GLY 4734 - COASTAL GEOMORPHOLOGY NOTES DR. PETER N. ADAMS 1. Introduction and Basics (Motivations, Classification, Landforms, Sediments, and Littoral Cells) This course is about a narrow, yet very dynamic and important portion of the Earth s surface - the coastal zone. The coast is a unique setting because it sits at the interface of two very different environments, the terrestrial surface and the ocean, each of which are governed by very different sets of physical processes. Because of this unique position in the landscape, the coast is shaped by physical processes native to both land and sea, and a major portion of the course will be devoted to understanding the workings and results of these physical processes. In this section, I ll outline some of the reasons for studying coastal geomorphology, recapitulate a brief history of some notable past thoughts on the subject, discuss some successful classification schemes, identify key features of passive and active margin coasts, and broadly overview the currency of the coastal realm (sediment), its movement through littoral cells, and its accounting (for) in budgets. 1.1. Importance of Studying Coastal Geomorphology. 1.1.1. Population, Development, and Infrastructure. 30 coastal states contain 62% of US population and 12/13 largest cities 53% of US pop. lives w/in 50 miles of the shore (83% in Australia) in 1973: 440,000 km of global coastline / 3.3 billion humans = 13 cm each today: 440,000 km of global coastline / 6.7 billion humans = 6.5 cm each Date: January 2010. 1
1.1.2. Natural Hazards, Climate Change, Global Biota. Hurricanes: 2004 and 2005, examples of coastal change on Dauphine Island after Ivan and Katrina Inundation from eustatic sea level rise Sea cliff retreat from climate induced changes in storminess Biota, habitat, and coastal ecology - the coastal ecosystem is diverse, unique, and valuable. Understanding of coastal processes assists our ability to protect this resource. 1.1.3. Geologic History and Building of the Stratigraphic Record. Coasts are modern depositional environments which illustrate the processes of erosion and sedimentation By understanding the distributions of facies created in these depo-environments, we are well equipped to interpret the stratigraphic record, and therefore, better explain the geologic history of a region 1.2. Historical Perspective. This comes largely from pp.9-19 of Woodroffe (2002). (1) Greeks (Herodotus) and the Nile delta (2) da Vinci and the Pontine Marshes in Italy - 15th century (3) Captain Cook s voyages shed light on many coastal reaches worldwide (4) Early geologists (Hutton, Lyell) recognized the coast as a modern depositional environment responsible for sedimentary rock generation. (5) Early geomorphologists (de Beaumont, Huxley) identified the link between process and form along the coast. (6) Darwin theorized on the origins and evolution of reefs and atolls. (7) Even Grove Karl Gilbert (our main man in American geomorphology) interpreted abandoned terraces in Utah to testify to the presence of Pleistocene lakes in the western U.S. (8) Early 20th century coastal evolution theories included Douglas Johnson s application of William Morris Davis s geographical cycle concept that interpreted landforms as initial, youthful, and mature, which he used to explain observations along the New England coast 2
(9) An enormous step forward in understanding coastal geomorphology and processes took place during World War II and in the Cold War era which followed. At this time, significant Naval resources were invested in detailed studies of wave shoaling, nearshore currents, sediment transport, and beach behavior. Much of this work was conducted by scientists at Scripps Institution of Oceanography (namely Harald Sverdrup, Walter Munk, Francis Parker Shepard, and Douglas Inman) during the middle of the 20th century. We transition by discussing a bit more of the history of coastal science in the next section on coastal classification. 1.3. Coastal Classification. An incomplete summary of coastal classification schemes is provided on pp. 38-43 of Komar (1998). 1.3.1. Submergence vs. Emergence. Johnson (1919, 1925), mentioned earlier for his application of Davisian principles, was also responsible for championing a coastal classification scheme based on relative rise (submergence) or fall (emergence) of the sea relative to the land. Submergent coasts display drowned features such as flooded coastal river valleys, abandoned barrier islands, or fjords. Emergent coasts exhibit raised features such as elevated marine terraces (formerly wave cut platforms), dry coral reefs above sea level, and wave cut notches in sea cliff walls. 1.3.2. Tectonic Classification of Coasts. Just like so many other disciplines in the Earth sciences, the subject of coastal geomorphology may be viewed in the context of our grand unifying theory of Plate Tectonics. The classic paper by Inman and Nordstrom (1971) sets up the organization scheme for large-scale coastal geomorphic character based on tectonic setting. Gross topographic character of coastal zones is related to position on plates of the tectosphere. First order classification consists of three classes: Collision edge (a.k.a. Leading edge), Trailing edge, and Marginal sea. Along active continental margins, the following scenarios can occur: (1) At continent-continent convergent margins, landlocked mountainous terrain forms (example: Himalayas and Tibetan plateau), so no coast forms to be classified. 3
