Estuaries Of Large Rivers Research Articles

Floods and Landslides in Natal and Notably the Greater Durban Region, September 1987: a Retrospective View

The low-lying and coastal areas of Natal, South Africa, were subjected to severe flooding in September 1987. This followed exceptionally heavy rainfall which in four days exceeded 600 mm. The resultant floods were the worst in South African history. Saturation of soils occurring on slopes gave rise to landslides, particularly mudflows. The floods and landslides caused damage estimated at over 600 million rands, and 380 lives were lost. The river gradients in Natal are steep and flow velocities of up to 7 m/sec occurred under these flood conditions, leading to rapid runoff and discharge of large volumes of water which caused severe erosion. The type and extent of erosion in estuaries of large rivers was governed by the type of sediments present in banks and beds, and rock outcrops. Gravel and sand were removed from the beds of large estuaries in preference to the cohesive materials of the banks. General scour depths at times exceeded 10 m, but around bridges total scour below the original river bed may have exceeded 20 m. This, as well as pressure from accumulating debris, caused the displacement of founding caissons of bridges and subsequent pier failure. Furthermore, approach roads to bridges suffered severe erosion. However, none of the newer bridges which were designed in accordance with modern flood magnitude prediction failed. In rural areas, river crossings, mostly fill-over culvert and drift type structures, failed primarily through erosion and overtopping. The heavy rainfall helped trigger a large number of landslides causing damage to property, particularly in the greater Durban area. The most notable slides occurred as mudflows in areas underlain by sandstones, which had been previously considered stable. These localities had thin soil cover which became saturated quickly. In other areas translational and rotational slides occurred, such as in soils overlying granite-gneiss. In rural areas slides were responsible for damage to informal developments on steep slopes. Sliding also occurred in soils above shales, and in recently weathered dune sand.

Osmoregulation of the Estuarine PrawnPalaemon Longirostris(Caridea: Palaemonidae)

The caridean prawnPalaemon longirostris(Milne-Edwards) (Palaemonidae) occurs predominantly in the upper regions of large river estuaries in Europe, and reaches its northern limit of geographical distribution along southern parts of Britain (Smaldon, 1979). All post-larval stages are thought to live under dilute and fluctuating salinity conditions, although there are conflicting reports as to whether ovigerous females need to migrate to the sea at times of egg hatching (Gurney, 1923; Fincham & Furlong, 1984). The species has been known to be extremely euryhaline for some considerable time (Gurney, 1923), yet the salinity tolerance limits of larval (Antonopoulou & Emson, 1989) and post-larval stages (Campbell & Jones, 1989) have been established only recently. Apart from studies on salinity tolerances, and some preliminary measurements of blood and urine osmolality (Parry, 1957), the osmoregulatory adaptations ofP.longirostristo its dynamic habitat have not been investigated previously. The present paper describes the osmoregulatory responses ofP.longirostrisacclimated to a wide range of salinities, and also reports on the ability of individuals to compensate for sudden acute salinity change.

Procedimiento para la supresión del empleo de cimbras en puentes de hormigón armado o sin armar

This paper gives a detailed description of the various procedures that may be used to avoid the need of scaffolding in the construction of arches and bridge vaults, made either of reinforced or unreinforced concrete. The scaffolding is expensive and cumbersome, and could be eliminated not only in the case of small spans on firm ground, but also in the case of large spans over seawater, i. e., over estuaries of large rivers, or narrows; for instance in the case of the «Pas de Calais».