landslide driven erosion and slope channel coupling in steep forested terrain ruahine ranges new zealand 1946 2011 abstract landslides are an important means of conveying sediment from slopes to channels in steepland environments but landsliding is a discontinuous process this means that any single assessment in time of their contribution to the sediment cascade is of limited value to better understand landscape dynamics and the contribution of landslides to slope channel coupling over time this paper quantifies connectivity over a time span of 65 years in small steep headwater catchments in the southern ruahine range new zealand temporal variability in landsliding and slope channel coupling was assessed using six sets of aerial photography flown between 1946 and 2011 from which over 6900 landslides were mapped in arcgis of which up to 78 connected with the stream network estimates of the volume of material delivered by landslide erosion to headwater channels were based on ground survey measurements of selected landslide scars and suggest that between 1946 and 2011 over 5 million m 3 of sediment was delivered from slopes to channels in the 221 km 2 study area forest cover is not sufficient to prevent this erosion these catchment systems are particularly vulnerable to high magnitude storm events which significantly elevate landslide intensity and enhance sediment delivery as occurred in the mid 1970s the legacy of these events remains in these headwater channels with ongoing consequences for stream and hazard management in and adjacent to the ranges

spatial modelling and uncertainty assessment of pyroclastic cover thickness in the sorrento peninsula assessing spatial variability of soil thickness is a critical issue for understanding and predicting slope processes the present work was aimed at estimating the spatial scales at which the variation of pyroclastic cover thickness occurs in a sample area in the sorrento peninsula italy stochastic simulation was used to understand the spatial variability of pyroclastic cover thickness on mount pendolo and to assess its spatial uncertainty in the study area covering about 0 7 km2 thickness measurements were collected using electrical resistivity tomography profiles continuous core drillings and steel rod penetrometric tests variographic analysis revealed the occurrence of an anisotropic behaviour along the n50 and n140 directions in the latter anisotropic direction a nested variogram was fitted including 1 a long range component which could be related to large scale factors like the curvature of the slope and contributing area and 2 a shorter scale variation which is probably associated with the occurrence of denudation processes or to the articulate cover bedrock interface to assess the spatial variability and uncertainty of pyroclastic cover thickness a stochastic simulation algorithm was used and 500 equally probable images of cover thickness were yielded the results showed that a better thickness distribution map can be drawn by simulating the data collected on the slope and at the footslope separately the approach also allowed delineating the areas characterized by greater uncertainty suggesting supplementary measurements to further improve the cover thickness distribution model thus reducing the uncertainty

landslide occurrence as a response to land use change a review of evidence from new zealand abstract vegetation cover is an important factor influencing the occurrence and movement of rainfall triggered landslides and changes to vegetation cover often result in modified landslide behaviour however it is difficult to relate the occurrence of landslides directly to variations in land use especially in some european countries in contrast new zealand provides a good opportunity to investigate geomorphic responses to anthropogenic land cover changes before european settlers first arrived in the 1840s hilly regions were only marginally influenced by human activity the maoris new zealand s first settlers lived largely on coastal plains or near lakes and rivers they influenced general vegetation cover only through localized burning practices in contrast european settlers moved into the back country and converted extensive hill areas from native forest and bush to pasture this reduced the strength of the regolith and rendered the slopes more susceptible to landslides pulses of natural sedimentation in the pre european period have been related to volcanic activity climatic variability including changes in frequency of cyclonic storms and wind erosion and fluvial erosion following forest fires initiated either by volcanic eruptions or lightning strikes since european deforestation began sediment production has largely been determined by landslide events on unstable slopes thousands of landslides were triggered by high magnitude low frequency climatic events during storms with estimated return periods in excess of 50 years in contrast low magnitude high frequency rainfall events have caused gully and channel erosion examples from different parts of new zealand indicate changes in sediment generating processes following land use modifications after deforestation landslides have contributed significantly to sediment sequences in depositional basins such as lakes swamps estuaries coastal wetlands and the nearshore and offshore zones of continental platforms

