the weir is not a very efficient sediment trap in this watershed.

URBANIZATION PRESSURE INCREASES POTENTIAL FOR SOILS-RELATED HAZARDS, DENTON COUNTY, TEXAS

The Dallas-Fort Worth region was the fastest growing metropolitan area in the US in the decade 1990–2000. Rapid urbanization accompanied this population growth. A GIS-based analysis of urban growth in Denton County revealed that 53% of new urban development was on soils rated of low suitability for urban uses by the Soil Conservation Service. This compares to only 15% of urban areas on low-suitability soils prior to 1990. These soils are considered poorly suited for urban uses because they are montmorillonitic expansive soils. Expansive soils are known to cause damage to structures, including slab foundations used extensively in new housing in the Dallas–Fort Worth region. Increased urban development on these soils has increased the potential for soils-related hazards.

SHORELINE EROSION AT MAD ISLAND MARSH PRESERVE, MATAGORDA COUNTY, TEXAS.

Mad Island Marsh Preserve, located on the shores of Matagorda Bay, Texas, has experienced considerable shoreline erosion along its boundary with the Gulf Intracoastal Waterway, since construction of the Waterway in the early 1940's. This study documents long-term shoreline change, based on the analysis of sequential aerial photographs dating from 1930, 1943, 1958, 1978 and 1991. The results indicate that the Gulf Intracoastal Waterway widened to about 3 times its original width over the 48 years studied, and that this widening has been accompanied by rates of shoreline retreat of up to 3.1 m per year on Mad Island Marsh Preserve. In addition to the loss of Preserve land, the erosion also threatens a number of wetland habitats on the Preserve. Erosion has diminished an area of Spartina marsh alongside the Waterway from 0.154 km2 in 1958 to 0.022 km2 in 1991. Erosive shortening of Mad Island Bayou - a tidal inlet connecting Mad Island Lake to Matagorda Bay - may cause increased salinity within the lake and the consequent loss of freshwater environments.

You should also review labs 7, 8,9 & 10.

Example Questions

1. Describe the effects of waves on a). steep coastlines, and b). gently-sloping coastlines.

2. Explain the difference between primary coasts and secondary coasts, giving 3 examples of primary coasts and 2 examples of secondary coasts.

3. Describe the major contrasts between landscapes of alpine glaciation and landscapes of continental glaciation (include typical glacial landforms in your answer).

4. What are outliers? Name three examples from the Denton area. How do you recognize outliers in the Denton area/how do they differ from their surroundings? Draw a cross section from west to east across Denton to illustrate local bedrock outcrops, outliers and their relationship to the underlying geology.

5. Explain the process of beach drift (alongshore sediment transport) and describe three resulting coastal landforms.

6. Explain the formation of a wave-cut platform; why is this analogous to formation of a desert pediment?

7. Explain, using examples, the classification of North American coastlines according the major divisions of TECTONIC SETTING and LOCAL PROCESSES, and the subdivisions of Primary Coasts and Secondary Coasts.

8. Outline the objectives, methods and main findings of the article on Metroplex soil hazards, focusing on where new urban growth is occurring and why this may be a problem.

9. With reference to the lab on Trinity River estuary marsh sedimentation, a. explain the relationship between total sedimentation-compaction (net elevation change) and elevation (include a sketch of the graph) b. why does total sedimentation-compaction fall (practically) to zero as elevation increases? c. explain the relationship between organic sedimentation rate and compaction rate (include a sketch of the graph).

10. Describe two examples of landforms created from direct deposition from a continental ice sheet and two examples of landforms created from meltwater deposition - what are their distinguishing features?

11. With reference to the paper on shoreline erosion at Mad Island Marsh Preserve: a. What is the cause and geomorphological evidence of shoreline erosion in this area? b. How might shoreline erosion increase salt-water intrusion into Mad Island Lake?

12. With reference to the paper, “Assessing the impact of weir construction on recent sedimentation using Cesium-137”: a. Explain, with the aid of diagrams, the cesium-137 dating technique. b. The study found evidence of declining sedimentation upstream of the weir and in Mad Island Lake - explain the likely cause of the reduction in sediment supply.

13. With reference to the paper on shoreline erosion at Mad Island Marsh Preserve: a. What is the Gulf Intracoastal Water Way? b. What is causing erosion of the preserve shoreline and what is the geomorphological evidence of the erosion? c. What technique was used to determine where and how much erosion is occurring?

14. With reference to the paper on shoreline erosion at Mad Island Marsh Preserve: explain why shoreline erosion is a threat to habitats in Mad Island Lake.

15. With reference to the article on Metroplex soil hazards: draw a sketch showing the four major bedrock outcrops in Denton County; explain the connection between bedrock, soils and urban potential.

16. Pecan Creek is a potential flood hazard for central Denton. Name and describe the function of two flood control structures that have been constructed to help prevent flooding (based on Lab 9).

17. Explain how stream erosion, bedrock outcrops and outliers have formed the major features of the landscape in the Denton area (based on lab 8).

18. Explain why hurricane impacts on the Gulf Coast (erosion and deposition) are greater to the east of landfall than to the west of landfall.

19. With reference to the paper on the magnitude of Hurricane Ike storm surge sedimentation, describe and explain a) the area with the greatest impacts, b) alongshore trends in the inland penetration and volume of deposits, c) the potential importance of storm surge deposits for coastal marshes.

20. What are Quaternary terraces? What are they made of? How did they form? Draw a topographic profile to illustrate a typical bluff, terrace and floodplain; indicate where bedrock and alluvium are found. What is a common commercial use of terraces? Name a terrace from the North Texas area (based on the field trip to Dallas).

Six of these questions will be on the final – you will select four to answer. All questions are worth the same.