Geomorphological Scales Research Articles

Deciphering Earth's Natural Hourglasses: Perspectives On Source-To-Sink Analysis

Abstract Production, transport, and deposition of siliciclastic sediments takes place across changing altitudes, physical processes, environments, and controls en route from source to sink commonly through a downstream narrowing and then broadening fairway of sediment grains, constituting giant “hourglasses” of nature, a fundamental unit of both geomorphology and sedimentology. Here we review the status of the rapidly evolving multidisciplinary source-to-sink approach, and compare it with the more mature sequence stratigraphic approach. The latter uses outcrop, well, and seismic data to gain information about elements of basin fills with less data coverage and is thus mainly a “sink-to-sink” approach. Source-to-sink on the other hand aims to understand the dynamics and budgets of the complete onshore–offshore sediment fairway, including elements of the transect that are no longer preserved. Furthermore, we summarize the spatial and temporal variability of source-to-sink systems, and discuss qualitative and semiquantitative methods for reconstruction of area, relief, and sediment supply from source terrains. The variability of source-to-sink systems is viewed in the framework of three end-member types, “steep, short, and deep,” “wide and deep,” and “wide and shallow,” where each is characterized by typical patterns of sediment partitioning and long-term preservation. Modern and sub-modern systems are keys to enhance our understanding of their ancient counterparts. Three different time-framework categories for source-to-sink analysis are presented: modern systems, pre-modern Quaternary systems, and pre-Quaternary systems, all of which have large differences when it comes to amount and type of data, controlling factors, accuracy in interpretation, and societal applications. Importantly, systems are evolving through time with the effect that estimations of source-area parameters for one period of time may change significantly into another when boundary conditions are different. Sink reconstruction can, with variable confidence, be established through the use of seismic-reflection, well, and outcrop data, whereas reconstruction of source relief, drainage, and sediment production is a more challenging task. Methods like landscape interpolation, sediment volume backfilling, geomorphological scaling relationships, sediment-load estimations from river data and from stratigraphy as well as geochemical data can, preferably in combination, be used to unravel past source terrains. These methods are presented along with a discussion on how they can improve models for basin fill. Sink and source reconstruction is a two-way process. Source reconstruction sheds light on sink understanding and vice versa. This integrated approach is a prerequisite for further advance in source-to-sink studies. The prognosis of source-area parameters may give additional insight into the complete erosional–depositional system in general and sediment supply in particular and hence enables us to arrive at more robust models and predictions for the sink where resources commonly are contained.

The normalized topographic method: an automated procedure for gully mapping using GIS

ABSTRACTGully delineation is a critical aspect of accurately determining soil losses but associated methodologies are rarely detailed. Here, we describe a new gully mapping method, the normalized topographic method (NorToM), based on processing digital elevation model (DEM) data, and we assess associated errors when it is applied over a range of geomorphological scales. The NorToM is underpinned by two gully detection variables (normalized slope and elevation) calculated over local windows of prescribed size, and a group of filtering variables. For four study sites, DEMs of gullies were obtained using field and airborne photo‐reconstruction and evaluated using total station and differential global positioning system (dGPS) survey. NorToM provided accurate areal and volume estimates at the individual gully scale but differences increased at the larger gully system and gully network scales. We were able to identify optimal parameters for using the NorToM approach and so confirm that is represents a useful scale‐independent means of gully mapping that is likely to be valid in other environments. Its main limitations are that the normalization process might be time‐consuming at regional scales and the need for a fixed window size when applied to landforms with extreme variations in dimensions. Copyright © 2014 John Wiley & Sons, Ltd.

Complex Interaction of Dendritic Connectivity and Hierarchical Patch Size on Biodiversity in River-Like Landscapes

Habitat fragmentation and land use changes are causing major biodiversity losses. Connectivity of the landscape or environmental conditions alone can shape biodiversity patterns. In nature, however, local habitat characteristics are often intrinsically linked to a specific connectivity. Such a link is evident in riverine ecosystems, where hierarchical dendritic structures command related scaling on habitat capacity. We experimentally disentangled the effect of local habitat capacity (i.e., the patch size) and dendritic connectivity on biodiversity in aquatic microcosm metacommunities by suitably arranging patch sizes within river-like networks. Overall, more connected communities that occupy a central position in the network exhibited higher species richness, irrespective of patch size arrangement. High regional evenness in community composition was found only in landscapes preserving geomorphological scaling properties of patch sizes. In these landscapes, some of the rarer species sustained regionally more abundant populations better tracking their own niche requirements compared to landscapes with homogeneous patch size or landscapes with spatially uncorrelated patch size. Our analysis suggests that altering the natural link between dendritic connectivity and patch size strongly affects community composition and population persistence at multiple scales. The experimental results are demonstrating a principle that can be tested in theoretical metacommunity models and eventually be projected to real riverine ecosystems.

