This page last updated on 13-Jul-2012 EENS 3050 Natural DisastersTulane University Prof. Stephen A. NelsonRiver Systems & Causes of FloodingFloodingThroughout history humans have found it desirable to construct cities along streams. Streams are sources of water for consumption, agriculture, and industry. Streams provide transportation routes, energy, and a means of disposal of wastes. Stream valleys offer a relatively flat area for construction. But, human populations that live along streams also have the disadvantage that the flow of water in streams is never constant. High amounts of water flowing in streams often leads to flooding, and flooding is one of the more common and costly types of natural disasters.A flood results when a stream runs out of its confines and submerges surrounding areas.z In less developed countries, humans are particularly sensitive to flood casualties because of high population density, absence of zoning regulations, lack of flood control, and lack of emergency response infrastructure and early warning systems.z Bangladesh is one of the most susceptible countries to flood disasters. About one half of the land area in Bangladesh is at an elevation of less than 8 meters above sea level. Up to 30% of the country has been covered with flood waters. In 1991 more 200,000 deaths resulted from flooding and associated tropical cyclones.z In industrialized countries the loss of life is usually lower because of flood control structures, zoning regulations that prevent the habitation of seriously vulnerable lands, and emergency preparedness. Still, property damage and disruption of life takes a great toll, and despite flood control structures and land use planning, floods still do occur. Causes of FloodingFrom a geological perspective, floods are a natural consequence of stream flow in a continually changing environment. Floods have been occurring throughout Earth history, and are expected so long as the water cycle continues to run. Streams receive most of their water input from precipitation, and the amount of precipitation falling in any given drainage basin varies from day to day, year to year, and century to century. The Role of PrecipitationWeather patterns determine the amount and location of rain and snowfall. Unfortunately the amount and time over which precipitation occurs is not constant for any given area. Overall, the water cycle is a balanced system. Water flowing into one part of the cycle (like streams) is balanced by water flowing back to the ocean. But sometimes the amount flowing in to one area is greater than the capacity of the system to hold it within natural confines. The result is a flood. Combinations of factors along with exceptional precipitation can also lead to flooding. For River Systems and Causes of Flooding7/13/2012Page 1 of 13example, heavy snow melts, water saturated ground, unusually high tides, and drainage modifications when combined with heavy rain can lead to flooding.Coastal FloodingAreas along coastlines become subject to flooding as a result of tsunamis, hurricanes (cyclonic storms), and unusually high tides. In addition, long term processes like subsidence and rising sea level as a result of global warming can lead to the encroachment of the sea on to the land. Dam & Levee FailuresDams occur as both natural and human constructed features. Natural dams are created by volcanic events (lava flows and pyroclastic flows), landslides, or blockage by ice. Human constructed dams are built for water storage, generation of electrical power, and flood control. All types of dams may fail with the sudden release of water into the downstream drainage. Spectacular and devastating examples of dam failures include that resulting in flooding downstream include:z The St. Francis Dam, near Saugus, California, failed in 1929 killing 450 people.z The Johnstown, Pennsylvania dam, built of earthen material (soil and rock) collapsed after a period of heavy rainfall in 1889. 2,200 people were killed by the flood.z The Vaiont Dam in Italy (discussed in a previous lecture on mass-wasting) did not fail in 1963, but the landslides that moved into the reservoir behind the dam caused water to overtop the dam killing over 3,000 people.As we have seen during Hurricane Katrina in New Orleans, levee systems designed to prevent flooding can also fail and lead to catastrophic flooding and loss of life. Stream SystemsA stream is a body of water that carries rock particles and dissolved ions and flows down slope along a clearly defined path, called a channel. Thus streams may vary in width from a few centimeters to several kilometers. Streams are important for several reasons z Streams carry most of the water that goes from the land to the sea, and thus are an important part of the water cycle.z Streams carry billions of tons of sediment to lower elevations, and thus are one of the main transporting mediums in the production of sedimentary rocks. z Streams carry dissolved ions, the products of chemical weathering, into the oceans and thus make the sea salty.z Streams are a major part of the erosional process, working in conjunction with weathering and mass wasting. Much of the surface landscape is controlled by stream erosion, evident to anyone looking out of an airplane window.z Streams are a major source of water and transportation for the world's human population. Most population centers are located next to streams.River Systems and Causes of Flooding7/13/2012Page 2 of 13Geometry and Dynamics of Stream ChannelsThe stream channel is the conduit for water being carried by the stream. The stream can continually adjust its channel shape and path as the amount of water passing through the channel changes The volume of water passing any point on a stream is called the discharge. Discharge is measured in units of volume/time (m3/sec). z Cross Sectional Shape - varies with position in the stream and discharge. The deepest parts of a channel occur where the stream velocity is the highest. Both width and depth increase downstream because discharge increases downstream. As discharge increases the cross sectional shape will change, with the stream becoming deeper and wider.z Long Profile - a plot of elevation versus distance. Usually shows a steep gradient near the source of the stream and a gentle gradient as the stream approaches its mouth.z Velocity - A stream's velocity depends on position in the stream channel, irregularities in the stream