Geography
Depositional Landforms
Depositional landforms are physical features on the Earth's surface formed by the deposition of sediment by wind, water, or ice. These landforms include deltas, sand dunes, beaches, and alluvial fans, and are created through the accumulation and settling of eroded material. They play a crucial role in shaping the Earth's landscapes and are often associated with the processes of erosion and sedimentation.
Written by Perlego with AI-assistance
6 Key excerpts on "Depositional Landforms"
Learn about this page
Index pages curate the most relevant extracts from our library of academic textbooks. They’ve been created using an in-house natural language model (NLM), each adding context and meaning to key research topics.
eBook - ePub- Simon Bell(Author)
- 2012(Publication Date)
- Routledge(Publisher)
- The places where the rock is prone to weathering over the timescale of the processes being considered. Soft rocks may be eroded quickly by water as in the badlands of the USA or along river valleys.
- Examples of unique geological formations that have exerted a major influence on the geomorphology. Igneous intrusions, volcanic plugs and lava flows can determine the structure and the character of a landscape. The city of Edinburgh in Scotland is a prime example of this (see Chapter Eight).
- Examples of geological formations, often deep down in the earth, such as oil, coal or minerals of economic value which have led to certain settlement or industrial patterns as well as large-scale disturbance and modification of the surface through mining or quarrying.
Maps of superficial deposits may already include references to the types and origins of landforms that comprise them, such as moraines, eskers, lacustrine or lake sediments and so on. If the Depositional Landforms are sufficiently large in size, they will register on the topographic map as contours, depending on the map scale. The correlation between landforms of erosional and depositional origin is important for later analysis of ecological and cultural patterns. Depending on the scale of analysis, it may be useful to delineate the main groupings of types, such as kame-and-kettle, basket-of-eggs, estuarine structures or flood plain morphology, as these groupings may define later subdivisions of landscape character and ecological types.Erosive structures carved out of the solid geology will display a number of forms, partly dependent on the characteristics of the parent rock. There will also be numerous places where the topographic map correlates with the map of geological structures when overlaid, because the weaknesses provided by jointing, faults or discontinuities are exploited by the erosive agent and show up as variations in topography. There will also be places where rock changes produce no noticeable alteration in the topography, soil or vegetation and therefore these can be ignored.
eBook - ePubLandscape Grading
A Study Guide for the LARE
- Valerie E. Aymer(Author)
- 2020(Publication Date)
- Routledge(Publisher)
Section TwoLandforms2.1 What are landforms?Landforms are the identifiable shapes that the earth naturally forms due to geological processes. As landscape architects, we mimic and manipulate these basic forms to create functional designs. The natural shapes we are most concerned with are plane surfaces, ridges, valleys, peaks and depressions. These are manifested differently throughout the world and are named by their vegetative and animal habitats. For example, a bog, a swamp, and a fen are different types of wetlands. The processes that form and sustain these different wetlands vary; and the types of bog, swamp, and fen vegetation varies, but the underlying shape they form is a depression, a low point in the topography. It is the underlying landform in all cases that we manipulate in landscape grading. These natural forms are most easily recognized on a larger scale (e.g. 100, 200, 500 scale). However, because these problems are at such a large scale, intricate grading is difficult. Identification of the landforms, knowing where to put the problem elements and calculating slope between contour lines becomes essential.2.2 WatershedsTogether, plane surfaces, ridges, valleys, peaks, and depressions form watersheds. A watershed is a natural drainage basin for a particular area. It carries surface runoff, water from rainfall events, from the highest regions of the watershed downhill until it collects in streams, rivers, ponds, and eventually the ocean.Watersheds can be divided into subdrainage basins where water flows to a particular stream or pond within a watershed or they can be linked together to form a larger watershed region. These larger watershed regions are not determined by state government boundaries, but often encompass several states or more than one country. Additional information about watershed processes can be found in the recommended reading. As illustrated in Figure 2.2–1
eBook - ePub- Ro Charlton(Author)
- 2007(Publication Date)
- Routledge(Publisher)
