Dominant Erosion Process Research Articles

Where along a river's length will vegetation most effectively stabilise stream banks?

Riparian vegetation has different impacts on stream processes depending upon its position in a catchment. Native riparian vegetation is increasingly becoming the favoured stream management tool but managers need to locate revegetation schemes where they will most effectively achieve ecological, geomorphological, or other, project goals. Using the Latrobe River in SE Australia as an example, this paper illustrates a structured decision-making approach for assessing the role of vegetation in stream bank erosion at different points throughout a catchment. Three bank-erosion process groups are identified: subaerial preparation, fluvial entrainment, and mass failure. Although these processes act on banks throughout the catchment there exists spatial zoning in the dominance of each process group over the others. Bank erosion in upper reaches is dominated by subaerial preparation, in mid-basin reaches by fluvial entrainment, and in the lower reaches by mass failure. We find that in upper reaches, windthrown trees are responsible for most bank sediment transfer to the flow. Where direct fluvial entrainment of bank material is the dominant erosion process, flow resistance due to vegetation becomes crucial. In reaches where bank slumping is the dominant erosion process, increased bank shear strength due to root reinforcement is the major role of vegetation in stabilising banks. Other effects, such as tree surcharge, and altered bank hydrology appear to exert only minor influences on the slumping process. Considering the above variables we are able to define a critical zone in which revegetation will be most effective in reducing bank erosion. On the Latrobe River, this zone occurs in that portion of the river where it first leaves the mountain front and meanders across a broad floodplain. This reach occupies the second quarter of the river's length. This information, combined with other scale analyses (e.g. ecological, hydrological), will assist river managers to plan physically based riparian revegetation strategies.

Landscape position, surface hydraulic gradients and erosion processes

Different hydraulic gradients, especially due to seepage or drainage, at different locations on a hillslope profile may have a profound effect on the dominant erosion processes. A laboratory study was designed to simulate hillslope processes and quantify effects of surface hydraulic gradients on erosion for a Glynwood clay loam soil (fine, illitic, mesic Aquic Hapludalf). A 5 m long, 1·2 m wide soil pan was used at 5 and 10 per cent slopes with an external watering tube to vary the soil bed's hydrological conditions. Different combinations of slope steepness with seepage or drainage gradients were used to simulate the hydrologic conditions on a 5 m segment of a hillslope profile. Runoff samples were taken during rainfall-only and rainfall with added inflow. Results showed that, under drainage conditions, interrill processes dominated and rilling was limited. The surface contained scattered crescent-shaped pits after the run. Under seepage conditions, rilling processes dominated and the inflow introduced at the top of the soil pan further accelerated the headward erosion of the rills. Erosion rates increased by as much as 60 times under seepage conditions representative of the lower backslope when compared to drainage conditions that generally occur at the upper backslope. This indicated that rills and gullies on backslopes and footslopes may be catalysed or enhanced by seepage conditions rather than form from flow hydraulic shear stress alone. An understanding of spatial and temporal changes that affect both hillslope hydrology and erosional processes is needed to develop accurate process-based erosion prediction models. This knowledge may lead to different management practices on landscape positions where seepage occurs. © 1998 John Wiley & Sons, Ltd.

Holocene fluvial-coastal interactions on a mixed sand and sand and gravel beach system, North Canterbury, New Zealand

Fluvio-coastal interactions are examined on a progradational sand, and mixed sand and gravel beach sequence between the Ashley and Kowai rivers, Pegasus Bay, New Zealand. This coastal system presents an example of a wave-dominated environment energetic enough to deal with the sediment supply derived from the rivers, but where the coast is still prograding. Progradation occurs because of the inability of the marine system to evacuate wave-eroded sediment from the bay floor. This results in gradual nearshore aggradation until nearshore storage is filled and sediment is finally transported landward to form a new beach ridge. This type of coastal system can be identified by a diagnostic morphological assemblage comprising a ‘small river’ coastal morphology (sensu Zenkovich (1967, Processes of Coastal Development, Oliver and Boyd, Edinburgh) backed by a sequence of beach ridges. Sediment fractionation (shape and size sorting) is identified as the primary effect of erosional processes on a mixed sand and gravel beach. Fractionation is capable of converting sandy beaches with minor gravel components to graveldominated beaches. Gravel is concentrated by the evacuation of sand from the shoreface. This occurs dominantly through storm sifting but also occurs in response to normal swash processes. This process is very similar to the process of chenier production on mixed mud and sand beaches, and fractionation is highlighted as a dominant erosional process on mixed beaches, irrespective of grain size.

