Douglas J. Sherman

A Review of the Effects of Surface Moisture Content on Aeolian Sand Transport

Over the past several decades, a number of studies have shown that intergranular cohesion associated with the presence of moisture significantly increases the critical shear velocity required to initiate motion in sand grains, and decreases transport rates. This paper examines currently available models of moisture effects and compares model predictions for several hypothetical situations. Model predictions exhibit considerable disagreement regarding the magnitude of moisture effects. For 0.27-mm sands, predicted increases in threshold shear velocity associated with a 1% moisture content ranged from about 8% to 148% of the expected dry threshold velocity, and with 4% moisture increased to 47%–206% of the dry value. Based on the predicted threshold shear velocities, the expected transport rates at a 1% moisture content under a 0.50 m s−1 shear velocity range from no transport to more than 100 kg m−1 hr−1.

An equilibrium relationship for shear velocity and apparent roughness lenght in aeolian saltation

Abstract An equilibrium relationship between shear velocity and apparent surface roughness is established for aeolian saltation. Velocity profiles of wind over blowing sand were digitized from eleven published data sets. The 106 profiles were analyzed to obtain estimates of shear velocity and apparent surface roughness lenghts. These values were used to test Charnocks relationship for suitability in aeolian environments. The results indicate that z 0 = 0.0091 u ∗ 2 /g . Modifying Charnocks model to describe wind over an equilibrium saltation field yields z 0 − (2D 50 /30) = 0.0252 (u ∗ −u ∗t ) 2 /g . The latter result is used to demonstrate that the focal point concept proposed by Bagnold stems from the increase in apparent roughness with increased shear velocity. The results demonstrate an excellent example of threshold-controlled equilibrium in dynamic systems.

Problems of scale in the modeling and interpretation of coastal dunes

Abstract Coastal dune systems are studied at time scales from seconds to millennia, and space scales from millimeters to kilometers. Present approaches to the study of coastal dunes make it difficult to integrate models and interpretations of these systems over these scale ranges and arrive at reasonable conclusions. It is argued that identification of key controls on dune development, measurement of those controls, and synthesis of data, describing past and present conditions and used as calibration points, will improve the viability of coastal dune models. Theoretical and empirical advances are necessary to improve the reliability of predictions across a range of geomorphological scales. Attempts at linking theoretical (systematic) models, process (synoptic) measurement, and historical or paleoenvironmental (synthetic) approaches make explicit the recognition that at time scales of more than a few hours, and space scales greater than a few hundred meters, deterministic models become unstable vis-a-vis prototype environments. Process “climatologies” provide one means to link process-based work with broader-scaled analysis. As scales increase, such climatologies will become less appropriate as the data become less reliable, or as the systems change, or as the scales become too large relative to the process record lengths. In these conditions, specific data (i.e. samples) representing points in time and space become check points for calibrating models. It should be possible, ideally, to integrate both up and down time and space scales. This is not yet possible.

Waves, currents, sediment flux and morphological response in a barred nearshore system

