A classification system for natural rivers is presented in which a morphological arrangement of stream characteristics is organized into relatively homogeneous stream types. This paper describes morphologically similar stream reaches that are divided into 7 major stream type categories that differ in entrenchment, gradient, width/depth ratio, and sinuosity in various landforms. Within each major category are six additional types delineated by dominate channel materials from bedrock to silt/clay along a continuum of gradient ranges. Recent stream type data used to further define classification interrelationships were derived from 450 rivers throughout the U.S, Canada, and New Zealand. Data used in the development of this classification involved a great diversity of hydro-physiographic/geomorphic provinces from small to large rivers and in catchments from headwater streams in the mountains to the coastal plains. A stream hierarchical inventory system is presented which utilizes the stream classification system. Examples for use of this stream classification system for engineering, fish habitat enhancement, restoration and water resource management applications are presented. Specific examples of these applications include hydraulic geometry relations, sediment supply/availability, fish habitat structure evaluation, flow resistance, critical shear stress estimates, shear stress/velocity relations, streambank erodibility potential, management interpretations, sequences of morphological evolution, and river restoration principles.

https://www.cpp.edu/~marshall/rs414/RS414_Readings/Rosgen_94_Classification_of_Natural_Rivers.pdf

A classification of natural rivers

Uniaxial compression of plates of brittle materials containing pre-existing planar cracks oriented at certain angles with respect to the direction of overall compression has revealed that the relative frictional sliding of the faces of the pre-existing cracks may produce, at their tips, tension cracks which deviate at sharp angles from the sliding plane. These tension cracks then continue to grow in a stable manner with increasing axial compression, curving toward an orientation parallel to the direction of axial compression. Within the framework of linear fracture mechanics, the out-of-plane extension of a pre-existing straight crack, induced by overall far-field compression, is analyzed, and various parameters which characterize the growth process are quantified. It is shown analytically that, for a wide range of pre-existing crack orientations, the out-of-plane crack extension initiates at an angle close to 70° from the direction of the pre-existing crack; the exact value of this angle, of course, depends on the friction factor and the orientation of the pre-existing crack. It is found that the growth process is stable initially, but the rate of increase of the length of the extended portion with respect to the increasing axial compression dramatically increases after a certain extension length is attained, and in fact, this length becomes unbounded if a small lateral tension also exists. Various limiting cases are examined and the corresponding analytical estimates are compared with the numerical results, arriving at good correlations. A series of qualitative experiments is performed on thin plates of Columbia Resin CR 39, arriving at excellent agreement with the analytical results. In light of the analysis, the phenomena of axial splitting, exfoliation (or sheet fracture), and rockburst are examined, and it is suggested that they may all be the results of the out-of-plane (tensile) extension of pre-existing cracks, induced by large overall far-field compressions. This assertion is then supported by a series of experiments which show that the relative frictional sliding of the faces of one or even an array of pre-existing cracks does not result in coplanar (sliding mode) crack growth, but rather leads to the formation of tension cracks which grow in the direction of maximum compression. Moreover, a pre-existing crack close to a free boundary grows in a similar manner under compression parallel to the boundary, and shows no tendency to move toward the free surface. Possible lateral buckling which may result, and which may cause further unstable crack extension, is illustrated experimentally, and discussed in an effort to shed light on the phenomena of rockburst and surface spalling.

Compression-induced nonplanar crack extension with application to splitting, exfoliation, and rockburst

The fluvial system is a major concern in modeling landform evolution in response to tectonic deformation. Three stream bed types (bedrock, coarse-bed alluvial, and fine-bed alluvial) differ in factors controlling their occurrence and evolution and in appropriate modeling approaches. Spatial and temporal transitions among bed types occur in response to changes in sediment characteristics and tectonic deformation. Erosion in bedrock channels depends upon the ability to scour or pluck bed material; this detachment capacity is often a power function of drainage area and gradient. Exposure of bedrock in channel beds, due to rapid downcutting or resistant rock, slows the response of headwater catchments to downstream baselevel changes. Sediment routing through alluvial channels must account for supply from slope erosion, transport rates, abrasion, and sorting. In regional landform modeling, implicit rate laws must be developed for sediment production from erosion of sub-grid-scale slopes and small channels.

