River channel change during the last 50 years in the middle Yangtze River, the Jianli reach

Intensive anthropogenic disturbances have affected the channel of the middle Yangtze River since the 1950s. This paper selects the Jianli reach as an example to examine human impact on channel change in the middle Yangtze River. 1:100,000 channel distribution maps from 1951, 1961 and 1975 and 1:25,000 navigation charts from 1981 and 1997 were employed to reconstruct channel change in the study reach. The result indicates that the channel, under the constraint of levees along the riverbanks, underwent a minor widening but frequent bank failure due to susceptible bank structure and increase in water discharge. The bank failure promoted bar growth in the channel. Cross-section changes and quantitative calculations of erosion and deposition based on the DEM derived from navigation charts present a pattern of over-bank sedimentation and riverbed incision. The stage and duration of floods have increased following levee construction and bank revetment.     

Response of small mid-mountain rivers to human impact with particular reference to the last 200 years; Eastern Sudetes, Central Europe

In the mid-mountains of Central Europe relict forms of periglacial morphogenesis occur, including debris slope covers. However, the Holocene forest succession limited sediment transfer towards valley systems.The eastern, mid-mountain part of the Sudetes, characterised by a dense stream network and by the local occurrence of metallic and gold ores, was already settled by humans in early medieval times. Timber from the forested slopes was used to produce charcoal and streams acted as energy sources for smelting plants and smithies. Agriculture developed on less inclined slopes. Temporal clearances on steep slopes and replacement of primeval beech forest by spruce trees, influenced water circulation on slopes. The exposed slope covers were transferred to valley floors during high precipitation events. Consequently, they were selectively redeposited within the floodplain, changing the former channel pattern, especially at confluences. The introduction of cultivation on slopes triggered intense soil erosion so that agricultural terraces were built in order to prevent surface wash. The terraces are often strengthened with stone walls up to 4.5 m high. Terraces acted as ‘sediment traps’, storing the washed material within the slope. The layers of fine-grained colluvial sediments, which can be linked with agriculture, cover the older, debris slope covers. The colluvium is especially thick at the terrace edges. Despite the protective role of terracing, part of the washed material was transferred to valleys, often becoming overbank deposits, covering older gravel and boulder alluvium. The decline of iron ore exploitation and processing, at the beginning of the 19th century, together with agricultural withdrawal, which was especially radical after the 2nd World War, resulted in successive renaturalisation of the environment being followed by slope stabilisation and deepening of bigger river channels.The results led to the conclusion that a strong human impact on the slope–valley system in the mid-mountain part of the Eastern Sudetes influenced changes in the dynamics and morphology of small rivers to a much higher extent than the climate variations, especially during the Little Ice Age.     

Sediment budget analysis of slope–channel coupling and in-channel sediment storage in an upland catchment, southeastern Australia

Slope–channel coupling and in-channel sediment storage can be important factors that influence sediment delivery through catchments. Sediment budgets offer an appropriate means to assess the role of these factors by quantifying the various components in the catchment sediment transfer system. In this study a fine (< 63 µm) sediment budget was developed for a 1.64-km² gullied upland catchment in southeastern Australia. A process-based approach was adopted that involved detailed monitoring of hillslope and bank erosion, channel change, and suspended sediment output in conjunction with USLE-based hillslope erosion estimation and sediment source tracing using ¹³⁷Cs and ²¹⁰Pb_ex. The sediment budget developed from these datasets indicated channel banks accounted for an estimated 80% of total sediment inputs. Valley floor and in-channel sediment storage accounted for 53% of inputs, with the remaining 47% being discharged from the catchment outlet. Estimated hillslope sediment input to channels was low (5.7 t) for the study period compared to channel bank input (41.6 t). However an estimated 56% of eroded hillslope sediment reached channels, suggesting a greater level of coupling between the two subsystems than was apparent from comparison of sediment source inputs. Evidently the interpretation of variability in catchment sediment yield is largely dependent on the dynamics of sediment supply and storage in channels in response to patterns of rainfall and discharge. This was reflected in the sediment delivery ratios (SDR) for individual measurement intervals, which ranged from 1 to 153%. Bank sediment supply during low rainfall periods was reduced but ongoing from subaerial processes delivering sediment to channels, resulting in net accumulation on the channel bed with insufficient flow to transport this material to the catchment outlet. Following the higher flow period in spring of the first year of monitoring, the sediment supplied to channels during this interval was removed as well as an estimated 72% of the sediment accumulated on the channel bed since the start of the study period. Given the seasonal and drought-dependent variability in storage and delivery, the period of monitoring may have an important influence on the overall SDR. On the basis of these findings, this study highlights the potential importance of sediment dynamics in channels for determining contemporary sediment yields from small gullied upland catchments in southeastern Australia.     