Abstract A shore-normal array of seven, bi-directional electromagnetic flowmeters and nine surface piercing, continuous resistance wave staffs were deployed across a multiple barred nearshore at Wendake Beach, Georgian Bay, Canada, and monitored for a complete storm cycle. Time-integrated estimates of total (ITVF) and net (INVF) sediment volume flux together with bed elevation changes were determined using depth-of-activity rods . The three bars, ranging in height from 0.10 to 0.40 m accreted during the storm (0.03 m), and the troughs were scoured (0.05 m). Sediment reactivation depths reached 0.14 m and 12% of the nearshore control volume was mobilized. However, the INVF value for the storm was less than 1% of the control volume revealing a near balance in sediment volume in the bar system. Landward migration of the inner, crescentic and second, sinuous bars occurred in association with an alongshore migration of the bar form itself; the outermost, straight, shore-parallel bar remained fixed in location. The surf zone was highly dissipative throughout the storm ( ϵ = 3.8 × 10 2 –192 × 10 2 ) and the wave spectrum was dominated by energy at the incident frequency. Spectral peaks at frequencies of the first harmonic and at one quarter that of the incident wave were associated with secondary wave generation just prior to breaking and a standing edge wave, respectively. The former spectral peak was within the 95% confidence band for the spectrum while the latter contributed not more than 10% to the total energy in the surface elevation spectrum even near the shoreline. During the storm wave height exceeded 2 m ( H s ) and periods reached 5 s ( T p k ): orbital velocities exceeded 0.5 m s −1 ( u rm s ) and were above the threshold of motion for the medium-to-fine sands throughout the storm. Shore-parallel flows in excess of 0.4 m s −1 were recorded with maxima in the troughs and minima just landward of the bar crest. The rate and direction of sediment flux is best explained by the interaction of antecedent bed slopes with spatial gradients in the mean and asymmetry of the shore-normal velocity field. These hydrodynamic parameters represent “steady” flows superimposed on the dominantly oscillatory motion and assumed a characteristic spatial pattern from the storm peak through the decay period. Increases spatially in the magnitudes of both the mean flows and flow asymmetries cause an increasing net transport potential (erosion); decreases in these values spatially cause a decreasing net transport potential and thus deposition. These transport potentials are increased or decreased through the gravity potential induced by the local bed slope. Shore-parallel flow was important in explaining sediment flux and morphological change where orbital velocities, mean flows and flow asymmetries were at a minimum.

Geomorphology and natural hazards

Abstract Natural hazards research was initiated in the 1960s by Gilbert White and his students who promulgated a research paradigm that involved assessing risk from a natural event, identifying adjustments to cope with the hazard, determining peoples perception of the event, defining the process by which people choose adjustments, and estimating the effects of public policy on the choice process. Studies of the physical system played an important role in early research, but criticismsof the paradigm resulted in a shift to a prominence of social science. Geomorphologists are working to fill gaps in knowledge of the physical aspects of individual hazards, but use of the information by social scientists will only occur if information is presented in a format that is useful to them. One format involves identifying the hazard according to seven physical parameters established by White and his colleagues: magnitude, frequency, duration, areal extent, speed of onset, spatial dispersion, and temporal spacing. Geomorphic hazards are regarded as related to landscape changes that affect human systems. The processes that produce the changes are rarely geomorphic in nature, but are better regarded as atmospheric or hydrologic. An examination of geomorphic hazards in four fields — soil erosion, mass movement, coastal erosion and fluvial erosion — demonstrates that advances in those fields may be evaluated in terms of the seven parameters. Geomorphologists have contributed to hazard research by focusing on the dynamics of the landforms. The prediction of occurence, the determination of spatial and temporal characteristics, the impact of physical characteristics on peoples perception, and the impact of physical characteristics on adjustment formulation. Opportunities for geomorphologists to improve our understanding of geomorphic hazards include research into the characteristics of the events particularly with respect to predicting the occurence, and increased evaluation of the impact of human activities on natural systems.

BEACH-STATE CONTROLS ON AEOLIAN SAND DELIVERY TO COASTAL DUNES

Characteristics of morphodynamically reflective, intermediate, and dissipative beach systems, from the data of Short (1984), are used with an iterative aeolian sediment transport model to predict beach-state controls on transport rates. The predictions are based on the transport model of White (1979), the moisture effect model of Belly (1964), and the slope-effect model of Hardisty and Whitehouse (1988). The model adjusts profile geometry continuously as sand is preferentially eroded and deposited across the model beaches. For an assumed shear velocity of 0.50 ms-1, sediment transport rates were about 20% larger for the dissipative system compared to the reflective beach. For a shear velocity of 0.75 ms-1, about 80% more sand is removed from the dissipative beach. These results suggest that the Short and Hesp (1982) conceptual model of beach state-dune form relationships is valid and can be quantified. [Key words: coastal dunes, coastal geomorphology, beach state, beach modeling.]

