Spatial Patterns Of Erosion Research Articles

Reach‐scale sediment transfers: an evaluation of two morphological budgeting approaches

AbstractThis paper compares two approaches used to derive measures of annual sediment transfers within a 1 km long piedmont reach of the gravel‐bed River Coquet in Northumberland, northern England. The techniques utilize: (i) channel planform and cross‐section surveys based on a theodolite/electronic distance measurement (EDM) survey of 21 monumented channel cross‐sections and channel and gravel bar margins; and (ii) theodolite‐EDM survey generating a series of x,y,z coordinates, from which digital elevation models (DEMs) of the reach were constructed. Calculating the difference between DEM surfaces provided a measure of volumetric change between surveys carried out during the spring of 1999 and 2000. The use of kriging in DEM generation and differencing permits computation of estimate variances and confidence intervals for sediment transfer. Error analysis, validating the DEMs using surveyed cross‐sections, indicated a mean error between surveyed and DEM‐generated cross‐sections of around twice the value of the D50 of the surface sediment in the reach. Comparison of sediment volumes derived from the two approaches suggests that, compared with the DEM method, monumented cross‐sections underestimate the magnitude of volumetric changes that occur within the reach. The cross‐section approach relies on a simplistic integration of the volumes, whereas DEM differencing provides an estimate at a resolution under the control of the analyst. Furthermore, the cross‐section approach does not permit a reliable estimate of the uncertainty of the volumes calculated. In addition, the DEM methodology based on the morphological unit scale provides an explicit identification of spatial patterns of erosion and deposition, a feature that cross‐section‐based approaches may fail to include. Copyright © 2003 John Wiley & Sons, Ltd.

Calibration of the LISEM model for a small Loess Plateau catchment

The Limburg Soil Erosion Model (LISEM) soil erosion model was calibrated for a 2-km 2 catchment on the Chinese Loess Plateau. The most important calibration factors were saturated conductivity and Manning's n. Calibration on catchment discharge was done by using the discharge peak (timing and discharge) followed by an adjustment for the total discharge to obtain the correct amount of sediment output. The results showed that LISEM can be successfully calibrated for a Loess Plateau catchment, and that small runoff events need to be calibrated separately from large runoff events. A separate calibration might even be needed for each event. The model performance was also evaluated using catchment wide spatially distributed data on rill erosion. Rill erosion intensity was mapped in the field and compared to spatial patterns of erosion predicted by LISEM. The simulated erosion patterns do show some resemblance with mapped erosion patterns in a general sense but they are very different in detail. The cause for this can be found in the extremely steep slopes and abrupt slope changes in the catchment. Some of the process descriptions in LISEM are not intended for such an environment, while the grid based kinematic wave routing cannot cope with the abrupt changes in flow conditions. The effects of this are amplified by inaccuracies in the input data and the DEM. For topographically complex catchments it will be very difficult to obtain data that are good enough for an accurate simulation of erosion patterns. This limits the use of a model such as LISEM as a predictive tool for future events. Simulation of different land use scenarios is less problematic, if a known event is used for all scenario simulations.

The case for rainfall on a warm, wet early Mars

Valley networks provide compelling evidence that past geologic processes on Mars were different than those seen today. The generally accepted paradigm is that these features formed from groundwater circulation, which may have been driven by differential heating induced by magmatic intrusions, impact melt, or a higher primordial heat flux. Although such mechanisms may not require climatic conditions any different than today's, they fail to explain the large amount of recharge necessary for maintaining valley network systems, the spatial patterns of erosion, or how water became initially situated in the Martian regolith. In addition, there are no clear surface manifestations of any geothermal systems (e.g., mineral deposits or phreatic explosion craters). Finally, these models do not explain the style and amount of crater degradation. To the contrary, analyses of degraded crater morphometry indicate modification occurred from creep induced by rain splash combined with surface runoff and erosion; the former process appears to have continued late into Martian history. A critical analysis of the morphology and drainage density of valley networks based on Mars Global Surveyor data shows that these features are, in fact, entirely consistent with rainfall and surface runoff. The necessity for a cold, dry early Mars has been predicated on debatable astronomical and climatic arguments. A warm, wet early climate capable of supporting rainfall and surface runoff is the most plausible scenario for explaining the entire suite of geologic features in the Martian cratered highlands.

Discharge and sediment supply controls on erosion and deposition in a dynamic alluvial channel

Research on dynamic alluvial channels has recognised the influence on river channel change of both discharge and sediment supply, although it has proved difficult to measure the latter. This paper presents the first accurate data from a dynamic alluvial channel that describe the interrelated effect on channel morphological change of both discharge and sediment supply variations over different timescales. Reliable information on the spatial patterns of erosion and deposition were obtained using combined photogrammetry and tacheometric survey of the meltwater stream of a glacierised catchment over a 5 week period. Over the full time-period, the morphological evidence suggested the passage of a wave of sediment. Counter-intuitively, the period of aggradation was associated with increasing diurnal peak and base flow discharge magnitude, while degradation occurred during a period of steady peak diurnal discharge. This suggests that the channel change was controlled by variation in upstream sediment supply. Over a shorter timescale, information on diurnal changes in morphology reveals that although discharge was responsible for some aspects of channel change, this was modified by the effects of upstream sediment supply. Consideration of the spatial patterns of erosion and deposition at the within-reach scale reinforces this point, with spatial aspects of morphological change being driven by discharge and sediment supply fluctuations, but modified by spatial feedbacks associated with internal channel morphology. The implications of these findings for fluvial geomorphology in general are considered.

