Topographic Thresholds Research Articles

Piecewise prediction model for watershed‐scale erosion and sediment yield of individual rainfall events on the Loess Plateau, China

AbstractEstablishing a universal watershed‐scale erosion and sediment yield prediction model represents a frontier field in erosion and soil/water conservation. The research presented here was conducted on the Chabagou watershed, which is located in the first sub‐region of the hill‐gully area of the Loess Plateau, China. A back‐propagation artificial neural model for watershed‐scale erosion and sediment yield was established, with the accuracy of the model, then compared with that of multiple linear regression. The sensitivity degree of various factors to erosion and sediment yield was quantitatively analysed using the default factor test. On the basis of the sensitive factors and the fractal information dimension, the piecewise prediction model for erosion and sediment yield of individual rainfall events was established and further verified. The results revealed the back‐propagation artificial neural network model to perform better than the multiple linear regression model in terms of predicting the erosion modulus, with the former able to effectively characterize dynamic changes in sediment yield under comprehensive factor conditions. The sensitivity of runoff erosion power and runoff depth to the erosion and sediment yield associated with individual rainfall events was found to be related to the complexity of surface topography. The characteristics of such a hydrological response are thus closely related to topography. When the fractal information dimension is greater than the topographic threshold, the accuracy of prediction using runoff erosion power is higher than that of using runoff depth. In contrast, when the fractal information dimension is smaller than the topographic threshold, the accuracy of prediction using runoff depth is higher than that of using runoff erosion power. The developed piecewise prediction model for watershed‐scale erosion and sediment yield of individual rainfall events, which introduces runoff erosion power and runoff depth using the fractal information dimension as a boundary, can be considered feasible and reliable and has a high prediction accuracy. Copyright © 2013 John Wiley & Sons, Ltd.

Subglacial sediment, proglacial lake-level and topographic controls on ice extent and lobe geometries during the Last Glacial Maximum in NW Russia

Abstract Investigations in sections along the rivers Severnaya Dvina and Vaga, and morphological mapping based on a new digital elevation model and Landsat imagery, allow for a reinterpretation of the extent of the Scandinavian Ice Sheet during the Last Glacial Maximum (LGM). The reconstruction provides a much better link between stratigraphical and morphological expression of glaciation than previous proposals. Most striking is the configuration of long, low-gradient ice-lobes (ice-streams) extending for some 300–400 km up the wide and smooth river valleys. Their extremely low surface gradients are evidenced by glacial trimlines that formed along the sides of the ice-lobes in contact with the gentle valley slopes. In the main valleys, end moraines marking terminal positions are present, whereas drumlins are rare in the peripheral areas of the ice sheet but found in some tributary valleys at somewhat higher elevations. Large drumlin fields, however, are found farther up-ice. The glacial sediment succession is composed of waterlain sediments and tills. The sediments provide evidence of distal ice damming, glacier overriding and retreat, and finally distally glaciolacustrine sedimentation and lake drainage. The diamicton unit associated with the LGM shows evidence of basal coupling, although more striking is the abundant evidence for ice–bed decoupling. These include in situ waterlain sediments within tills, and clastic sills running along the till/substrate contact showing lift-off. These features indicate that the weight of the glacier lobes was, to a large extent, carried by pore-water pressure in subglacial sediments, and that subglacial shear and erosion were moderate. Thus, fast ice flow in the lobes is envisaged. The balance between glacier weight and pore-water pressure was probably maintained over time by the buoyancy effect of glaciers advancing into proglacial lakes. Subglacial lakes may have formed by capture of proglacial lakes during glacial advance. The combination of low-gradient ice and decoupled beds indicate that glacier advance and extent was largely controlled by lake levels and topographic thresholds. This is taken to indicate that steeper-gradient ice much farther upstream was providing the gravitational push. A presumably abrupt change in ice-surface profile is supported by the mapped distribution of glacial bedforms. The model presented herein potentially has wide applicability for large parts of northern Russia as the topographic setting and glacier advance in lakes across fine-grained low-permeable sediments provide very similar conditions.

