Ian J. Walker

Numerical modelling of airflow over an idealised transverse dune

Abstract The general flow structure over transverse aeolian dunes is now well documented through both field studies and wind tunnel experiments. Research on windward (stoss) slopes of dunes is extensive and has recently been complemented by research on the lee-side flow structure. However, a number of technical deficiencies in wind tunnel instrumentation and a lack of detailed resolution in and appropriate turbulence instrumentation for field research have resulted in an incomplete quantified characterisation of the flow structure over aeolian dunes. This study applies a two-dimensional numerical model with an RNG-modified κ-ϵ turbulence model to simulate the time-averaged flow field over an idealized aeolian dune. The model is successfully validated with wind tunnel experimental data. Results indicate that the model accurately predicts the flow patterns over the dune, producing regions of flow stagnation at the toe, acceleration up the stoss slope and a region of flow separation and reversal in the lee. Further development and application of this model will allow examination of flow-form interaction, the testing of more complex isolated dune morphologies, and characterisation of flow over multiple dunes.

Topographic Steering of Alongshore Airflow over a Vegetated Foredune: Greenwich Dunes, Prince Edward Island, Canada

Abstract High-frequency measurements of airflow from ultrasonic anemometers and time-averaged cup anemometer profiles were taken during an oblique alongshore sand-transporting event (6.7 m/s) over a vegetated foredune in May 2002 as part of a larger study on the sedimentary dynamics of a beach-dune complex in Greenwich Dunes, Prince Edward Island National Park, Canada. Local flow and sand transport pathways deviated significantly from the regional wind because of topographic steering of oblique alongshore airflow on the beach back toward the foredune in the backshore. Flow decelerated on the lower seaward slope of the foredune because of flow stagnation and vegetation-induced roughness effects. On the upper seaward slope, flow veered crest-parallel because of secondary flow effects, including potential flow reversal, acceleration, and interaction with faster regional flow. An inflection point in velocity profiles indicates a momentum sink at plant canopy height, and above this height exists a distinct shear layer. Flow acceleration occurs only on the upper slope above the plant canopy as the shear layer intensifies. Flow steadiness, as indicated by the coefficient of variation in horizontal (U) and vertical (W) velocity, also declines up the seaward slope, in contrast to measurements of flow over unvegetated dunes, perhaps because of increasing vegetation-induced drag and turbulence. Gusting and vertical bursting was evident as honami motion in the beachgrass and was responsible for maintaining intermittent saltation into the foredune. Temporal correlations exist between U, W, and flow angle. As flow shifted onshore, both U and W increased because of enhanced topographic forcing and increased vertical lift on the lower seaward slope, whereas when flow shifted alongshore, U and W declined because of reduced forcing and acceleration effects because the dune is effectively less steep to flow. Although these dunes align well with the vector of the regional resultant sediment drift potential, sand transport pathways followed local flow vectors of varying magnitude and direction. Offshore to oblique alongshore winds are typical of the summer wind regime, and, although frequently incompetent, they contribute to dune maintenance by cycling sand to the backshore for incipient foredune growth, scarp infilling, or both. To date, the role of secondary flows under alongshore winds in foredune morphodynamics has not been well documented.

Artificial modifications of the coast in response to the Deepwater Horizon oil spill: Quick solutions or long-term liabilities?

The Deepwater Horizon oil spill threatened many coastal ecosystems in the Gulf of Mexico during the spring and summer of 2010. Mitigation strategies included the construction of barrier sand berms, the restriction or blocking of inlets, and the diversion of freshwater from rivers to the coastal marshes and into the ocean, in order to flush away the oil, on the premise that these measures could reduce the quantity of oil reaching sensitive coastal environments such as wetlands or estuaries. These projects result in changes to the ecosystems that they were intended to protect. Long-term effects include alterations of the hydrological and ecological characteristics of estuaries, changes in sediment transport along the coastal barrier islands, the loss of sand resources, and adverse impacts to benthic and pelagic organisms. Although there are no easy solutions for minimizing the impacts of the Deepwater Horizon disaster on coastal ecosystems, we recommend that federal, state, and local agencies return to the ...

