Physical conditions and forces of nature

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1.2.1 Physical conditions and forces of nature

Physical Conditions Geology, Geomorphology and Topography : There is a complex interaction between the various elements defining the coastal environment. Coastal physical features such as topography and bathymetry, in conjunction with geological properties of the nearshore sediments result in different erosion and accretion patterns. The near-shore bathymetry and topography determine the way in which waves and currents approach the shoreline. The response of the shoreline will in turn depend on the geological properties of the beach material. Hydrographic conditions: This includes bathymetry, ocean and physical properties of seawater salinity and temperature. For coral reefs and atolls, increasing seawater temperature could be important because this could adversely affect the growth potential of the coral, which will in turn reduce the protective ability from the wave force and processes associated with sea level rise. Since the EACC flows almost parallel to the coastline, a slope perpendicular to the direction of the current will be created. Due to the Coriolis effect, water mass will be deflected to left in the southern hemisphere, when looking in the direction of the current Lisitzin, 1974. As a result of this deflection and considering that the current passes in the Zanzibar and Pemba channels, locations on Mainland Tanzania will experience sea level rise while those on Zanzibar will experience a fall in sea level. This phenomenon, though theoretically possible, has not yet been investigated in Tanzania coastal waters and can have significant practical implications. In summary, the information on spatial and temporal variations of hydrographic conditions of the coastal area of Tanzania is necessary for a proper evaluation of its vulnerability to erosion. Forces of nature These are shoreline-induced changes that occur as a result of the response of the shoreline to the forces of nature. The forces include changes in the climate, hydrology and coastal processes. For coastal areas, it is always worthwhile to consider climatic changes over a long period of time to avoid the disastrous century events. Climate-induced causes include changes in meteorological conditions winds intensity and direction, rainfall, barometric pressure and storms, El-Nino related changes and accelerated sea level rise Klein and Nicolls, 1998 is just an example of what can happen due to these type of changes. Hydrologic changes include variability in water discharge and sediment transport. these can have a myriad of effect on the nearshore zone. The nearshore waves and currents in essence control the coastal processes. Climate change The wind has multiple effects on ocean circulation, waves and sea level. It is important in the generation of wind- induced currents, waves, wave-induced currents, water levels and the translation of coastal dunes. Wind determines the predominant or resultant wave direction. In near-shore zones, waves and currents play a major role in sediment movement and the resulting natural beach geometry. The wave climate at any given location depends on the wind speed, fetch and duration. It is wind that determines the wave height, wave period and the predominant wave direction. Steepness and direction of the incident waves are the major parameter in the generation of longshore currents and corresponding longshore drift. When wind blows over the sea surface, it induces a steady movement of the upper ocean. This movement is generally known as wind-induced current, which flows in the same general direction as the wind. Surface currents are typically 3 of the wind speed. So, 10 ms wind is likely to generate surface flows of up to 0.3 ms Bowden, 1983. This type of water movement often influences the drifting of floating matter such as oil slick, buoys and sewage solids. Tangential stress on surface water leads to accumulation if winds blow towards land or depletion if winds blow away from the coast of water and the development of surface slopes. The surface slopes in turn create horizontal pressure gradients that drives local currents. The effect of wind shear stress associated with strong winds and 10 storms is very important when considering water levels and the associated shoreline changes. In shallow seas, storm surges due to wind shear stress can be particularly dangerous. Since wind set-up is directly proportional to the square of the wind speed and inversely proportional to water depth Silvester, 1974. A drop in barometric pressure often results in a temporary increase in the water level. The piling of water at the coast as a result of this process is known as wave set-up. The steep waves generated by coastal storms can transport sand offshore with a temporary storage in sandbars or shoals. The sediment material deposited offshore in this way may later be recovered by the action of longer period processes, but in most cases the material may be lost permanently. The tangential wind shear stress also leads into the generation of waves. The growth of wind-generated waves is not well understood. During the Second World War, the forecasting of wave conditions on beaches was very important for planning the operations. It was at this time when Sverdrup and Munk derived an empirical