Critical topographic threshold of gully erosion in Yuanmou Dry-hot Valley in Southwestern China

Gully initiation has been a subject of much discussion in the field of gully erosion. Gully initiation could be examined by the S–A relationship (S = aA − b ) between the critical slope of the gully head (S) and the upstream drainage area (A). Gully erosion plays an important role in sediment yield and land degradation in the Dry-hot Valley region of Southwest China; however, little research has focused on gully development in this region. This study investigated gully morphology and the critical topographic threshold of permanent gullies in Yuanmou Dry-hot Valley. Data for 36 gully heads were derived from the gully digital elevation model created by high-precision real-time kinematic GPS (RTK GPS). A significant relationship between gully length and volume (r 2 = 0.77, p < 0.01) was found to exist, indicating that gully length can be used to estimate the gully erosion rate. Additionally, as vegetation coverage inside the gully increases, the gully morphology tends to be stable. The S–A relationship was S = 0.5195A −.0899, while AS 2 values ranged between 4.2 and 758.5 m2. The low value of constant b (0.0899) indicates that collapse of the gully wall is the dominant process of gully erosion in Yuanmou Dry-hot Valley.

Predicting gully initiation: comparing data mining techniques, analytical hierarchy processes and the topographic threshold

ABSTRACTPredicting gully initiation at catchment scale was done previously by integrating a geographical information system (GIS) with physically based models, statistical procedures or with knowledge‐based expert systems. However, the reliability and validity of applying these procedures are still questionable. In this work, a data mining (DM) procedure based on decision trees was applied to identify areas of gully initiation risk. Performance was compared with the analytic hierarchy process (AHP) expert system and with the commonly used topographic threshold (TT) technique. A spatial database was used to test the models, composed of a target variable (presence or absence of initial points) and ten independent environmental, climatic and human‐induced variables. The following findings emerged: using the same input layers, DM provided better predictive ability of gully initiation points than the application of both AHP and TT. The main difference between DM and TT was the very high overestimation inherent in TT. In addition, the minimum slope observed for soil detachment was 2°, whereas in other studies it is 3°. This could be explained by soil resistance, which is substantially lower in agricultural fields, while most studies test unploughed soil. Finally, rainfall intensity events &gt;62.2 mm h‐1(for a period of 30 min) were found to have a significant effect on gully initiation. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Glacial/interglacial ice-stream stability in the Weddell Sea embayment, Antarctica

The resilience of the Antarctic Ice Sheet and its effect on global sea level depends on the dynamics of ice streams. Antarctic ice streams are known to be responsive to changes at the ocean interface and, as expected, most have thinned in response to ocean warming and sea-level rise since the Last Glacial Maximum (LGM). Here we provide direct and unexpected evidence that points toward the glacial/interglacial stability of the Slessor and Recovery glaciers, ice streams of the East Antarctic Ice Sheet (EAIS) which merge with the West Antarctic Ice Sheet (WAIS) to form the Filchner Ice Shelf in the Weddell Sea embayment. Cosmogenic-nuclide measurements in the Shackleton Range suggest that the Slessor and Recovery ice streams were not significantly thicker than today during the LGM. We hypothesise that the glaciers did not thicken because the grounding line was not able to migrate seaward beyond the deep Thiel/Crary Trough beneath the Filchner Ice Shelf immediately offshore. This discovery reveals how a topographic threshold can affect the dynamics of ice streams. It also reduces uncertainties on the thickness, extent and volume of the Antarctic Ice Sheet in a large but unknown sector of the Antarctic Ice Sheet; it constrains the potential sea-level rise from Antarctica; it helps explain observed anomalies in glacio-isostatic adjustment; above all it suggests that the behaviour of the Atlantic-facing Weddell Sea sector of the WAIS contrasts with that of the Pacific-facing Ross and Amundsen Sea sectors.

