Module 2 Floods Flood-producing processes Swiss Virtual Campus “Dealing with Natural Hazards” Module 2 Floods Flood-producing processes “Dealing with Natural Hazards and Risks” 1 Module 2 Floods Flood-producing processes CONTENTS INTRODUCTION.........................................................................................................3 THE MYSTERY OF STORM FLOW........................................................................3 PROCESSES GENERATING WATER DISCHARGE..........................................5 THE RIVER FLOW COMPONENTS ................................................................................5 PROCESSES GENERATING SURFACE FLOW ...............................................................6 VARIABLE SOURCE AREA CONCEPT...........................................................................8 PROCESSES GENERATING SUBSURFACE FLOW.........................................................9 PROCESSES GENERATING GROUNDWATER FLOW ...................................................11 THE RIVER FLOOD HYDROGRAPH...................................................................13 DOMINANT PROCESSES - SPACE AND TIME SCALES...............................14 BIBLIOGRAPHY AND INTERNET RESSOURCES...........................................16 Lausanne, le 26.11.2002 C. Picouet & A. Beney A. Musy EPFL / HYDRAM “Dealing with Natural Hazards and Risks” 2 Module 2 Floods Flood-producing processes Introduction Apart from the rare effects of Jökulhaups, ice jam, or dam failures, floods in most river basins are caused almost entirely by excessively heavy and/or excessively prolonged rainfall, or in areas of snow and ice accumulation, by periods of prolonged and/or intense melt. In each case the operative processes result in a large volume of water which, under certain conditions, produce more runoff than: i) an area can store or, ii) a stream can carry within its normal channel. This learning unit attempt to clarify the nature of this flood-producing processes - from Horton ideas to those of today - responding to the key question of what are the hydrological rainfall-runoff processes. This course will also examine the storm hydrograph and its main parameters (hydrograph peaks and lag time) and the importance of the spatial scale in determining the dominant flow generation mechanism. The mystery of storm flow For a long time, human beings have been wondering about the origin of streams. Further questions arose from the observation that rivers continue to flow even after rain has stopped. What feed the rivers? The first recorded theories on the origin of water and its natural cycle were postulated by the Greeks, who regarded water to be one of the four elements (along with earth, wind and fire). Their main concern was about the source of water flowing in rivers. They imagined that in addition to rains, rivers were fed from some sort of subterranean body of fresh water. Aristotle (384-322 BC) developed the fanciful notion that river flow resulted in part from the condensation of vapour of groundwaters, itself produced by the flux and desalinisation of seawater in the ground. This misunderstanding persisted through Roman times. When Roman civilization faded, there was literally no original thinking about natural phenomena for about the next 1500 years. Even the great Leonardo da Vinci (1452 - 1519) maintained the concept of a hydrological cycle in which underground veins of water rose from the sea to the mountains, where they issued forth as rivers. During this underground passage the salt was filtered out. Figure 1 illustrates a typical thought of Europeans during the 17th century. This concept and those of da Vinci were identical to some of the concepts put forth by Greeks. Even though these ideas look humorous in the modern view, at that time many scholars seriously believed and tried to prove them. “Dealing with Natural Hazards and Risks” 3 Module 2 Floods Flood-producing processes Figure. 1. Origin of streams (Kircher, 1678). Athanasius Kircher contended that water is transported from sea-bed through under-ground canals and stored in huge hollow spaces inside mountains, where streams originate. The first person to provide a correct written explanation of the hydrologic cycle was a Frenchman, Bernard Palissy (1510 - 1590). Pierre Perrault (1608- 1680), french physicist, made the first quantitative measurements of hydrologic phenomena. He measured the average annual rainfall over a small part of the upper Seine basin as well as the annual discharge of the river from that catchment. He found that rainfall was six times the amount that flowed in the Seine, thus proving that precipitation was more than enough to supply the water in the Seine. However, he thought that the springs were fed by the rivers and