/pdf/modeling-fluvial-erosion-on-regional-to-continental-scales-idabqrjrv8.pdf

Modeling fluvial erosion on regional to continental scales

We study the interplay of various factors causing vertical grain‐size changes in alluvial basins using a simple coupled model for sediment transport and downstream partitioning of grain sizes. The sediment‐transport model is based on the linear diffusion equation; by deriving this from first principles we show that the main controls on the diffusivity are water discharge and stream type (braided or single‐thread). The grain‐size partitioning model is based on the assumption that the deposit is dominated by gravel until all gravel in transport has been exhausted, at which point deposition of the finer fractions begins. We then examine the response of an alluvial basin to sinusoidal variation in each of four basic governing variables: input sediment flux, subsidence rate, supplied gravel fraction, and diffusivity (controlled mainly by water flux). We find that, except in the case of variable gravel fraction, the form of the basin response depends strongly on the time‐scale over which the variation occurs. There is a natural time‐scale for any basin, which we call the ‘equilibrium time’, defined as the square of basin length divided by the diffusivity. We define ‘slow’ variations in imposed independent variables as those whose period is long compared with the equilibrium time. We find that slow variation in subsidence produces smoothly cyclic gravel‐front migration, with progradation during times of low sedimentation rate, while slow variation in sediment flux produces gravel progradation during times of high sedimentation rate. Slow variation in diffusivity produces no effect. Conversely, we define ‘rapid’ variations as those whose period is short compared with the equilibrium time. Our model results suggest that basins respond strongly to rapid variation in either sediment flux or diffusivity; in both cases, deep proximal unconformities are associated with abrupt gravel progradation. This progradation occurs during times of either low sediment flux or high diffusivity. On the other hand, basin response to variation in subsidence rate gradually diminishes as the time scale becomes short relative to the equilibrium time. Each of the four variables we have considered ‐ input sediment flux, subsidence, gravel fraction, and diffusivity ‐ is associated with a characteristic response pattern. In addition, the time scale of imposed variations relative to the equilibrium time acts in its own right as a fundamental control on the form of the basin response.

The large-scale dynamics of grain-size variation in alluvial basins, 1: Theory

https://cirp.usace.army.mil/products/files/doc/Hallermeier1981-ProfileZonation.pdf

A profile zonation for seasonal sand beaches from wave climate

Modern and ancient wave-cut platforms on Ben Lomond Mountain in central California are broadly similar in shape. They have a seaward slope composed of two segments: a steeper, slightly concave inshore segment, with gradients of generally 0.02 to 0.04 (20 to 40 m/km), and a flatter, planar offshore segment with gradients of 0.007 to 0.017 (7 to 17 m/km). The flattest inshore and offshore gradients measured were, respectively, 0.015 (15 m/km) and 0.005 (5 m/km), suggesting that these are close to minimum gradients for erosional platforms in central California. The inshore segments are generally 300 to 600 m wide and extend to a depth of 8 to 13 m. Platforms are widest in areas where soft sandstone crops out and where there has been least uplift. Major storm waves now break in water 7 to 12 m deep. We conclude that inshore platform segments were associated with storm-wave surf zones and that offshore segments were associated with the zone of deep-water wave transformation. A gradient of 0.005 for the offshore segment would keep wave energy at the bottom constant (Zenkovich, 1967). A steeper gradient for the inshore segment would enable backwash undertow to counteract the strong onshore movement of surf, so that available coarse sediment could be moved laterally. Slopes less than the minimum would so dissipate wave energy in offshore areas that the surf zone would not be able to provide the needed longshore transport for coarse sediment, and beach progradation would result. Thus, platforms have a shape that allows efficient conversion of wave energy into erosion and longshore transport; their seaward gradient is not used for the downhill transport of sediment. Platform gradients decrease with time, at least until the minimum is achieved. Whether the offshore segments were eroded at their existing depths or were eroded by surf zones as sea level rose remains a matter of controversy. Ben Lomond platforms have been uplifted and progressively tilted in a seaward direction, indicating that late Tertiary domical uplift has continued into Quaternary time. Uplift rates have ranged from 0.16 m/1,000 yr near Santa Cruz to 0.26 m/1,000 yr near Greyhound Rock. Tilts have varied from 0.001 (1 m/km) for the lowest prominent platform to 0.009 (9 m/km) for the highest platform (which may be as old as 10 6 yr). Because of uplift, platforms must have been cut at times of eustatically high sea level.

Form, genesis, and deformation of central California wave-cut platforms

Precambnan outcrops in the Llano Uplift of central Texas furnish the Colorado River with a distinctive granitic load which changes systematically as it moves downstream. In 160 miles between Austin and the Columbus-Eagle Lake area, coarse granitic gravel diminishes in size by about 50 percent and changes from a dommantly granite-gneissaplite assemblage to a dominantly pegmatite-graphic granite assemblage. Coarse quartz and chert gravel diminish in size by about 30 percent and 20 percent, respectively. In this reach, the Colorado flows on a flood plain whose alluvial width and depth stay fairly constant, a situation unfavorable for significant down-valley sorting (Mackin, 1963). Field observations generally lead to the same conclusion.