Prediction of spatial patterns of collapsed pipes in loess-derived soils in a temperate humid climate using logistic regression

Abstract Soil piping (tunnel erosion) has been recognised as an important erosion process in collapsible loess-derived soils of temperate humid climates, which can cause collapse of the topsoil and formation of discontinuous gullies. Information about the spatial patterns of collapsed pipes and regional models describing these patterns is still limited. Therefore, this study aims at better understanding the factors controlling the spatial distribution and predicting pipe collapse. A dataset with parcels suffering from collapsed pipes (n = 560) and parcels without collapsed pipes was obtained through a regional survey in a 236 km² study area in the Flemish Ardennes (Belgium). Logistic regression was applied to find the best model describing the relationship between the presence/absence of a collapsed pipe and a set of independent explanatory variables (i.e. slope gradient, drainage area, distance-to-thalweg, curvature, aspect, soil type and lithology). Special attention was paid to the selection procedure of the grid cells without collapsed pipes. Apart from the first piping susceptibility map created by logistic regression modelling, a second map was made based on topographical thresholds of slope gradient and upslope drainage area. The logistic regression model allowed identification of the most important factors controlling pipe collapse. Pipes are much more likely to occur when a topographical threshold depending on both slope gradient and upslope area is exceeded in zones with a sufficient water supply (due to topographical convergence and/or the presence of a clay-rich lithology). On the other hand, the use of slope-area thresholds only results in reasonable predictions of piping susceptibility, with minimum information.

Regional mapping and characterisation of old landslides in hilly regions using LiDAR-based imagery in Southern Flanders

AbstractAnalysis of LiDAR-derived imagery led to the discovery of more than 330 pre-Holocene to recent landslides in Southern Flanders (4850 km2). The morphology of three landslides, including the 266.5 ha deep-seated gravitational slope deformation in Alden Biesen, was investigated in more detail. The analysis of the morphological and topographical characteristics (width–length relation, frequency–area distribution and topographical threshold) of the landslides revealed important differences compared to the characteristics reported in other landslide studies, and helped understanding possible landslide triggering mechanisms. Especially the possibility of a seismic origin of the landslides was investigated. Finally, a heuristic model for region-wide landslide susceptibility mapping was successfully tested. The susceptibility model and map allow prediction of future landslide locations and contribute to better understanding the role of individual causal factors on landslide location and spatial density. The results suggest that landslides on low-gradient, soil-mantled hills are a more important contributor to landscape evolution of hilly areas than was hitherto thought. The morphology of all hilly regions of Flanders is clearly marked by landslide processes and higher landslide densities often coincide with the presence of quaternary active faults. This study further shows that high-resolution topographical data such as LiDAR significantly contributes to a better detection of old, previously unknown landslides.

Factors related to gully erosion in woody encroachment in south-eastern Australia

Abstract Gullies initiate when the equilibrium within a drainage line is upset by increased discharge or decreased soil resistance to detachment and transport of soil particles, and is triggered and accelerated by human-induced changes to vegetation cover. Woody encroachment is the increase in density, cover, extent and biomass of woody plants, and can result in low ground cover, which increases the potential of soil erosion. This paper reports the relationships between gully erosion and site characteristics such as topography, vegetation cover and road infrastructure in a 120-km² study area of semi-arid woodland subject to woody encroachment in south-eastern Australia. This study characterised gully morphology and activity, identified the factors related to gully volume, and estimated the topographic thresholds for sub-catchments subject to woody encroachment. Firstly, relationships between gully volume, activity, topography, vegetation cover and road infrastructure were explored. Secondly, site characteristics of 32 sub-catchments with and without gullies were compared. Finally, the topographic thresholds for unstable and stable sub-catchments with and without gullies, respectively, were estimated using the slope – drainage area relationship (S = aA− b). All gullies were active and high gully volume was related to large sub-catchments, long gullies and short distances to roads. Stable sub-catchments (without gullies) had gentler slopes, higher foliage projective cover and ground cover than unstable sub-catchments (with gullies). When data for sub-catchments with and without gullies were combined, the main factors related to gully volume were drainage area at the gully or stream heads and their local slope, and ground cover. The mean topographic threshold for gully development provided insights into the drainage area and slope conditions needed for gully development. Gully erosion in the study area is most likely the result of interactions among topography, vegetation and human-made structures (roads) in space and time. The results of the study advance our understanding of gully erosion dynamics in areas of woody encroachment, and can be used to identify areas susceptible to gullying.