validation of a region wide model of landslide susceptibility in the manawatu wanganui region of new zealand since european settlement 160 years ago much of the indigenous forest in new zealand hill country has been cleared for pastoral agriculture resulting in increased erosion and sedimentation to prioritise soil conservation work in the manawatu wanganui region we developed a model of landslide susceptibility it assigns high susceptibility to steep land not protected by woody vegetation and low susceptibility everywhere else following the commonly used approach for identifying inappropriate land use a major storm on 15 16 february 2004 that produced many landslides was used to validate the model the model predicted hills at risk to landsliding with moderate accuracy 58 of erosion scars in the february storm occurred on hillsides considered to be susceptible the model concept of slope thresholds above which the probability of landsliding is high and below which the probability is low is not adequate because below 308 the probability of landsliding is approximately linearly related to slope thus reforestation of steep slopes will need to be combined with improved vegetation management for soil conservation on moderate slopes to significantly reduce future landsliding d 2005 elsevier b v all rights reserved

channel bed adjustments following major aggradation in a steep headwater setting findings from oyabu creek kyushu japan abstract a typhoon in 1993 induced major aggradation along oyabu creek a steep gravel bed mountain stream in kyushu japan processes of sediment reworking are inferred from a 7 year monitoring program that measured adjustments to channel cross sections the longitudinal profile and the extent distribution of bedrock outcrops along a 3 km study reach over time the reach adopted a riffle and pool structure with notable increase in the area of exposed bedrock on the bed this adjustment process was characterised by progressive reduction in sediment storage change per unit flow the relaxation pathway following disturbance induced by the typhoon was shaped by the magnitude and frequency of subsequent rainfall events the capacity of these events to transport available sediments and physical linkages between reaches adjacent subreaches demonstrated differing relaxation pathways in response to these influences induced by spatial and temporal variability in threshold conditions along the channel longer term evidence indicates that responses to major disturbance such as the 1993 typhoon occur as cycles of around 20 year duration a relaxation period of 7 years is required to attain a quasi equilibrium bed configuration and rate of sediment flux the timeframe of cycles is considered to reflect changes to hillslope channel bed coupling marking the period required to generate sufficient sediment stores to reactivate phases of aggradation and subsequent degradation

post deforestation soil loss from steepland hillslopes in taranaki new zealand soil erosion on steepland hillslopes in taranaki new zealand where landsliding is the dominant erosion form was investigated by comparing mean regolith depths between first order basins that have had their forest cover removed for different periods of time regolith depth and slope angle data were collected along 19 profile lines and 30 profile lines from steepland basins that had been deforested for 10 and 85 years respectively these profile lines were subdivided into a total of 236 profile segments of relatively linear slope angle and uniform regolith depth that averaged 17 5 m in length the depth of pre existing regolith on post deforestation landslide sites is estimated from a regression of regolith depth on slope angle for undisturbed non landslide profile segments regolith depletion on landslide sites is in turn estimated by subtracting the depth of regolith on landslide sites from the estimate of pre existing regolith depth regolith depletion by post deforestation landslides averaged over the entire length of profile lines gives an estimate of average surface lowering for the area deforested for 85 years average surface lowering by post deforestation landslides is 0 15 0 04 m and is the same as the difference in mean depth of 0 15 0 11 m between this area and the area deforested for 10 years erosion of regolith from hillslopes by processes other than landsliding appears to be minimal the 0 15 m average surface lowering represents a regolith depletion rate of 1 8 0 5 mm yr 1 for hillslopes steeper than 28 where all post deforestation landslides occur average surface lowering is 0 20 0 05 m and the regolith depletion rate is 2 4 0 6 mm yr 1 average surface lowering is greatest at 0 23 0 07 m on hillslopes steeper than 32 where most post deforestation landslides occur here the regolith depletion rate is 2 7 0 8 mm yr 1 a large magnitude low frequency storm in march 1990 produced an average surface lowering of 0 041 m there were proportionately more landslides in the area deforested for 10 years illustrating the importance of previous erosion history of hillslopes on the spatial distribution of landslides there were also comparatively few landslides on steeper hillslopes because previous lower magnitude storms had already removed much of the deeper regolith

