DR-Methods

Journal of Geophysical Research, 2006

2017

As one of the most devastating natural hazards, flash floods are responsible for major and abrupt geomorphic effects in the fluvial system as well as significant loss of life and socio-economic damages. Flash floods are characterized by strong spatio-temporal rainfall variability and therefore show variations in discharge and energy expenditure: associated geomorphic effects depend on geological controls on channel geometry and sediment characteristics, as well as on variations in flood intensity. Geomorphic effects usually take the form of erosional and depositional modification of the pre-flood channel. The central question of this thesis is to evaluate why flash floods of similar magnitudes and intensities sometimes produce dissimilar geomorphic results. The use of peak instantaneous hydraulic flow parameters such as discharge, velocity, shear stress, and stream power to quantify geomorphic changes has commonly been non-deterministic. This thesis aims at investigating how factors...

2007

The basic patterns and processes of steep channels remain poorly known relative to lower-gradient channels. In this analysis, characteristics of step-pool, plane-bed, and pool-riffle channels are examined using a data set of 335 channel reaches from the western United States, Nepal, New Zealand, and Panama. We analyzed differences among the three channel types with respect to hydraulics, channel geometry, boundary roughness, and bedforms. Step-pool channels have significantly steeper gradients, coarser substrate, higher values of shear stress and stream power for a given discharge, and larger ratios of bedform amplitude/wavelength (H/L). Pool-riffle channels have greater width/depth ratios and relative grain submergence (R/D84) than the other channel types. Plane-bed channels tend to have intermediate values for most variables examined. Relative form submergence (R/H) is statistically similar for step-pool and pool-riffle channels. Despite the lesser relative grain submergence and greater bedform amplitude of step-pool channels, mean values of Darcy–Weisbach friction factor do not change in response to changes in relative grain submergence. These patterns suggest that adjustments along mountain streams effectively maximize resistance to flow and minimize downstream variability in resistance among the different channel types.

Canadian Journal of Civil Engineering, 2001

Geomorphology, 2019

Catena, 2020

Three pairs of small, fourth-to sixth-order catchments (approximately 5-80 km 2), draining the margin of the Darjeeling Himalayas into the piedmont, were selected for a comparison of the land-use impact on the morphology and sedimentology of their stream channels. Each pair experienced similar annual rainfall and comprised similar metamorphic bedrock, steep topography, and brown soils; the members of each pair contrast with respect to their land-use structure as they comprise large forest cover (forest > 90%) and a significant contribution of agricultural land with tea plantations (forest < 63%), respectively. A set of rainfall, cross-sectional, and sediment data were collected for the characterization of the stream channels. The obtained results revealed that, under extreme rainfall and high denudation rates as well as frequent flash floods, the geomorphic response to land-use changes involving agricultural expansion at the expense of forest cover was less pronounced in the mountain channels than in the piedmont. Significant differences between the studied pairs of mountain streams draining forested and agricultural catchments were observed only at bankfull width and baseflow depth in fourth-order streams and baseflow depth in fifth-order streams. Local environmental factors such as bedrock channel boundaries, steep topography, high material supply to river networks, and selective sediment mobilization during extreme rainfall and floods override the effects of land use and exert dominant control over stream channels in mountain catchments. However, the effects of agricultural activity in mountains are propagated downstream. In the agricultural catchment in the piedmont, a local rapid decrease in river slope and an increase in water infiltration into alluvia facilitates the deposition of eroded material. In addition, the stream in the agricultural catchment exhibited a rise in the riverbed with the expansion of a braided channel over a dozen kilometers below the Himalayan front in the piedmont, while the streams in the forested catchment revealed a distinct tendency towards incision. The present-day channel morphologies and sediment patterns observed in forested catchments are most likely similar to those that existed in the studied rivers prior to the development of tea plantations and settlements in the mid-19th century.

On sait les remous qu'a causés dans la communauté chrétienne la récente Déclaration publiée par le Dicastère pour la Doctrine de la Foi, intitulée Fiducia Supplicans, et sous-titrée « sur la signification pastorale des bénédictions » 1. L'attention sourcilleuse des critiques s'est jusqu'ici majoritairement concentrée sur le danger que, selon certains théologiens, ce texte fait courir à la doctrine du sacrement de mariage, lequel, estiment-ils, semble ouvert désormais aux couples non-mariés canoniquement et aux homosexuels. Étonnamment, ils n'ont pas paru surpris par la phrase-problématique selon moi-, qui figure au paragraphe 27 de Fiducia Supplicans...