incised channels and gully erosion in northern iberian peninsula controls and geomorphic setting abstract gully erosion has scarcely been reported in the cantabrian cordillera northern iberian peninsula even though this phenomenon is relatively frequent in this mountain range a total of 133 permanent channels with evidences of gully erosion have been mapped in a 71 8 km2 study area on the northern slope of this mountain range the underlying ordovician unit comprises quartzite sandstone and slate which are partially mantled by different surficial deposits the relief is fairly steep with average slopes of 27 and hillsides with level drops of the order of 700 m and generally vegetated by atlantic heaths a result of the degradation suffered by the natural vegetation following forest fires the mapped channels present different morphological features shallow and deep channels some of them with associated ravines and are usually related to headcut in first and second order pre existing channels although some of them are located on unchanneled hillsides with no evidence of pre existing channels moreover lower reaches of first and second order channels are often incised into bedrock a field description and morphometric study has been carried out on channels affected by gully erosion based on their cartography digital terrain models and several layers of thematic mapping this study describes some of the relations between relief factors the type of substratum and the development of gully erosion i there are robust correlations between morphometry relief and erosion intensity and ii gullying is strongly influenced by the presence of a deep regolith the erosion forms observed and the described relations suggest that gully development is associated to landsliding processes which are ultimately controlled by ground water movement through a porous regolith regional uplift and deforestation are argued to be the most likely causes of channel incision and gullying

slope area thresholds of road induced gully erosion and consequent hillslope channel interactions pikes peak highway is a partially paved road between cascade colorado and the summit of pikes peak significant gully erosion is occurring on the hillslopes due to the concentration of surface runoff the rearrangement of drainage pathways along the road surface and adjacent drainage ditches and the high erodibility of weathered pikes peak granite that underlies the area as a result large quantities of sediment are transported to surrounding valley networks causing significant damage to water quality and aquatic wetland and riparian ecosystems this study establishes the slope drainage area threshold for gullying along pikes peak highway and a cesium 137 based sediment budget highlighting rates of gully erosion and subsequent valley deposition for a small headwater basin

geomorphic threshold conditions for ephemeral gully incision abstract ephemeral gully erosion in cultivated land is an important source of sediment that is frequently being overlooked and not accounted for in soil erosion studies furthermore little is known about the factors controlling ephemeral gully erosion in this paper the available information on the initiation and location of ephemeral gullies is summarised focusing on the relationship between the upslope drainage area a and the critical slope gradient s cr for ephemeral gully initiation by plotting the non linear relationship between critical slope gradient measured immediately upstream of the incision head and drainage area at the incision head for gullied sites it was possible to draw a straight line on log log paper through the lower most points for each of the available datasets representing a critical slope area relationship for incision consequently below this line no incision occurs this line or critical relation can also be written as a power function between critical slope and area although many factors vary between the different datasets the exponent of the drainage area relationship 0 4 showed very little variation the observed critical slope area relationship can be related to a simple model of channel initiation by overland flow furthermore this relationship can be used to identify potentially unstable sites where control measures should be undertaken