method for predicting wave height and for a given wind speed, duration and fetch. Later, with increased number of observations, Bretschneider improved the method, which became to be known as the Sverdrup-Munk- Bretschneider SMB method. Under certain assumptions, simplified wave growth formulas have been derived to give quick estimates of wave parameters in deep and shallow water. Simplified versions of the wave growth are found in Shore Protection Manual SPM of 1984 and Vincent 1984. For proper evaluation of wind-generated wave conditions, it is necessary to collect information on wind speed intensity, direction and fetch. This information is useful for hindcasting and statistical analysis of extreme wind events. Sea level rise : Over the last 100 years, the global sea level rose by 1.0-2.5 mmyr Warrick et al., 1996. Estimates of future sea level rise as presented in the IPCC Second Assessment Report range from 20-86 cm for the year 2100. This effect is about 2-5 times the rate experienced over the last century. When it comes to assessing the impact of sea level rise, it is the local change in relative sea level that matters, not the global average. Relative or observed sea level is the level of the sea relative to the land. Regional sea level changes are highly uncertain and could therefore best be considered zero until more concrete evidence emerges Warrick et al., 1996. Values for vertical land movements can be assessed from a number of different sources, including geological analysis, geodetic surveys and analysis of long-term tide-gauge records. A natural coastal system that experiences sea level rise can be affected in several ways. From an erosion perspective, the most important physical effects are: A slow, long-term recession of the shoreline: This is due to direct flooding and partly to profile adjustment to the higher level. Bruun 1962 suggested that a 1-cm rise in sea level would result in a 1-m shoreline retreat. Representative recession rates for exposed shores like those of the Atlantic, the Pacific and the Indian Oceans were found to be Sea level rise per year cm Shoreline recession per year m 1 1-1.5 2 2-3 3 3-4.5 10 10-15 Increasing inundation and flood-frequency probability: Low lying coastal areas will experience the risk of flooding. The degree to which a coastal land is at risk depends on morphological and meteorological factors, including coastal slope, wind and wave characteristics. Information on these factors can be used to plot a flood- frequency probability curve, from which the design water levels can be plotted on a topographic map. The design water levels are contour lines that indicate with which probability a particular area could be flooded Hoozemans et al., 1993. Other effects of sea level rise, which are not directly related to erosion are rising water tables, salt water intrusion and biological effects. The potential socio-economic impact of sea level rise includes: direct loss of economic, ecological, cultural and subsidence values through the loss of land, infrastructure and coastal habitats, increased flood risk of people, land and infrastructure, and impacts related to change in water management, salinity and biological effects. 11 Hydrology Rainfall : Rainfall in Tanzania comes in two regimes Mwandosya et al., 1998. One sector of the country receives bimodal rainfall normally referred as long rains or Masika during the months of March, April and May and short rains or Vuli during the months of October, November and December. The areas which receive bimodal rainfall are confined to the north eastern parts of the country comprising of Arusha, Moshi, Same; the north western areas including Lake Victoria basin and the northern part of the coastal belt comprising of Tanga, Morogoro and Dar es Salaam. The other rainfall regime has a unimodal pattern and covers the southern, central, western and south- eastern parts of the country. These areas receive most of their seasonal rainfall during the months of December, January, February, March and April. Changes in global weather patterns can result in the variability of water discharge and sediment supply to the coastal zone. Droughts will result in the reduction of sediment supply to the coastal zone through rivers. Information on both changing rainfall pattern and seawater temperature should therefore be obtained from meteorological stations and also from general circulation models Klein and Nicolls, 1999 in order to design warming and forecasting systems. Coastal processes When waves reach the coast, they generate a variety of currents in the nearshore zone. Wave-generated currents tend to dominate water movements in the nearshore zone and therefore are important in the dispersal of sediments, pollutants and biological nutrients. These currents can also carry sediments if their speed reach the threshold speed at which sediments on the seabed can be moved. If the quantity of sediment carried away by such currents exceeds the amount naturally supplied to the beach, erosion will occur. In addition to dispersing sediments, these currents interact directly with the wave orbital motion to influence the resultant direction of the movement of beach sediments, and as a result act to mould the nearshore topography. The beach topography in turn becomes an important factor in controlling the current patterns.

1.2.2 Man-induced factors