Erosion rates in badland areas recorded by collectors, erosion pins and profilometer techniques (Ebro Basin, NE-Spain)

In badland areas of the Ebro Basin, in a semiarid climate, two erosion plots (257 m 2; 5° slope and 128 m 2; 23° slope) on exposed Tertiary clays were monitored over two years (Nov. 1991–Nov. 1993). This material is characterized by high sodium absorption ratios which lead to high soil dispersivity. The dominant erosion processes in both plots are rilling and sheet erosion. Rainfall intensity was recorded at a weather station, connected to a data-logger, sediment production for single events was collected in tanks, and ground lowering was measured every six months by erosion pins and microtopographic profile gauge techniques. Significant runoff was produced only by rainfall events above 5 mm. Another threshold at 20 mm rain was noted. For rainfalls higher than 20 mm, the 23° slope plot shows a greater runoff response than the 5° one. Rainfall events exceeding this threshold showed a higher sediment production for the steeper slope. In the relationship between precipitation and sediment concentration, an envelope curve can be drawn indicating that any rainfall event of a given amount and intensity has a maximum sediment concentration which we speculate to be a function of the runoff sediment transport capacity. Runoff response and sediment yield in the studied plots are controlled by the rainfall and soil characteristics and their seasonal variations. In both plots, the erosion pins show that erosion rates in rill areas are 25–50% higher than in the interrill areas. Sediment yield recorded by collector devices was higher than the rates measured by erosion pins. The erosion rates based on rill cross-sections by profilometers were higher than the ones recorded by collectors.

Soil Erosion and the Archaeological Landscape of Methana, Greece

An attempt is made to assess the effect of soil erosion on the archaeological record of Methana, Greece, by detailed analysis of the distribution and quality of sherds in relation to soil conditions and land morphology. Sherds have been concentrated through soil removal caused by surface wash, the dominant erosion process, at least on terraced slopes. The chief agent of sherd displacement is probably livestock. Sherd distribution is therefore thought in general to be a reliable archaeological indicator. Certain of the agricultural terraces are likely to be of Classical age, and judging from their condition and from recent erosion processes affecting them, they appear to have been largely stable throughout the period of their existence.

Application of the Caesium-137 Technique in a Study of Soil Erosion on Gully Slopes in a Yuan Area of the Loess Plateau Near Xifeng, Gansu Province, China

The caesium-137 technique has been used to derive hitherto unavailable information concerning erosion rates on gully slopes in a yuan area of the Loess Plateau in Gansu Province, China. The patterns shown by the caesium- 137 inventories were used to interpret the dominant erosion processes operating on cultivated and uncultivated land. These patterns demonstrate the importance of tillage redistribution and water erosion on cultivated terraces and suggest that both wind and water erosion processes operate on grassland areas. Rates of erosion derived from the caesium-137 inventories for the sites on uncultivated land indicate that the estimate of 61 km-2 year-1 provided by the local soil conservation station may be a serious underestimate, although the data do confirm the importance of full vegetation cover in controlling erosion. The erosion rates derived for terraced cultivated sites are in line with published estimates. Two terrace types were examined and these were characterized by widely divergent net erosion rates, indicating that narrow-strip terraces cultivated parallel to the contour should be used where possible, because these offer a clear soil conservation benefit.

Late cenozoic history of the eastern llanos of venezuela: Geomorphology and stratigraphy of the Mesa Formation

The study area corresponds to the Mapire River basin in the southern part of the State of Anzoátegui and constitutes a region of low relief covered by the Pleistocene Mesa Formation. This formation consists of horizontally bedded alluvial sediments, represented by light gray, coarse- to fine-grained quartz sand, with cross-bedding marked by ochre, red and pink alteration, and to a minor extent by marsh deposits, represented by laminated and spotted shale. The typical topography consists of mesas north of the left margin of the Orinoco River (the Musinacio and Mapire Mesas). To the north of the mesas there is a topography of low hills covered by blocks of ferruginous crusts. A typical tropical savanna climate exists in the region, with a strong dry season which lasts 4–6 months, followed by a rainy season with excess humidity. The dominant erosional process is scarp retreat, due to the action of the savanna climate on the lithology of the partly unconsolidated sediments; it has formed deep canyons in the headwaters, and is magnified by tectonic uplift and the lowering of the regional base level. However, some topographic forms have been preserved due to the hardening of the sands by iron oxides. Preliminary thermoluminescence dating of sand from two stratigraphic sections confirms the Pleistocene age of the Mesa Formation (0.5–1 Ma BP), previously suggested by its stratigraphic position and geomorphological interpretation.