modelling landslide hazard soil redistribution and sediment yield of landslides on the ugandan footslopes of mount elgon in this study the lapsus ls landslide model together with a digital terrain analysis of topographic attributes is used as a spatially explicit tool to simulate recent shallow landslides in manjiya county on the ugandan slopes of mount elgon manjiya county is a densely populated mountainous area where landslides have been reported since the beginning of the twentieth century to better understand the causal factors of landsliding 81 recent landslides have been mapped and investigated through statistical analysis it was shown that steep concave slopes high rainfall soil properties and layering as well as human interference were the main factors responsible for landslides in the study area lapsus ls is used to construct a landslide hazard map and to confirm or reject the main factors for landsliding in the area the model is specifically designed for the analysis of shallow landslide hazard by combining a steady state hydrologic model with a deterministic infinite slope stability model in addition soil redistribution algorithms can be applied whereby erosion and sedimentation by landsliding can be visualized and quantified by applying a threshold critical rainfall scenario the model is tested in the manjiya study area for its ability to delineate zones that are prone to shallow landsliding in general and to group the recent landslides into a specific landslide hazard category the digital terrain analysis confirms most of the causal topographic factors for shallow landsliding in the study area in general shallow landslides occur at a relatively large distance from the water divide on the transition between steep concave and more gentle convex slope positions which points to concentration of sub surface flow as the main hydrological triggering mechanism in addition lapsus ls is capable to group the recent shallow landslides in a specific landslide hazard class critical rainfall values of 0 03 0 05 m day 1 by constructing a landslide hazard map and simulating future landslide scenarios with the model slopes in manjiya county can be identified as inherently unstable and volumes of soil redistribution can yield four times higher than currently observed more than half of this quantity can end up in the stream network possibly damming rivers and causing major damage to infrastructure or siltation and pollution of streams the combination of a high population density land shortage and a high vulnerability to landslides will likely continue to create a major sustainability problem 2007 elsevier b v all rights reserved

the topographic thresholds of hillslope incisions in southwestern rwanda abstract this article presents new quantitative evidence that land use in rwanda contributes to the development of hillslope incisions two types of hillslope incisions can be distinguished in southern rwanda incisions of the first type drain an area depending on the form and extension on the natural topography and geology the runyinya gully 25 and the rugabano soil slippage 39 are two examples on a logarithmic plot of critical slope inclination at the incision head versus drainage area towards the incision head both incisions lay sensibly to the right of the montgomery dietrich m d envelope the latter gives the range of these topographical thresholds for gully and mass wasting incision in parts of north america the runyinya and rugabano cases obey the linear equation s cr 0 6 a 0 6 where scr critical slope gradient tangent of slope in at the gully head or the scar and a the area ha drained towards the incision head hillslope incisions of the second group rely on a run on area larger than normal because they are localised at the outlet of artificially runoff collecting systems like roads soil conservational contour trenches tracks and other linear landscape elements such systems often drain a surface much larger in extension than the natural run on area to the outlet these hillslope incisions taking into account their artificially big drainage area concentrate more or less along the line s cr 0 3 a 0 6 this line is about in the center of the montgomery dietrich envelope if however only the natural drainage area of these outlet incisions is taken into account all points fall close to the left border or even to the left of the montgomery dietrich envelope this indicates a much higher probability for incision in those localities receiving supplementary runoff or interflow from outside the natural drainage area in the case of a soil slippage at rwaza hill detailed stability calculations show that the slope failure should be due to excessive water infiltration into the bottom of a trench the digging of the trench provoked an increase in the area drained to the slippage head by a factor of 6 the phenomenon of forward erosion is compatible with the existence of threshold combinations of slope and drained area for slopes steeper than 7 8 the phase of regressive erosion does often follow the forward incision event with a delay of several years or more finally the scanty data set now available for rwanda suggests that the drainage area critical to hillslope incision on the red brown ferrallitic soils in rwanda might be nearly twice as big as those in north america

landslides in a densely populated county at the footslopes of mount elgon uganda characteristics and causal factors manjiya county on the ugandan slopes of mount elgon is a densely populated mountainous area where landslides have been reported since the beginning of the twentieth century the numerous fatalities and the damage done during the extreme rainfall events of 1997 to 1999 drew attention to this phenomenon in order to better understand the causal factors of these landslides 98 recent landslides in the study area mostly debris slumps were mapped and investigated together they displaced 11 millions m 3 of slope material statistical analysis shows that landslides dominate on steep concave slope segments that are oriented to the dominant rainfall direction northeast and at a relatively large distance from the water divide based on landslide occurrence and impact four different zones can be distinguished within the study area causal factors as well as landslide characteristics differ greatly between the four zones besides the fact that steep slopes high rainfall and typical soil properties and stratification turn manjiya into an inherently unstable area human interference cannot be neglected whereas deforestation has reduced the stability of the shallow soils on the eastern slopes of the study area the excavation of slopes mainly for house building is an important destabilizing factor for the western slopes the growing population density not only increases the risk on damage but hampers the search for solutions for the landslide problem as well d 2005 elsevier b v all rights reserved