Factors controlling the spatial distribution of soil piping erosion on loess-derived soils: A case study from central Belgium

Abstract Collapsible loess-derived soils are prone to soil piping erosion, where enlargement of macropores may lead to a subsurface pipe network and eventually to soil collapse and gully development. This study aims at understanding the main factors controlling spatial patterns of piping in loess-derived soils under a temperate climate. To map the spatial distribution of piping and identify the environmental controls on its distribution, a regional survey was carried out in a 236 km2 study area in the Flemish Ardennes (Belgium). Orthophotos taken at optimal field conditions (winter) were analyzed to detect piping in open landscapes and ground thruthing was systematically done through field surveys. In total, 137 parcels having 560 collapsed pipes were mapped. Dimensions of the sinkholes and local slope gradient were measured in the field and topographical variables were derived from LiDAR data. Land use plays an important role as 97% of the sites with piping are found under pasture. The probability of piping increases rapidly on hillslopes with gradients exceeding 8% and with a concave profile and plan curvature, enhancing subsurface flow concentration. The zones with soil profiles on shallow loess over a relatively thin layer of homogeneous blue massive clays (Aalbeke Member) are most prone to piping. Soil characteristics are of less importance to explain piping occurrence. Furthermore, the topographical threshold line indicating the critical slope gradient for a given contributing drainage area was determined. This threshold line (negative power relation) is similar to the threshold line for shallow gully initiation.

Catchment scale analysis of the effect of topography, tillage direction and unpaved roads on ephemeral gully incision

AbstractHigh‐resolution airphotographs and digital elevation models (DEMs) were used to study areas prone to gully incision in a small agricultural catchment in northern Israel. Data are analyzed to understand better the integrated effect of environmental and human factors on gully incision. The effect of flow accumulation, slope, unpaved roads density and tillage direction on gully width and length is found discontinuous and topography – in the form of the topographic threshold – was found to be the most dominant factor on gully incision in the catchment scale. The topographic threshold coefficients observed in Yehezkel catchment were lower than those found in previous studies, due to the effect of large rainfall events during the early winter season, when the soil in fields is still unprotected by crops. In addition, a survey of previous studies shows that, in cultivated areas, there is relationship between the topographic threshold coefficients a (multiplier) and b (exponent). The influence of unpaved roads and tillage direction on gully incision was also found significant and consistent during the two seasons observed, while the non‐parametric test showed that it added to the prediction in ∼15% of the gullies. Incision points density plot show that of the two parameters, the unpaved roads were found to have greater influence on gully‐head location than tillage direction. Studying the phenomenon on the catchment scale allows better understanding of the effect of spatial variation on gully‐head location. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Topographic thresholds for plant colonization on semi‐arid eroded slopes

AbstractSoil erosion plays an important role in plant colonization of semi‐arid degraded areas. In this study, we aimed at deepening our knowledge of the mechanisms that control plant colonization on semi‐arid eroded slopes in east Spain by (i) determining topographic thresholds for plant colonization, (ii) identifying the soil properties limiting plant establishment and (iii) assessing whether colonizing species have specific plant traits to cope with these limitations.Slope angle and aspect were surrogates of erosion rate and water availability, respectively. Since soil erosion and water availability can limit plant establishment and both can interact in the landscape, we analysed variations in colonization success (vegetation cover and species number) with slope angle on 156 slopes, as a function of slope aspect. After determining slope angle thresholds for plant colonization, soil was sampled near the threshold values for soil analysis [nitrogen, phosphorous, calcium carbonate (CaCO3), water holding capacity]. Plant traits expressing the plant colonizing capacity were analysed both in the pool of species colonizing the steep slopes just below the threshold and in the pool of species inhabiting gentler slopes and absent from the slopes just below the threshold.Results show that the slope angle threshold for plant colonization decreased from north to south. For the vegetation cover, threshold values were 63°, 50°, 46°, 41° for the north, east, west and south slope aspect classes, respectively, and 65°, 53°, 49° and 45° for the species richness and the same aspect classes. No differences existed in soil properties at slope angle threshold values among slope aspects and between slope positions (just below and above the threshold) within slope aspect classes. This suggests that variations between slope aspect classes in the slope angle threshold result from differences in the colonizing capacity of plants which is controlled by water availability. Long‐distance dispersal and mucilage production were preferably associated with the pool of colonizing species.These results are discussed in the perspective of a more efficient ecological restoration of degraded semi‐arid ecosystems where soil erosion acts as an ecological filter for plant establishment. Copyright © 2009 John Wiley &amp; Sons, Ltd.

