Greatest Point and Areal Rainfalls

Abstract

Weather systems of the tropics and extra tropics generate heavy to very heavy rainfall for periods of days in various parts of the world, as discussed in Chapter 5. The heavy rainfall events may last 3 to 4 days and are responsible for causing floods, landslides, levee breaches, dams overtopping, sedimentation, erosion and other such occurrences. Flooding from some of the abnormally heavy rainfall events have destroyed dams, causing enormous loss of life and property damage. This happened in India in 1979 when a heavy rainfall event over the Machhu River catchments totally destroyed the Machhu-2 dam on 11 August. The disaster killed as many as 2000 people and totally devastated urban and rural property downstream of the dam (Purohit et al., 1993). Another instance is the catastrophic flood generated from a heavy rainfall event caused by typhoon Nina of August 1975 in the Hongru River basin in China which destroyed the Banqiao and Shimantan dams (Tan and Lu, 1994). There are many dam failures (see Table 8.11)

Table 8.11 Major dam failures

caused by flooding due to heavy rains in various parts of the world (Lemperiere, 1993). In fact, the first recorded dam failure in the USA occurred on 16 May, 1874 in Williamsburg, Massachusetts. It is evident from the above examples that tropical and extra tropical storms and associated heavy rainfalls cause floods which are undoubtedly one of the mightiest and most devastating forces of nature. However, there are ways to prevent or minimize the impacts of flood disasters from heavy rainfall on people and property by adopting the following measures:

• Constructing flood walls and levees to constrict the overflow of rivers

• Increasing capacity of existing channels by river improvement and cutoff

• Constructing flood control dams on rivers to store runoff and reduce flooding downstream

• Constructing diversion channels for flood waters

• Monitoring spillway gates in times of heavy rainfall

• Creating and enforcing effective building codes to prevent property from tropical storms.