ABSTRACT
For example the Río Segura in Murcia Spain has been studied in detail by López Bermúdez (1976). At Murcia city, the river has a catchment area of 6,960km2. The monthly flows of the Guadalentín, the main tributary (2,780km2 at Totana) are shown in Figure 5.5 and range from a mean monthly flow of 0.4m3 s1 in July to 2.58m3 s1 in January. The average annual precipitation in the basin is 271.6mm. The maximum recorded flow is 547.1m3 s1 in the flood of October 1948, the minimum 0m3 s1 in August 1945. The rivers in this region are broad, braided, gravel-bedded streams known by the name rambla. In smaller catchments, flow is rare and even intermittent and these are called ephemeral channels. Again, even these smaller channels can be subject to very great variations in flow. Because they are an important source of irrigation and building material (gravel and sand) for roads and concrete structures, they have been studied quite intensively (Thornes, 1977, 1993a, b). At tributary junctions, there are big
Box 5.2 Return periods The concept of return period is very important. The essential idea is that floods of a given size occur an average distance apart in time. This is not the same as saying that they do occur this fixed time interval apart, though. Bigger floods are less frequent, smaller floods more frequent. Size is usually measured by total rainfall, rainfall intensity, or flow peak volume. A plot of size against return period for the Sierra Nevada in Spain (after Cirugeda, 1973) is shown in Figure 5.3. From the graph one can read the return period for an event of a given magnitude or, conversely, the magnitude of an event of a given return period. In designing flood-prevention works, engineers have to decide what event they are working to. Often this is the one hundred year flood, i.e. the flood peak flow that occurs on average once every one hundred years. The quality of these estimates improves as the history of flows increases.
