Long-term Recession Rates Research Articles

Three-dimensional bluff evolution in response to seasonal fluctuations in Great Lakes water levels

Effects of lake level fluctuations on coastal bluff erosion in the Great Lakes are pervasive and of significant concern to policy-makers and property owners. Previous studies of bluff erosion have primarily estimated two-dimensional bluff recession rates over years to decades. However, bluff erosion is an irregular process in time and space so averaging coarse resolution observations over long time periods may neglect important processes driving erosion. To address this we use photogrammetric surveys of a Lake Michigan bluff to create four high resolution (10 cm) digital elevation models (DEMs) that span one year with images collected from small unoccupied aerial systems (sUAS). The elevation models were differenced to compare variations in sediment loss to modelled time-series of wave data, atmospheric temperature and lake level. The sub-annual erosion rates calculated from sUAS surveys were compared to long-term recession rates obtained by digitizing historic aerial imagery. The rate of erosion of the bluffs is a time dependent function of two factors: (1) the supply of sediment to lower regions on the bluffs from mass wasting and (2) increased wave erosion resulting from above-average lake levels and intensified onshore wave energy. These factors combine in the early spring when wave energy is elevated and atmospheric temperatures increase, causing reduction in bluff sediment strength by thawing, as well as an increase in porewater pressure from snow melt. Most importantly, above-average lake levels sustained in the longer term (multiple years) are required for the combination of (1) and (2) to result in substantial bluff erosion.

Interaction of yttrium disilicate environmental barrier coatings with calcium-magnesium-iron alumino-silicate melts

Reactions between molten calcium-magnesium-iron alumino-silicate (CMFAS) deposits and yttrium disilicate (Y2Si2O7, YDS) based environmental barrier coatings (EBC) on SiC/SiC ceramic matrix composites (CMCs) were investigated at 1300 °C. The coating readily dissolves into the melt from which an apatite phase, nominally Ca2Y8(SiO4)6O2, precipitates. These reactions are sufficiently fast to consume the majority of the approximately 275 μm thick coating in 24 h. Liquid phase separation, producing an essentially pure SiO2 second phase, occurs near the reaction front suggesting dissimilar rates of CaO and SiO2 exchange with the overlaying deposit. The rise of large bubbles through the melt above the coatings appears to disrupt the reaction layer and distributes apatite throughout the residual deposit. Channel cracks were found in the deposits and the reaction layers; after longer exposures, the cracks branch and extend laterally through the Si bond coat and into the underlying CMC. Complementary experiments performed on monolithic YDS pellets yielded long-term recession rates similar to those of the coatings, although some differences were evident in recession rates and reaction layer morphologies in the early stages. Thermodynamic calculations were used to understand the evolving driving force for the YDS-to-apatite conversion. The agreement between the simulated and experimentally observed behaviors suggests that such calculations could be used to predict the influence of temperature and deposit composition on EBC degradation.

Bluff evolution and long-term recession rates, southwestern Lake Michigan

Where eroding cohesive sediments are present, Lake Michigan bluffs range up to 40 m in height, exposing multiple glacial stratigraphic units. Following the model presented here, bluffs form as a wave-cut terrace erodes inland from a point near the original shoreline. The erosion plane is nearly horizontal, in contrast with the eastward dip of the glacial units inherited from underlying bedrock. Therefore, terraces eroding inland produce progressively higher bluffs and expose successively older units at the toe and beneath the lake. This process was repeated several times as lake levels sequentially dropped to their modern stage. The initial modern shoreline, and hence the width of the wave-cut terrace, was determined from four offshore seismic and bottom-sampling profiles. It was picked as an inflection point in the lake bed, occurring offshore of dipping reflectors intersecting the lake bottom. The calculated average recession rate over a 2500-year duration of the modern stage is 1.5 m/yr in contrast to average rates of approximately 0.6 m/yr measured over the last century. Thus rates decrease through time as the terrace widens and wave energy is dampened. By correlating bluff height to recession distance, a third rate of approximately 2.7 m/yr for the first 940 years of recession is calculated from relict Nipissing bluffs. The three rates define a steeply decaying exponential curve in early stages of bluff retreat, flattening into a nearly linear function after 1000 years.

Lake Michigan Bluff Recession

ABSTRACT Long-term recession rates of Lake Michigan's shorezone bluffs were investigated at 118 sites by comparing modern measurements with nineteenth-century Government Land Office surveys. These rates were evaluated spatially and related to selected shorezone characteristics. One hundred and six of the bluff sites are experiencing retreat that averages 0.4 m (1.3 ft) per year. Although amounts vary significantly, average values show that (1) rates are similar for opposite sides of the lake but (2) values differ between northern and southern Wisconsin. In addition, dune-encompassed bluffs tend to recede at lower long-term rates than nondune bluffs. Groundwater seepage, shoreline orientation and fetch, and shorezone protective structures are some of the factors found to influence bluff crest recession. Data from this study indicate that extensive Lake Michigan shorezone tracts will continue to experience bluff recession measurable in meters per decade unless lake levels are relatively low or unless protec...

Geologic approaches to the determination of long-term coastal recession rates, matagorda peninsula, Texas

The degree to which human modifications in the coastal zone have increased or decreased coastal erosion rates is difficult to determine owing to the short time period for which shoreline-position data are available. This limitation is circumvented in areas where long-term recession rates can be determined from geologic data. Three such areas from Matagorda Bay have been examined to determine the temporal variation in recession rates over the past several thousand years. Preliminary results indicate that recession rates over the past century may be 30% to 40% greater than those of prehistoric time. Although additional data are needed, it is suggested that accelerated rates result from human modification of the coastal zone, and that in the future increased recession rates can be anticipated.