topographic and road control of mega gullies in kinshasa dr congo abstract diachronic mapping 1957 1967 2007 and 2010 shows an exponentially growing mega gully network since roads were constructed through in the forests and plantations which occupied the sandy soils of the high town of kinshasa we found that the spatial occurrence of the mega gullies width 5 m in this newly urbanized environment is controlled by two factors first there is a topographic control given by the relation s 0 00008 a 1 459 with s being the slope gradient m m 1 of the soil surface at the gully head and a the drainage area ha above the head there is also a road control expressed by s 22 991 lc 1 999 with lc being the cumulated length of roads in the basin above the gully head the co existence of both controls reflects the fact that the local sands are highly permeable and hence roads are more important generators of continuous runoff the s a relation noted above should not be applied outside the town where the road network is less dense in contrast the s lc relation may be used in both the town and rural areas underlain by porous soils where roads are the only generators of continuous runoff we further conclude that the high town of kinshasa is one of the most vulnerable places for gullying and gullying can potentially transform the town into a badland artisanal gully treatment is more successful than generally believed and the s lc relation can be a tool for mega gully prevention

active deformation in old oceanic lithosphere and significance for earthquake hazard seismic imaging of the coral patch ridge area and neighboring abyssal plains sw iberian margin recently acquired high resolution multichannel seismic profiles together with bathymetric and sub bottom profiler data from the external part of the gulf of cadiz iberia africa plate boundary reveal active deformation involving old mesozoic oceanic lithosphere this area is located 180 km offshore the sw iberian peninsula and embraces the prominent ne sw trending coral patch ridge and part of the surrounding deep horseshoe and seine abyssal plains e w trending dextral strike slip faults showing surface deformation of flower like structures predominate in the horseshoe abyssal plain whereas ne sw trending compressive structures prevail in the coral patch ridge and seine hills although the coral patch ridge region is characterized by subdued seismic activity the area is not free from seismic hazard most of the newly mapped faults correspond to active blind thrusts and strike slip faults that are able to generate large magnitude earthquakes mw 7 2 8 4 this may represent a significant earthquake and tsunami hazard that has been overlooked so far

does the topographic threshold concept explain the initiation points of sunken lanes in the european loess belt abstract sunken lanes or road gullies are a common geomorphic feature in the european loess belt these landforms are usually created by various processes over time water erosion mass movements and traffic they are important for ecology and biodiversity for recreation and transport as well as from a hydrological and geomorphological perspective e g by enhancing the runoff and sediment connectivity of the landscape nonetheless little is known about their origin and evolution the formation of sunken lanes is typically only understood in qualitative terms while no studies so far have provided a quantitative analysis of their initiation conditions to address this research gap and to improve our geomorphic understanding of this landform we investigated to what extent sunken lanes can be treated and described as a gully erosion process more specifically we explored to what extent their position in the landscape and their characteristics can be explained by the topographic slope area sa threshold that typically controls gully initiation s k a b with k and b empirical constants based on field surveys and lidar data analyses we determined the slope gradient of the soil surface s and contributing area a for 132 representative sunken lane heads in the belgian loess belt and collected data on several other characteristics position of initiation and sedimentation points width depth and length of each sunken lane our results show a very large scatter on the s and a values moreover the morphological characteristics of sunken lanes e g width and length showed no clear relation with these values however a very weak sa threshold relation with a low b exponent 0 016 and a low k coefficient 0 022 could be identified overall our results show that the formation of sunken lanes is not in line with the topographic threshold conditions that govern the initiation of gullies across different environments the low k coefficient and the lack of a strong correlation between initiation slope and contributing area point towards an overall higher susceptibility to the formation of sunken lanes than the formation of gullies this supports the conclusion that not only concentrated flow detachment and transport but especially human activities mainly traffic erosion play a major role in the initiation and development of sunken lanes