Abrasion and its relationship to weathering was studied by running fresh and weathered specimens of Colorado River granitic gravel in a Kuenen-type abrasion tank at the University of Texas. Degree of weathering was slight to moderate, with biotite and feldspar the primary and secondary targets. Abrasion of fresh granitic gravel during 160 miles of travel produced only a 10 percent reduction in size. Abrasion of weathered gravel produced a reduction of more than 50 percent and was clearly related to lithology; the biotite-bearing rocks (granite, gneiss, and some aplite) were the least durable, the biotite-free rocks (pegmatite and graphic granite) most durable. Abrasion of quartz and chert produced a reduction in size of less than 10 percent.

Changes in size and lithology of coarse granitic gravel along the lower Colorado River are best explained by abrasion of particles tha t weather slightly during periods of temporary alluvial storage. Reduction in size of quartz and chert are not satisfactorily explained for the reach between Austin and Columbus-Eagle Lake, but downstream from Eagle Lake, sorting appears to be in control.

Effect of Weathering on Abrasion of Granitic Gravel, Colorado River (Texas)

Knik River of southern Alaska is a glacial river with a valley train which was, until recently, swept each summer by a brief large flood resulting from the sudden draining of an ice-dammed lake (Lake George). These flows were 10 to 20 times larger than the postflood summer discharge of approximately 20,000 cfs and were responsible for the morphological and alluvial characteristics of the valley train. Flooding stopped after the 1966 season. During its first 16 mi of travel, where alluvial contamination from side streams is unimportant, the coarse fraction of the valley train changes systematically in size and shape. Grain size decreases by 94 percent, although this figure pertains to two populations: oversized particles and coarse particles which are readily moved by Knik flood flows. If the latter population alone is considered, reduction is 87 percent. At the same time, specimens of quartz and graywacke become progressively more elongated, and foliated specimens show the same change initially, but then deviate to become progressively more platy. Downstream from mile 16, Knik River alluvium is contaminated by detritus from an old outwash fan of the nearby Matanuska River. Sixteen miles of travel in a Kuenen-type abrasion tank reduced the size of Knik River gravel by only 8 percent and failed to change its shape. We conclude that downvalley changes in size and shape of coarse gravel are caused chiefly by sorting processes, aided by frost action which splits foliated particles into platy fragments. During shape-sorting, platy specimens are the most transportable, elongated specimens are intermediate, and compact (equidimensional) specimens are the least transportable. Our data support Russell9s (1939) contention that sorting is favored by aggradation and extreme variations in discharge.

Coarse Sediment Transport by Flood Flows on Knik River, Alaska

According to classical theory, wave-cut platforms are developed by deep submarine abrasion while sea level is stationary. Modern writers, however, believe that wave erosion takes place only in shallow water. Fragile, subaerially etched pyroxene sand grains are mechanically unstable at depths of less than about 30 feet in the Santa Cruz area of California but are mechanically stable at greater depths, supporting the conclusion that significant wave-produced submarine abrasion is restricted to the surf zone with a maximum depth of about 30 feet. Most sediment in transit beyond the surf zone is too fine-grained to act as an abrasive. If sea level is stationary, marine planation may produce a wave-cut platform up to about one-third of a mile wide; a wider platform can be cut only during slow submergence. The modern and former platforms in the Santa Cruz area were carved while sea level was rising.

Submarine abrasion and wave-cut platforms

Massive igneous and metamorphic rocks commonly display a near-surface sheet jointing that is unrelated to other structures and textures but is related to topography in that it exists only at shallow depths and tends to parallel the surface of the ground. Most authors believe that this large-scale exfoliation accompanies expansion when erosion releases confining pressure on rocks which were once deeply buried. Confining pressure is released unequally, enabling residual compressive stresses parallel to the surface to cause the splitting. Little has been written about these structures in sedimentary rocks. Massive sandstones on the Colorado Plateau flake off in sheets bounded by joints which are unrelated to other structures and textures, which are found only at shallow depths, and which are oriented parallel to the outcrops. This large-scale exfoliation is similar in all important aspects to that found in crystalline terranes and it has been produced in the same way—by differential residual stresses resulting from erosion, particularly canyon cutting, of rocks that were once buried. Exfoliation has contributed to canyon development and has led to such distinctive topographic features as exfoliation caves and domes. Most existing exfoliation joints are probably of Pleistocene age.

William C Bradley

Papers

Form, genesis, and deformation of central California wave-cut platforms

Effect of Weathering on Abrasion of Granitic Gravel, Colorado River (Texas)

Coarse Sediment Transport by Flood Flows on Knik River, Alaska

Submarine abrasion and wave-cut platforms

Large-Scale Exfoliation in Massive Sandstones of the Colorado Plateau