Changes In Slope Gradient Research Articles

Unidirectional and combined transitional flow bedforms: Controls on process and distribution in submarine slope settings

ABSTRACTMixed grain‐size bedforms comprise alternating sand‐rich and poorly sorted mud‐rich laminae and bands. These bedforms have been identified in distal submarine settings formed underneath unidirectional flows. This study documents mixed grain‐size bedforms in a proximal submarine slope setting formed beneath both unidirectional and combined flows. Core and outcrop data with well‐constrained palaeogeographical context are used to describe two types of mixed grain‐size bedform. Type A bedforms comprise mud‐rich current ripples and low‐amplitude bed‐waves with alternating concave and planar sandstone–mudstone foresets that pass into mud‐rich troughs, and aggradational sinusoidal laminasets. Type B bedforms consist of sandstone–mudstone laminasets that comprise rounded, biconvex ripples with sigmoidal‐shaped foresets and swale and hummock‐like laminasets and banded sets. These bedforms occur in channel‐margin, internal‐levée and external‐levée, intraslope and disconnected lobe environments, and represent 27 to 63% by stratigraphic thickness of the studied successions. They are interpreted as deposits of clay‐rich transitional flows, whose depositional style is governed by the balance of cohesive and turbulent forces, and the rate of flow deceleration. Type B bedforms are further interpreted as combined transitional flow deposits, resulting from flow deflection and ponding processes by seabed topography. Upward and lateral transitions between different bedforms create distinct bedform sequences, demonstrating progressive spatio‐temporal transformations in flow properties and their topographic interactions. By using a well‐constrained palaeogeographical setting, mixed grain‐size bedforms are shown to be situated close to sites of erosion into muddy substrates, abrupt losses in confinement, and/or changes in slope gradient. These bedforms demonstrate that flow transformation and transitional flow behaviour are not restricted to distal submarine settings. Furthermore, mixed grain‐size bedforms are not a diagnostic criterion for bottom currents, because such flows cannot account for the high mud content in laminasets, or the interlamination of sand and mud.

End-point determination of heterogeneous formulations using inline torque measurements for a high-shear wet granulation process

In this study, the torque profiles of heterogeneous granulation formulations with varying powder properties in terms of particle size, solubility, deformability, and wettability, were studied, and the feasibility of identifying the end-point of the granulation process for each formulation based on the torque profiles was evaluated. Dynamic median particle size (d50) and porosity were correlated to the torque measurements to understand the relationship between torque and granule properties, and to validate distinction between different granulation stages based on the torque profiles made in previous studies. Generally, the torque curves obtained from the different granulation runs in this experimental design could be categorized into two different types of torque profiles. The primary factor influencing the likelihood of producing each profile was the binder type used in the formulation. A lower viscosity, higher solubility binder resulted in a type 1 profile. Other contributing factors that affected the torque profiles include API type and impeller speed. Material properties such as the deformability and solubility of the blend formulation and the binder were identified as important factors affecting both granule growth and the type of torque profiles observed. By correlating dynamic granule properties with torque values, it was possible to determine the granulation end-point based on a pre-determined target median particle size (d50) range which corresponded to specific markers identified in the torque profiles. In type 1 torque profiles, the end-point markers corresponded to the plateau phase, whereas in type 2 torque profiles the markers were indicated by the inflection point where the slope gradient changes. Additionally, we proposed an alternative method of identification by using the first derivative of the torque values, which facilitates an easier identification of the system approaching the end-point. Overall, this study identified the effects of different variations in formulation parameters on torque profiles and granule properties and implemented an improved method of identification of granulation end-point that is not dependent on the different types of torque profiles observed.

