Quaternary stratigraphy of the Koratallaiyar-Cooum basin,Chennai

In this paper we present Quaternary stratigraphy of the area around Chennai based on archaeological findings on the ferricrete surface, geomorphological observations supplemented by radiocarbon dating. The coastal landscape around Chennai, Tamil Nadu, has preserved ferruginised boulder gravel deposits, ferricretes and fluvial deposits of varying thickness. The area studied is approximately 150 km east to west and 180 km north to south with a broad continental shelf towards the seaward. Several rivers enter the Bay of Bengal along its shores like the Koratallaiyar, Cooum and the Adyar. Precambrian charnockite and Upper Gondwana sandstone and shale bedrock rim the shelf margin. For the most part, the Upper Pleistocene-Holocene fluvial sediments overlie an erosion surface that has cut into older Pleistocene sediments and ferricrete surface. Incised valleys that cut into this erosion surface are up to 5–6 km wide and have a relief of at least 30 m. The largest valley is that cut by the Koratallaiyar River. Holocene sediments deposited in the incised valleys include fluvial gravels, early transgressive channel sands and floodplain silts. Older Pleistocene sediments are deposited before and during the 120-ka high stand (Marine isotope stage 5). They consist of ferricretes and ferricrete gravel formed in nearshore humid environments. Muddy and sandy clastic sediments dated to the ca. 5 ka highstand suggest that the climate was semi arid at this time with less fluvial transport. The coarsening up sequence indicates deposition by high intensity channel processes. Pedogenic mottled, clayey silt unit represents an important tectonic event when the channel was temporarily drained and sediment were sub aerially exposed. Uplift of the region has caused the local rivers to incise into the landscape, forming degradation terraces.     

Boulder arrangement on a rocky ramp fishway based on the swimming behavior of fish

A rocky ramp fishway is one of the Nature like fishway, comprises a long sloping channel and boulders on a slope (Katopodis et al., 2001). The turbulence and backwater around the boulders creates refuges of decreased local flow where fish can rest. However, recent studies have shown that turbulent flow may influence fish migration (Smith et al., 2006). This paper is focused on boulder arrangement and height of boulder on rocky ramp fishway. The swimming performance of char (Salvelinus richardson) and freshwater sculpin (Cottus pollux) around boulders on experimental channel were observed using ultra-high-speed cameras, and the appropriate arrangement of boulders for each species was considered. In total, 27 hydraulic conditions were assessed considering variable distributions, shapes, heights of the boulders as well as variable flow depths by observing swimming behavior for each condition. Laboratory measurement shows that sculpin, which are bottom fish, require a density, and several linear arrangements of boulders to allow migration. As increasing the vertical low-velocity area, which is created behind high boulders does not always result in a good outcome for sculpin. However, this solution is not suitable for char; therefore, a diverse arrangement of boulders is required on individual rocky ramp fishways to allow the movement and migration of multiple species. We propose to add cloister in the fishway for free migration of weak swimmer.     

Triassic carbonate buildup flanks in the Dolomites, northern Italy: breccias, boulder fabric and the importance of early diagenesis

The flanks of Middle Triassic carbonate buildups in the Dolomites show well‐developed clinostratification, with typical angles of 30–40°. This paper focuses on the metre‐scale fabric of these clinoforms and sets these within the context of their large‐scale and microscopic features. Clinoform stratification is caused by fibrous cement crusts, by stylolites paralleling a vague stratification and by pelagic limestone interbedded with the lower portions of the clinoforms. These parts of the clinoforms locally exhibit a boulder fabric. A fracture system, subvertical to clinostratification, is filled by fibrous cements and marine internal sediment. The analysis of clinoform fabric indicates that parts consist of in situ automicrite. Other parts of the clinoforms are made up of breccia. The breccias occur as isolated pockets and lenses with random orientation and dimensions ranging from a few square decimetres to hundreds of square metres. The breccias have gradual contacts with the unbrecciated host rock. Breccia components are mostly angular, show a microfacies similar to that of the unbrecciated host rock and are composed of millimetre‐ to decimetre‐sized particles that generally float in radiaxial‐fibrous cement. Matching grain boundaries are common. Calcite cement typically makes up 20 and 40 vol.% of the brecciated areas. Clinostratification, the fracture system, brecciation and boulder fabric point to gravity induced deformation of in situ flank deposits rather than gravity induced depositional processes. Brecciation appears to result from translational sliding (millimetres to metres) on the steep buildup flanks, which caused fracturing of the vaguely stratified automicrite, followed by displacive growth of fibrous cement. Cementation occurred in a (shallow) burial, marine phreatic environment, because cement clasts are virtually absent from the flank‐derived gravity flows in the adjoining basinal sediments. Displacive cement growth indicates a volume increase of the clinoforms during diagenesis of up to 20–40 vol.% and can account for the local drag of buildup interior limestones. Similarly, the boulder fabric appears to be a diagenetic feature, which resulted from differential settling of incompletely lithified boundstone and grainstone, and the interbedded pelagic limestone.     

