River and Costal Floods 1- River Floods The basic cause of river flooding is the incidence of heavy rainfall.
Other factors are: -The promotion of hydraulic surcharge in water levels -The presence of natural or man-made obstructions in the flood path such as bridges piers, floating debris, weirs, etc. -Also included are the generally unforeseen river-surge events caused by sudden dam failure, land slip or mudflow.
The slow-developing characteristic of flood flows in many large rivers is in sharp contrast to flash floods more commonly but not exclusively associated with small catchments. There are so many small catchments within a given region, that efficient surveillance, warning and protection against the eventuality proves difficult. In other regions, flash floods occur each year on the same river. Warning in these cases is more a problem of timeliness.
Floods Hazards Information needed by Planers The dangers of flood water are associated with a number of different parameters:- -Depth of water -Duration -Velocity -Rate of rise -Frequency of occurrence -Seasonality
Flood Hazards Assessment Flood Maxima- “the regional flood” By far the most effective way of determining extreme flood levels (and discharges thereafter) is through exhaustive survey of riverside conditions and questioning of local inhabitants.
I Flood class Topographic situationFlood susceptibility 1Hilly zones and other high areas Above highest flood level 2Areas of medium heightSubject to exceptional flooding 3Medium height to low areasInfrequent flooding, in part seasonal shallow flooding 4Low-lying areasSubject to annual flooding. flooding, in part frequently and deeply flooded 5Local depressionsDeep flooding, often moist 6Local depressions, lowest parts Frequently very deeply flooded and in parts waterlogged throughout the year
II Flood classDischarge m³/sec Water level(m) Frequency in 100 yrs Flood/war ning Up to 14.700 Up to 8.7050 – 100 Dangerous flood Up to 2.200 Up to 10.30 10 – 20 Emergent flood Up to 37.300 Up to 11.69 2 – 4 Exceptiona l flood Up to 37.600 Up to 12.75 0.5 - 1
Floods and Forests -A key driving force in the yearly increase in flood disasters in the rapid rate of deforestation in the tropics.
-According to an FAO/UNEP study in 1981, tropical forests are disappearing at the rate of 7.3 million hectare (18 million acres)per year:.2 million hectares (10.4 million acres) a year in Latin America; 1.8 million hectares (4.4 million acres) a year in Asia; and 1.3 million hectares (3.2 million acres) a year in Africa.
The environmentalist Erik Eckholm wrote in Down to Earth (pluto Press;Norton, 1982): “Decades of research have proved that the deforestation of watersheds, especially around smaller rivers and streams, can increase the severity of flooding, reduce stream flows and dry up springs during dry seasons, and increase the load of sediment entering waterways.
Yet most efforts to combat such problems have entailed engineering measures –dams, embankments, dredging- that address symptoms and not causes. The exact contribution of deforestation to flood trends is probably impossible to pinpoint, but as flooding worsens in country after country, new attention is being given to watersheds”
Interventions: Reforestation and other soil conservation measures could decrease floods. The sponge effect of trees, grass and crops, whereby the land absorbs rainfall and then releases it slowly over a long period of time, reduces flooding
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