River and landslide dynamics on the western Tanganyika rift border, Uvira, D.R. Congo: diachronic observations and a GIS inventory of traces of extreme geomorphologic activity

Uvira occupies a series of narrow alluvial fans squeezed between the NW corner of Lake Tanganyika (±710 m asl) and the W-shoulder of the Tanganyika rift, the Itombwe–Mitumba Plateau (±3,000 m asl). In 50 years, the fans progressed into the lake over distances up to some hundreds of metres. This happened during a few catastrophic flash floods issued from the torrents which cascade from the rift shoulder with a mean longitudinal gradient of 0.2 m m−1. The last event in 2002 led to the destruction of parts of the town and to some 50 casualties. Landslides occurred in the hills. On the base of stereoscopic interpretation of aerial photographs from 1959, complemented with data from 2000 ETM and 2004 IKONOS imagery, a geographical inventory has been made of strongly incising (10−1 to 0 m in 43–45 years) river sections, of all types of landslides and of all tectonic structures, visible in the rugged hinterland of the fans. Traces of active N–S as well as E–W trending faults are present. Some of these faults and some surfaces, interpreted as degraded fault facets dip at angles of 40° or less and are probably remnants of formerly active lystric extension faults, originally at a depth of some 2 km, but now at the surface as a result of posterior uplift and erosion. Sixty landslides could be identified. Six slides fall far below the topographic threshold envelope, where the slope at the incision head is expressed as a function of drained surface. Therefore, they are considered to be seismic in origin. Most of the other landslides are located along strongly incising river sections. Temporary landslide barriers contribute to irregular river hydrographs. It is concluded that Uvira is threatened by landsliding, potentially massive (>18 × 106 m3 debris), in the case of heavy seismicity. It is further discussed that the regularisation of the river regime depends on soil and water conservation strategies, to be developed in the headwaters of the torrents Kavimvira, Mulongwe and Kalimabenge.

THE ACQUISITION OF LOCAL TOPOGRAPHIC RELIEF AND ITS APPLICATION: AN SRTM-DEM ANALYSIS

Topographic analysis has been widely used in the study of landscape evolution and surface process,and the researches in this field have become more and more quantitative.Local topographic relief,which is defined as the difference between local maximum elevation and local base level,is one of the key methods in topographic analysis.Based on a spatial analysis of the SRTM-DEM,this paper puts forward two methods for the acquisition of local topographic relief,i.e.,Swath Profile and Threshold Value.The quantitative analysis of the erosion within a typical area of the Loess Plateau has been preliminarily realized by extending the Threshold Value method.

Gully erosion, land use and topographical thresholds during the last 60 years in a small rangeland catchment in SW Spain

AbstractGully erosion plays an important role in degradation processes of Mediterranean environments. In this paper aerial orthophotographs were used for (i) analysing the evolution of a valley bottom gully and its relation with land use and vegetation cover, (ii) exploring the role of land use and vegetation cover on the coefficients of the equation S = aA −b (where S is slope at the headcut and A is drainage area), which is based on the topographical threshold concept and is commonly used to predict gully initiation. The study was carried out in a small catchment (99·5 ha) located in the southwest of the Iberian Peninsula. Gullies and headcuts were mapped together with land use and vegetation cover using aerial photographs for the years 1945, 1956, 1989, 1998, 2002 and 2006, which had to be digitized and orthorectified in advance. The results showed an increase of the area affected by gullying from 695 m2 in 1945–1009 m2 in 2006, reaching a maximum of 1560 m2 in 1956. Gullying was closely related with land use, especially with the amount of cultivated areas within the catchment and also with grazing intensity. No clear relationship was found between the evolution of the gullied area and rainfall amounts. Finally, the values of the exponent b obtained for different headcuts and different dates (close to 0·4) were similar to those proposed by other authors for gully erosion caused by Hortonian overland flow in semiarid environments. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Topographic thresholds in gully development on the hillslopes of communal areas in Ngqushwa Local Municipality, Eastern Cape, South Africa