slope gradient controls soil thickness and chemical weathering in subtropical brazil understanding rates and timescales of regional soilscape evolution through a combination of field data and modeling

topographic controls on evolution of shallow landslides in pastoral wairarapa new zealand 1979 2003 abstract this study aims to identify topographic influence on shallow landslides and their evolution during 1979 2003 in the pastoral lower north island new zealand landslide affected sites were mapped from aerial photographs and an aster satellite image the two landslide maps were overlaid in a geographic information system arcgis to detect the changes that have taken place in landslide affected areas the change map and the landslide distribution maps were overlaid with three topographic layers of slope gradient slope position and slope aspect that had been derived from a digital elevation model in arcgis the overlay analysis revealed that landslides affected area increased from 23 75 ha in 1979 to 110 97 ha in 2003 the distribution of landslide affected sites is inversely correlated with slope gradient but positively with slope position northern aspects are the most vulnerable to landsliding containing about 78 of the total affected sites moderate slope gradients suffered the most severe landsliding in the lower to middle slopes the overall magnitude and frequency of landsliding is also related to northern and eastern octants on the moderate slope gradients in the low to moderate slope positions although the same topographic setting is conducive to landsliding in both 1979 and 2003 the rate of landsliding varies with topographic setting hence topography is critical not only to landsliding but also to the evolution of sites already affected by landslides such an understanding over large spatial scales is possible only with the use of multitemporal remote sensing data and gis that are able to speed up data collection and analysis

approaches to dynamic equilibrium in theoretical simulations of slope development the model slop3d is used to identify characteristic aspects of slope development under different conditions a series of slopes along the kall valley in the northern eifel is reproduced by a developmental sequence of model slopes the spatial and temporal variations of the components of slope development systems relief slope form weathering rates denudation rates are discussed by means of mass movement and wash denudation models the negative feedbacks between system components create a tendency towards the establishment of a dynamic equilibrium which is reached however only if the endogenic and exogenic conditions remain constant for the required relaxation time the latter is shown to be a function of slope length of the rate of uplift of the denudational process type and of the intensity of exogenic inputs e g precipitation varying rock resistance leads to the development of shield inselbergs only if the rate of denudation becomes higher than the maximum possible weathering rate of the resistant bedrock

erosion rates in the southern oregon coast range evidence for an equilibrium between hillslope erosion and sediment yield the relationship of hillslope erosion rates and sediment yield is often poorly defined because of short periods of measurement and inherent spatial and temporal variability in erosion processes in landscapes containing hillslopes crenulated by alternating topographic noses and hollows estimates of local hillslope erosion rates averaged over long time periods can be obtained by analysing colluvial deposits in the hollows hollows act as local traps for a portion of the colluvium transported down hillslopes and erosion rates can be calculated using the age and size of the deposits and the size of the contributing source area analysis of colluvial deposits in nine oregon coast range hollows has yielded average colluvial transport rates into the hollows of about 35 cm3cm 1yr 1 and average bedrock lowering rates of about 007 mm yr for the last 4000 to 15000 yr these rates are consistent with maximum bedrock exfoliation rates of about 0 09 mm yr calculated from six of the hollows supporting the interpretation that exfoliation rates limit erosion rates on these slopes sediment yield measurements from nine coast range streams provide similar basin wide denudation rates of between 0 05 and 0 08 mm yr 1 suggesting an approximate steady state between sediment production on hillslopes and sediment yield in addition modern sediment yields are similar in basins varying in size from 1 to 1500 km2 suggesting that erosion rates are spatially uniform and providing additional evidence for an approximate equilibrium in the landscape

