Comprehensive Risk Assessment for Natural Hazards - Planat

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Comprehensive risk assessment for natural hazards or urbanized watersheds are associated with rainfall intensities that are greater than the surface infiltration rate. 3.3.4 Coastal and lake flooding The causes of lake flooding are similar to the causes of river flooding, except that flood volumes have a greater influence on high water levels than do flood discharge rates. As a first approximation, the increase in lake volume is equal to inflow rate (sum of flow rates from tributary streams and rivers) minus outflow rate (determined by water surface elevation and characteristics of the lake outlet). In large lakes, either large volumes of inflow or storm surge may cause flooding. Coastal flooding can be caused by storm surge, tsunami, or river flooding exacerbated by high astronomical tide or storm surge. High astronomical tide can exacerbate storm surge or tsunami. Storm surge occurs when tropical cyclones cross shallow water coastlines. The surge is caused by a combination of winds and variations in atmospheric pressure (Siefert and Murty, 1991). The nearshore bathymetry is a factor in the level of the surge, and land topography determines how far inland the surge reaches. Water levels tend to remain high for several days. The Bay of Bengal is particularly prone to severe surges; several surges in excess of 10 m have occurred in the last three centuries (Siefert and Murty, 1991). Surge heights of several metres are more common. Chapter 2 contains more information on this phenomenon. Tsunamis are great sea waves caused by submarine earthquakes, submarine volcanic eruptions or submarine landslides. In the open ocean they are scarcely noticeable, but their amplitude increases upon reaching shallow coastlines. Tsunami waves undergo refraction in the open ocean, and diffraction close to shore; coastal morphology and resonance affect the vertical run up (Bryant, 1991). The largest recorded tsunami was 64 m in height, but heights of less than 10 m are more typical (Bryant, 1991). Other factors of importance include velocity and shape of the wave, which also reflect the destructive energy of the hazard. Often a succession of waves occurs over a period of several hours. Depending on the topography, very large waves can propagate several kilometres inland. At a given shoreline, tsunami may be generated by local seismic activity or by events tens of thousands of kilometres away. Tsunamis are particularly frequent in the Pacific Basin, especially in Japan and Hawaii. Chapter 5 contains more information on the seismic factors leading to the generation of a tsunami. 3.3.5 Anthropogenic factors, stationarity, and climate change It is widely acknowledged that people’s actions affect floods and flood hazards. Land use can affect the amount of runoff for a given storm and the rapidity with which it runs off. Human occupancy of floodplains increases their vulnerability due to exposure to flood hazards. Dams, levees and other channel alterations affect flood characteristics to a large degree. These factors are discussed in section 3.8.1. It is customary to assume that flood hazards are stationary, i.e. they do not change with time. Climate change, anthropogenic influences on watersheds or channels, and natural watershed or channel changes have the potential, however, to change flood hazards. It is often difficult to discern whether such changes are sufficient to warrant reanalysis of flood hazards. The impact of climate change on flooding is discussed in section 3.8.2. 3.4 PHYSICAL CHARACTERISTICS OF FLOODS 3.4.1 Physical hazards The following characteristics are important in terms of the physical hazard posed by a particular flood: (a) the depth of water and its spatial variability; (b) the areal extent of inundation, and in particular the area that is not normally covered with water; (c) the water velocity and its spatial variability; (d) duration of flooding; (e) suddenness of onset of flooding; and (f) capacity for erosion and sedimentation. The importance of water velocity should not be underestimated, as high velocity water can be extremely dangerous and destructive. In the case of a flood flowing into a reservoir, the flood volume and possibly hydrograph shape should be added to the list of important characteristics. If the flood passes over a dam spillway, the peak flow rate is of direct importance because the dam may fail if the flow rate exceeds the spillway capacity. In most cases, however, the flow rate is important because it is used, in conjunction with the topography and condition of the channel/floodplain, in determining the water depth, velocity and area of inundation. Characteristics such as the number of rivers and streams involved in a flood event, total size of the affected area, duration of flooding and the suddenness of onset are related to the cause of flooding (section 3.3). Usually, these space-time factors are determined primarily by the space-time characteristics of the causative rainstorm (section 3.3.1) and secondarily by watershed characteristics such as area and slope. Because of the seasonality of flood-producing storms or snowmelt, the probability of floods occurring in a given watershed can differ markedly from season to season. On a given river, small floods (with smaller discharges, lower stages and limited areal extent) occur more frequently than large floods. Flood-frequency diagrams are used to illustrate the frequency with which floods of different magnitudes occur (Figure 3.2). The slope of the flood-frequency relation is a measure of the variability of flooding. 3.4.2 Measurement techniques In order to understand the characteristics and limitations of flood data, it is helpful to understand measurement techniques (WMO,1980).Streamflow rates can be measured directly (discharge measurement) or indirectly (stage measurement or slope-area measurement). Direct measurements can be taken 21
