Modeling the sediment yield from landslides in the Shihmen Reservoir watershed,Taiwan

Landslides generate enormous volumes of sediment in mountainous watersheds; however, quantifying the downstream transport of landslide‐derived sediment remains a challenge. Landslide erosion and sediment delivery to the Shihmen Reservoir watershed in Taiwan was estimated using empirical landslide frequency–area and volume–area relationships, empirical landslide runout models, and the Hydrological Simulation Program‐ FORTRAN (HSPF). Landslide erosion rates ranged from 0.4 mm yr^‐1 to 2.2 mm yr^‐1 during the period 1986–2003, but increased to 7.9 mm yr^‐1 following Typhoon Aere in 2004. The percentage of landslide sediment delivered to streams decreased from 78% during the period 1986–1997 to 55% in 2004. Although the delivery ratio was lower, the volume of landslide sediment delivered to streams was 2.81 × 10⁶ Mg yr^‐1 in 1986–1997 and 8.60 × 10⁶ Mg yr^‐1 in 2004. Model simulations indicate that only a small proportion of the landslide material was delivered downstream. An average of 13% of the landslide material delivered to rivers was moved downstream during the period 1986–1997. In 2004, the period including Typhoon Aere, the annual fluvial sediment yield accounted for approximately 23% of the landslide material delivered to streams. In general, the transfer of sediment in the fluvial system in the Shihmen Reservoir watershed is dominantly transport limited. The imbalance between sediment supply and transport capacity has resulted in a considerable quantity of landslide material remaining in the upper‐stream regions of the watershed. Copyright © 2012 John Wiley & Sons, Ltd.     

Reefs at the edge: coral community structure around Rapa,southernmost French Polynesia

Rapa (27°36′ S, 144°20′ W) is a small (~40 km²) volcanic island isolated in the Southern Austral Archipelago, where direct anthropogenic stressors are extremely limited. Here, we present the results of the first quantitative survey of coral community structure across habitats and depths around the island. Despite its geographical isolation in the depauperate South Central Pacific, its small size and unfavourable environmental conditions (competition with macroalgae, low sea surface temperatures, reduced reef accretion), the diversity of scleractinian corals at Rapa is particularly high (112 species from 32 genera, including 37 species of Acropora) in comparison to other French Polynesian islands and subtropical Pacific locations. Our results indicate that the abundance (>100 colonies per 10 m² recorded at nine of the 17 sampling stations) and cover (>40% at four stations) of corals are relatively high for a marginal reef location. Strong spatial heterogeneity was found, with high variation in diversity, abundance, cover and community composition among stations. Variation in community composition was related to habitat types, with distinct assemblages among fringing reefs within bays, reef formations at bay entrances, and those on the submerged platform surrounding the island. On the platform, a depth gradient was detected, with generic richness, abundance and cover generally greater at deeper stations (18–20 m depth) compared with medium‐depth (10–12 m) and shallow (1–3 m) stations. A gradient was also recorded along bays, with increasing coral diversity and abundance from the bay heads to the bay entrances. The coral community at Rapa was characterized by the presence of several taxa not found in other French Polynesian archipelagos and the rarity of others that are common and abundant in the Society and the Tuamotu islands. Another distinctive feature of reef communities at Rapa is the high cover and dominance of macroalgae, particularly in the shallower parts of the surrounding platform, which probably explains the lower densities of coral colonies recorded there. These characteristics of the diversity and biogeographical composition of coral assemblages at Rapa provide considerable ecological grounds for its conservation.     

Soft‐sediment deformation structures in a Pleistocene glaciolacustrine delta and their implications for the recognition of subenvironments in delta deposits

The development of soft‐sediment deformation structures in clastic sediments is now reasonably well‐understood but their development in various deltaic subenvironments is not. A sedimentological analysis of a Pleistocene (ca 13·1 to 15 ¹⁰Be ka) Gilbert‐type glaciolacustine delta with gravity‐induced slides and slumps in the Mosty‐Danowo tunnel valley (north‐western Poland) provides more insight, because the various soft‐sediment deformation structures in these deposits were considered in the context of their specific deltaic subenvironment. The sediments show three main groups of soft‐sediment deformation structures in layers between undeformed sediments. The first group consists of deformed cross‐bedding (inclined, overturned, recumbent, complex and sheath folds), large‐scale folds (recumbent and sheath folds) and pillows forming plastic deformations. The second group comprises pillar structures (isolated and stress), clastic dykes with sand volcanoes and clastic megadykes as examples of water‐escape structures. The third group consists of faults (normal and reverse) and extensional fissures (small fissures and neptunian dykes). Some of the deformations developed shortly after deposition of the deformed sediment, other structures developed later. This development must be ascribed to hydroplastic movement in a quasi‐solid state, and due to fluidization and liquefaction of the rapidly deposited, water‐saturated deltaic sediments. The various types of deformations were triggered by: (i) a high sedimentation rate; (ii) erosion (by wave action or meltwater currents); and (iii) ice‐sheet loading and seasonal changes in the ablation rate. Analysis of these triggers, in combination with the deformational mechanisms, have resulted – on the basis of the spatial distribution of the various types of soft‐sediment deformation structures in the delta under study – in a model for the development of soft‐sediment deformation structures in the topsets, foresets and bottomsets of deltas. This analysis not only increases the understanding of the deformation processes in both modern and ancient deltaic settings but also helps to distinguish between the various subenvironments in ancient deltaic deposits.     