Response of erosion reduction effect of typical soil and water conservation measures in cropland to rainfall and slope gradient changes and their applicable range in the Chinese Mollisols Region, Northeast China

Mollisols are rich in organic matter, which makes them suitable for cultivation and for enhancing global food security. Mollisols are experiencing severe soil erosion due to overfarming and a lack of maintenance. Thus, suitable soil and water conservation measures (SWCMs) are needed to protect Mollisols. However, how SWCMs respond to changes in slope gradient and rainfall and their effective application area remain ambiguous. Using a long-term field observation dataset, meta-analysis, and statistical test methods, we reveal the typical erosion reduction effect of SWCMs and their responses to changes in slope gradient and rainfall. Next, we calculated a coupling factor, P·S, by slope gradient and rainfall in flood season to determine the effective SWCMs application threshold. Compared with bare land, no-tillage (NT), contour ridge tillage (CT), ridge hedgerows (RH), and terraced fields (TF) had an average erosion reduction coefficient of 91.5%. There was a significant exponential increase in the correlation between P·S and the soil erosion amount in plots with typical measures. According to this correlation and soil loss tolerance (200 t/km2), the upper limits of P·S (NT: 564 mm; CT: 885 mm; RH: 1135 mm) were determined and utilized to determine the effective application areas (NT: 311.40 103 km2; CT: 320.86 103 km2; RH: 323.72 103 km2) at the plot scale. In wet years, the applicable area of SWCMs declined toward the low-elevation foothills and flat terrain. All the results are applicable when slope length within 20 m. These results provide a basis for the precise allocation of SWCMs in Mollisols area and promote the scientific utilization of Mollisols resources.

Contrasting mixed siliciclastic-carbonate shelf-derived gravity-driven systems in compressional intra-slope basins (southern Hikurangi margin, New Zealand)

Along active margins, the combination of predominant tectonic activity and shallow-marine mixed siliciclastic-carbonate source systems developing upon and around actively growing structures challenges traditional source-to-sink models. This study aims to investigate the implications of mixed siliciclastic-carbonate shelfal domains located in contrasting geotectonic settings (thrust forelimb and backlimb) for the development of the concomitant gravity-driven systems beyond the shelf edges. Here, we document the vertical and lateral stratigraphic variabilities of the shelf-derived turbidites and mass-transport deposits (MTDs) at outcrop-scale through the integrated interpretation of photogrammetry, field and taphonomic data from the emerged southern portion of the Hikurangi subduction margin. Results highlight the role and importance of varying structural setting of the sediment source, whereby the different morphologies of the source regions (continent-attached forelimb, continent-detached backlimb) control the development of highly varied shelf-derived gravity-driven depositional systems that interact with the structures across the same confined intra-slope basin. The deposits are tens to a few hundred of meters in thickness and have a lateral extent of several kilometers. The depositional systems are characterized by durations of 1 to 2 Ma and were primarily controlled by the geometries and tectonic motion of the underlying structures at the shelf edges. Shelf-derived mass-wasting systems occurred on both sides of the actively growing thrust structures and were sourced from both shelfal domains that were attached or detached from the continental domain. When sourced from the backlimbs however, the subsequent MTDs exhibit more complex internal architectures, ultimately recording the dynamic changes in slope gradient, and can therefore be used as proxies for unraveling the tectonic activity of an individual structure. Our study provides new insights to better predict mixed siliciclastic-carbonate depositional settings along active margins, sourced from thrust forelimb and backlimb. These results may be important for deep-marine exploration and tectonostratigraphic reconstruction of fold-and-thrust belts.

Influence of successive landslides on topographic changes revealed by multitemporal high-resolution UAS-based DEM

Landslides are recognized as dominant geomorphic events of morphological evolution in most mountainous and hilly landscapes. However, the lack of multitemporal high-resolution topographic data has resulted in a lack of quantitative estimates of topographic changes influenced by successive landslides. Taking a typical hillslope with successive loess landslides in the Heifangtai loess tableland, China, as an example, we conducted four unmanned aerial system (UAS) surveys and created corresponding high-resolution digital elevation models (HRDEMs) and orthophotos. We found that multitemporal UAS surveys have become a powerful new approach for addressing local topographic changes and evolution over a relatively long time series. Moreover, landslides can leave persistent geomorphic imprints on hillslope topography. The frequency distributions of topographic indexes are significantly influenced by successive landslides. The mean slope gradient, roughness and local relief decreased with successive landslide occurrences, whereas the mean topographic wetness index (TWI) increased. However, the mean slope aspect was almost unchanged by successive landslides. Furthermore, analysis of the coefficient of variation demonstrates that the frequency distribution of the slope gradient becomes more dispersed with landslide occurrences, while the slope aspect and TWI become more concentrated. The slope gradient changes with elevation. More broadly, this study provides new insights into the prediction of the local topographic feature changes and morphology evolution trends caused by successive landslides.

