Marwan A. Hassan

Stabilizing self-organized structures in gravel-bed stream channels: Field and experimental observations

Stable reticulate structures, which we call “stone cells,” have been observed in cobble-gravel channel beds with low bed material transport rates. Experiments show that such structures develop simultaneously with the armor layer during an extended period when flows do not exceed the Shields threshold by more than ∼2 times, so that bed material transport is low. They are constructed by particles moving from less stable positions into more stable configurations against each other. Intermediate developments include clusters and stone lines. They reduce sediment transport by orders of magnitude and are evidently a major stability-promoting mechanism in gravel channels. The timescale for their development suggests that the boundaries of many gravel-bed channels are not in equilibrium with recent competent flows but reflect the history of recent “dominant” flows.

DISTANCE OF MOVEMENT OF COARSE PARTICLES IN GRAVEL BED STREAMS

Distributions of distance of bedload particle movement were examined in two gravel bed streams using several hundred magnetically tagged cobbles and pebbles. The compound Poisson model of Einstein-Hubbell-Sayre and a simple gamma function model were compared with observed distributions of moved particles, and of all particles. Both models fit the data reasonably well for small mean displacements, but notable misfits occurred in an event with large mean displacement. When mean particle travel distance approaches the scale of bar spacing, trapping in the bars interrupts particle progress and the dispersion process. The data remain very noisy, so definitive discrimination of suitable models will require trials with more than 103 particles.

Landslide inventory in a rugged forested watershed: a comparison between air-photo and field survey data

Abstract Landslide inventories are routinely compiled by means of aerial photo interpretation (API). When examining photo pairs, the forest canopy (notably in old-growth forest) hides a population of “not visible” landslides. In the present study, we attempt to estimate how important is the contribution of landslides not detectable from aerial photographs to the global mass of sediment production from mass failures on forested terrain of the Capilano basin, coastal British Columbia. API was coupled with intensive fieldwork for identification and measurement of all landslides. A 30-year framework was adopted. We show that “not visible” landslides can represent up to 85% of the total number of failures and account for 30% of the volume of debris mobilised. Such percentages display high sub-basin variability with rates of sediment production varying by one order of magnitude between two sub-basins of the study area. This is explained qualitatively by GIS-based analysis of slope frequency distributions, drainage density, and spatial distribution of surficial materials. Such observations find further support in the definitions of transport-limited and supply-limited basins. As a practical consideration to land managers, we envisage that supplementary fieldwork for landslide identification is mandatory in transport-limited systems only. Fieldwork has demonstrated that gully-related failures have a greater importance than one could expect from API.

Experiments on surface structure and partial sediment transport on a gravel bed

Eight flume experiments were conducted to study the development of bed surface texture and structure in the presence of partial bed material transport. The experiments have two phases, a no-feed degradational phase followed by a feeding phase. A surface structure of irregular, reticulate stone nets and clusters was developed before sediment feeding commenced. Bed load transport equaled or slightly exceeded the fed supply, except at the highest feed rate. The bed structure was maintained, but bed surface texture fined with increasing sediment load. The two phenomena may coexist because the largest grains on the bed moved only very sporadically. The actual sediment transport rates were much less than the expected rate calculated from the ratio of bed surface grain size to transported grain size. The difference reflects the increase in bed stability introduced by the bed structure. Between 17% and 47% of the bed shear stress is estimated to be carried by the structure, <4% being absorbed by the load, while the bed grains carried the balance of the stress. Bed material transport is exceedingly sensitive to bed surface structure and grain size, which raises concerns about the realizable precision of grain size measurements and characterization of the structure.

Size and distance of travel of unconstrained clasts on a streambed

We have examined the relation between the mean distance of movement during individual flood events and the mean particle size of unconstrained clasts in gravel bed rivers. Distance of movement was scaled using the mean distance of movement of the median size group of the surface material; size was scaled using the median b axis diameter of the subsurface material. The scaled data are fitted with a nonlinear function. Clasts larger than the median of surface material exhibit a steep decline of mobility, with an upper limit for mobility near Di/D50sub = 10. Smaller materials exhibit a less sensitive relation. We suppose that trapping of small clasts affects their travel distance. These results indicate that the movement of free surface stones is size preferred. A subordinate analysis of free, constrained and buried particles reveals that bed surface structure acts to reduce differential movement.