(2) At continent-oceanic convergent margins, subduction takes place forming mountainous terrain on the overriding continental plate, a narrow continental shelf, and a deep trench at the plate interface (example: west coast of South America). This configuration forms the first sub-class of Collision coasts known as Continental collision coasts. (3) At oceanic-oceanic convergent margins, island arcs form (examples: Japan, Aleutian Islands). This configuration forms the second sub-class of Collision coasts known as Island-arc collision coasts. Along passive continental margins, the following scenarios can occur: (1) New trailing-edge coasts may form near a young (recently emerged) spreading center or rift (examples: Gulf of California, Red Sea). This configuration forms the first sub-class of Trailing-edge coasts known as Neo-trailing-edge-coasts. (2) Where a passive continental margin has another passive margin on the opposite side of the same continent, the potential for terrestrial erosion and deposition is low, because there is no major elevated sedimentary source terrain (mountain belt) whose erosional products could contribute a sediment load to enhance the dynamics of the coastal environment (examples: East and west coasts of Africa). This configuration forms the second sub-class of Trailing-edge coasts known as Afro-trailing-edgecoasts. (3) Where a passive margin is opposite an active margin (Continental collision coast), significant sedimentary load is provided to the trailingedge-coast due to abundant source terrain and disproportionately large drainage basins from the skewed distribution of continental topography (examples: East coast of North America). These coasts can build wide continental shelves and may exhibit extensive barrier island systems, due to the large sediment load present.this configuration forms the third sub-class of Trailing-edge coasts known as Amero-trailing-edge-coasts. Along continental margins fronted by an Island arc, significant protection from the open ocean exists, thereby changing the inherent characteristics of the coast. These are Marginal sea coasts and form a separate first-order class. 1.3.3. Shepard s Classification. As noted byinman and Nordstrom (1971), secondorder features of local geologic influence, and erosion/deposition (of spatial scale 100km), can also be used to classify the coastal landscape. Francis Parker Shepard, another celebrated coastal researcher from Scripps, offered the scheme where primary 4
agents (geologic) provide the framework for coastal configuration, and secondary agents (marine processes) modify that framework. A valuable website which offers more information on Shepard s classification scheme is: http://geology.uprm.edu/morelock/morphol.htm 1.4. Depositional Coastal Features. As stressed in lecture, depositional features are not the only type of coastal landforms along tectonically passive margin coasts, but there is a preponderance of them there. It is not uncommon to see barrier islands, spits, cuspate forelands, capes, and chenier plains (beach ridge sets) along passive margins. Here we explore some of the theories of the formation of these sedimentary, constructional landforms. This material is covered on pp. 25-38 of Komar (1998). 1.4.1. Cuspate Forelands and Capes. Large-scale example: Carolina Outer Banks - Cape Hatteras, Cape Lookout, Cape Fear Small-scale example: Rhythmic Cuspate Features of Nantucket Harbor 1.4.2. Spits and Tombolos. 1.4.3. Barrier Islands and Inlets. Barrier island formation theories (1) debeaumont (1845) presented a theory based on cross shore sediment transport, whereby continued deposition of a submarine bar (originating from wave breaking) eventually accreted to an elevation above sea level, creating a barrier island. (2) In the famous USGS Monograph 1 by Gilbert (1890), on Lake Bonneville a theory is presented which offers an explanation that barrier islands were the result spit growth from longshore sediment transport. (3) McGee (1890) argued that the drowned river valleys along the east coast of North America testified to the submergence of the shoreline, which he claimed explained the formation of barrier islands unconnected to the mainland. Hoyt (1967) also supported this theory. 5