Abstract The relationships between the spatial distribution of gully erosion and topographic thresholds in the form of slope angle, position and configuration, as well as land use change in the form of abandoned lands were examined in several affected catchments of the Eastern Cape Province, South Africa. Land use and permanent gullies were mapped, digitized from orthophoto maps in Arc/info 3.5.2 GIS and converted to shapefiles using ArcView 3.2 GIS. Relationships between the mapped phenomena and topographic variables were sought using a Digital Elevation Model (DEM) in Idrisi Kilimanjaro GIS. A comparison between areas with a high potential for gullying and actual gully erosion was made using the Stream Power Index (SPI) as a surrogate for critical flow shear stress. Field surveys were also conducted to assess the present condition of the gullied sites as well as to validate DEM derivations. Seventy five percent of the gullied area was noted to lie on abandoned lands. A predominance of gullying in concave bottom lands was also identified. The SPI values highlighted a distinct preferential topographic zone for gully location. A conceptual model depicting the interaction between land use and topographic parameters to induce gully erosion was developed. This should assist local authorities to develop a policy regarding management of abandoned lands.

Effect of incipient gullying mechanisms on topographic threshold conditions for gully initiation in southwestern Iran (Boushehr-Samal watershed).

Gully erosion is widespread process of land degradation in dryland regions and has attracted the interest of a large number of researches. Although much research has carried out on soil erosion in arid and semi arid regions of Iran, few studies have been conducted to understand the threshold conditions of gully incipient. This study was conducted on gullying mechanism and topographic threshold conditions of gullies in an arid region of Iran. Since the dynamics of gully development channel initiation and extension are controlled by a variety of processes and factors, the data collection and analysis consist of field studies to the determine predominance process responsible for incipient gullying and Digital Model Elevation (DEM) analysis to examine slope-area relationship. Soil attributes (EC and SAR) and landuse practices were known as major factors to initiate piping and the potential of this process to initiate gully is very significant for bank gullies. It seems that topography and soil attributes are the major factors determining the effective process for incipient gullying in our study area. In the other hand all of gullies occurred in the main drainage streams and consequently are the main processes for landscape evolution. Correlation analysis showed an inverse linear relationship in log-log plot of upslope area and local slope dataset related to permanent gully heads. However for dataset associated with landsliding the S-A relation was not significant. Results of S-A relation can be used to determine vulnerable areas to gullying.

Thresholds for channel initiation at road drain outlets

Abstract Slope–Area relationships have been widely used to identify channel initiation points in the landscape. More recently, the importance of road-related rill and gully erosion has been highlighted. Previous studies indicate that channel initiation at road drain outlets can be predicted from the contributing road area and hillslope gradient at the drain outlet. In this study the validity and usefulness of channel initiation thresholds for road runoff management has been evaluated using field data from three catchments in south-east Australia. None of these catchments show a clear separation between channelised and non-channelised sites based on contributing road area and hillslope gradient. Many drains without channel have similar contributing road area and slope gradients as drain with a channel at the outlet. This demonstrates a relatively large variability within our dataset and limits the identification of a slope–area threshold for these catchments. Logistic regression using road area and slope gradient shows that both variables contribute significantly to channel initiation in the Albert and Tyers River catchments, while for the Sandy Creek catchment only slope gradient is significant at the 0.05 level. Stepwise logistic regression on a wider range of variables shows the significance of cut-batter height in two of the catchments, indicating that flow interception at cut-and-fill roads may be important. Comparison of road-related channel initiation thresholds to previously published topographic thresholds, indicates that channels at road outlets tend to form at relatively small contributing areas and steep slopes. Current forest management guidelines do not consider the risk of channel initiation at road drain outlets in road design and drain spacing. The slope–area thresholds are still considered useful in minimising the risk of gully erosion, especially in forestry areas where it appears most likely that channel initiation occurs during a significant rainfall event immediately following hillslope disturbance or road construction.