Development of a one-parameter variable source area runoff model for ungauged basins

This research develops a one-parameter model of saturated source area dynamics and the spatial distribution of soil moisture. The single required parameter is the maximum soil moisture deficit within the catchment. The concept behind the development of the model comes from the fact that the complexity of topographically-driven runoff generation can be reduced through the use of geomorphological scaling relations. The scaling formulation allows the prediction of the dynamics of saturated source areas as a function of basin-wide soil moisture state. This model offers a number of potential advantages. Firstly, the model parameter is independent of topographic index distribution and its associated scale effects. Secondly, it may be possible to measure this single parameter using field measurements or perhaps remote sensing, which gives the model significant potential for application in ungauged basins. Finally, the fact that this parameter is a physical characteristic of the basin, estimation of this parameter avoids regionalization and parameter transferability problems. The model is tested using rainfall–runoff data from the 10.4 ha experimental catchment known as Tarrawara in Australia, the 37 km 2 Town Creek catchment in U.S.A., and the 620 km 2 Balaphi and the 850 km 2 Likhu sub-catchments of the Koshi river in Nepal. In sub-catchments of Koshi river, the simulation results compare favorably against the calibrated TOPMODEL both in terms of direct runoff and the spatial distribution of soil moisture state. In the Tarrawara and Town Brook catchments, simulation results compare favorably against observed storm runoff using all observed data, without calibration.

Engineering geomorphological input to ground models: an approach based on Earth systems

The engineering geomorphological input to ground (‘geo’) models as part of standard site investigations tends to lack a systematic approach and is often restricted only to morphological mapping, the naming of landform features and usage of outdated morphoclimatic concepts. Ground models require more sophisticated geomorphological approaches and this is particularly apparent in regions such as Europe and North America where ground conditions can be strongly influenced by geomorphological processes linked to Quaternary or older environmental changes. Within this paper it is proposed that an ‘Earth system science’ approach underpinned by the ‘rock cycle’ can provide a framework for a systematic and more detailed geomorphological input into ground models. Such a framework lends itself to recognizing the fact that landscapes comprise multiple ‘fragments’ of process–response landforms, which, under contemporary conditions, may be: relict and inactive; relict but reactivated; subject to change once certain threshold events occur; or fully active. Using the rock cycle, a simple basis for landform categorization is outlined starting with a four-fold subdivision into structural, weathering, erosional and depositional landforms that allows the user to focus initially on the historical development of the landscape. Each category comprises a system of landforms that occur over standard geomorphological scales (pico-, nano-, micro-, meso-, macro- and mega-scale) as part of a process–response system. Macro-scale landforms are readily identifiable within a landscape, typically occurring as six principal terrain types: hills, ridges, mountains, plains, valleys and basins. These in turn comprise a series of meso-scale features that, once identified, can inform about the geomorphological processes that have resulted in their formation. The geomorphological input into ground models benefits primarily from recognition of these meso-scale landforms and the processes responsible for their formation, although smaller-scale landforms may have importance for specific engineering structures.

Scale invariance analysis of channel network width function and possible implications for flood behaviour

An understanding of the scaling properties of precipitation and landforms (hillslopes, channel network) is, on its own, of theoretical and predictive significance. However, it is also expected that it should lead to the development of a physically meaningful flood scaling theory. The objective of this study is to identify catchment geomorphological scaling descriptors useful in establishing a criterion for similarity in regional flood behaviour. The focus is on the channel network. The nature of the scaling structure of the channel network width function is investigated. An attempt is made to determine the strength of the association between the scaling structure of the width function and at-site flood peaks. For catchments with drainage area greater than 200 km2, located east of Melbourne and the Great Dividing Range, this work established an association between the Levy index of the generators of the cascade structure of the width function and at-site flood behaviour. It is further suggested that such an index will be a useful candidate in the development of a criterion, which predicts similarity in regional flood behaviour.

Complexity and scale in geomorphology: Statistical self-similarity vs. characteristic scales

Two models of the relationship between complexity and scale of geomorphic lines are compared, one based on statistical self-similarity (in which complexity is invariant for some range of scale), and the other on the concept of characteristic scales (in which complexity changes continuously with scale). Two corresponding techniques are used in the comparison, fractal analysis utilizing the divider method, and an angle measure technique. These techniques are applied to three types of coastlines: fiord, volcanic, and tectonic, in order to ascertain which model, statistical self-similarity or characteristic scales, is more useful in understanding variations in coastline complexity for scale. Apparently linear log-log plots of number of steps against steplength produced by fractal analysis display slight but significant curvature. Upon closer examination, it is determined that using fractal dimension to compare even the same types of features is unreliable because of the dependency of fractal dimension on scale of measurement, even if the same steplengths are used throughout the study. These results are corroborated by the use of the angle measure technique, a method based on measuring angles between points along a digitized line. It is concluded that the coastlines examined display no evidence of statistical self-similarity and that the characteristic scales model is more useful in investigating complexity and scale in geomorphology.