main types of soil mass failure and characteristics of their impact factors in the yuanmou valley china abstract gully erosion is a dominant process causing soil loss in the yuanmou valley of yunnan province china soil mass failure is a common process involved in gully development the development of the gully and the occurrence of mass failure are interdependent and mutually promotive but mass failure types have not been very well understood in this study we identify the main types of soil mass failure and the characteristics of their impact factors in the yuanmou valley a detailed field survey was conducted in may 2010 in a representative area of the studied region and selected soil parameters were measured in the field and laboratory results showed that there are four different soil mass failure types falling sliding exfoliating and toppling each having unique characteristics of geology topography soil and climate examination of these characteristics indicates that the occurrence of mass failure is controlled by four factors 1 the typical geological structure of the yuanmou group represented by alternating layers of sand and clay 2 the different terrain and landforms 3 the different gully development stages and 4 the particular soil properties under special weather conditions including a dry hot climate with distinct wet and dry seasons

multi agent gully processes evidence from the monaro volcanic province australia and in terra cimmeria mars abstract comparison of the similarities and differences between terrestrial and martian hillside gullies promotes understanding of how surface processes operate on both planets here we tested the viability of subsurface flow of water as a process affecting gully evolution we compared gullies within the monaro volcanic province near cooma new south wales australia to gullies possessing strong structural control near gasa crater terra cimmeria mars although cursory examination of the monaro gullies initially suggested strong evidence for aquifer erosion detailed field surveys showed the evidence to be ambiguous instead a complex regime of erosion dependent on multiple conditions and processes such as local geology surface runoff dry mass wasting and animal activity emerged we found the morphology of gullies near gasa crater to be consistent with erosion caused by liquid water while also being heavily influenced by the local environment including slope and geology additionally erosion at the martian site was not consistent with evidence of subsequent smaller scale erosion and channel modification by dry mass wasting local conditions thus play an important role in gully evolution further highlighting that processes forming martian gullies may be more diverse than initially thought

regional mapping and characterisation of old landslides in hilly regions using lidar based imagery in southern flanders abstract analysis of lidar derived imagery led to the discovery of more than 330 pre holocene to recent landslides in southern flanders 4850 km 2 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

ravine formation on steep slopes forward versus regressive erosion some case studies from rwanda summary observations on steep slopes in rwanda reveal that human interventions on surficial drainage can introduce gullying on places where runoff normally leads to diffuse or ephemeral rill erosion in such cases an initial runoff incision or slide scar respectively due to an artificial concentration of runoff or underflow grows in downslope direction mainly by runoff incision this type of progressive or forward gully development contrasts with the more classical regressive development here only observed in places where the flow gradient is less than 7 8

land use changes on the slopes of mount elgon and the implications for the occurrence of landslides abstract a reconstruction of land use changes and the implications thereof for landslide occurrence on critical slopes of mount elgon in eastern uganda were undertaken aerial photographs taken in 1960 formed the benchmark for the analysis of respective land use changes between 1995 and 2006 using 30 m landsat tm and 20 m spot ms images landslide sites were mapped using a mobilemapper and terrain parameters were derived using a 15 m digital elevation model a supervised classification approach was employed to generate land cover maps from which the areas of three land cover classes agricultural fields woodlands and forests were calculated a post classification comparison change detection technique revealed different trends in land cover change between the periods 1960 1995 and 1995 2006 whereas there were minimal land use changes between 1960 and 1995 the period 1995 2006 marked a considerable loss of woodlands and forest cover particularly on steep concave slopes 36 58 of the national park the encroachment onto the critical slopes was noted to have induced a series of shallow and deep landslides in the area all the mapped landslides were noted to lie on steep concave slopes of a northerly orientation which had been opened up for cultivation deforestation and cultivation alter the soil hydrological conditions on steep concave slopes rendering them susceptible to saturation this may trigger debris flows during rainfall events there is a need to restore forest cover on the fragile steep slopes and restrain local communities from opening up new areas for cultivation on critical slopes particularly within the protected area