Gully erosion in South Eastern Tanzania: spatial distribution and topographic thresholds

Summary. Though gully erosion is often mentioned as a major process of land degradationin South Eastern Tanzania, little information is available on its distribution. The Makondeplateau and adjacent inland plains are of particular concern as they are densely populated andare major areas of cashew nuts production. The occurrence of gully erosion was assessed in 66villages selected by stratified random sampling in an area of 13,000 km 2 . Difference in suscep-tibility to gully erosion between landscape units was assess by determining topographicthreshold parameters of 22 gullies on the Makonde plateau and 14 in the inland plains. Over-all, gullies are common and spread equally over the different landscape units. Their occurrenceis positively associated with terrain roughness (Cramer’s V = 0.30; P = 0.05) and negatively withpopulation density ( V = 0.44; P 0.01). On the Makonde plateau occurrence of gully erosionis associated with the presence of roads, while on the inland plains it is predominantly foundin fields (

Geomorphology and urban geology of Bukavu (R.D. Congo): interaction between slope instability and human settlement

Abstract The city of Bukavu, on the south coast of Lake Kivu in the Democratic Republic of Congo, suffers from frequent landsliding, which leads to continual damage and destruction of buildings, roads, waterworks and sewerage infrastructure. Thirty-one landslides identified on aerial photographs of 1954 and 1973 are located outside the Bukavu micro-rift and are related to actively incising rivers. Their origin is thought to be due to increases in hydrostatic pressure. Six landslides occur within the Bukavu micro-rift, four of which are much larger and wider than other slope failures in the study region. These large landslides are adjacent to, or crossed by, active faults. They fall far below the topographic threshold envelope, a relationship of slopes at the head of the slide and the surface area drained into it, indicating seismic or anthropomorphic triggering. The Bukavu landslides still reactivate occasionally. Deforestation, followed by a large increase in the population, have been indirect causes of the reactivated mass wasting processes. On the steep slopes in the south of the city the high density of newly built houses has led to reduced water infiltration and enhanced runoff, causing landslides and mudflows. Very high spatial resolution IKONOS satellite images have recently been used as base maps for geohazard assessments of Bukavu. A geographical information system (GIS) has been developed for Bukavu's planners: this shows the locations of buildings, roads and tracks, the river network, the water distribution system and the sewerage infrastructure, as well as areas of slope instability.

Gully systems under undisturbed indigenous forest, East Coast Region, New Zealand

Gully erosion is commonly associated with agricultural landscapes where vegetation clearance and farming practices increase runoff, leading to fluvial incision. However, gully erosion can also occur in forests that have undergone some form of disturbance, either natural or resulting from human impacts. This paper reports on recent gully development within areas of undisturbed indigenous forest as a result of a high magnitude rainfall event on the East Coast of New Zealand's North Island. This region, through a combination of crushed and sheared rock types, steep topography, and tectonic and climatic setting, has high natural rates of erosion, exacerbated by European deforestation in the late 19th and early 20th centuries. Sequential air photographs, spanning a 58 year period between 1939 and 1997 were used to classify and document the growth and recovery of gully systems in the 14.1 km 2 headwaters of the Mangaoporo catchment. Following a severe cyclone in 1988, with a rainfall of 535 mm, there were 21 active gully systems within the indigenous forest. On photography prior to 1988 only four gully systems were present. During this period there were 8 major rainfall events (150–250 mm). Despite further 5 rainfall events of 150–250 mm between 1988 and 1997 all gully systems showed signs of recovery, with a combined reduction in active area of 37%. The nature and location of these features is strongly influenced by lithology (orientation of jointing and bedding), and to a topographic threshold defined by catchment slope and catchment area.