FREQUENCY AND MAGNITUDE OF BEDLOAD TRANSPORT IN A MOUNTAIN RIVER

Magnitude and frequency of bedload transport was examined in the Lainbach River, Bavaria, using magnetically tagged particles. During the study, 18 small to moderate events and one major event occurred. About 1 per cent of the flow period exceeded the entrainment threshold and at least once every year about 50 per cent of the tagged particles were mobile. Themajor event which occurred during the study period resulted in the deposition of a thick layer of sediment in parts of the channel and passive burial of most of the recovered particles. The step–pool pattern, which characterized the study site prior to the large event, was obliterated. However, the channel recovered quickly and has returned to a new step–pool pattern. The event changed the boundary conditions by increasing the availability of loose sediment and creating higher river-bed slopes in reaches between breached check dams. As a result, movement of individual particles measured for events of both the same magnitude and duration, occurring before and after the large event, yielded different values. For events which occurred after the large event, the range and the mean distance of movement were about ten times higher.

Sensitivity of bed load transport in Harris Creek: seasonal and spatial variation over a cobble-gravel bar

Bed load sediment was caught in pit traps at several locations on a bar in Harris Creek, a cobble-gravel stream with a nivally dominated hydrograph, a structurally highly organized bed, and very low rates of bed material transport. Observations were made during two spring freshets. In order to obtain representative grain size distributions of the material in transport, the traps were left for periods of up to 24 hours, so that samples of up to 30 kg were recovered. We examine the sensitivity of bedload flux to flow variations via trap-specific ratings for narrowly defined textural subranges. All the ratings are very sensitive, indicating that bed load flux remains in the regime of “partial transport.” The ratings also exhibited seasonal hysteresis and varied from trap to trap and from year to year. At one trap the ratings for large material are distinctly segmented, with no strong correlation for the highest flows. Significant transport for material >8 mm in size begins at ∼7 m3 s−1, considerably higher than for sand. At flows competent to move the local bed gravel the portion of the sand load between 0.25 and 0.50 mm goes into suspension (finer material being dominantly suspended at all flows). Most of the sand transported at <7 m3 s−1 is washed in from upstream while the local bed remains stable. Auxiliary tracer studies demonstrate that at no observed stage (up to mean annual flood level) was the local bed generally mobilized.

Vertical mixing of coarse particles in gravel bed rivers: A kinematic model

Burial of transported material in gravel bed rivers was examined using several hundred magnetically tagged particles. Burial depth of individual particles varies with no simple relation to particle size. Distributions of burial depth of particles were examined by considering observed burial depth of individual size groups of all mobile particles and of all tagged particles (including static ones). “Simple” (single-peak) flow events yielded exponential distributions of burial depth of mobile particles. Distributions with secondary peaks were obtained from large and multipeak events. Season long results were skew peaked with a tendency toward uniformity. An m-layer model is developed to account for the variation of frequency of burial depth of particles, based on the assumptions that the burial depth of particles in a simple event is exponentially distributed and the proportion of particles which change vertical position from a given layer is constant from event to event. The model is elaborated for varying mixing rates, varying scour depths, and variable behavior of different size classes. After many events the material is uniformly distributed throughout all layers, in conformity with field experience.

Displacement characteristics of coarse fluvial bed sediment

[1] Previous work highlights the need for data collection to identify appropriate models for temporal evolution of tracer dispersal in rivers. Results of 64 gravel-bed field tracer experiments covering a wide range of flow and sediment supply regimes are compiled here to determine the probabilistic character of gravel transport. We focus on whether particle travel distances and waits are thin- or heavy-tailed. While heavy-tailed travel distance distributions are observed between successive monitoring events in different hydrological and sediment supply regimes, heavy-tailedness does not persist through total travel distance over multiple monitoring events, suggesting that individual monitoring events occur before particle travel distance exceeds the characteristic correlation length for the channel (such that particles that start in fast paths remain in fast paths and particles in slow paths remain in slow paths). After a large number of transport events, super-diffusive spreading was not observed at any of the gravel bed streams. Continuous-time tracking of x, y, z coordinates of tracers in natural streams is necessary to capture exact step and waiting time distributions.

Spatial and temporal variation of sediment yield in the landscape: Example of Huanghe (Yellow River)

Regional scale relations between specific sediment yield and drainage basin area in the Huanghe basin reveal varying landscape sedimentary dynamics. Declining specific sediment yield with area is typical of disturbed agricultural areas, while increasing yields indicate the remobilization of stored sediment by bank and floodplain erosion along major tributaries. Using the developed relations, areally-adjusted sediment yield maps are created over the entire Huanghe basin. Maps of the Loess Plateau for periods before and after 1970 reveal major changes in sediment yield, indicating that land management may have a major effect on sediment dynamics within a short period of time. Copyright 2008 by the American Geophysical Union.