a data based landslide susceptibility map of africa abstract our understanding of the spatial patterns of landslides in africa is limited with available landslide studies typically focusing on only one or a few study areas moreover africa is clearly underrepresented in terms of available landslide inventories this study aims to produce a first continent wide landslide susceptibility map for africa calibrated with a well distributed landslide dataset we reviewed the literature on landslides in africa and compiled all available landslide inventories ca 10 800 landslides supplemented by additional landslide mapping using google earth imagery in underrepresented regions ca 7250 landslides this resulted in a dataset of approximately 18 050 landslides various environmental variables were investigated for their significance in explaining the observed spatial patterns of landslides to account for potential mapping biases in the dataset we used monte carlo simulations that selected different subsets of mapped landslides to test the significance of the considered environmental variables based on these analyses we constructed two landslide susceptibility maps for africa one for all landslide types and one excluding the known rockfalls in both maps topography is by far the most significant variable we evaluated the performance of the fitted multiple logistic regression models using independent subsets of landslides selected from the total dataset overall both maps perform very well in predicting intra continental patterns of landslides in africa and explain about 80 of the observed variance in landslide occurrence to further test the robustness and sensitivity to mapping biases we also modelled landslide susceptibility while excluding regions with arid climates as landslides in these environments are expected to be better preserved over time and therefore likely relatively overrepresented despite this potential bias the effect on the landslide susceptibility model is limited based on the constructed database and our analyses we further discuss potential research gaps for landslide prediction in africa and at continental scales for example analysis of the african countries mean landslide susceptibility shows a lack of landslide research in various countries prone to landsliding e g guinea gabon lesotho madagascar apart from the intrinsic value of this landslide susceptibility map as a natural hazard risk management tool the map and compiled database are highly promising for other applications for example we explored the potential significance of landslides as a geomorphic process by confronting our landslide susceptibility map with an available database of measured catchment sediment yield for 500 rivers in africa overall a significant positive but relatively weak relation between landslide susceptibility and sediment yield is observed

spatial patterns of old deep seated landslides a case study in the northern ethiopian highlands abstract during the last decade slope failures were reported in a 500 km2 study area in the geba werei catchment northern ethiopia a region where landslides were not considered an important hazard before field observations however revealed that many of the failures were actually reactivations of old deep seated landslides after land use changes therefore this study was conducted 1 to explore the importance of environmental factors controlling landslide occurrence and 2 to estimate future landslide susceptibility a landslide inventory map of the study area derived from aerial photograph interpretation and field checks shows the location of 57 landslides and six zones with multiple landslides mainly complex slides and debris flows in total 14 8 of the area is affected by an old landslide for the landslide susceptibility modelling weights of evidence wofe was applied and five different models were produced after comparison of the models and spatial validation using receiver operating characteristic curves and kappa values a model combining data on elevation hillslope gradient aspect geology and distance to faults was selected this model confirmed our hypothesis that deep seated landslides are located on hillslopes with a moderate slope gradient i e 5 13 the depletion areas are expected on and along the border of plateaus where weathered basalts rich in smectite clays are found and the landslide debris is expected to accumulate on the amba aradam sandstone and upper antalo limestone as future landslides are believed to occur on inherently unstable hillslopes similar to those where deep seated landslides occurred the classified landslide susceptibility map allows delineating zones where human interventions decreasing slope stability might cause slope failures the results obtained demonstrate that the applied methodology could be used in similar areas where information on the location of landslides is essential for present day hazard analysis

