School of Community and Regional Planning

Dryland vegetation is often spatially patchy, and so affects wind flow in complex ways. Theoretical models and wind tunnel testing have shown that skimming flow develops above vegetation patches at high plant densities, resulting in little or no wind erosion in these zones. Understanding the dynamics of skimming flow is therefore important for predicting sediment transport and bedform development in dryland areas. However, no field-based data are available describing turbulent airflow dynamics in the wake of vegetation patches. In this study, turbulent wind flow was examined using high-frequency (10 Hz) sonic anemometry at four measurement heights (0.30 m, 0.55 m, 1.10 m and 1.65 m) along a transect in the lee of an extensive patch of shrubs (z = 1.10 m height) in Namibia. Spatial variations in mean wind velocity, horizontal Reynolds stresses and coherent turbulent structures were analysed. We found that wind velocity in the wake of the patch effectively recovered over $12 patch heights (h) downwind, which is 2–5 h longer than previously reported recovery lengths for individual vegetation elements and two-dimensional wind fences. This longer recovery can be attributed to a lack of flow moving around the obstacle in the patch case. The step-change in roughness between the patch canopy and the bare surface in its wake resulted in an initial peak in resultant horizontal shear stress (s r) followed by significant decrease downwind. In contrast to s r , horizontal normal Reynolds stress (u 02) progressively increased along the patch wake. A separation of the upper shear layer at the leeside edge of the patch was observed, and a convergence of s r curves implies the formation of a constant stress layer by $20 h downwind. The use of s r at multiple heights is found to be a useful tool for identifying flow equilibration in complex aerodynamic regimes. Quadrant analysis revealed elevated frequencies of Q2 (ejection) and Q4 (sweep) events in the immediate lee of the patch, which contributed to the observed high levels of shear stress. The increasing downwind contribution of Q1 (outward interaction) events, which coincides with greater u 02 and wind velocity, suggests that sediment transport potential increases with greater distance from the patch edge. Determining realistic, field-derived constraints on turbulent airflow dynamics in the wakes of vegetation patches is crucial for accurately parameterising sediment transport potential in larger-scale dryland landscape models. This will help to improve our understanding of how semi-vegetated desert surfaces might react to future environmental and anthropogenic stresses.

Knowledge of the changing rate of sediment flux in space and time is essential for quantifying surface erosion and deposition in desert landscapes. Whilst many aeolian studies have relied on time-averaged parameters such as wind velocity (U) and wind shear velocity (u ⁎) to determine sediment flux, there is increasing field evidence that high-frequency turbulence is an important driving force behind the entrainment and transport of sand. At this scale of analysis, inertia in the saltation system causes changes in sediment transport to lag behind de/accel-erations in flow. However, saltation inertia has yet to be incorporated into a functional sand transport model that can be used for predictive purposes. In this study, we present a new transport model that dynamically balances the sand mass being transported in the wind flow. The 'dynamic mass balance' (DMB) model we present accounts for high-frequency variations in the horizontal (u) component of wind flow, as saltation is most strongly associated with the positive u component of the wind. The performance of the DMB model is tested by fitting it to two field-derived (Namibia's Skeleton Coast) datasets of wind velocity and sediment transport: (i) a 10-min (10 Hz measurement resolution) dataset; (ii) a 2-h (1 Hz measurement resolution) dataset. The DMB model is shown to outperform two existing models that rely on time-averaged wind velocity data (e.g. Radok, 1977; Dong et al., 2003), when predicting sand transport over the two experiments. For all measurement averaging intervals presented in this study (10 Hz–10 min), the DMB model predicted total saltation count to within at least 0.48%, whereas the Radok and Dong models over-or underestimated total count by up to 5.50% and 20.53% respectively. The DMB model also produced more realistic (less 'peaky') time series of sand flux than the other two models, and a more accurate distribution of sand flux data. The best predictions of total sand transport are achieved using our DMB model at a temporal resolution of 4 s in cases where the temporal scale of investigation is relatively short (on the order of minutes), and at a resolution of 1 min for longer wind and transport datasets (on the order of hours). The proposed new sand transport model could prove to be significant for integrating turbulence scale transport processes into longer-term, macro-scale landscape modelling of drylands.

