Abstract Surface roughness plays a key role in determining aerodynamic roughness length ( z o ) and shear velocity, both of which are fundamental for determining wind erosion threshold and potential. While z o can be quantified from wind measurements, large proportions of wind erosion prone surfaces remain too remote for this to be a viable approach. Alternative approaches therefore seek to relate z o to morphological roughness metrics. However, dust‐emitting landscapes typically consist of complex small‐scale surface roughness patterns and few metrics exist for these surfaces which can be used to predict z o for modeling wind erosion potential. In this study terrestrial laser scanning was used to characterize the roughness of typical dust‐emitting surfaces (playa and sandar) where element protrusion heights ranged from 1 to 199 mm, over which vertical wind velocity profiles were collected to enable estimation of z o . Our data suggest that, although a reasonable relationship ( R 2 > 0.79) is apparent between 3‐D roughness density and z o , the spacing of morphological elements is far less powerful in explaining variations in z o than metrics based on surface roughness height ( R 2 > 0.92). This finding is in juxtaposition to wind erosion models that assume the spacing of larger‐scale isolated roughness elements is most important in determining z o . Rather, our data show that any metric based on element protrusion height has a higher likelihood of successfully predicting z o . This finding has important implications for the development of wind erosion and dust emission models that seek to predict the efficiency of aeolian processes in remote terrestrial and planetary environments.
Comprehensive empirical data of the response of unstable streams over a range of environmental conditions are unavailable. In this study, as a substitute for empirical data, a physically based numerical model of channel evolution is used in a range of numerical simulation experiments designed to predict the sensitivity of channel response to changes in control variables. The scope of the study is limited by the scope of the numerical model which applies to straight, sand-bed streams with cohesive bank materials that have been destabilized by sediment starvation and evolve towards equilibrium through bed degradation followed by channel widening. Results are presented for stable and unstable channel conditions. Stable channel depths are most sensitive to channel discharge, though the critical threshold shear stress for the entrainment of cohesive bank materials and discharge are both significant in determining the width. The sediment load, channel gradient, bank material cohesion, size of failed bank material aggregates and the initial bank height have sensitivities an order of magnitude smaller than discharge for both width and depth. Variations in bed material characteristics within the sand-size range are found to have little impact on simulated stable channel morphology. For unstable channels, the relative dominance of parameter sensitivities is examined in the context of an empirical-conceptual model of channel evolution proposed by Thorne and Osman (1988), to highlight the relationships between parameter dominance, time, and the processes and forms characterizing individual stages of channel evolution. Rates of change with time of width and depth sensitivity parameters for five tested independent variables (discharge, sediment supply, channel gradient, bank material cohesion and bed material size) are found to vary as a function of time, such that different stages of channel evolution are characterized by variations in the relative dominance of tested variables. The results support the hypothesis proposed by Thorne and Osman (1988) that the critical bank height required to initiate mass-wasting and widening may be regarded as a geomorphic threshold.
Few sources exist to draw generalizations about the incredibly diverse international river restoration community. Generalizations in the restoration literature tend to be grounded on individual experiences or logical conjecture. To fill this perceived gap, an international web‐based survey was launched. Over 500 respondents from 37 different countries participated. The results, posted on the web, act as a database of perceptions and individual experiences, from which the restoration community can make their own interpretations. With three examples, we contrast scientific conjecture with the perceptions of the restoration community who participated in this survey.
Abstract Riverbank erosion, associated sedimentation and land loss hazards are a land management problem of global significance and many attempts to predict the onset of riverbank instability have been made. Recently, Osman and Thorne (1988) have presented a Culmann‐type analysis of the stability of steep, cohesive riverbanks; this has the potential to be a considerable improvement over previous bank stability theories, which do not account for bank geometry changes due to toe scour and lateral erosion. However, in this paper it is shown that the existing Osman‐Thorne model does not properly incorporate the influence of tension cracking on bank stability since the location of the tension crack on the floodplain is indirectly determined via calculation or arbitrary specification of the tension crack depth. Furthermore, accurate determination of tension crack location is essential to the calculation of the geometry of riverbank failure blocks and hence prediction of land loss and bank sediment yield associated with riverbank instability and channel widening. In this paper, a rational, physically based method to predict the location of tension cracks on the floodplain behind the eroding bank face is presented and tested. A case study is used to illustrate the computational procedure required to apply the model. Improved estimates of failure block geometry using the new method may potentially be applied to improve predictions of bank retreat and floodplain land loss along river channels destabilized as a result of environmental change.
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