that in general there was little infiltration of rainwater into the ground. Further investigation of the rainfall - runoff cycle was made in France by Edmé Mariotte (1620 - 1684). He confirmed Perrault’s work on the Seine and studied also infiltration of rainwater into the basement of the Paris basin. He observed that infiltrating precipitation penetrated the pores of the earth and accumulated in underground reservoirs. Edmond Halley (1656 - 1742) the noted English astronomer made an important contribution to hydrology by studying evaporation. He concluded that there was sufficient water evaporating from the ocean to supply all of the rivers and springs on earth. In the 18th Century progress was made in utilizing the correct knowledge of the hydrologic cycle to develop ground water supplies. In the 19th Century with the progress in geology (development of a more comprehensive understanding of the relationship of ground water with the geological formations) and meteorology, the water hydrology begun as a modern science. With the 20th century and the new scientific measurements and theories, the debate about “mystery of storm flow” is related to the process of stormflow “Dealing with Natural Hazards and Risks” 4 Module 2 Floods Flood-producing processes generation. The two essential questions of modern hydrology are "what becomes the water of rains?" (Penmann, 1963) and "where does the water of the river come from?" (Hewlett, 1961). The aim of the next chapter is to describe general and modern theory relating to flow generating mechanisms, from Horton ideas (1933) - innovative at the time - to those of today which are based on preferential flows. Processes generating water discharge The river flow components Commonly, the river flow (river runoff) is the amount of three separate sources of runoff, which may, at a given time, occur separately or simultaneously with varying magnitude (Figure 2): Figure 2. Hydrologic movement of water beneath the Earth's surface. Water usually enters the surface as precipitation. When rain falls to the ground, some water flows along the land surface to streams or lakes (surface flow), some water evaporates into the atmosphere, some is taken up by plants, and some seeps into the ground. This water then percolates into the soil layer. Some of this water flows horizontally as subsurface flow. Water continuing to flow downward eventually reaches a permanent store of water known as groundwater. The groundwater movement is called groundwater flow. § Surface flow. If rainfall is not intercepted, it will evaporate, infiltrate or lie in depression storage. Surface runoff is the rainfall surplus that flows over the surface to the nearest stream channel. Surface flow is generally referred to as overland flow, and may occur as interrill overland flow (also known as sheet flow or sheet wash) or rill flow. § Subsurface flow (or interflow). The subsurface flow is the portion of infiltrated rainfall, which has not passed down to the water table, but that moves laterally through the upper soil horizons until reaches the stream channel. “Dealing with Natural Hazards and Risks” 5 Module 2 Floods Flood-producing processes § Groundwater flow (or baseflow). Groundwater flow is the portion of infiltrate water that reaches the water table and then discharges into the stream. Ground water is water held within the interconnected openings of saturated rock beneath the land surface. We can also mention direct precipitations on the surface of the river. However, this process is quite marginal, considering that the catchment’s surface concerned is generally very small. The contribution rises nevertheless when precipitations are long-lasting or when hydrological networks are composed of lakes and marshes. Snowmelt runoff can be added too to this list of sources; this specific process will be discuss in the section "Snow-melt and movement processes". Processes generating surface flow There are two basic concepts that explain the production of surface runoff during rainfall events. § Hortonian overland flow In 1933, a hydrologist named Horton proposed a process for stormflow generation, named the process of Hortonian overland flow later, using the concept of infiltration capacity and neglecting the interception processes (Figure 3). The soil’s infiltration capacity is defined as the maximum rate at which water can be absorbed by a given soil per unit area under given conditions. Figure 3. Hortonian Overland flow. Overland flow occurs when rainfall intensity exceeds the soil's infiltration capacity (Figure 4). This capacity, characterized by the soil’s infiltrability, is supposed to decrease exponentialy as a function of time until a constant value. In a homogeneous soil with a deep water table, this final constant