Developments in monitoring and modelling small‐scale river bed topography

AbstractRecent research in fluvial geomorphology has emphasized the spatially distributed feedbacks amongst river channel topography, flow hydraulics and sediment transport. Although understanding of the behaviour of dynamic river channels has been increased markedly through detailed within‐channel process studies, less attention has been given to the accurate monitoring and terrain modelling of river channel form using three‐dimensional measurements. However, such information is useful in two distinct senses. Firstly, it is one of the necessary boundary conditions for a physically based, deterministic modelling approach in which three‐dimensional topography and river discharge drive within‐channel flow hydraulics and ultimately spatial patterns of erosion and deposition and therefore channel change. Secondly, research has shown that an alternative means of estimating the medium‐term bedload transport rate can be based upon monitoring spatial patterns of erosion and deposition within the river channel. This paper presents a detailed assessment of the distributed monitoring and terrain modelling of river bed topography using a technique that combines rigorous analytical photogrammetry with rapid ground survey. The availability of increasingly sophisticated terrain modelling packages developed for civil engineering application allows the representation of topographic information as a landform surface. Intercomparison of landform surfaces allows visualization and quantification of spatial patterns of erosion and deposition. A detailed assessment is undertaken of the quality of the morphological information acquired. This allow some general comments to be made concerning the use of more traditional methods to monitor and represent small‐scale river channel morphology.

Gullying Within Wetlands in Zimbabwe

Abstract Legislation in Zimbabwe restricts use of dambos, seasonally waterlogged botomlands characteristic of the central watershed region, because of fears of erosion and drying out of these wetlands. Erosion, especially gullying, is widespread today within dambos in peasant farmlands but not in areas of commercial farming, although most or this wetland type occurs in the latter areas. This historical context of the dambo gully problem is outlined in the context of the land use and conservation history of the commercial and peasant farming system. The incidence of erosion at national and local scales is described and evaluated in relation to selected human and physical factors. At a national level, population density is a key influence on erosion, whereas at a local level the incidence of livestock grazing is an imponant ractor influencing gully erosion in dambos. Physical factors are found to have limited significance in explaining the spatial patterns of erosion at national and local scales.

A model of the redistribution of disaggregated soil particles by rainsplash

AbstractA model of the dispersion of splash droplets from a single raindrop impact on a sloping soil surface is combined with a theory of the entrainment of mineral particles from a disaggregated mixture in splash droplets to obtain a model of the dispersion of such particles by a raindrop impact. Stochastic modelling techniques extend this further to a model of the spatial redistribution of soil on a plot after a period of rainfall.Since the model is probabilistic and physically based it enables the incorporation of further advances in the understanding of splash erosion at all stages and can simulate the effect of the stochastic nature of rainfall and soil properties on the process. Several different situations are simulated. These include the movement of marked soil particles from point sources and the spatial patterns of erosion on a sloping plot. The model can also simulate the differential erosion of different soil particle size fractions.

Random field modelling of spatial variations in erosion and deposition in flat alluvial landscapes in arid central Australia

Little work has been done on spatial patterns of erosion and deposition making it difficult to extrapolate point measurements of erosion rate. This paper describes how it is possible to detect, describe and possibly forecast such patterns on a broad scale using remotely sensed data from Landsat which have been transformed to produce an index of erosion and deposition intensity. A stochastic approach to modelling is used because natural systems are too complex for the currently available analytical model. The stochastic process used is a simultaneous autoregressive spatial model defined on a finite lattice. Methods for fitting this model, generating synthetic data and obtaining model input data by deconvolution for use in subsequent forecasting with another model are described. Studies of 100 × 100 pixel (45 km 2) test areas in central Australia indicate that a first-order eight-neighbourhood model structure is the most appropriate. They also indicate that there are consistent changes in model parameter values as the amount of erosion in a landscape increases. This suggests that if the input series for a particular area may be derived, the model for a more eroded condition may be applied to obtain a forecast of the spatial patterns which would occur if the area should suffer more intense erosion.

Effects of benthos on sediment transport: difficulties with functional grouping

No consistent functional grouping of organisms as stabilizers vs destabilizers, respectively decreasing or enhancing erodibility, is possible. Benthic organisms can affect erodibility in particular—and sediment transport in general—via alternation (1) of fluid momentum impinging on the bed, (2) of particle exposure to the flow, (3) of adhesion between particles, and (4) of particle momentum. The net effects of a species or individual on erosion and deposition thresholds or on transport rates are not in general predictable from extant data. Furthermore, they depend upon the context of flow conditions, bed configuration, and community composition into which the organism is set. Separation of organism effects into these four categories does, however, allow their explicit incorporation into DuBoys-type and stochastic sediment dynamic models already in use and thus permits the specification of parameters whose measurement will enhance predictability of sediment transport modes and rates in natural, organism-influenced, marine settings. If the variable of prime concern is the total amount of sediment transported, rather than the frequency of transport events or the spatial pattern of erosion and eposition, and if most transport occurs in rare but intense bouts (e.g., winter storms on boreal continental shelves), then it may be possible to ignore organism effects without major sacrifices in accuracy or precision. Under high transport rates, suspended load effects override organism-produced bottom roughness, abrasion removes adhesives from transporting grains, and transport rates (normalized per unit width of the channel or bed) exceed feeding and pelletization rates. Moreover, at high rates most material transports as suspended load, effectively out of reach of the benthos. The transport rates at which organism effects are overridden, however, remain to be determined. For lower transport rates, foraging theory promises to provide insights into organism effects.