bajada. Fans are commonly found in dry mountain regions, where an abundant sediment supply is associated with extreme discharges and frequent mass movements. Frequent shifts are often seen in the position of the braided channels that cross the fan surface, although only part of the fan surface may be active during a major flood event. In long profile, the slope is steepest at the fan head, progressively decreasing along the length of the fan. There is also a downslope reduction in sediment size, although deposits are coarse and poorly sorted. Incision and fan head trenching is associated with decreases in sediment supply, or increases in slope. Such changes can be caused by tectonics, climatic variations, a fall in regional or local base level, or human activity. In the absence of external change, the progressive lowering of the landscape will also result in a decline in sediment yield over time.Arid fans are generally smaller and steeper than those found in humid regions, a large-scale humid example being the Kosi Fan on the southern Himalayan mountain front. This covers an area of 15,000 km2 and formed where the Kosi River descends onto the wide alluvial plain of the Indus. It has a very low gradient, only averaging 1 m km-1 at its head, with further decreases downstream (Summerfield, 1991).Deltas are found where sediment-charged flowing water enters a body of still water. They extend outwards from shorelines where rivers enter lakes, inland seas and oceans. In coastal areas deltas form where the supply of sediment is greater than the rate of marine erosion, although sediment is redistributed by coastal processes. The influence of fluvial processes tends to dominate in the case of lake deltas.CHAPTER SUMMARY
Work is involved in transporting water and sediment through the channel. The rate at which work is carried out is defined by the stream power, which is controlled by channel slope and flow discharge. Material is eroded from bedrock reaches by various processes, including block quarrying, abrasion and corrosion. The erosion of alluvial banks also provides a significant proportion of a river’s load and is carried out by a combination of pre-weakening mechanisms, fluvial action and mass wasting. Whether or not a given flow is able to entrain a stationary particle of sediment on the bed depends on the balance between driving fluid forces (lift and drag), and the resisting force imposed by the particle’s submerged weight. The effects of neighbouring particles are also important and include inter-particle friction, sheltering, packing and armouring. Rates of bedload transport are strongly influenced by bed sediment characteristics and shear stress distributions, which are highly variable over space and time. Rates of bedload transport can therefore be very difficult to measure or predict accurately. Rates of suspended load transport are influenced mainly by the supply of fine sediment, which is controlled by a number of factors, including climate. Suspended sediment is transported by processes of advection and turbulent diffusion. A related process, convection, refers to suspended sediment entrainment by large-scale vortices. Bedload is deposited when the local bed shear stress falls below the critical shear stress for a given particle. The deposition of suspended load occurs when the fall velocity dominates over turbulent diffusion. The fall velocity of a given particle is determined by its density, size and shape, and by the density and viscosity of the fluid. As a result, finer material tends to be deposited first, leading to sediment sorting. Various circumstances lead to sediment deposition, including reductions in slope and discharge, increases in flow resistance, and the divergence of flow around obstructions. Depositional Landforms range in size from small bedforms to vast floodplains.
eBook - ePub- Rattan Lal(Author)
- 2017(Publication Date)
- CRC Press(Publisher)
]A genetic classification, or the mode of origin, has some overlap with descriptive systems because different manner of origin may produce different forms. Examples of genetic systems include the classification of volcanic rocks,[ 18 ] the classification of glacial landforms,[ 19 , 20 ] and landslide types and processes.[ 21 ]LANDFORM DEVELOPMENTLandforms may be erosional or constructional in form, or a composite of both. Most landforms are the products of erosion, but many are formed by the deposition of sediments, by volcanic activity, or by movements from within the earth’s crust. Examples of depositional features include dunes, moraines, and spits. Examples of erosional features are water gaps, deflation basins, or arêtes. Lahars, lava plains, and volcanoes result from volcanic activity. The formation of many alluvial fans and subsequent episodes of erosion and sedimentation in the basin and range of the Western United States are triggered by tectonic uplift.LANDFORMS AND MAPPINGLandforms are part of a continuum of near surface features, soils, and surficial sediments. Bloom[ 22 ] describes a landform in a continuum as a unit of systematic analysis. When mapping, distinctive landform populations need to be delineated. The patterns are identifiable in the field with aids such as topographic maps, aerial photography, geophysical surveys, land use maps, and other available information.Delineating landforms can sometimes simplify soil mapping. For example, in traversing a hillslope from its summit to its toe, one might recognize several differing soil map units. The sequence may repeat itself on adjacent hillslopes resulting in a predictable pattern of soils that can be related back to a distinctive landform population.
