Andrew Barkwith

Exploring the sensitivities of crenulate bay shorelines to wave climates using a new vector-based one-line model

We use a new exploratory model that simulates the evolution of sandy coastlines over decadal to centennial timescales to examine the behavior of crenulate-shaped bays forced by differing directional wave climates. The model represents the coastline as a vector in a Cartesian reference frame, and the shoreface evolves relative to its local orientation, allowing simulation of coasts with high planform-curvature. Shoreline change is driven by gradients in alongshore transport following newly developed algorithms that facilitate dealing with high planform-curvature coastlines. We simulated the evolution of bays from a straight coast between two fixed headlands with no external sediment inputs to an equilibrium condition (zero net alongshore sediment flux) under an ensemble of directional wave climate conditions. We find that planform bay relief increases with obliquity of the mean wave direction, and decreases with the spread of wave directions. Varying bay size over 2 orders of magnitude (0.1–16 km), the model predicts bay shape to be independent of bay size. The time taken for modeled bays to attain equilibrium was found to scale with the square of the distance between headlands, so that, all else being equal, small bays are likely to respond to and recover from perturbations more rapidly (over just a few years) compared to large bays (hundreds of years). Empirical expressions predicting bay shape may be misleading if used to predict their behavior over planning timescales.

Past changes in the North Atlantic storm track driven by insolation and sea-ice forcing

Changes in the location of Northern Hemisphere storm tracks may cause significant societal and economic impacts under future climate change, but projections of future changes are highly uncertain and drivers of long-term changes are poorly understood. Here we develop a late Holocene storminess reconstruction from northwest Spain and combine this with an equivalent record from the Outer Hebrides, Scotland, to measure changes in the dominant latitudinal position of the storm track. The north-south index shows that storm tracks moved from a southern position to higher latitudes over the past 4000 yr, likely driven by a change from meridional to zonal atmospheric circulation, associated with a negative to positive North Atlantic Oscillation shift. We suggest that gradual polar cooling (caused by decreasing solar insolation in summer and amplified by sea-ice feedbacks) and mid-latitude warming (caused by increasing winter insolation) drove a steepening of the winter latitudinal temperature gradient through the late Holocene, resulting in the observed change to a more northern winter storm track. Our findings provide paleoclimate support for observational and modeling studies that link changes in the latitudinal temperature gradient and sea-ice extent to the strength and shape of the circumpolar vortex. Together this evidence now suggests that North Atlantic winter storm tracks may shift southward under future warming as sea-ice extent decreases and the mid- to high-latitude temperature gradient decreases, with storms increasingly affecting southern Europe.

Investigating the maximum resolution of µXRF core scanners: A 1800 year storminess reconstruction from the Outer Hebrides, Scotland, UK

Micro x-ray fluorescence (µXRF) core scanning is capable of measuring the elemental composition of lake sediment at sub-millimetre resolution, but bioturbation and physical mixing may degrade environmental signals at such fine scales. The aim of this research is to determine the maximum possible resolution at which meaningful environmental signals may be reconstructed from lake sediments using this method. Sediment from a coastal lake in the Outer Hebrides, Scotland, has been analysed using calibrated element measurements to reconstruct storminess since AD 200. We find that a Ca/K ratio in lake-core sediments reflects the presence of fine calcium carbonate shell fragments, a constituent of sand in the catchment that is washed and blown into the lake. Variations in this ratio are significantly correlated with instrumental records of precipitation and low pressures, suggesting it is a proxy for storminess. Furthermore, identification of a c. 60-year cycle supports a climatic influence on Ca/K, as this cycle is frequently identified in reconstructions of the North Atlantic Oscillation and North Atlantic sea-surface temperature. Comparison with weather records at different resolutions and spectral analysis indicate that µXRF data from Loch Hosta can be interpreted at sub-decadal resolutions (equivalent to core depth intervals of 3–5 mm in this location). Therefore, we suggest that sub-centimetre sampling using µXRF core scanning could be beneficial in producing environmental reconstructions in many lake settings where sediments are not varved.

Spatio-temporal Variability in the Tipping Points of a Coastal Defense

ABSTRACT Brown, J.M.; Prime, T.; Phelps, J.J.C.; Barkwith, A.; Hurst, M.D.; Ellis, M.A.; Masselink, G., and Plater, A.J., 2016. Spatio-temporal Variability in the Tipping Points of Coastal Defense. 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. 1042 - 1046. Coconut Creek (Florida), ISSN 0749-0208. To enable effective adaptive management, early warning of when a ‘tipping point’ within a systems defense may occur is vital. A tipping point is a critical threshold at which the state of a system is altered, perhaps irreversibly. After the extremity of the UKs 2013/2014 winter, many coastal systems have undergone a change in state. For example, the conversion of a sandy beach into a rocky platform or an increase/decrease in flood hazard due to a defense breach or new intervention. Coastal monitoring networks around the UK have enabled data collection of these extreme events to drive model applications to assess plausible changes in coastal conditions that trigger a sudden change in a systems state and conditions that enable recovery. Using available UK monitoring networks and a numerical approach, we focus on Dungeness and Rye Bay, a region of high value in terms of habitat and energy, to assess (i) how the natural variability within the profile of the gravel barrier modifies the overwash rates that can occur and (ii) how ambitious human intervention that re-scape the geomorphic character of the shoreline could impact the critical point at which overwash occurs.