Wind erosion is a key component of land degradation in vulnerable dryland regions. Despite a wealth of studies investigating the impact of vegetation and windbreaks on windflow in controlled wind-tunnel and modelling environments, there is still a paucity of empirical field data for accurately parameterizing the effect of vegetation in wind and sediment transport models. The aim of this study is to present a general parameterization of wind flow recovery in the lee of typical dryland vegetation elements (grass clumps and shrubs), based on their height (h) and optical porosity (θ). Spatial variations in mean wind velocity around eight isolated vegetation elements in Namibia (three grass clumps and five shrubs) were recorded at 0.30 m height, using a combination of sonic and cup anemometry sampled at a temporal frequency of 10 seconds. Wind flow recovery in the lee of the elements was parameterized in an exponential form, u ref À u 0 ð Þ : 1 À e Àb x h À Á þ u 0. The best-fit parameters derived from the field data were u 0 = u ref (0.0146θ À 0.4076) and b = 0.0105θ + 0.1627. By comparing this parameterization to existing models, it is shown that wind recovery curves derived from two-dimensional wind fence experiments may not be suitable analogues for describing airflow around more complex, three-dimensional forms. Field-derived parameterizations such as the one presented here are a crucial step for connecting plant-scale windflow behaviour to dryland bedform development at landscape scales.

Dryland regions are characterized by patchy vegetation, erodible surfaces, and erosive aeolian processes. Understanding how these constituent factors interact and shape landscape evolution is critical for managing potential environmental and anthropogenic impacts in drylands. However, modeling wind erosion on partially vegetated surfaces is a complex problem that has remained challenging for researchers. We present the new, coupled cellular automaton Vegetation and Sediment TrAnsport (ViSTA) model, which is designed to address fundamental questions about the development of arid and semiarid landscapes in a spatially explicit way. The technical aspects of the ViSTA model are described, including a new method for directly imposing oblique wind and transport directions onto a cell-based domain. Verification tests for the model are reported, including stable state solutions, the impact of drought and fire stress, wake flow dynamics, temporal scaling issues, and the impact of feedbacks between sediment movement and vegetation growth on landscape morphology. The model is then used to simulate an equilibrium nebkha dune field, and the resultant bed forms are shown to have very similar size and spacing characteristics to nebkhas observed in the Skeleton Coast, Namibia. The ViSTA model is a versatile geomorphological tool that could be used to predict threshold-related transitions in a range of dryland ecogeomorphic systems.

Drylands are home to over 2 billion people globally, many of whom use the land for agricultural and pastoral activities. These vulnerable livelihoods could be disrupted if desert dunefields become more active in response to climate and land use change. Despite increasing knowledge about the role that wind, moisture availability and vegetation cover play in shaping dryland landscapes, relatively little is known about how drylands might respond to climatic and population pressures over the 21 st century. Here we use a newly developed numerical model, which fully couples vegetation and sediment-transport dynamics, to simulate potential landscape evolution at three locations in the Kalahari Desert, under two future emissions scenarios: stabilising (RCP 4.5) and high (RCP 8.5). Our simulations suggest that whilst our study sites will experience some climatically-induced landscape change, the impacts of climate change alone on vegetation cover and sediment mobility may be relatively small. However, human activity could strongly exacerbate certain landscape trajectories. Fire frequency has a primary impact on vegetation cover, and, together with grazing pressure, plays a significant role in modulating shrub encroachment and ensuing land degradation processes. Appropriate land management strategies must be implemented across the Kalahari Desert to avoid severe environmental and socioeconomic consequences over the coming decades. Drylands as non-equilibrium environments Dryland environments are extreme in their nature, typified by non-equilibrium conditions in climate, vegetation and geomorphology 1–3. The strong interannual variability in precipitation that is characteristic of drylands 4 often results in the formation of dynamic vegetation patterning 5. These patterns, which range from 'gaps' to 'labyrinths' , 'stripes' and 'spots' , likely arise from adaptation to resource limitation (specifically of water), with plant-plant facilitation occurring at short distances and competition acting at larger distances 6–9. In turn, the patchy nature of semi-arid vegetation makes it an important variable in shaping the rate and extent to which geomorphological processes operate, notably wind-blown sediment movement 10, 11. The interactions between vegetation, geomorphology and, increasingly, humans, often result in shifts in the composition and patterning of dryland ecosystems. Ecosystem regime shifts involve the transition from one stable state to an alternative stable state 12–15 , and can broadly be divided into two categories: (i) small, slow and reversible; and (ii) large, abrupt and irreversible. Large, abrupt shifts can be detrimental to dryland ecosystems because they cause rapid and widespread loss of bioproductivity and biodiversity 16, 17. This affects ecosystem function and stability 18, 19 and thus a broad range of ecosystem services, from economic resources to recreation, firewood and food 17, 20 , which help to sustain a large majority of dryland livelihoods. There is particular concern about how land degradation, particularly through bush encroachment and associated losses of grass species 8, 21–24 , may intersect with increasing rural poverty levels 25. Whilst large-scale vegetation shifts are known to result from variations in external (climatic and CO 2) forcing 26 , the potential impacts of 21 st century climatic change on dry-land vegetation cover and composition remain unclear 27. Abrupt ecosystem shifts in drylands can also be triggered or exacerbated by a range of human disturbances, such as agriculture, grazing and fire 28–32. Modelling studies have shown that such disturbances can lead to hys-teretic ecosystem recovery 6, 16, 33 , whereby the state of the landscape depends on past as well as current conditions. The potential existence of vastly different vegetation covers in the same climatic context has important

