Davin J. Wallace

Coastal Impact Underestimated From Rapid Sea Level Rise

A primary effect of global warming is accelerated sea level rise, which will eventually drown low-lying coastal areas, including some of the worlds most populated cities. Predictions from the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) suggest that sea level may rise by as much as 0.6 meter by 2100 [Solomon et al., 2007]. However, uncertainty remains about how projected melting of the Greenland and Antarctic ice sheets will contribute to sea level rise. Further, considerable variability is introduced to these calculations due to coastal subsidence, especially along the northern Gulf of Mexico (see http://tidesandcurrents.noaa.gov/sltrends/sltrends.shtml).

Evidence of similar probablility of intense hurricane strikes for the Gulf of Mexico over the late Holocene

Hurricane magnitude and frequency have been linked to numerous mechanisms, including the steady rise in annual sea-surface temperatures, El Nino–Southern Oscillation (ENSO) variations, and atmospheric changes. In order to better understand those factors that control hurricane magnitude and frequency, a long-term record spanning the entire Gulf of Mexico coast is needed. Here we present a detailed record from ca. 5300–900 yr B.P. of past intense hurricane impacts for cores collected from Laguna Madre, Texas, United States. Relative storm intensities were calculated for each event, and the average intense storm impact probability for south Texas was determined to be ∼0.46% (annual landfall probability). Previous field studies in Western Lake, Florida, and Lake Shelby, Alabama, reveal similar probability intense hurricane strikes of ∼0.39%. Although high-frequency oscillations between warm and dry and cool and wet climate conditions have occurred in Texas through the late Holocene, there has been no notable variation in intense storm impacts across the northwestern Gulf of Mexico coast during this time interval, implying no direct link between these changing climate conditions and annual hurricane impact probability. In addition, there have been no significant differences in the landfall probabilities of storms between the eastern and western Gulf of Mexico during the late Holocene, suggesting that storm steering mechanisms have not varied during this time.

Palaeohurricane reconstructions from sedimentary archives along the Gulf of Mexico, Caribbean Sea and western North Atlantic Ocean margins

Abstract Hurricanes annually threaten the Atlantic Ocean margins. Historical hurricane records are relatively short and palaeohurricane sedimentary archives provide a geological and climatic context that sheds light on future hurricane activity. Here we review palaeo-trends in hurricane activity elucidated from sedimentary archives. We discuss dating methods, site selection and statistics associated with previously published records. These archives have been useful for understanding the long-term evolution of coastal systems and the response of intense hurricane activity to climatic changes. Regional shifts in hurricane overwash on centennial to millennial timescales have been linked to various climatic modes of variability, including El Niño/Southern Oscillation and the North Atlantic Oscillation, but could also reflect regional-scale controls on hurricane activity.

Transgressive Ravinement versus Depth of Closure: A Geological Perspective from the Upper Texas Coast

Abstract The upper Texas coast is one of the most populated areas along the Gulf of Mexico. Three dynamic barriers along this section of coastline (Bolivar Peninsula, Galveston Island, and Follets Island) have a well-documented history of shoreline change. Numerous engineering studies incorporating both sedimentological data and numerical models have been established for this system to understand sediment fluxes. However, rarely have previous studies examined sediment fluxes for the upper Texas coast in light of certain fundamental concepts of coastal geology. Here, we discuss the current theory and understanding of barrier island dynamics from a geologic standpoint as they relate to sediment budgets for the upper Texas coast. From sediment cores, we quantify both shoreface and washover sand fluxes, which previously were not incorporated as sand sinks into sediment budgets for this system. Shoreface sand fluxes represent a sizable portion of the total budget, whereas modern washover sand fluxes are minimal. Until now, a depth of closure (beyond which sediment transport is negligible) of 4 m has typically been used; however, our data suggest a depth of at least 8 m would be more appropriate. We show that the combined upper and lower shoreface has the potential to sequester ~160,000 ± 39,000 m3/y of sand, equaling ~17% of the entire calculated sediment flux and ~37% of the total longshore transport flux for the upper Texas coast, based on previous studies. Therefore, we recommend revised approaches to future sediment budget studies in order to establish more robust analyses. Ultimately, it will be crucial to use both engineering principles and geologic concepts to construct an accurate and realistic scenario for coastal restoration projects.

Unprecedented erosion of the upper Texas coast: Response to accelerated sea-level rise and hurricane impacts

The upper Texas coast is an ideal location to examine coastal response to global change over geologic and historic time. Here we quantify the long-term sequestration for sand eroded from an island into two main sinks, offshore Galveston Island and San Luis Pass Tidal Delta, in order to compare long-term and short-term erosion. We determine the average storm-related offshore sand fl ux for the middle part of the Holocene (ca. 5240–5040 [2σ] cal yr B.P. to present) to be ~4200–4400 ± 670 m 3 /yr, with a decrease in the offshore sand fl ux to ~920–970 ± 270 m 3 /yr during the latter part of the Holocene (ca. 2730–2610 [2σ] cal yr B.P. to present). The tidal delta initially formed ca. 2100 (1σ median) cal yr B.P., when the rate of sealevel rise slowed from 2.0 mm/yr to 0.60 mm/yr. We calculate the sand fl ux from Galveston Island into San Luis Pass from ca. 2100 (1σ median) cal yr B.P. to 200 yr ago to be ~4700 m 3

Follets Island: A Case of Unprecedented Change and Transition from Rollover to Subaqueous Shoals

Follets Island, a transgressive island located on the upper Texas coast, is an ideal location to study barrier island transition from a rollover subaerial barrier to subaqueous shoals. This system also allows for an examination of coastal response to accelerated sea-level rise, storms, and sediment supply. The landward shoreline retreat rate during historical time is similar to the landward retreat rate of the bay shoreline, hence its current classification as a rollover barrier. However, the island has a limited and diminishing sand supply, which makes it even more vulnerable to erosion during storms and relative sea-level rise. Four core transects that extend from the upper shoreface to the back barrier bay are used to constrain the thickness of washover, barrier and upper shoreface deposits and to estimate sediment fluxes in the context of the overall sand budget for the island over centennial timescales. Stratigraphic architecture reveals two prominent transgressive surfaces. A lower flooding surface separates red fluvial-deltaic clay from overlying bay mud and an upper erosional surface separates back-barrier deposits from overlying shoreface and foreshore deposits.