Input data resolution‐induced uncertainty in watershed modelling

The temporal‐spatial resolution of input data‐induced uncertainty in a watershed‐based water quality model, Hydrologic Simulation Program‐FORTRAN (HSPF), is investigated in this study. The temporal resolution‐induced uncertainty is described using the coefficient of variation (CV). The CV is found to decrease with decreasing temporal resolution and follow a log‐normal relation with time interval for temperature data while it exhibits a power‐law relation for rainfall data. The temporal‐scale uncertainties in the temperature and rainfall data follow a general extreme value distribution and a Weibull distribution, respectively. The Nash‐Sutcliffe coefficient (NSC) is employed to represent the spatial resolution induced uncertainty. The spatial resolution uncertainty in the dissolved oxygen and nitrate‐nitrogen concentrations simulated using HSPF is observed to follow a general extreme value distribution and a log‐normal distribution, respectively. The probability density functions (PDF) provide new insights into the effect of temporal‐scale and spatial resolution of input data on uncertainties involved in watershed modelling and total maximum daily load calculations. This study exhibits non‐symmetric distributions of uncertainty in water quality modelling, which simplify weather and water quality monitoring and reducing the cost involved in flow and water quality monitoring. Copyright © 2011 John Wiley & Sons, Ltd.     

Location,location, location: changes in the diversity of animal resources exploited by Tardiglacial humans in northern Spain

Prey diversity of Homo sapiens in north Iberia exhibited shifts associated with climate warming after the Last Glacial Maximum, as deduced from animal remains obtained from Upper Palaeolithic caves in Asturias (northern Spain). Significant association of prey diversity and river proximity was found during adverse climate conditions, indicating opportunistic predation. A high increase of prey diversity was found when small animals (including fish) were taken into account in archaeological records, highlighting the importance of this type of prey in Palaeolithic diets. Copyright © 2009 John Wiley & Sons, Ltd.     

Flood‐based analysis of high‐magnitude sediment transport using a non‐parametric method

Upland erosion and the resulting reservoir siltation is a serious issue in the Isábena catchment (445 km² Central Spanish Pyrenees). During a three‐month period, water and sediment fluxes have been monitored at the catchment outlet (Capella), two adjacent subcatchments (Villacarli, 41 km²; Cabecera, 145 km²) and the elementary badland catchment Torrelaribera (8 ha). This paper presents the results of the monitoring, a method for the calculation of a sedigraph from intermittent measurements and the derived sediment yields at the monitored locations. The observed suspended sediment concentrations (SSCs) demonstrate the role of badlands as sediment sources: SSCs of up to 280 g l^?1 were encountered for Villacarli, which includes large badland areas. SSCs at the Cabecera catchment, with great areas of woodland, barely exceeded 30 g l^?1. SSCs directly at the sediment source (Torrelaribera) were comparable to those at Villacarli, suggesting a close connection within this subcatchment. At Capella, SSCs of up to 99 g l^?1 were observed. For all sites, SSC displayed only a loose correlation with discharge, inhibiting the application of a simple sediment rating curve. Instead, ancillary variables acting as driving forces or proxies for the processes (rainfall energy, cumulative discharge, rising/falling limb data) were included in a quantile regression forest model to explain the variability in SSC. The variables with most predictive power vary between the sites, suggesting the predominance of different processes. The subsequent flood‐based calculation of sediment yields attests high specific sediment yields for Torrelaribera and Villacarli (6277 and 1971 t km^?2) and medium to high yields for Cabecera and Capella (139 and 410 t km^?2) during the observation period. In all catchments, most of the sediment was exported during intense storms of late summer. Later flood events yield successively less sediment. Relating upland sediment production to yield at the outlet suggests considerable effects of sediment storage within the river channel. Copyright © 2008 John Wiley & Sons, Ltd.     

Impact of climate change on hydrological processes over a basin scale in northern Taiwan

This study investigates the impact of climate change on rainfall, evapotranspiration, and discharge in northern Taiwan. The upstream catchment of the Shihmen reservoir in northern Taiwan was chosen as the study area. Both observed discharge and soil moisture were simultaneously adopted to optimize the HBV‐based hydrological model, clearly improving the simulation of the soil moisture. The delta change of monthly temperature and precipitation from the grid cell of GCMs (General Circulation Models) that is closest to the study area were utilized to generate the daily rainfall and temperature series based on a weather generating model. The daily rainfall and temperature series were further inputted into the calibrated hydrological model to project the hydrological variables. The studies show that rainfall and discharge will be increased during the wet season (May to October) and decreased during the dry season (November to April of the following year). Evapotranspiration will be increased in the whole year except in November and December. Copyright © 2009 John Wiley & Sons, Ltd.     

Experimental study of extreme shear stress for shallow flow under simulated rainfall

The raindrop impact and overland flow are two major factors causing soil detachment and particle transportation. In this study, the turbulent characteristics of the shallow rain‐impacted water flow were investigated using a 2‐D fibre‐optic laser Doppler velocimetry (FLDV) and an artificial rainfall simulator. The fluctuating turbulent shear stress was computed using digital data processing techniques. The experimental data showed that the Reynolds shear stress follows a probability distribution with heavy tails. The tail probability increases with an increase of rainfall intensity or raindrop diameter, and it decreases with an increase of Reynolds number. A modified empirical equation was derived using both the raindrop diameter and rainfall intensity as independent variables to provide a better prediction of the Darcy‐Weisbach friction coefficient f under rainfall conditions. Copyright © 2009 John Wiley & Sons, Ltd.