This study challenges the paradigm that salt marsh plants prevent lateral wave-induced erosion along wetland edges by binding soil with live roots and clarifies the role of vegetation in protecting the coast. In both laboratory flume studies and controlled field experiments, we show that common salt marsh plants do not significantly mitigate the total amount of erosion along a wetland edge. We found that the soil type is the primary variable that influences the lateral erosion rate and although plants do not directly reduce wetland edge erosion, they may do so indirectly via modification of soil parameters. We conclude that coastal vegetation is bestsuited to modify and control sedimentary dynamics in response to gradual phenomena like sea-level rise or tidal forces, but is less well-suited to resist punctuated disturbances at the seaward margin of salt marshes, specifically breaking waves.coast ͉ hurricane ͉ wave attenuation ͉ wetland
The water column of Lake Baikal is extremely weakly-but permanently-stratified below 250 m. Despite the thickness of this relatively stagnant water mass of more than 1000 m, the water age (time since last contact with the atmosphere) is only slightly more than a decade, indicating large-scale advective exchange. In the stratified deep water, the fate of water constituents is determined by the combined action of advective processes (deep-water intrusions) and small-scale turbulent vertical diffusion.Here, vertical diffusivity is addressed through the analysis of 25 temperature microstructure profiles collected in the three major basins of Lake Baikal to a depth of 600 m. In addition, in the 1,432-m deep south basin, monthly CTD profiles and a two year record of near-bottom currents were analyzed. Balancing turbulent kinetic energy and small-scale temperature variance leads to the conclusions that (1) vertical diffusivity in the stratified deep water ranges from 10-90 ϫ 10 Ϫ4 m 2 s Ϫ1 (between 600 and 250 m), which is three orders of magnitude more than estimated by Killworth et al. (1996), (2) the mixing efficiency is ϳ0.16, comparable to that found in stronger stratification (e.g., the ocean interior), (3) turbulence under ice decays with a time scale of 40 Ϯ 2 d and (4) the interior of the permanently stratified deep water below 250 m and the bottom boundary layer contribute roughly equally to the TKE production. The latter implies, that mixing in the deep water of Lake Baikal's three sub-basins is dominated by bottom boundary mixing as found in smaller lakes and ocean basins. Lake Baikal, located in the Great Baikal Rift zone in eastern Siberia, is by volume (23,015 km 3 ) and by depth (maximum: 1,632 m) the earth's largest freshwater body (Shimaraev et al. 1994). Since Lake Baikal is one of the oldest freshwater basins (Golubev et al. 1993) and because it faces extreme conditions, including several months of ice cover, large depth, a long water residence time (ϳ350 years), and low nutrient concentration, it developed to a unique ecosystem, which gives habitat to more than 1,500 endemic species (Martin, 1994). Topographically, the lake consists of three main basins (south, middle, and north) which are formed by sills reaching up to about 300 m (Fig. 1). The south basin (maximum depth: 1,432 m) is the main focus of this study.As with most temperate natural waters, during the summer the vertical temperature gradient of Lake Baikal is positive throughout the entire water column leading to stable density stratification with surface temperature above 14ЊC. Since the temperature of maximum density T md [ЊC] of near-surface AcknowledgmentsWe are indebted to Mike Schurter and Ruslan Gnadovsky for their help with the field campaign. We thank Michael Sturm and Rolf Kipfer for logistic and organizational support, the Swiss Federal office for Education and Science (BBW) for financial support, and Daniel Schenker for mathematical assistance in analysis of the velocity spectra. Current and thermister data were obta...
We studied cold, deep‐water intrusions in the South Basin of Lake Baikal on the basis of 2 yr of data (December 1995‐November 1997) from near‐bottom and near‐surface thermistor strings, monthly conductivity‐temperature‐depth (CTD) profiles, and a near‐bottom current meter, all collected near the South Basin maximum depth of 1,461 m. The data show intrusions into the greatest depths with temperatures of 0.08–0.20°C below ambient (~3.33 to ~3.38°C at maximum depth). The intrusions were observed three times per year between January and June, when surface water is always cooler than deep water, with durations of a few (at least 1–3) days. The estimated water input ranged from 1 to 10 km3 per event, and the annually accumulated volume was estimated to be 10–30 km3, which is significantly less than the steady‐state indirect estimates of 30–70 km3 yr−1 to the permanently stratified deep water (depth ≫ 300 m). This indicates that not all of the cold intrusions reach the deepest area. Because the cooling of the near‐bottom waters was not accompanied by a significant increase in ion or particle content and because deep sediment traps did not contain significant enrichments of minerogenic particles, we concluded that Selenga River inflow is not a possible source of the cold intrusions. Two CTD profiles in June 1996 and 1997 showed lower temperature throughout the deep water, as expected from thermobaric instabilities. The required downwelling is definitely not occurring in the pelagic interior but most probably by near‐coast counterclockwise currents. The source of the regularly occurring deep intrusions is clearly cold surface water, but the actual mechanism is uncertain.
A predictive, coastal erosion/shoreline change model has been developed for a small coastal segment near Drew Point, Beaufort Sea, Alaska. This coastal setting has experienced a dramatic increase in erosion since the early 2000's. The bluffs at this site are 3-4 m tall and consist of ice-wedge bounded blocks of fine-grained sediments cemented by ice-rich permafrost and capped with a thin organic layer. The bluffs are typically fronted by a narrow (∼5 m wide) beach or none at all. During a storm surge, the sea contacts the base of the bluff and a niche is formed through thermal and mechanical erosion. The niche grows both vertically and laterally and eventually undermines the bluff, leading to block failure or collapse. The fallen block is then eroded both thermally and mechanically by waves and currents, which must occur before a new niche forming episode may begin. The erosion model explicitly accounts for and integrates a number of these processes including: (1) storm surge generation resulting from wind and atmospheric forcing, (2) erosional niche growth resulting from wave-induced turbulent heat transfer and sediment transport (using the Kobayashi niche erosion model), and (3) thermal and mechanical erosion of the fallen block. The model was calibrated with historic shoreline change data for one time period (1979-2002), and validated with a later time period (2002-2007).
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