Long-term effectiveness of indigenous and introduced soil and water conservation measures in soil loss and slope gradient reductions in the semi-arid Ethiopian lowlands

Across the world, on-site and off-site effects of soil erosion accelerated by anthropogenic land-use/cover changes have been partly offset by the implementation of soil and water conservation (SWC) measures. However, studies evaluating the effectiveness of SWC over longer periods are currently rare. The dimensions of the terraced fields and SWC were measured at 242 sample fields that had slope gradients between 2.0 and 6.5% in three villages in the semi-arid Ethiopian Rift Valley (Ethiopian lowlands). In the study area, both indigenous contour weed hedgerows (denber) and introduced bund techniques, such as soil and stone bunds, are observed as major SWC measures. The establishment of the oldest SWC dated back to 65 years ago. Long-term effectiveness of the SWC was assessed in terms of the reduction in soil loss rates and slope gradient change of the terraced fields. Long-term quantitative assessment of SWC across the world has been conducted only in highlands; however, no case studies were conducted in lowlands. It is partly because the model that assessed the long-term effectiveness of SWC measures for the highlands appears to be inapplicable to lowlands. In the lowland model used in this study, the mid-point of the field was connected with that of the upper and lower fields by straight lines, which were assumed to represent the original slopes before the SWC establishment. The difference in slope gradient between the present slope and the original slope of the field without SWC was used for the estimates of slope reduction. The measured mass of the soil loss zone of the field was used to determine the long-term mean intra-field soil loss rate, while that of the sediment deposition zone was used to determine the long-term mean intra-field sediment accumulation rate behind the lower SWC. The mean net soil loss from the terraced fields was 19.6 Mg ha−1 yr−1, which was higher than that in the semi-arid northwestern highlands (Tigray); however, that from the stone bund fields was 13.7 Mg ha−1 yr−1, which was lower than that in Tigray. The soil and stone bunds retained 52–72% of the intra-field soil loss, compared to 34% retained by the denber. Slope gradients were reduced annually by 0.03% grade for the denber fields and 0.17–0.28% grades for the soil and stone bunds fields. Uncertainty analysis proved these estimates had relative uncertainty of <10% in general. All the SWC measures investigated in this study were effective in reducing soil loss from cropland and in reducing slope gradients of cropland in Ethiopian lowlands. Farmers’ efforts to place eliminated weeds on the lower hedgerows and bunds and a subsequent regeneration mechanism of weed hedgerows sustain the long-term effectiveness of not only denbers but also other bunds even without any extra maintenance efforts by farmers in the Rift Valley.

Impacts of soil and water conservation practices after half of a generation age, northwest highlands of Ethiopia