sediment budget to assess the geomorphic effect of a cyclonic storm new zealand abstract a short term sediment budget was constructed to assess the erosion response of a soft rock hill country watershed to an intense rainstorm event the watershed is located in a landslide prone area on the east coast of the north island new zealand during march 1988 753 mm of rain was recorded over a four day period with 320 mm and 329 mm on successive days known as cyclone bola this was the largest rainstorm event in the tutira watershed in the 93 year rainfall record the budget quantifies the total sediment generated during the storm the relative contribution of erosion processes involved the amount of sediment held in storage and the amount discharged into two lakes within the watershed a landform map of the watershed was constructed and the contribution of each landform type to the total budget was calculated a total of 1 35 0 13 million m3 of sediment was generated during the storm at an average of 420 m3 ha this is equivalent to a denudation value of 42 mm for the total watershed 3208 ha and 83 mm for the landforms severely affected by landsliding 1427 ha of the sediment generated 21 remained on hillslopes 22 was deposited on valley floors 51 was deposited on the lakebeds and the remaining 6 was discharged from the watershed via the lake outlet most of the sediment generated during the storm was from primary source areas on hill slopes with sediment in secondary storage providing only a small contribution landslide erosion was the main process accounting for 89 of the sediment generated channel tunnel gully and sheet erosion were only minor contributors to the budget six hillslope landforms which occupy only 44 of the watershed generated 90 of the sediment the results of this sediment budget when put in context with the storm magnitude frequency history being analysed from lake cores contribute to the identification of sustainable land use and management of soft rock hill country

slope control on the frequency distribution of shallow landslides and associated soil properties north island new zealand the extrapolation of results from field trials to larger areas of land for purposes of regional impact assessment is an important issue in geomorphology particularly for landform properties that show high stochastic variability in space and time such as shallow landslide erosion it is shown in this study that by identifying the main driver for spatial variability in shallow landslide erosion at field scales namely slope angle it is possible to develop a set of generic functions for assessing the impact of landslides on selected soil properties at larger spatial scales and over longer time periods research was conducted within an area of pastoral soft rock tertiary hill country in the north island of new zealand that is subject to infrequent high intensity rainfall events producing numerous landslides most of which are smaller than several hundred square metres in size and remove soil to shallow depths all landslides were mapped within a 0 6 km2 area and registered to a high resolution 2 m slope map to show that few landslides occur on slopes 24 the areal density of landslides from all historical events showed an approximately linear increase with slope above 24 integrating landslide densities with soil recovery data demonstrates that the average value of a soil property fluctuates in a saw tooth fashion through time with the overall shape of the curve controlled by the frequency of landslide inducing storm events and recovery rate of the soil property between events despite such fluctuations there are gradual declines of 7 5 in average total carbon content of topsoil and 9 5 in average soil depth to bedrock since the time of forest clearance results have application to large scale sediment budget and water quality models and to the new zealand soil carbon monitoring system cms copyright 2012 john wiley sons ltd

soil depth age relationship of landslides on deforested hillslopes taranaki new zealand a soil chronosequence was examined on landslide scars of different ages in the taranaki hill country this area underlain by tertiary silty sandstone was deforested 90 years ago sequential aerial photographs historical terrestrial photographs and documented erosion events were used to date landslide scars formed since forest removal and establish age control for the chronosequence soil depth increased with landslide age and was used as an index of soil formation mean soil depth increased from 5 cm on 15 year old scars to 20 cm on 82 year old scars measures soil depths were attributed to rafted soil colluvium and bedrock weathering a chronofunction was derived by regressing mean soil depth against the logarithm of known scar age similarly a second chronofunction was derived by excluding depths of rafted soil from the calculation of mean soil depth to describe soil accumulation on exposed bedrock within the landslide scar this chronofunction showed a better correlation r2 0 92 compared with r2 0 79 and can give an estimate of the age of other landslide scars to within 27 for ages up to 90 years the rate of soil depth increase averaged 3 5 mm yr 1 over the first 40 years after slipping but dropped to 1 2 mm yr 1 over the following 50 years the logarithmic chronofunction suggests that the rate of soil formation further decreases beyond 90 years soil formation is primarily a result of bedrock weathering and accumulation of colluvium derived from surface fragmentation of exposed bedrock and crumbling scar margins