- Gerd Masselink, Michael Hughes, Jasper Knight(Authors)
- 2014(Publication Date)
- Routledge(Publisher)
Patterns of erosion and deposition can be assessed using the concept of net sediment balance. If the sediment balance is positive (i.e. more sediment is entering a coastal region than exiting) deposition will occur, while a negative sediment balance (i.e. more sediment is exiting a coastal region than entering) results in erosion. The sediment balance can be qualitatively assessed using sediment budgets or quantitatively using the sediment continuity equation (Box 1.1). • Morphology – The three-dimensional shape of a landform is referred to as its morphology. Changes in morphology are caused by erosion and deposition. Mapping spatial and temporal changes in landform morphology enables a better understanding of how formation and destruction of landforms relates to changes in sediment budget. • Stratigraphy – As landforms develop over time, their evolutionary history can be preserved within the layered stratigraphy of sediments within the landform. As such, sediment stratigraphic sequences allow insight into past behaviour of coastal systems, but are only a partial record of depositional history because erosional or non-depositional events may form gaps or truncations in the record. Box 1.1 Sediment budgets and sediment balance Morphological change is the net result of sediment transport processes. Sediment budgets can be used to gain an understanding of the sediment inputs (sources) and outputs (sinks) involved, which are measured in sediment volume (Figure 1.10). Key components of the sediment budget are sediment fluxes, which are vectors representing the direction and amount of sediment transport, expressed as the quantity of sediment moved per unit of time (e.g. kg s -1 or m 3 yr 1, etc). Whether a sediment flux is considered input or output depends on the point of view. For example, the transport of sediment from beach to dunes entails a sediment loss (output) for the beach, but a sediment gain (input) for the dunes
eBook - ePub- I. Cojan, M. Renard(Authors)
- 2020(Publication Date)
- CRC Press(Publisher)
2. Wind bedfomnsDue to variation in wind direction, aeolian ripples and dunes exhibit greater diversity of form than bedforms developed in water.Characteristics common to different aeolian structures are the existence of a gentle slope on the stoss side and a steep slope on the lee side. During their migration, these structures preserve their forms as long as the wind parameters remain stable. The surfaces between successive bedforms are most often wavy and correspond to ancient sand dunes. When the surface is truncated or even semi-planar, this phenomenon is sometimes interpreted as due to the influence of a phreatic aquifer close to the surface11 .Fig. III.13 Storm deposits (after Brenchley, 1985).a: Ideal sequence.b: Distribution of facies along a profile perpendicular to the coastline.In aeolian dunes, sandy beds accumulate through two mechanisms: avalanche deposits and accretionary deposits. Avalanche deposits represent the essential process of migration.With the effect of grain creeping along the stoss side, height of the dune increases. When the critical shearing value is reached, the crest of the dune collapses. Consequently, the net rate of accumulation of particles is zero on the crest, attains a maximum on the lee side and decreases towards the foot of the latter. This differential accumulation along the lee side results in an increase in slope with time. Avalanches are triggered when the critical angle of repose (34°) is reached12 . The succession consists of thick, inclined (25–30°) layers in the form of tongues in which the sand is not well packed. These are in general planar inclined beds and the rare trough-structures are interpreted as fillings of erosional forms due to changes in wind direction.Accretionary deposits or horizontal layers