Soil and water conservation practices (SWCPs) have been implemented in many parts of Ethiopia for the past 4–5 decades, however, farmers’ are destructing the constructed SWCPs claiming lack of short-term benefits like minimizing slope gradient, reducing soil erosion, improving soil fertility and consequently increasing crop productivity. The aim of this study was to investigate the impacts of SWCPs age on maize grain yield, sediment trapping, soil moisture conservation and slope gradient change in Koga catchment, in the northwest highland of Ethiopia. Physical (fanya juu and soil bund) and biophysical (fanya juu integrated with grass) SWCPs were investigated using field experiment. SPSS version 24 software was used for data analysis. Three treatments were designed in three replications: (i) Fields treated with Fanya Juu (FJ), (ii) Fields treated with soil bund (SB), and (iii) Fields treated with a combination of FJ and Grass (G). The investigated SWCPs were grouped into three age classes (<5 years, 5−10 years and >10 years). The result shows that significantly higher maize grain yield was recorded in fields treated with aged SWCPs than recently constructed SWCPs. Maize grain yield was also higher in fields treated with biophysical SWCPs than fields treated with physical SWCPS alone. Unlike aged SWCPs, significantly higher amount of sediment was recorded behind recently constructed SWCPs because of soil erosion from the newly constructed unstable soil ridges in addition to soil erosion from upslope fields. The sediment trapping efficiency of SWCPs decreases as age increases, because the area of sediment deposition was filled by sediment due to lack of maintenance. Aged SWCPs significantly reduced slope gradient due to the long year trapped sediment and man-made ridge lines made during construction. In general, although the investigated SWCPs improved crop grain yield, maintained soil moisture and reduced slope gradient; the effects were insignificant compared with expectations after 4–5 decades. For instance, slope gradient didn’t change to its minimum. Hence, researchers and experts are expected to develop alternative approaches like ‘’Slope Reversing Terrace (SRT)’’ or modify the existing practices, after half of a generation age experience.

Upper slope processes and seafloor ecosystems on the Sabrina continental slope, East Antarctica

This study applies detailed seafloor bathymetry data and seafloor images to understand upper slope features and how these influence the distribution of seafloor biota on the East Antarctic margin. The East Antarctic slope on the Sabrina margin has been shaped by diverse processes related to repeated glaciation. Differences in the morphology of gullies on the upper slope enable an understanding of the likely processes that have been active on this margin. Gully morphology varies according to changes in slope gradient, which may have driven variations in sedimentation. Areas of lower slope angles may have led to rapid sediment deposition during glacial expansion to the shelf edge, and subsequent sediment failure. Typically, gullies in these areas are U-shaped, initiate well below the shelf break, are relatively straight and long, and have low incision depths, consistent with formation due to mass wastage. Areas of higher slope angles likely experienced enhanced flow of erosive turbidity currents during glaciations associated with the release of sediment-laden basal meltwaters. Sediment-laden subglacial meltwater flows typically create gullies such as those we observe that initiate at, or near, the shelf break; are V-shaped in profile; and have high sinuosity, deep incision depths and a relatively short downslope extent. The short downslope extent reflects a reduced sediment load associated with increased seawater entrainment as the slope becomes more concave in profile. These differences in gully morphology have important habitat implications associated with differences in the composition and beta-diversity of the seafloor communities. This upper slope region also supports seafloor communities that are distinct from those on the adjacent shelf, highlighting the uniqueness of this environment for biodiversity. Conservation strategies therefore need to consider slope and shelf communities as distinct and equally important components of the Antarctic ecosystem.

The Ogooue Fan (offshore Gabon): a modern example of deep-sea fan on a complex slope profile

Abstract. The effects of changes in slope gradient on deposition processes and architecture have been investigated in different deep-sea systems both in modern and ancient environments. However, the impact of subtle gradient changes (&lt; 0.3∘) on sedimentary processes along deep-sea fans still needs to be clarified. The Ogooue Fan, located in the northeastern part of the Gulf of Guinea, extends over more than 550 km westwards of the Gabonese shelf and passes through the Cameroon volcanic line. Here, we present the first study of acoustic data (multibeam echosounder and 3.5 kHz, very high-resolution seismic data) and piston cores covering the deep-sea part of this West African system. This study documents the architecture and sedimentary facies distribution along the fan. Detailed mapping of near-seafloor seismic-reflection data reveals the influence of subtle slope gradient changes (&lt; 0.2∘) along the fan morphology. The overall system corresponds to a well-developed deep-sea fan, fed by the Ogooue River sedimentary load, with tributary canyons, distributary channel–levee complexes and lobe elements. However, variations in the slope gradient due to inherited salt-related structures and the presence of several seamounts, including volcanic islands, result in a topographically complex slope profile including several ramps and steps. In particular, turbidity currents derived from the Gabonese shelf deposit cross several interconnected intra-slope basins located on the low gradient segments of the margin (&lt; 0.3∘). On a higher gradient segment of the slope (0.6∘), a large mid-system valley developed connecting an intermediate sedimentary basin to the more distal lobe area. Distribution and thickness of turbidite sands is highly variable along the system. However, turbidite sands are preferentially deposited on the floor of the channel and the most proximal depositional areas. Core description indicates that the upper parts of the turbidity flows, mainly composed of fine-grained sediments, are found in the most distal depocenters.