new insight into the factors leading to the 1998 flank collapse and lahar disaster at casita volcano nicaragua during unusually high rainfall associated with hurricane mitch in 1998 a flank collapse leading to a disastrous lahar occurred at casita volcano nicaragua the lack of a similar failure during a rainfall event of comparable magnitude in 1982 tropical storm alleta suggests that additional factors influenced the 1998 collapse we have investigated the potential contribution of erosional flank undercutting seismic activity and anthropogenic land cover change at the collapse site except for seismic events prior to mitch which may have increased flank instability none of these factors appears significant instead for failure to occur the following conditions were required 1 highly fractured slope material allowing deep and rapid infiltration of meteoric water 2 a less permeable underlying layer to obstruct drainage 3 strong continuous antecedent rainfall to build up high pore water pressure and 4 episodic high intensity precipitation during mitch to generate recurrent pressure waves prerequisite 1 was provided by edifice wide deformation towards the south east seismic activity and proximity to a prominent fault and local subsidence condition 2 was met by clay rich layers resulting from hydrothermal alteration more than 1 900 mm of rain fell in the 6 months prior to mitch without significant interruption while intense episodic precipitation occurred during the hurricane satisfying conditions 3 and 4 respectively the main difference with alleta was that it occurred at the beginning of the rainy season and therefore without sufficient antecedent rainfall anthropogenic activity including land cover change did not affect slope stability i e the hazard however vulnerability was generated when two towns were established in the lowlands south of casita on top of previous lahar deposits it was greatly increased when approximately 4 km of forest between the collapse site and the towns were cleared paving the way for a largely unobstructed debris flow deforestation also facilitated erosion along the flanks to provide about 78 of the material contained in the lahar when it destroyed the towns killing more than 2 500 people

geomorphic change implied by regolith slope relationships on steepland hillslopes taranaki new zealand abstract slope angle and regolith depth were measured along profile lines covering a range of hillslopes in several small neighbouring drainage basins in taranaki hill country new zealand slope measurements show a cyclic variation along the length of broadly rectilinear profiles variation in slope on the lower portion of profiles adjacent to stream channels indicates that hillslopes have undergone phases of accelerated downcutting and hillslope rejuvenation a recent phase of increased erosion and hillslope rejuvenation is suggested for hillslopes steeper than 31 by the steep lowerslope and incised stream channel hillslope evolution occurs primarily by parallel retreat of steeper swale sites and slope decline during pauses in the rate of downcutting all slope profiles show a large range in regolith depth mean regolith depth of swale sites is highly correlated with mean profile slope angle mean regolith depth decreases and the frequency of landslides increase with increasing mean profile slope angle microtopographic variation in regolith depth is attributed mainly to vegetation effects recurrent landsliding and in part to bedrock curvature hillslope position and slope angle shallowing of regolith is caused by recurrent landsliding on steep swale sites fluvial erosion at the base of steep profiles and surface erosion on extremely convex spur sites the very shallow soil on the steep lowerslope portion of profiles suggests a phase of soil removal consistent with hillslope rejuvenation average depth of regolith is least at 97 cm and variability greatest on hillslopes with a profile mean exceeding 31 erosion rates are greater on these sites where soil is derived predominantly from underlying tertiary sandstone this is attributed to a greater frequency of landsliding compared to hillslopes with a profile slope less than 31 where a more regular and deeper average regolith of 153 cm infers a longer period of hillslope stability consistent with the high volcanic ash content of soils airfall andesitic volcanic ash deposited throughout the holocene has largely been removed from steeper sites the work demonstrates the need to interpret the frequency and distribution of landslides in context with natural geomorphic changes in order to assess the effect that landuse changes has on erosion rates

