Variations In Mean Water Level Research Articles

Effect of Mean Water Level Variations on the Design and Performance of Rubble-Mound Coastal Protection Structures

Gel-git, mevsimsel değişiklikler, dalga kabarması/alçalması, fırtına kabarması ve küresel ısınmaya bağlı su seviyesi değişimleri sonucunda ortalama su seviyesinde gözlenen değişimler, taş dolgu kıyı koruma yapılarının tasarımlarının ve performanslarının değerlendirilmesi ile doğrudan ilgilidir. Bu tip yapılar için en kritik su seviyesi, yaygın olarak en yüksek su seviyesi tanımı ile kullanılmaktadır. Ancak, Kıyı Yapıları Planlama ve Tasarım Teknik Esasları’nda [1] koruma yapısında kullanılacak taşların kütlelerinin belirlenmesi için en kritik su seviyesinin en düşük su seviyesi ile en yüksek su seviyesi arasında ortaya çıkabileceği belirtilmiştir. Bu çalışmada taş dolgu kıyı koruma yapılarının tasarım derinliğinin belirlenmesinde kullanılan farklı yaklaşımların koruma tabakası taş kütlesi ile serbest kret kotuna olan etkisi incelenmiştir. Bu amaçla Karadeniz, Ege Denizi ve Akdeniz’de birer proje alanı seçilmiş ve bu projelerdeki yapıların ekonomik ömürleri boyunca gözlenebilecek tüm su seviyelerinde koruma tabakası taş kütlesi ile serbest kret kotu hesaplanmıştır. Seçilen projeler için daha düşük su seviyelerinde daha yüksek su seviyelerine göre %60’a varan oranlarda daha büyük taş kütlesi bulunmuştur. Çalışma sonuçları, en yüksek su seviyesinden daha kritik bir su seviyesinin, düşük su seviyesi ile en yüksek su seviyesi arasındaki herhangi bir su seviyesinde de oluşabileceğine örnek oluşturmaktadır. Buna bağlı olarak, seçilen kritik su seviyesi değeri ile koruma tabakası taş kütlesi hesaplama yöntemlerinin ilişkisi tartışılmış ve yapının ekonomik ömrü boyunca oluşabilecek tüm su seviyelerinin tasarım derinliği belirlenirken göz önünde bulundurulması önerilmiştir.

The driving forces behind the Caspian Sea mean water level oscillations

Closed basins such as the Caspian Sea rapidly respond to variations in atmospheric and geological events. This study has been conducted to deduce the role of natural and anthropogenic influences on the Caspian Sea mean water level fluctuations for the period of 1998–2005. It is recognized that climate is the primary mechanism of the Caspian Sea mean water level variations based on the relatively small differences of the Caspian Sea hydrologic budget residuals and the Caspian Sea mean water level fluctuations. This is further supported by the similarity in water-level variations of the Caspian Sea with those of Lake Van and Lake Urmia. On the other hand, the Caspian Sea needed to lose some of its water to attain water balance equilibrium in 2000 and 2001. The year 2000 showed anomalous seismic activity particularly in the southwestern part of the Caspian Sea. Two significant earthquakes with magnitudes of 6.8 and 6.5 Mw occurred on November 25, 2000. The focal mechanisms of these earthquakes and numerous aftershocks indicated normal faulting and, therefore, caused the Caspian Sea lake level to decline in 2000. The contribution of submarine mud volcano eruptions to the Caspian Sea lake level could be insignificant based on the comparison of water budget residuals and the mean water level variations. Neither crustal deformation (based on the GPS measurements) nor the offshore oil and natural gas production activities in the Caspian Sea are responsible for noticeable changes to the level of the Caspian Sea.

Long term water level and surface temperature changes in the lagoons of the southern and eastern Baltic

The paper studies variations in the water level and surface temperature of coastal lagoons along the southern and south-eastern shores of the Baltic Sea: the Curonian Lagoon, Vistula Lagoon, and Darss-Zingst Bodden Chain. Linear regressions for annual mean water level variations showed a positive trend in water level, but at different rates. The highest rate during the period between 1961–2008 was recorded for the Curonian and Vistula lagoons (~ 4mm year−1), the lowest for the DarssZingst Bodden Chain (approximately ~ 1mm year−1). The warming trend of the mean surface water temperature in the lagoons was 0.03°C year−1 in the period 1961–2008. Moreover, the variability in annual water temperature and sea level as well as their extreme values have increased most dramatically since the 1980s.

The influence of tidal forcing on groundwater flow and nutrient exchange in a salt marsh-dominated estuary

Data from salt marshes in the U.S. Southeast show that long-term variations in mean water level (MWL) correlate strongly with salt marsh productivity and porewater salinity. Here we used numerical models of tidally-driven groundwater flow to assess the effect of variations in tidal amplitude and MWL on porewater exchange between salt marshes and tidal creeks. We modeled homogeneous and layered stratigraphy and compared flat and sloped topography for the marsh surface. Results are consistent with field observations and showed that increases in tidal amplitude increased groundwater flushing, particularly when increasing the tidal amplitude caused the marsh platform to be inundated at high tide. Increases in MWL caused groundwater flushing to increase if that rise caused greater areas of the marsh to be inundated at high tide. Once the marsh was fully inundated at high tide, further increases in MWL caused groundwater flushing to decrease. Results suggest that small increases in MWL associated with sea level rise could increase nutrient export significantly in marshes with elevations that are equilibrated near mean high water, but rising sea level could decrease the export of nutrients to, and thus fertility in, estuaries adjacent to marshes that are equilibrated lower in the tidal frame. Likewise, macrotidal estuaries are predicted to be subject to much larger groundwater and nutrient exchange than similar microtidal estuaries. We speculate that the early stages of rising relative sea level may significantly impact water quality in estuaries that are not river-dominated by raising the discharge of nutrients from coastal wetlands.