Bank Retreat and Streambank Morphology of a Meandering River during Summer and Single Flood Events in Northern Norway

In recent years, advanced methods for measuring riverbank migration have been used to understand the process of river planform evolution. However, the role of the so-called outer secondary cell in the hydraulic pattern in bank erosion remains unclear. For this purpose, a natural river meander with high curvature bends and steep riverbanks was chosen to quantify bank migration by high-resolution terrestrial laser scanning of three patches along two river bends in four time intervals. The first two time intervals were seasonal, from spring to autumn, and with relatively few water level changes, whereas the third and fourth time intervals were short, just before and after single flood peak events. The yielded point clouds were filtered and digital elevation models (DEMs) were created. These DEMs were used to analyze bank retreat, riverbank morphology, and slope gradient changes in order to understand the role of the outer secondary cell in these processes. In addition, it is shown that storm events causing short peaks in river discharge are less important for river migration than longer-lasting medium discharge.

A new slope unit extraction method for regional landslide analysis based on morphological image analysis

In landslide analysis it is necessary to first define and extract slope units from remotely sensed data. When this is done using hydrological process analysis, there can be complications caused by apparent sudden changes in slope gradient, resulting in the assumption of homogeneity of regional slope stability not being met. This issue is addressed in this paper with the proposal of a novel slope unit extraction method, which is derived from morphological image analysis (MIA) and results in the extraction of homogeneous slope units (HSU). It is thus referred to as the MIA-HSU method. In this method, slope units are defined as small, closed and homogeneous in 3D space. The MIA method uses logical algorithms, such as expansion and erosion, to extract slope units. Jiangjia Gully, Yunnan Province, China was selected to test the MIA-HSU method. Results indicate that slope units extracted by this method overcome defects of sudden changes in slope gradient and are able to identify geographical features meaningful for further landslide analysis. The MIA-HSU method presents a more uniform slope and aspect, and is better suited to regional landslide analysis than data obtained using the hydrological process analysis method.

Land use and land cover change on slope in Qiandongnan prefecture of Southwest China

This study uses DEM (Digital Elevation Model) data and remote sensing maps of the study area in 1993, 1999, and 2009 to analyze the slope gradient change of land use patterns in Qiandongnan Prefecture, Guizhou province, China. The land use data were classified into five types, forest, farmland, grassland, water and built-up, the slope gradients were divided into four grades. Indices for analyzing land use features were defined by their proportions, transformation matrixes, land use degree and changes. The results showed that all land use types can be found at every gradient. Generally, with the slope degree increased, the area of forest being augmented as well, while the area of the other land use types (farmland, grassland and build-up) declined. Moreover, a mass of farmland were shifted from other land use types from 0° to 25°, while a quantity of forest were transformed from the other land use types on >25° from 1993 to 2009. In terms of land use degree and changes, the area of farmland and buildup land use types decreased when slope degree increased. Finally, we calculated the five landscape pattern metrics: Patch Density (PD) value, Largest Patch Index (LPI), Shannon’s Diversity Index (SHDI), Area-Weighted Mean Shape Index (AWMSI) and Contagion Index (CONTAG). The results of metrics analysis showed that PD values, SHDI values and CONTA values had a similar variation trend, that is, they decreased when slope degree increased. There was no obvious variation trend on LPI value.