Washover - Landward migration of barrier islands in response to sea level rise occurs during intervals of raised water surface elevation (usually during a storm surge, at high tide, when large waves provide high wave set-up). Sustained, landward migration of the barrier form will eventually cause previous back-barrier sediments to become exposed in the foreshore, such as the peats exposed at Cape Hatteras (Komar (1998), Fig. 2-21) or at Cape Canaveral, FL. Transgressive vs. Regressive sequences are visible in the stratigraphic record and document the seaward or landward migration of depositional environments, respectively. Beach ridge sets - These ubiquitous landforms lack a clear explanation of their formation, preservation, and evolution. Otvos (1969) proposed that changes in wave directions and sediment supply were primarily responsible for the production of beach ridge sets in the SE Louisiana chenier plain. Discussion of Inlet dynamics and relationship of spacing to headland or sedimentary source 1.5. Erosional Coastal Features. Features that are characteristic of tectonically active coasts, are often considered erosioal features, meaning that the sedimentary cover has been stripped away to expose the bedrock. However, these coasts tend to accumulate sediment in specific locations, such as the characteristic sedimentary carapace on the top of an uplifted marine terrace, or within the sheltered recess of a pocket beach, features discussed further below. This material is covered on pp. 16-25 of Komar (1998). 1.5.1. Sea Cliffs. 1.5.2. Uplifted Marine Terraces. Often appearing as flights of stairs occupying several 10 s of km adjacent to an active margin coastline, these features commonly form from the interplay of surface uplift and oscillating sea level. The process of marine terrace generation and degradation (by fluvial and hillslope processes) has been investigated by Anderson et al. (1999), whose animation we view in class. 1.5.3. Arches and Sea Stacks. 1.5.4. Pocket Beaches. 1.6. Beach Sediment. 6
1.6.1. Composition. The mineralology (chemical makeup) of individual grains of sediment on beaches is largely a reflection of the source region. An area of the terrestrial landscape where felsic rocks are eroding (granites as an igneous example and quartz sandstone as a sedimentary example), will produce siliciclastic beach sediment composed of quartz and feldspar grains, though feldspars are notoriously vulnerable to decimation during transport, so the quartz is typically all that remains. In contrast, mafic source terrain will produce beach sediments rich in heavy minerals such as hornblende, garnet, epidote, tourmaline, zircon and magnetite, which often accumulate as concentrated dark layers interbedded among quartz rich beaches. The process of determining the integrated source terrain from a small sample of sedimentary product, accumulated on a beach for example, is known as a provenance study. This material is covered on pp. 48-53 of Komar (1998). 1.6.2. Texture. This topic is covered in detail on pp. 53-56 of Komar (1998). Here s the relationship for calculating phi size: (1) φ = log 2 d d o and the Udden-Wentworth scale: [insert grain size table here - table 3.2 on p. 53 of Komar (1998)] 1.6.3. Sorting of Sediment. Sorting is a useful component of sediment analysis due to the relationship that exists between grain size and the energy of the depositional environment. High energy environments typically leave coarse-grained deposits, whereas low energy environments are identified by a fine sediment character. A review of sorting processes and distributions is provided on pp. 56-66 of Komar (1998). Cross Shore Distribution of Sediment Size - Sediment size is strongly related to cross-shore position on the beach, through the interaction of waves and water depth. The coarsest grains (with highest standard deviation and most negative skewness) are typically found at the plunge point (where wave breaking occurs) on the foreshore. Examples of cross shore grain sized distributions from Lake Michigan and Duck FRF in North Carolina are provided on pp. 56-57 of Komar (1998). Longshore Distribution of Sediment Size - A fundamental study which related longshore increase in grain size to a parallel longshore increase in wave energy was done by Bascom (1951) at Halfmoon bay, California. In this study, it was shown that a headland shelters the north portion of the bay from large waves resulting in a gently-sloped beach composed of fine sediment. 7
The beach profile is increasingly more steeply-sloped, and the mean sediment size coarsens, with distance from the headland, reflecting the more intense nearshore wave climate witnessed by the more exposed portion of coast. Longshore Sorting by Shape - Roundness or angularity of grains can also be used to deduce the amount of abrasion that sediments have undergone in the surfzone, and the longshore distribution of shape can illustrate the longterm direction of longshore sediment transport as discussed on pp. 62-66 of Komar (1998). 1.6.4. Settling Velocity and Stokes Law. When sediment size was discussed earlier, we presented a fairly straightforward scale with cutoff diameters distinguishing boulders from cobbles from pebbles from granules from sand from silt from clay. However, we note that in the coastal environment, where nearshore currents are often turbulently swirling about, it is the hydraulic equivalent size that is important, because that is what will determine whether a grain will be held aloft, or settle to the bed. This means we must understand something about how sedimentary grains settle. Get ready for the derivation of Stokes Law: As an aside, think about what happens to a skydiver: After leaping from the plane the skydiver accelerates downward, but not indefinitely. At some point, the skydiver s downward gravitational force is balanced by an upward directed drag force on his body. When this occurs, the velocity is constant and no further acceleration occurs - this is known as Constant Terminal Settling Velocity, or Fall Velocity. The same thing happens to sediment grains. Imagine we have a spherical object, of diameter d, that is settling through a column of fluid (e.g. air, water, ketchup, etc.). Newton s 2nd Law (F=ma) suggests that the sum of the forces acting on a body are equal to the time rate of change of momentum: (2) (mv) t = (F ) In this relationship, the sum of the forces are zero, so we get: (3) F downward = F upward or stated another way, (4) F grav = F drag 8