Boulder weathering and erosion associated with a wildfire, Sierra Ancha Mountains, Arizona

An April–May 2000 “Coon Creek Fire” burned 37.5 km² of the Sierra Ancha Mountains, 32.3 km miles north of Globe, AZ—including 25 sandstone and 19 diorite boulders surveyed in 1989 and resurveyed after the burn, after the summer 2000 monsoon season, and after the winter 2001 season. When viewed from the perspective of cumulative eroded area, both sandstone and diorite displayed bimodal patterns with 79% of sandstone boulder area and 93% of diorite boulder area undergoing either no fire-induced erosion or fire-induced erosion >76 mm. When stretched over cumulative boulder areas, erosion due to the fire averaged >26 mm for sandstone and >42 mm for diorite. Post-fire erosion from thunderstorm summer rains averaged <1 mm for 5 diorite and 1 mm for 10 sandstone boulders. While only a single diorite boulder eroded an average of 1.2 mm after the winter, winter erosion removed an average of 5.5 mm from 14 sandstone boulders. Thus, fire appears to increase a rock's susceptibility to post-fire weathering and erosion processes, as predicted by Goudie et al. [Earth Surf. Process. Landf. 17 (1992) 605]. In contrast to experimental research indicating the importance of size in fire weathering, no statistically significant relationship exists between erosion and boulder height or boulder surface area—a result similar to Zimmerman et al. [Quat. Res. 42 (1994) 255]. These data exclude 12 original sites and 85 boulders at sites impacted by the fire that could not be relocated, with a reasonable cause for the lack of relocation being boulder obliteration by the fire. Given evidence from ¹⁰Be and ²⁶Al cosmogenic nuclides [Earth Planet. Sci. Lett. 186 (2001) 269] supporting the importance of boulders in controlling evolution of nonglaciated, bouldered landscapes [Geol. Soc. Amer. Bull. 76 (1965) 1165], fire obliteration of boulders could be an important process driving drainage evolution in nonglaciated mountains.     

Local scouring and morphological adjustments in steep channels with check-dam sequences

This paper describes bed profile and grain size distribution adjustments in a mountain river (Maso di Spinelle River, Italian Alps) stabilized by a sequence of boulder check-dams. The control works were originally designed to simulate the geometry of natural step-pool channels, where tumbling flow is the dominant hydraulic regime. Local scouring downstream of 29 drop structures is analysed through the use of nondimensional parameters where maximum scour depth and scour length are normalised to the drop height. Prior laboratory data reveal a pattern similar to field scours, where complex interactions occur between drop height, critical flow depth, and step spacing. The linkage between scour length and depth is also discussed. There seems to be a maximum step height for impinging jets that is approximately twice the drop height; this maximum may explain the upper limit of the steepness factor found in high-gradient step-pool streams. If such a maximum upper limit is confirmed by further studies, this may aid designs of foundation heights for transverse control works in steep channels.     

Estimating depth‐averaged velocities in rough ­channels

Profiles of streamwise velocity obtained from North Boulder Creek, Colorado, typically are non‐logarithmic in form and exhibit the strong influence of form drag associated with coarse bed roughness. The spatially averaged profile is consistent with recent theoretical profile forms suggested for rough channels that are based on a partitioning of the total stress between a fluid part and a part associated with form drag on bed particles. Estimates of local depth‐averaged velocity using algorithms that are based on several measurements in the flow column improve with explicit Riemann averaging, versus simple averaging, of the measurements. Estimates based on a single‐point measurement at 0·6 of the flow depth, assuming a logarithmic or approximately logarithmic velocity profile, are the least reliable. Copyright © 2000 John Wiley & Sons, Ltd.     

Magnitude of deviatoric terms in vertically averaged flow equations

The depth‐integrated momentum and kinetic energy equations contain velocity correlation terms that involve products of local deviations in velocity components about depth‐averaged values. Based on velocity data obtained from North Boulder Creek, Colorado, a simple scaling analysis suggests that certain of these terms, which normally can be neglected in the case of smooth channels, can be significant parts of the momentum and energy balances in steep, rough channels owing to the occurrence of non‐logarithmic velocity profiles. A linearized version of the kinetic energy equation suggests that, for flow accelerations over small‐amplitude bed forms, the energy of the mean motion is spatially partitioned between a form involving the depth‐averaged velocity and a form involving the deviatoric part of the velocity profile; this partitioning is associated with spatial variations in the uniformity of the vertical profile of the streamwise velocity. These points are consistent with published flume measurements involving flow over sand‐roughened dunes, and with published field measurements of flow over a gravel bar. Copyright © 2000 John Wiley & Sons, Ltd.     

黄山第四纪泥砾沉积物分形结构特征与沉积环境分析

黄山第四纪泥砾沉积物粒度组成的分维值介于２．７０８￣２．７５３之间,非常接近古泥石流堆积物粒度组成的典型分维值２．７５。结合沉积特征,表明该类沉积物属第四纪间冰缘环境下的稀性泥石流沉积,其形成过程与季风型冰缘环境有关,是两种环境综合作用的结果。