Influence of Slope Gradient on Distribution Rule of Geostress Field in River Valleys

During the construction of hydropower station, the change of slope gradient in river valleys often takes place. In order to study influence of slope gradient change on distribution rule of geostress field, the three dimensional unloading models under different slope gradients were established by finite difference software (FLAC3D). After numerical simulation, the results were as follows: (1) The phenomenon of stress concentration at the bottom of river valleys was obvious, which appeared the typical stress fold. Both the depth of stress concentration zone and the principal stress values significantly increased with the increment of slope gradient. (2) Maximum principal stress values increased less in shallow part of upper bank slope (low stress zone) but increased more in the nearby slope foot with the increment of slope gradient, causing great difference in geostress field of bank slope. (3) There was some difference in released energy of bank slope due to slope gradient change in river valleys. In order to distinguish the difference, stress relief zone was further divided into stress stably released zone and stress instability released zone. Finally, take Ada dam area of the western route project of South-to-North Water Transfer as an example, the results by numerical simulation were reliable through comparing the distribution rule of geostress field for the dam, which could provide important reference for stability of the design and construction of steep and narrow river valleys.

Differences in Landsat TM derived lava flow thermal structures during summit and flank eruption at Mount Etna

The simultaneous solution of the Planck equation (the so-called “dual-band” technique) for two shortwave infrared Landsat Thematic Mapper (TM) bands allows an estimate of the fractional area of the hottest part of an active flow and the temperature of the cooler crust. Here, the dual-band method has been applied to a time series of Mount Etna eruptions. The frequency distribution of the fractional area of the hottest component reveals specific differences between summit and flank lava flows. The shape of the density function shows a trend consistent with a Gaussian distribution and suggests a relationship between the moments of the distribution and the emplacement environment. Because flow composition of Etnean lavas generally remains constant during the duration of their emplacement, it appears that the shape of any particular frequency distribution is probably related to fluid mechanical aspects of flow emplacement that affect flow velocity and flow heat loss and thus the rate of formation of the surface crust. These factors include the influence of topographical features such as changes in slope gradient, changes in volume effusion rate, and progressive downflow increases in bulk or effective viscosity. A form of the general theoretical solution for the ‘dual-band’ system, which illustrates the relationship between radiance in TM bands 5 and 7, corresponding to hot fractional area and crust temperature, is presented. Generally speaking, it appears that for a given flow at any point in time, larger fractional areas of exposed hot material are correlated with higher temperatures and that, while the overall shape of that distribution is common for the flows studied, its amplitude and slope reflect individual flow rheological regimes.

Turbidity current sediment waves on irregular slopes: observations from the Orinoco sediment–wave field

The Orinoco sediment–wave field covers an area of at least 29 000 km2 on the southern margin of the Orinoco Valley, at a water depth of 4400–4825 m. Wave dimensions are highly variable across the wave field, with wavelengths of 110–2600 m, and wave heights of 1–15 m. Slope gradients are also very variable, with values of 0.14–0.48°. Overall, the sedimentary sequence on the upslope wave flanks is about 40% thicker than that on the downslope flanks, leading to upslope wave migration in the manner of antidunes. In addition, reflectors on the upslope flanks generally display higher reflectivity than those on the downslope flanks, suggesting that a higher proportion of coarser sediment occurs on the upslope flank. An unconfined turbidity current origin is proposed for the Orinoco sediment waves, based upon detailed analysis of regional stratigraphic/seismic facies, and sediment wave distribution, morphology and dimensions. Sediment waves are not related to flows passing along (or spilling out of) the Orinoco Valley, or to bottom currents flowing parallel to the slope. Turbidity currents responsible for wave generation are interpreted as originating from slope failures on the adjacent Venezuela, Guyana and Suriname continental margins. Simple numerical modelling has enabled turbidity current flow characteristics across the Orinoco sediment waves to be estimated: internal Froude number=0.7–1.1, flow thickness=24–645 m, and flow velocity=31–82 cm s−1. A key finding of this study is that there appears to be a close relationship between the changes in slope gradient and those in wave dimensions across the wave field. The irregular gradient of the present-day wave field is partly a reflection of the irregular bounding surface of the sediment waves, which is represented by mass-flow deposits and associated mud diapirs. The changes in slope gradient along this lower boundary lead to variations in the flow thickness and flow velocity of passing turbidity currents, which in turn control the wave dimensions. Generally, on lower gradients beyond minor breaks of slope, flow thickness increases and flow velocity decreases, leading to an increase in wavelength and a decrease in height.