prediction of landslide occurrence based on slope instability analysis and hydrological model simulation summary hillslope failure usually occurs as soil resistance deteriorates in the presence of the acting stress developed by a rising groundwater level during rainstorms the present study adopted a slope instability analysis and a hydrological model for landslide prediction during heavy rainstorms variation of the groundwater table on hillslope was simulated by using the hydrological model and then the temporal groundwater level at each grid was substituted into the slope instability analysis to determine the instability of the grids in watersheds for prediction of massive landslides hydrological records from two landslide prone areas in northern taiwan were collected digital elevation model was adopted to obtain the geomorphologic factors required for the slope instability analysis and the hydrological model the spatial distribution of soil thickness required for performing the infinite slope model was estimated by using a wetness index results showed that the temporal variation of the percentage of unstable grids in the study watersheds basically followed the variation of rainfall hyetographs the percentage of the unstable grids reached a maximum value when the centroid of the hyetograph passed a comparison between the landslide records and the model analytical results revealed that a massive landslide might occur if more than 50 of the grids in the subwatershed were classified as unstable in the study areas the predicted time and location of landslide occurrence were consistent with those obtained from field investigations it is therefore considered promising to apply the developed analytical method for landslide warning to alleviate the loss of lives and property

geomorphic events and landform response in south eastern taranaki new zealand abstract south eastern taranaki is a tectonically rising area of hill country and marine terraces cut into plio pleistocene marine sediments in the north island of new zealand it is argued that the geomorphic development of the region is more satisfactorily explained with resort to concepts of dynamic metastable equilibrium and unsteady behaviour than by concepts of steady equilibrium behaviour the region is constantly adjusting in part or in whole to a spectrum of geomorphic events including sea level change tectonic uplift aeolian deposition large landslide triggering earthquakes deforestation and landslide triggering rainstorms the record of these events in terms of frequency and magnitude is established and the consequential landform response is discussed twelve high sea level events within the last 700 ka are recorded in the landscape each with associated phases of aeolian deposition large earthquakes with terrain forming capability occur on average once every 1000 to 10 000 years as a result of slope destabilization by recent deforestation erosion rates appear to have increased by three times their previous rate the agents involved in this process are rainstorms with a return period of about 10 years which during a single event can strip the regolith from 5 to 15 percent of hillslopes over areas of up to 100 km2

decreasing landslide erosion on steeper slopes in soil mantled landscapes slope stability models predict that steeper hillslopes require smaller hydrological triggers for shallow landslides to occur due to the added downslope pull of gravity which should result in more frequent landslides and faster erosion however field observations indicate that landslide frequency does not consistently increase on steeper hillslopes here we use measurements of 1 096 soil landslides in california and switzerland and a compilation of landslide geometries to show that steeper hillslopes typically have thinner soils and that thin soils inhibit landslides due to enhanced roles of cohesion and boundary stresses we find that the landscape averaged landslide erosion depth peaks near the threshold slope for instability and it drops to half that value on hillslopes that are just 5 to 10 steeper we propose that faster rates of soil creep on steeper slopes cause thin and more stable soils which in turn reduces landslide erosion despite the added pull of gravity

a process based model for colluvial soil depth and shallow landsliding using digital elevation data a model is proposed for predicting the spatial variation in colluvial soil depth the results of which are used in a separate model to examine the effects of root strength and vertically varying saturated conductivity on slope stability the soil depth model solves for the mass balance between soil production from underlying bedrock and the divergence of diffusive soil transport this model is applied using high resolution digital elevation data of a well studied site in northern california and the evolving soil depth is solved using a finite difference model under varying initial conditions the field data support an exponential decline of soil production with increasing soil depth and a diffusivity of about 50 cm 2 yr the predicted pattern of thick and thin colluvium corresponds well with field observations soil thickness on ridges rapidly obtain an equilibrium depth which suggests that detailed field observations relating soil depth to local topographic curvature could further test this model bedrock emerges where the curvature causes divergent transport to exceed the soil production rate hence the spatial pattern of bedrock outcrops places constraints on the production law the infinite slope stability model uses the predicted soil depth to estimate the effects of root cohesion and vertically varying saturated conductivity low cohesion soils overlying low conductivity bedrock are shown to be least stable the model may be most useful in analyses of slope instability associated with vegetation changes from either land use or climate change although practical applications may be limited by the need to assign values to several spatially varying parameters although both the soil depth and slope stability models offer local mechanistic predictions that can be applied to large areas representation of the finest scale valleys in the digital terrain model significantly influences local model predictions this argues for preserving fine scale topographic detail and using relatively fine grid sizes even in analyses of large catchments