Feasibility of an Offshore Wind Farm in the Gulf of Gela: Marine and Structural Issues

Some of the marine and structural issues related to the installation of an offshore wind farm three miles away from the shoreline of Gela (Italy) are discussed. In particular, the characterization of the maritime climate has been carried out by considering the winds, the waves, the mean water level variations and the current regime. The results of such an analysis have been used in order to determine the environmental loads on the foundation structure. A gravity based structural type has been proposed as foundation of the wind turbines and the design of a foundation prototype has been carried out by trying to implement the most updated international standards to the Sicilian case. An original automatic optimization procedure has been developed in order to determine the dimensions of the design foundation, which satisfied the prescribed static verifications and provided the lowest costs. The scour process at the basis of the foundation and the wave run-up on the structure have been also taken into account. Finally, problems related to the laying and the maintenance of electrical connection cables are also discussed.

Sea-level rise and coastal marsh sustainability: geological and ecological factors in the Mississippi delta plain

Chronostratigraphic approaches to coastal geomorphology frequently include consideration of salt marsh deposits as indicators of past sea-level positions. Continuous horizons of such deposits can be used to infer that salt marshes were keeping pace with local rates of relative sea-level rise (RSLR). Rates of past accumulation, estimated using dating techniques, are then used to hindcast the rate of sea-level rise in that area. Estimates of contemporary sea-level rise rates are often derived from tide gauge records. This approach allows identification of subdecadal variations in mean water level. Accumulation rates of both organic and inorganic sediments can also be derived at these time scales and studies from many coastal marshes demonstrate the episodic nature of inorganic sediment deposition. The frequency and spacing of these events does not necessarily coincide with periods of increased local sea level. In addition, short-term increases in sea level could result in marsh deterioration as soils become excessively waterlogged. A conceptual model of changes in geomorphic and ecological processes contributing to marsh sustainability during the Holocene has been developed for the Mississippi delta plain (MDP). The survival of some marshes in this area, despite high rates of subsidence, indicates that the combined effect of organic and inorganic accumulation processes can be adequate to sustain coastal marshes in the face of sea-level rise.

Wave-Induced Mean Magnitudes in Permeable Submerged Breakwaters

The influence of wave reflection and energy dissipation by breaking and by porous flow induced by a permeable submerged structure on second-order mean quantities such as mass flux, energy flux, radiation stress, and mean water level is analyzed. For this purpose, analytical expressions for those mean quantities in terms of the shape functions are obtained. The dependence of those quantities on the incident wave characteristics, structure geometry, and permeable material characteristics is modeled, extending the writers' previous work including wave breaking. Two models for regular waves are presented: a 2D model to be applied to submerged trapezoidal breakwaters and a 3D model for submerged permeable rectangular breakwaters. Both models are able to reproduce experimental wave height transformation as well as mean water level variations along the wave flume with reasonable accuracy. Results give useful information for engineering applications of wave height evolution and set-up and set-down evolution in th...

Processes reshaping the Ebro delta

Due to the limited supply of sediment from the Ebro river, its delta is mainly subject to reshaping processes in the form of alongshore alternation of erosive and accretive coastal stretches, without a significant change in the overall subaerial surface of the delta. The main agents presently forcing these processes (waves and mean water-level oscillations) are analysed in detail as a function of their contribution to coastal processes. In this manner, waves are characterised in terms of short-term/seasonal changes, longshore sediment transport pattern, long-term cross-shore transport at the shoreface and storm impacts. Although the study area is a microtidal one, and waves are the main agent forcing coastal evolution at yearly scales, mean water-level oscillations (meteorological tides and long-term relative mean sea-level variations) play a non-negligible role. Additionally, a joint analysis of waves and storm surges has been done in order to characterise conditions inducing an ‘impulsive’ type of coastal response. At last, the classical ternary classification of deltas has been applied in a ‘dynamical way’ by considering the river vertex in terms of the variation in sediment supply. Moreover, the effects of considering mean water-level variations (other than tides) on the deltaic classification has also been tested. The inclusion of RSLR influence becomes relevant for ‘river-independent’ deltas since it indicates the inability of the delta to withstand it. The main problem to include this agent in the ternary classification is that RSLR is more of a ‘passive’ factor, whereas the other considered agents are ‘active’ in the sense that they induce more perceivable morphological processes.

SET-UP DUE TO IRREGULAR WAVES

Energy losses in breaking irregular waves are estimated on the assumption that a wave, while breaking, loses only that portion of its height which would be in excess of the breaker height for the given wave period and the mean local depth. This leads to expressions for the magnitude of the radiation stresses as a function of the distance offshore. From this the variations in mean water level and the longshore current velocity are calculated with existing methods. Laboratory measurements of set-up in two dimensional irregular waves are described. The data appear to some extent to be internally inconsistent; this may be due to enclosed air bubbles.