MIDA, a distributed hydrological model: Its behavior under some different parameter and data discretization conditions

MIDA is a distributed model for hydrological processes at the basin scale, the structure of which is suited to small basins of mountainous Mediterranean coastal areas. The underlying philosophy of the model is a distributed conceptual approach in which geomorphological scaling relationships are used, fully supported by a Geographical Information System. The sensitivity of the model response to some variable conceptual and global parameters and its performance across some different land-use and climatological conditions are analyzed. The analysis reveals a high sensitivity to some parameters related to infiltration and runoff generation (soil cover thickness on hillslope and saturated hydraulic conductivity) and to land-use features, while change in discretization levels seems to have less influence on predicted discharge.

Problems of scale in the modeling and interpretation of coastal dunes

Coastal dune systems are studied at time scales from seconds to millennia, and space scales from millimeters to kilometers. Present approaches to the study of coastal dunes make it difficult to integrate models and interpretations of these systems over these scale ranges and arrive at reasonable conclusions. It is argued that identification of key controls on dune development, measurement of those controls, and synthesis of data, describing past and present conditions and used as calibration points, will improve the viability of coastal dune models. Theoretical and empirical advances are necessary to improve the reliability of predictions across a range of geomorphological scales. Attempts at linking theoretical (systematic) models, process (synoptic) measurement, and historical or paleoenvironmental (synthetic) approaches make explicit the recognition that at time scales of more than a few hours, and space scales greater than a few hundred meters, deterministic models become unstable vis-à-vis prototype environments. Process “climatologies” provide one means to link process-based work with broader-scaled analysis. As scales increase, such climatologies will become less appropriate as the data become less reliable, or as the systems change, or as the scales become too large relative to the process record lengths. In these conditions, specific data (i.e. samples) representing points in time and space become check points for calibrating models. It should be possible, ideally, to integrate both up and down time and space scales. This is not yet possible.

Flow separation—a physical process for the concentration of heavy minerals within alluvial channels

Flow separation is a common feature within alluvial channels that occurs at abrupt changes in the bed geometry. At a wide range of scales, it is a process that can exert considerable control over the segregation and deposition of heavy mineral grains. Fluid separation generates a region of high bed shear stress that can entrain heavy minerals, and a region of low velocity that is a preferred site for the deposition of denser particles. Significant concentrations of heavy minerals, sometimes up to eight times the background value, may be formed by flow separation processes. A review of the occurrence of flow separation illustrates that it may be responsible for the accumulation of heavy mineral grains at a range of geomorphological scales.

On size and scale in geomorphology

On size and scale in geomorphology

Temporal Scales in Coastal Erosion Systems

Three coastal cliff erosion systems on the east coast of England are used to illustrate the importance of spatial and temporal scales. These systems are described in terms of location, dominant processes and erosion rates. The time scales suggested by Schumm and Lichty (1965) are applied to these systems, and the existence of dynamic equilibrium conditions is emphasized. The relative importance of the catastrophic event (storm surge) and the moderate storm is discussed, and is shown to be a function of the time scale under consideration. SCHUMM and Lichty (1965) have examined the concept of scale in geomorphology, and have shown that the distinction between cause and effect in the development of landforms is a function of time and space. They applied these concepts to the study of river systems, and showed that the factors that determine the character of landforms can be either dependent or independent variables as the limits of time and space change. It is the purpose of this paper to apply the time scales and related concepts proposed by Schumm and Lichty to coastal erosion systems. The soft rock cliffs on the east coast of England will be used as examples; these cliffs provide a good starting point, for their short-term changes are appreciable. Current emphasis in geomorphology has indicated that systems analysis can provide a useful framework in which to set our abstractions of the real world (Harvey, 1969; Chorley and Kennedy, 1971). One of the many advantages is that a particular system may be divided into sub-systems which may be studied separately. This paper concentrates on a particular type of coastal system, the coastal cliff erosion system. This particular system may be divided into three sub-systems: the cliff, the beach and the offshore sub-system; all these will be discussed but particular emphasis will be placed on the cliff sub-system. The cliff sub-system may be defined as the zone extending io m landward of the cliff-top edge to the cliff toe, and the beach subsystem extends from the cliff toe to the mean low-water mark of spring tides. The offshore sub-system has been taken as extending from mean low-water mark of spring tides to a depth of 20 m, a distance of approximately 7 km along most of this coast. The main variable in the present study is the retreat of the cliff-top edge over a period of years. Inevitably the actual method used to measure the retreat rate is dependent upon the time scale; for instance in the short term (i to 2 years) cliff retreat may be measured in the field, while over the longer term (ioo years) cliff retreat rates may be obtained by comparing different map editions. Since sea level rose to near the present position (approximately 5000 years B.P., Jelgersma, 1971) no long-term value for cliff retreat exists. A study of the retreat rates for any length of cliff will show considerable variation in both spatial and temporal dimensions, and it is this variation that has yet to be explained in geomorphology. Location of the coastal systems The areas studied are all lengths of unconsolidated Quaternary cliffs (the term consolidation here is not used in the strict soil mechanics sense where it refers to a specific overburden weight), and are all on the east coast of England, bordering the North Sea (see Fig. i). The Holderness