Rainfall Detachment and Deposition: Experiments with Low Slopes and Significant Water Depths

Rainfall impact is probably the dominant erosion process on very low slopes where water depths can be significant. This study was conducted to investigate the concentrations and settling velocity characteristics of sediment eroded under simulated rainfall for situations where very low slope and significant constant depths of water prevail. Two contrasting soils (Vertisol and Aridisol) were subjected to rainfall at two constant rates of 56 and 100 mm h−1 in a 5.8 by 1 m tilting-flume apparatus. Experiments were carried out with three different mean depths of ponded water (2, 5, and 10 mm), sustained by adjusting the flume slope (0.1 to 1%). Sediment concentrations and the settling velocity characteristics of the eroded sediment were determined from samples of runoff water. Sediment concentration was higher for the Aridisol than for the Vertisol, increased with rainfall rate and decreased with mean water depth in all experiments. For each rainfall rate and mean depth of water, sediment concentration decreased with time until a steady-state concentration was reached. Settling velocity characteristics indicated that eroded sediment was initially finer than the original soil, but became coarser with time, approaching an equilibrium distribution. The temporal change in experimental data is thought to be due to the development with time of a coarser deposited layer that shields an increasing fraction of the original soil surface until an equilibrium fraction less than unity is achieved. The concepts presented provide a basis for understanding the interaction of rainfall detachment and deposition of cohesive soils composed of a range of sizes and densities of aggregates and particles.

Evaluation of the CREAMS model. II. Use of rainulator data to derive soil erodibility parameters and prediction of field soil losses using derived parameters

The first paper of this series identified several parameters to which predictions of the CREAMS erosion model were sensitive. Two of these, soil erodibility parameters nbov and K, cannot be measured directly. Therefore, this paper reports procedures for deriving nbov and K for the CREAMS erosion model from rainulator data. The procedures identify confidence regions for best fit combinations of nbov and K. Fairly specific values of nbov were obtained, but a wide range of K values provided similar near-optimal predictions. Optimum regions of nbov and K for plots where rilling was the dominant erosion process were significantly different to those for plots dominated by rain-flow erosion for two of the three soils studied. The parameters derived from rainulator data were used to predict soil losses for field catchments on two clay soils, for conditions where surface cover by stubble was 110%. Measured rainfall and runoff were used as inputs to the model. For an Irving clay site, the model gave excellent prediction of field soil losses. For a Moola clay site, the larger soil losses were considerably underpredicted, because resistance to rilling on the rainulator plots resulted in lower K values than required for erosion events where rilling is fully developed. The need for parameters to be derived from data reflecting the erosion processes of importance to field soil loss is illustrated. Nonetheless, this study shows that parameters for the CREAMS model can be obtained satisfactorily from rainulator data, provided that the erosion processes studied are relevant, and that there is sufficient replication within the rainulator studies.

Bank conditions and erosion along selected reservoirs

This analysis was done as the preliminary step in an ongoing study of reservoir bank erosion processes that are active in the northern U.S. The objectives of this analysis were to observe and document bank characteristics, conditions, and changes along reservoirs with eroding banks, to estimate the amounts of historical bank recession and to discuss its possible causes. Aerial photographs were used to observe the historical bank changes and estimate bank recession. Site reconnaissance, discussions with on-site personnel, and published reports were used to evaluate possible relationships between the local erosion and bank conditions. As part of this analysis linear regressions were done to determine if the estimated recession rates correlate with selected bank and climatic conditions and with physical characteristics of the reservoirs. The regression results, however, were generally not useful because they suggested relationships that are contrary to field observations and published results. Dominant bank erosion processes were wind-wave erosion, capillary wave erosion during high-water periods, groundwater-induced sliding, freeze-thaw processes, rain splash and rainwash, and boat waves. However, because of the complexity of the interrelationships of these and many other bank erosion processes and the variability of the processes at and between sites, it would be necessary to make site-specific measurements and observations year-round to evaluate the processes that are active along a particular bank.

SUBSURFACE DRAINAGE ERODES FORESTED GRANITIC TERRANE

Solution and landsliding, the dominant erosion processes in undisturbed forested mountainous watersheds, are both influenced by subsurface drainage. Biological processes that generate organic acids accelerate loss of dissolved solids by promoting the dissolution of primary minerals in granitic rock. These organic acids can also disperse the secondary minerals, kaolinite and ferric hydroxide. This accelerates erosion by subsurface drainage through suffusion and piping, particularly in well-drained, gap-graded soils. Solution, suffusion, and piping on steep slopes can promote conditions suitable for debris avalanches by forming weak, permeable zones. Consequently, subsurface drainage shapes topography over time, from the slow continual dissolution of rock to rapid episodic mass failures. Trees reduce the ratio of physical to chemical erosion while timber harvesting increases it.