Spatial analysis of rockfall activity, bounce heights and geomorphic changes over the last 50 years – A case study using dendrogeomorphology

Tree-ring series have been used to reconstruct 50 years of rockfall behavior on a slope near Saas Balen (Swiss Alps). A total of 796 cores and 141 cross sections from 191 severely injured conifer trees (Larix decidua Mill., Picea abies (L.) Karst. and Pinus cembra L.), combined with a series of aerial photographs, were used to investigate the evolution of the forest stand so as (i) to reconstruct past rockfall rates; (ii) to analyze the spatial behavior of maximum bounce heights; and (iii) to analyze the spatial comportment of rockfall activity over the last five decades.Tree-ring analysis permitted the reconstruction of the age distribution at the study site; results were in perfect agreement with the afforestation process shown in the aerial photographs. The oldest are located in the lower, central part of the study site; the youngest individuals at its uppermost lateral boundaries. Reconstructed rockfall rates reveal strong interannual variations and single event years with increased activity, namely in 1960/1961 and 1995. Spatial analysis of the maximum bounce heights indicate highest values at the lateral boundaries and lowest heights in the lower central part of the forest stand, where a big boulder seems to shield trees growing below it. The spatial analysis of past rockfall activity shows high active zones at the uppermost north-facing boundaries of the forest and least active zones in the lowermost central part of the studied stand. The high rockfall activity at the slope is expressed by a mean rockfall rate of > 1 event m^− 1 y^− 1.     

Short-term changes in the magnitude, frequency and temporal distribution of floods in the Eastern Mediterranean region during the last 45 years — Nahal Oren, Mt. Carmel, Israel

Short-term changes in Eastern Mediterranean precipitation affecting flow regime were documented in Nahal Oren, a 35 km² ephemeral stream in Mt. Carmel, a 500 m high mountain ridge located at the NW coast of Israel. The rainy winter of the Mediterranean type climate (Csa) in Mt. Carmel is characterized by average annual rainfall of 550 mm at the coastal plain to 750 mm at the highest elevation while the summer is hot and dry. Stream flow generates after accumulated rainfall of 120–150 mm while “large floods”, defined as flows with peak discharge of > 5 m³ s^− 1 and/or > 150,000 m³ in volume, are generated in response to rainfall of over 100 mm. Hence, large floods in Nahal Oren stream occur not earlier than December. Precipitation and flow data were divided into two sub-periods: 1957–1969 and 1991–2003 and compared to each other. The results indicate a clear increase in the frequency of large floods, their magnitudes and volumes during the second period with no parallel change in the annual precipitation. Similarly, an increase in storm rainfall–runoff ratio from < 5% to > 15% and a decrease in the threshold rainfall for channel flow by 16–25% were documented. These short-term variations in flooding behavior are explained by the clear decrease in the length of the rainy season and by the resulting significant shortening in the duration of the dry-spells. The increase in the number of large rainfall events and the large floods in each hydrological year together with the increasing number of years with no floods indicate strengthening of their uncertainty of behavior.     

Deep-seated failure propagation in a fractured rock slope over 10,000 years: The La Clapière slope, the south-eastern French Alps

The “La Clapière” area (Tinée valley, Alpes Maritimes, France) is a typical large, complex, unstable rock slope affected by Deep Seated Gravitational Slope Deformations (DGSD) with tension cracks, scarps, and a 60 × 10⁶ m³ rock slide at the slope foot that is currently active. The slope surface displacements since 10 ka were estimated from ¹⁰Be ages of slope gravitational features and from morpho-structural analyses. It appears that tensile cracks with a strike perpendicular to the main orientation of the slope were first triggered by the gravitational reactivation of pre-existing tectonic faults in the slope. A progressive shearing of the cracks then occurred until the failure of a large rock mass at the foot of the slope. By comparing apertures, variations and changes in direction between cracks of different ages, three phases of slope surface displacement were identified: 1) an initial slow slope deformation, spreading from the foot to the top, characterized by an average displacement rate of 4 mm yr^− 1, from 10–5.6 ka BP; 2) an increase in the average displacement rate from 13 to 30 mm yr^− 1 from the foot to the middle of the slope, until 3.6 ka BP; and 3) development of a large failure at the foot of the slope with fast displacement rates exceeding 80 mm yr^− 1 for the last 50 years. The main finding of this study is that such a large fractured slope destabilization had a very slow displacement rate for thousands of years but was followed by a recent acceleration. The results obtained agree with several previous studies, indicating that in-situ monitoring of creep of a fractured rock slope may be useful for predicting the time and place of a rapid failure.