Sea level responses to climatic variability and change in Northern British Columbia

Abstract Sea level responses to climatic variability (CV) and change (CC) signals at multiple temporal scales (interdecadal to monthly) are statistically examined using long‐term water level records from Prince Rupert (PR) on the north coast of British Columbia. Analysis of observed sea level data from PR, the longest available record in the region, indicates an annual average mean sea level (MSL) trend of +1.4±0.6 mm yr−1 for the period (1939–2003), as opposed to the longer term trend of 1±0.4 mm yr−1 (1909–2003). This suggests a possible acceleration in MSL trends during the latter half of the twentieth century. According to the results of this study, the causes behind this acceleration can be attributed not only to the effects of global warming but also to cyclic climate variability patterns such as the strong positive Pacific Decadal Oscillation (PDO) phase that has been present since the mid‐1970s. The linear regression model based on highest sea levels (MAXSL) of each calendar year showed a trend exceeding twice that (3.4 mm yr−1) of MSL. Previous work shows that the influence of vertical crustal motions on relative sea level are negligible at PR. Relations between sea levels and known CV indices (e.g., the Multivariate ENSO Index (MEI), PDO, Northern Oscillation Index (NOI), and Aleutian Low Pressure Index (ALPI)) are explored to identify potential controls of CV phenomena (e.g., the El Niño Southern Oscillation (ENSO), PDO) on regional MSL and MAXSL. Linear and non‐linear statistical methods including correlation analyses, multiple regression, Cumulative Sum (CumSum) analysis, and Superposed Epoch Analysis (SEA) are used. Results suggest that ENSO forcing (as shown by the MEI and NOI indices) exerts significant influence on winter sea level fluctuations, while the PDO dominates summer sea level variability. The observational evidence at PR also shows that, during the period 1939–2003, these cyclic shorter temporal scale sea level fluctuations in response to CV were significantly greater than the longer term sea‐level rise trend by as much as an order of magnitude and with trends over twice that of MSL. Such extreme sea level fluctuations related to CV events should be the immediate priority for the development of coastal adaptation strategies, as they are superimposed on long‐term MSL trends, resulting in greater hazard than longer term MSL rise trends alone.

Analysis of coastal dune dynamics, shoreline position, and large woody debris at Wickaninnish Bay, Pacific Rim National Park, British Columbia

Large woody debris (LWD) and colonizing vegetation alter the sediment budgets and stability of coastal dune systems. In British Columbia, LWD on beaches consists largely of historical escape logs from the coastal logging industry. In areas with strong wind regimes and high sand supply, LWD can trap appreciable amounts of windblown sand in the backshore, which can enhance foredune development and stabilization (roles typically played by vegetation) on stable or prograding shorelines. This additional store of sediment provides an important buffer that reduces erosion of established foredunes and backshore ecosystems. This study examines trends in LWD and vegetation coverage and associated geomorphic changes within the Long Beach unit of Pacific Rim National Park Reserve derived from aerial photography since the early 1970s. Over this time LWD has been reworked seasonally to interannually and, at Wickaninnish Bay, has declined in areal coverage by 61%. Despite this decline, LWD is found extensively within es...

Flow within a Trough Blowout at Cape Cod

ABSTRACT Hesp, Patrick A.; Smyth, Thomas A.G.; Walker, Ian J.; Gares, Paul A., and Wasklewisz, Thad, 2016. Flow within a trough blowout at Cape Cod. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 288–292. Coconut Creek (Florida), ISSN 0749-0208. The Province Lands dunefield at Cape Cod is characterised by large parabolic dunes on which have developed very significant numbers of active saucer and bowl blowouts. Blowouts occur across the entire dune landscape but many are initiated on the high lee margins of large erosional ridges and bowl blowouts within the larger parabolic dune landscape. Evolution of these blowouts is characterised by multiple stages. The first few evolutionary stages are described. In addition, the aerodynamics and flow structure within a trough blowout and former saucer blowout developed within a high ridge crest is elucidated from 2D sonic anemometer data, smoke bombs and videography. The flow is characterised by decreasing then increasing flow up the blowout centreline, and marked, highly turbulent opposed flow separation immediately inside and around the marginal entrance walls of the blowout.