Profile Concavity Research Articles

Holocene profile changes along a California coastal stream

AbstractWalker Creek in Marin County, California is a coastal stream draining to Tomales Bay, which lies in the San Andreas Rift Zone. Its valley contains an alluvial fill with a basal gravel dated at 5000 years BP. In upstream parts of the watershed, channels are incised arroyo‐like in the fill leaving the valley floor standing as a high terrace averaging 5·5 m (18 ft) high. Below this terrace is an inner terrace of historic age that stands 2·4 m (8 ft) above the streambed. The stratigraphy and morphology of this valley are seen in others nearby, and indicate that in the last half of Holocene time in this region a single episode of valley alluviation was followed by two episodes of valley cutting.The second episode of valley cutting is occurring in the present time. During the last 60 years the flow has become seasonal, the stream has incised 1·5 m (5 ft) below the inner terrace in upstream reaches, aggraded 1·2 m (4 ft) in downstream reaches, and extended its estuary. Incision upstream has begun to re‐expose the bedrock valley floor and is associated with aggradation downstream that has caused the flood plain to overtop both terraces. This has decreased the stream's gradient.Using a stream that is currently effecting major changes in its valley and channel morphology, two aspects of hydraulic adjustment in fluvial systems are examined. The changes in the average slope of the longitudinal profile are small but measureable. Profile concavity has not changed measurably. The various profiles that have existed in Holocene time show that stream gradient can be, but is not necessarily, slightly adjusted during valley filling and cutting. Flow measurements at a high discharge show that the channel has begun to assume the hydraulic geometry of an ephemeral channel. Adjustments of depth, velocity, and roughness appear to be hydraulic adjustments in response to changing watershed conditions.

A scree slope rockfall model

AbstractCertain observed characteristics of scree slopes; namely concavity of profile, straight slope angle less than the angle of repose, and good size sorting of particles, are not consistent with an angle of repose model for accumulation. An alternative model is proposed based upon rockfall and surface stone movement and is tested against experimental data of particle movement in the field. It is found that the mechanical model of stone movement generated adequately explains the motion of particles on scree slopes and that it is in keeping with the characteristics of many screes. The static features of some Isle of Skye screes were also measured and the straight‐concave slope form with good downslope sorting of material, characteristic of the rockfall process, was found to be present.

Surface Stone Movement and Scree Formation

The accumulation of rockfall talus slopes has been studied for conditions in which stones are added relatively slowly, so that dynamic interactions between them may be ignored, and stone movements modeled "one-at-a-time." Laboratory scale models (1 m long) are compared with the results of mathematical and computer simulation. Mathematical analysis assumes that stone movement down the talus is equivalent to a "sliding-only" movement, and laboratory results support this assumption. Grain-size grading is explained in terms of the relative roughness of different material sizes to stones moving down the talus. Profile concavity is ascribed to the run-out of some material, corresponding to the tail of the frequency distribution for travel distances. Growth of the talus profile is modeled as a kinematic wave. The lower concavity obtained on both stochastic and scale models relative to deterministic models is thought to result from local accumulations of stones in the upslope parts of the talus which then produce small mass-movements at a constant slope.

IV.—Further Contributions to British Carboniferous Palæontology

Sp. Chars.—Tooth bidentioulated with a re-entering angle between the denticles; sides sloping down to the lateral angles; anterior profile convex; posterior profile concave. The most prominent portion of the anterior surface is at the centre of the inferior margin of the crown, where it becomes bowed-out, whence the surface gradually recedes to the cutting edge of the crown on the one hand, and more rapidly towards the root on the other.