Drylands are characterised by patchy vegetation, erodible surfaces and erosive aeolian processes. Empirical and modelling studies have shown that vegetation elements provide drag on the overlying airflow, thus affecting wind velocity profiles and altering erosive dynamics on desert surfaces. However, these dynamics are significantly complicated by a variety of factors, including turbulence, and vegetation porosity and pliability effects. This has resulted in some uncertainty about the effect of vegetation on sediment transport in drylands. Here, we review recent progress in our understanding of the effects of dryland vegetation on wind flow and aeolian sediment transport processes. In particular, wind transport models have played a key role in simplifying aeolian processes in partly vegetated landscapes, but a number of key uncertainties and challenges remain. We identify potential future avenues for research that would help to elucidate the roles of vegetation distribution, geometry and scale in shaping the entrainment, transport and redistribution of wind-blown material at multiple scales. Gaps in our collective knowledge must be addressed through a combination of rigorous field, wind tunnel and modelling experiments.

Rock and boulder surfaces are often exposed to weathering and/or rock-breakdown processes for extremely long time periods. This is especially true for arid environments on Earth and on planetary bodies such as Mars. One important, but largely unexplored, gap in knowledge is the influence of past stress histories on the operation of present rock-breakdown processes. Do rocks in the same area with different stress histories respond equally to newly imposed environmental conditions? This study investigates the influence of different physical and chemical stress histories on the response of basalt to salt weathering. We designed a four-stage approach of pre-treatment, field exposure, weathering simulation, and post-treatment: (1) physical, chemical, or no pre-treatment in the laboratory; (2) 3 yr exposure in either a hyper-arid sandy or salt-pan environment in the Namib desert (Namibia); (4) 60 cycles of a hot desert salt weathering simulation; and (4) desalination. Salt uptake and rock breakdown was assessed at each stage through comparison with baseline observations of mass, internal strength (Dynamic Young's modulus) and surface morphology (three-dimensional microscopy). Clear differences in block responses were found. Physically pre-treated blocks (especially those left in the salt-pan environment) experienced the highest loss of strength overall, chemically pre-treated blocks showed the greatest mass loss in the sandy environment, and freshly cut blocks gained strength during exposure in the desert and maintained this during the experiment. These results imply that stress history matters for predicting breakdown rates, with humid, arid, and saline legacies influencing subsequent breakdown in distinctive ways.

In June 2013 a rare weather event on the eastern slopes of the Rocky Mountains resulted in significantly above normal
rainfall and accelerated snowmelt in the headwaters of the Bow and Elbow Rivers of Alberta, Canada. The resulting
increase in water flowing to these two rivers led to flooding in downstream communities throughout Southern Alberta, the
likes of which had not been seen in over one hundred years. This event presented a rare opportunity for dam operators in the region to test their operating rules for large inflow events. For The City of Calgary, which operates the Glenmore Reservoir,
this meant drawing down the reservoir in anticipation of the peak inflows. The current paper explores the ‘What if?’ scenario in which the peak flow had not been attenuated by the reservoir. We show that in this case the extent of the flood would have been 0.6 km2 (3%) larger in extent, and would have had social impacts on one additional downstream residential community within the city’s boundaries, affecting 2,018  additional dwellings and 2,400 additional residents.

Our team conducted a study exploring how residents and officials perceived the tsunami warning and evacuation in the Alberni Valley on Vancouver Island on January 23rd, 2018. We conducted a door to door survey of residents living in the inundation zone which was supplemented with an online survey. The paper discusses whether households indicated they evacuated, how they evacuated, and what barriers they may have experienced during their travels. We also look at whether this event had any impacts on the public's perceived trust in emergency planners and managers.