High‐resolution (2–7 kHz) acoustic and geometric characters of submarine creep deposits in the South Korea Plateau, East Sea

A synthesis of high‐resolution (Chirp, 2–7 kHz) seismic profiles in the South Korea Plateau reveals that large masses of wavy stratified sediment (≈60–90 m thick) cover broad, gently sloping (&lt;0·5°) ridges in water depths of 1000–2000 m. The wavy stratified sediment (WSS) is characterized by wavy (0·2–5 km in wavelength and &lt;15 m in relief), continuous reflective layers with a basal deformed zone that overlies undeformed, strong reflectors. The WSS exhibits systematic variation in wave dimensions and thickness of internal reflective layers with changes in slope gradient. The troughs of the waves are commonly associated with internal growth faults, and wave amplitude generally increases with subbottom depth. On steep slopes around the ridges, the WSS masses are bounded downslope by slide and slump deposits including slightly translated or rotated WSS blocks. The acoustic and geometric characters, and association with downslope slides and slumps on the steeper slopes, suggest that the WSS masses were most probably formed by slow creep movement before slope failure. In the absence of significant sediment input to the South Korea Plateau, the deep (1000–2000 m in water depth) mass movements were probably triggered by earthquakes that have occurred frequently in this region. Some slightly displaced, intact WSS blocks in the associated slides and slumps downslope reflect a progressive evolution from submarine creep into slide and slump.

Estimations of danger of landslides based on a geomorphological investigation of the landslides in Amakusakamishima Island, Kumamoto Prefecture

The Geographical Survey Institute made an investigation of the landslides in Amakusakamishima Island caused by the heavy rainfall of July, 1972, mainly from geomorphological viewpoints and made a quantitative estimation of danger of landslides by a statistical method. The report is summarized as follows.1. The heavy rainfall caused about 4, 600 small debris avalanches and very few large slumps of bedrock on mountain slopes of the eastern part of the island. In the aerial photographs taken in 1965, 240 scars are observed and 70 of those were scarred again due to this rainfall.2. These debris avalanches originated a large number of debris flows. Where a hard arkose sandstone of Palaeogene or volcanic rocks are present, debris flows became large and destructive owing to the very large blocks of these rocks.3. Most of the debris avalanches and flows occurred when the rainfall was extremely intensive, between 11 and 13 o'clock of July 6, while the large slumps seems to have occurred after the rain.4. The relation between distribution of debris avalanches and the age of mountainslope was studied. Knick-lines on which the slope gradient changes discontinuously, from the upper gentle to the lower steep, were read in a stereo-airphotographs and were used as erosion fronts for classifying the slope age. An erosion front indicates the borderline of an erosion area which spreads itself with time. The age of those in the south of Amakusa is divided into four.1st, originated in the middle and late Pleistocene.2nd; originated in the late Pleistocene.3rd; originated during the regression of the late Pleistocene and early Recent.4th; originated during the Recent transgression.Debris avalanches intensively occurred in the 2nd and 3rd erosion areas, while not so many occurred in the 4th. It is probable that the 4th area had not had much detritus to move down due to its short periodicity of erosional activities.5. The smaller limit of a watershed above a scar and its average gradient were found to be 40-50m2 and around 1/4 by measuring 1/2, 000 topographic maps.6. An equation for estimating the magnitude of debris avalanche was built by means of multivariate analysis in the area A where the precipitation was very high. Six factors were adopted as the predictor variables, i.e. rock, slope, drainage density, thickness of detritus, vegetation, and orientation of slope. The equation was applied to an estimation for another area B in the island, where the precipitation was much less than in A. The estimated magnitude is much larger than the real one.