Coastal geomorphology through the looking glass

Abstract Coastal geomorphology will gain future prominence as environmentally sound coastal zone management strategies, requiring scientific information, begin to supplant engineered shoreline stabilization schemes for amelioration of coastal hazards. We anticipate substantial change and progress over the next two decades, but we do not predict revolutionary advances in theoretical understanding of coastal geomorphic systems. Paradigm shifts will not occur; knowledge will advance incrementally. We offer predictions for specific coastal systems delineated according to scale. For the surf zone, we predict advances in wave shoaling theory, but not for wave breaking. We also predict greater understanding of turbulent processes, and substantive improvements in surf-zone circulation and radiation stress models. Very few of these improvements are expected to be incorporated in geomorphic models of coastal processes. We do not envision improvements in the theory of sediment transport, although some new and exciting empirical observations are probable. At the beach and nearshore scale, we predict the development of theoretically-based, two- and three-dimensional morphodynamical models that account for non-linear, time-dependent feedback processes using empirically calibrated modules. Most of the geomorphic research effort, however, will be concentrated at the scale of littoral cells. This scale is appropriate for coastal zone management because processes at this scale are manageable using traditional geomorphic techniques. At the largest scale, little advance will occur in our understanding of how coastlines evolve. Any empirical knowledge that is gained will accrue indirectly. Finally, we contend that anthropogenic influences, directly and indirectly, will be powerful forces in steering the future of Coastal Geomorphology. “If you should suddenly feel the need for a lesson in humility, try forecasting the future…” (Kleppner, 1991, p. 10).

Boundary roughness and bedforms in the surf zone

Hydrodynamical models of the nearshore system frequently assume that a single friction coefficient is sufficient to represent flow conditions at a point in the surf zone. Furthermore, models attempting to relate bed configuration to surf zone flows have relied primarily upon the wave orbital velocity as an indicator of potential bedforms, and thus as the control on boundary roughness. The data presented here point out potential errors arising from either of these approaches. The results of a field experiment conducted at Wendake Beach, Ontario, show that at a single location in an active surf zone, the Darcy-Weisbach friction coefficient, f, varied by approximately 250% (in this case between 0.016 and 0.041). It is also shown that existing bedform models, based upon primary wave motions alone, do not accurately predict conditions at this study site. For a relatively constant wave orbital velocity and velocity asymmetry, it is found that changes in bed roughness, as a result of bedform development, are reflected mainly in the vertical profile of the longshore current velocity. A sequence of bedforms, from oscillatory ripples through flat bed, is inferred from the data, and found to be supported by diver observations and preserved primary sedimentary structures.

Predicting Aeolian Sand Transport: Revisiting the White Model

The derivation and history of the frequently cited aeolian transport model of White are considered in light of the continued replication of an error in the original expression. The error may have escaped notice because the expression is still dimensionally correct and it yields predictions that appear reasonable in comparison with both the predictions of other models with field data. The incorrect expression has come to be identified as a distinct model. However, the correct formulation of the ‘White model’ is, in fact, a rearrangement of the Kawamura model with a slightly smaller (c.6%) empirical coefficient.

AEOLUS II : an interactive program for the simulation of aeolian sedimentation

Abstract A computer program has been developed to simulate two- or three-dimensional topographic change associated with aeolian sediment transport. Because predictions generated by available models of aeolian transport can vary by about an order of magnitude for a given set of environmental conditions, selection of an appropriate model is problematic. This program includes a number of models and provides for interactive selection of a desired model combination(s), to allow for comparison between model predictions. Topography, represented by a set of coordinates in two or three dimensions, is defined by ‘bins’ of sediment. Differential rates of transport between adjacent bins, resulting from variations in grain size, slope, and surface moisture content, are used to determine mass flux and to adjust the topography in an iterative fashion. Sub-routines allow the simulation of temporally varying levels of surface moisture resulting from evaporation, and periodic restoration of upwind boundary conditions. The wind field can be represented by a constant shear velocity, by a spatial (cross-shore) distribution of shear velocities, or by a time series of values representing multiple wind events. Output is in the form of a graphic display or tabulated results written to disk. Example results are presented which (qualitatively) suggest that simulated topographic changes are reasonable. Areas where further refinement is needed to improve correspondence with reality are identified and discussed.