So now all we have to do is define the gravitational and drag forces. easy: Gravity is (5) F grav = mg = ρv g = (ρ g ρ f )V g defined as a function of the difference in particle density and fluid density (accounting for the buoyancy force), particle volume (sphere, in this case), and gravitational acceleration. The drag force is a bit more complicated. This quantity can be thought of as the time rate of change of kinetic energy of the fluid intersected by the settling particle. This volume, and mass, of water coming to rest by colliding with the particle, per unit time, are, respectively: (6) V f = va (7) m f = ρ f va Therefore the time rate of kinetic energy loss, or power loss, is: (8) (KE) t = P = 1 2 (ρ fva)v 2 Stated another way, the time rate of change of kinetic energy is equal to the power loss which is equal to the product of the drag force (as now defined) and the velocity of the settling particle: (9) P = F d v At this point we can solve for the drag force, and by recognizing that we made an assumption of fluid stopping at intersection with the particle, rather than moving around the particle, we introduce a constant called the drag coefficient, which must be determined empirically. (10) F d = 1 2 C dρ f Av 2 We are ready to equate the gravitational and drag forces. 9
(11) (ρ g ρ f )V g = 1 2 C dρ f Av 2 At this point, we simply define the volume (V ) and the cross-sectional area (A) of the settling particle, plug in these definitions, and solve for settling velocity, v. (12) V = 4 3 πr3 = πd3 6 (13) A = πr 2 = π( d 2 )2 (14) v 2 = 2(ρ g ρ f )V g C d ρ f A = 2(ρ g ρ f )πd 3 g 6C d ρ f π( d 2 )2 = 4(ρ g ρ f )dg 3ρ f C d And finally, (15) v = 4(ρ g ρ f )dg 3ρ f C d But we aren t quite there yet. The coefficient of drag is still relatively undefined in the above equation, so we turn to the experiments of fluid mechanicians to understand that coefficient s behavior. We introduced the dimensionless number, representing the ratio of inertial forces to viscous forces, as the Reynolds Number (Re), which was a function of dynamic viscosity (η), and several parameters already defined: (16) Re = ρ fdv η Recall from our in-class discussion of the relationship between C d and Re, that in the laminar flow range (Re < 20), the drag coefficient follows the roughly linear inverse relationship: (17) C d = 24 Re This simplifies our equation, within the Stokes Range for sediment, to: 10
(18) v = 1 18 gd2 ρ g ρ f η 1.6.5. Applications of the Settling Velocity. Bagnold s concept of autosuspension and the Dean number, consider the particle size, as describd by its settling velocity and the incoming wave field. If a particle is of sufficient diameter, to have a rapid enough settling velocity to travel down through the fluid column in the time permitted between fluid motions of successive waves, then deposition will occur. This important consideration is covered in more detail on pp. 55-56 of Komar (1998). 1.7. Littoral Cells and Sediment Budgets. The final introductory section covers the behavior of sediment routing throughout the littoral system. A littoral sediment budget is simply an application of the principle of conservation of mass. The balance between the rate of sediment entering a leaving a control volume within the littoral system will dictate whether erosion or accretion is occurring within that control volume. The various credits and debits in a littoral sediment budget are covered in detail on pp. 66-72 of Komar (1998). References Anderson, R., A. Densmore, and M. Ellis (1999), The generation and degradation of marine terraces, Basin Research, 11 (1), 7 20. Bascom, W. N. (1951), The relationship between sand size and beach-face slope, Transactions of the American Geophysical Union, 32, 866 874. debeaumont, L. E. (1845), Lecons de geologie pratique, 7me Lecon-Levees de Sables et Galets. Paris. Gilbert, G. (1890), Lake bonneville, US Geol. Survey Monograph 1. Hoyt, J. (1967), Barrier island formation, Bulletin of the Geological Society of America. Inman, D., and C. Nordstrom (1971), On the tectonic and morphologic classification of coasts, The Journal of geology, 79 (1), 1 21. Johnson, D. W. (1919), Shore processes and shoreline development, p. 584. 11
Johnson, D. W. (1925), The new england-acadian shoreline, p. 608. Komar, P. D. (1998), Beach processes and sedimentation, 2nd ed., p. 429 pp. McGee, W. J. (1890), Encroachments of the sea, The Forum, 9, 437 449. Otvos, E. (1969), A subrecent beach ridge complex in southeastern louisiana, Bulletin of the Geological Society of America. Woodroffe, C. D. (2002), Coasts: form, process and evolution, p. 623. 12