22 Discharge (m 3 /s) 4 000 3 000 2 000 Annual exceedance probability 0.99 0.98 0.95 0.90 0.80 0.70 0.50 0.30 0.20 0.10 0.05 0.02 0.01 Chapter 3 — Hydrological hazards the suddenness of its onset due to a lack of coordinated and complementary systematic records for various aspects of the hydrological cycle. In addition, bank erosion, sediment transport and floodplain sedimentation are important topics in their own right. Quantitative prediction of sedimentrelated phenomena for specific floods and river reaches is very difficult because of the complexity of the phenomena involved and the lack of appropriate data. 3.5 TECHNIQUES FOR FLOOD HAZARD ASSESSMENT 1 000 1.01 1.1 2 5 10 20 50 100 Return period (years) Figure 3.2 — Example of a flood-frequency diagram plotted on log-probability paper. Symbols represent data points; the line represents the cumulative probability distribution, which has been fitted to the data by lowering a device into the water that measures water depth and velocity. These are measured repeatedly along a line perpendicular to the direction of flow. For any reasonablysized river a bridge, cableway or boat is necessary for discharge measurement. Discharge (m 3 /s) through each cross-section is calculated as the product of the velocity and the crosssectional flow area. Most gauging stations are located such that there is a unique or approximately unique relation between flow rate, velocity and stage. The flow-rate measurements may, therefore, be plotted against stage measurements to produce a rating curve. Once the rating curve is established, continuous or periodic stage measurements, made either automatically or manually, can be converted to estimates of discharge. Because measurements of discharge during floods are difficult to make when the water levels and flow velocities are high, it is common to have records of only stage measurement during major floods. Consequently, the estimation of discharges for such floods relies on extrapolating the rating curve, which may introduce considerable error. However, recent advances in the development and application of acoustic Doppler current profilers for discharge measurement have facilitated discharge measurement during floods on large rivers, e.g., on the Mississippi River during the 1993 Flood (Oberg and Mueller, 1994). Also, stage measuring equipment often fails during severe floods. In cases where no discharge measurements have been made, discharge can be estimated using the slope-area technique, which is based upon hydraulic flow principles and the slope of the high water line. The high water line is usually discernable after the event in the form of debris lines. The area of inundation can be measured during or immediately after a flood event using ground surveys (local scale), aerial photographs (medium scale), or satellite techniques (large scale). With remote-sensing techniques, it is best to verify the photo interpretation with ground observations at a few locations. In general, it may be difficult to establish the cause of flooding (cyclone, snowmelt, etc.) or 3.5.1 Basic principles Well-established techniques are available for the assessment of flood hazards (WMO, 1976 and 1994). The most comprehensive approach to hazard assessment would consider the full range of floods from small (frequent) to large (infrequent). Such comprehensive treatment is rarely, if ever, practical. Instead, it is customary to select one size of flood (or a few sizes) for which the hazard will be delineated. The selected flood is called the target flood (commonly referred to as a design flood in the design of flood-mitigation measures) for convenience. Often the target flood is a flood with a fixed probability of occurrence. Selection of this probability depends on convention and flood consequences. High probabilities are used if the consequence of the flood is light (for example, a 20-year flood if secondary roads are at risk) and low probabilities are used if the consequence of the flood is heavy (for example, a 500-year flood if a sensitive installation or large population is at risk). In some countries the probability is fixed by law. It is not strictly necessary, however, to use a fixed probability of occurrence. The target flood can be one that overtops the channels or levees, a historical flood (of possibly unknown return interval), or the largest flood that could conceivably occur assuming a maximum probable precipitation for the region and conservative values for soil moisture and hydraulic parameters (sometimes known as the probable maximum flood). In the simplest case, hazard estimation consists of determining where the hazard exists, without explicit reference to the probability of occurrence; but ignorance of the probability is a serious deficiency and every effort should be made to attach a probability to a target flood. In terms of specific techniques, several methods and combinations of methods are available for assessing flood hazards. What follows is, therefore, a list of possible approaches. In any practical application, the exact combination of methods to be used must be tailored to specific circumstances, unless the choice is prescribed by law or influenced by standard engineering practice. Determination of the most suitable methods will depend on: (a) the nature of the flood hazard; (b) the availability of data, particularly streamflow measurements and topographic data; (c) the feasibility of collecting additional data; and (d) resources available for the analysis. Even with minimal data and resources, it is generally possible to make some type of flood hazard assessment for
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CHAPTER 8 STRATEGIES FOR RISK ASSES
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