spatial patterns causes and consequences of landslides in the gilgel gibe catchment sw ethiopia abstract the gilgel gibe catchment in sw ethiopia is one of the areas in east africa affected by landslides to better understand the patterns and the causal factors of these landslides all landslides in a small study area 14 km in the hilly parts of the gilgel gibe catchment were mapped and analyzed in total 60 landslides were mapped these landslides caused a displacement of 1 million m slope material which corresponds to a mean displaced volume of 50 ton ha 1 y 1 in the last 20 years moreover many landslides deliver directly sediment to the rivers and hence increase the sediment load in the rivers this soil loss to the rivers was estimated at 11 ton ha 1 y 1 during the same period high annual rainfall ca 2000 mm y 1 lithological and pedological properties and to a lesser extent steep 16 slopes turn the area into an inherent unstable situation and can be indicated as preconditions for the landslides in the study area distance to rivers is significantly the most important precondition as slopes near rivers are less stable than slopes further away from the rivers this is mainly caused by river incision and bank erosion which often occur in the area and which can be attributed to increased runoff due to deforestation over the past 20 years therefore recent deforestation caused more shallow landslides but also indirectly more deep seated landslides close to the rivers heavy rainfall is indicated as the main triggering factor for almost all landslides

a geomorphological assessment of landslide origin at bukavu democratic republic of the congo abstract bukavu on the south coast of lake kivu in the democratic republic of the congo suffers from slow but never ending landsliding this leads to the continuous degradation and destruction of houses buildings and of the roads waterworks and sewerage infrastructure in several districts of the town research of mass wasting processes is hampered by the limited geotechnical and hydrogeological information concerning the deeply weathered tertiary and quaternary lavas there is also a disagreement about the exact location of active faults believed to play a role in the mass wasting additionally there is little information about the exact location and the rate of soil movements and whether ground instability is caused by tectonics and seismics or by increase in hydrostatic pressure based on aerial photographic interpretation landslides cover more than 15 of the town of bukavu thirty one landslides occur outside the bukavu microrift and do not contact active faults rather they occur in actively incising river basins ostensibly caused by neotectonic activity four from the six landslides within the micrograben are bigger and wider than the others apparently governed by pre existing tectonically induced landforms they are adjacent to or crossed by active faults they also fall far below the envelope of topographic thresholds for landslides established for north america and verified in rwanda therefore this threshold a combination of slope at the head of the slide and surface drained to it seems a promising tool to discriminate hydrologically from tectonically seismically induced landslides recent soil movements in bukavu generally correspond to landslide distribution portrayed on the geomorphologic map most bukavu soil movements occur on previous slides therefore from an engineering geologic standpoint old landslides should be avoided or if economically feasible be mitigated

channel network extent in the context of historical land use flow generation processes and landscape evolution in the north carolina piedmont intensive agricultural land use in the 18th to early 20th centuries on the southeastern piedmont resulted in substantial soil erosion and gully development today many historically farmed areas have been abandoned and afforested and such landscapes are an opportunity to study channel network recovery from disturbance by gullying channel initiation mapping watershed area slope relationships and field monitoring of flow generation processes are used to identify channel network extent and place it in hydrologic historical and landscape evolution context in six study areas in the north carolina piedmont 100 channel heads were mapped in fully forested watersheds revealing a channel initiation relationship of 380 as1 27 where a is contributing area m2 and s is local slope m m flow in these channels is generated by subsurface and overland flow the measured relative slope exponent is lower than expected based on literature values of 2 for forested watersheds with subsurface and overland flow suggesting that the channel network extent may reflect a former hydrological regime however geomorphic evidence of recovery in channel heads within fully forested watersheds is greater than those with present day pasture present day channel heads lie within hollows or downslope of unchanneled valleys which may be remnants of historical gullies and area slope relationships provide evidence of colluvial aggradation within the valleys channel network extent appears to be sensitive to land use change with recovery beginning within decades of afforestation channel initiation mapping and area slope relationships are shown to be useful tools for interpreting geomorphic effects of land use change copyright 2012 john wiley sons ltd

basin rebound origin for the tuscarora unconformity in southwestern virginia and its bearing on the nature of the taconic orogeny