Flood Modeling, Prediction and Mitigation

Floods are among the major natural extreme and dangerous events that cause loss of life and property, and they are the most frequent extreme occurrences in different parts of the world. A broad definition of floods and their types are explained with meteorological and hydrological causative triggers and the consequences. Ordinary and flash flood features are presented in a comparative manner so that the reader can appreciate the difference between them. Flood hazards are exposed with recommendations and human pre-flood preparation procedures. It is emphasized that the floods are although natural phenomenon, but skewed settlements especially along the main watercourse such as the flood plain are also effective in the flood losses.

There are different types of rainfall depending on elevation, temperature, or pressure differences that generated by composition of various meteorological factors such as light, moderate, or extreme rainfall occurrences with or without flood consequences. In arid regions, elevation (orographic) and temperature (convective) differences may cause to floods, but in humid regions, pressure difference (frontal) rainfalls are the major factors. In order to calculate various rainfall characteristics such as the intensity recording, raingauge records are necessary, but with accurate measurements. Different sources of measurement errors are explained in the text with their correctional actions in the field and office. Various areal average rainfall calculation methodologies, especially innovative percentage-weighted methodology, are presented in comparison with the classical and about 100-year-old Thiessen approach. New concepts such as the dimensionless intensity–duration curves are explained, and their application to annual maximum rainfall amounts is presented with actual data processing. The importance of the intensity–duration–frequency curves is explained with the concepts of different risk levels. The significance of probable maximum rainfall and its connection with probable maximum flood calculation is presented through the applications to a set of drainage basins from the western part of the Kingdom of Saudi Arabia. Efficiency factor is defined the first time in the text for distinction of the climate change impact on the rainfall occurrences in the region.

Floods occur not only because of the rainfall but also the surface features, which collects, converges, and diverges of the after rain runoff flows through natural or artificial channels. The significance of topographic maps, satellite images, and digital elevation model (DEM) data, if available. The treatment of these data sources for the purpose of flood evaluation and prediction of various softwares are used, which take into consideration a set of morphological (physiographic) features. The basic definitions of various surface features are presented in a conceptual and rational manner so that the reader can appreciate the significance and the partial effect of each feature to overall flood discharge. The significance of cross sections with their hydraulic properties so as to calculate the surface water flow speed is described in detail with the cooperation of the rating curve concepts and their practical importance in any flood and early flood system setups. The most important part of the flood modeling is to reach flood maps in terms of inundation maps, which are missing in many parts of the world. Standard hypsographic curves and their relationship to the flood discharge calculation and classification are explained. Finally, early flood discharge formulations that are related simply to the drainage area are presented in the forms of equations and graphical representations with comparisons and restrictive limitations.

In any flood prediction study, the most important tools are hydrographs that reflect the change of surface water flow discharge by time after each rainfall occurrence. These hydrographs are in the form of single peak curves, where the peak discharge value is one representation of the historical flood occurrences. In flood calculations, hydrograph concept and its various properties and features are important for successful applications. Various basic hydrograph concepts such as natural, unit, dimensionless, instantaneous, and synthetic are presented with relationships among them and the significance of each one for peak flood discharge calculations. Furthermore, starting from the simplest rational method for flood peak discharge calculation, various comparatively more sophisticated alternatives are also explained in detail among which are Snyder, the Soil Conservation Service (SCS), and geomorphological approaches. Finally, the most commonly used rational method irrationality is explained leading to innovative rational methodology for the first time in this book.

There are different flood discharge calculation methodologies that have started more than 100 years ago, and most of the first ones are based on logical and rational thinking. The very early ones do not include any information because there were no rain gauges established at that time. As already mentioned in the previous section, they were all related to drainage area nonlinearly without any further consideration of the surface features as soil type, land use, etc., or rainfall characteristics. Reliability of these early methodologies is discussed with comparison to the present most advanced techniques. For flood discharge estimations and preliminary appreciation of the magnitude, flood envelop curves are presented, and their usage procedures are given for different parts of the world again in comparative manner. For the application of rational methods (RMs) in arid regions, empirical runoff coefficient formulation is presented with applications to some of the drainage basins in the southwestern corner of the Arabian Peninsula. Irrationality of the RM is explained, and instead, a new and innovative modification of it is presented with application example. Practical surface water discharge calculation methods are presented on the basis of different ration based on the drainage area, mean flow, and standard deviation of the flow records.

In flood frequency and discharge calculations, there are two different treatment procedures as either probabilistic or deterministic approaches. So far, the previous chapters are concerned with hydrological deterministic methods, but this chapter provides information about the probabilistic, statistical, and stochastic uncertain methods. The very bases of these approaches are the annual, partial, or hybrid selections from a given time series of extreme discharge magnitudes. The selected flood discharges are fitted to the most suitably representative probability distribution functions for risk-level calculations. Most often, the flood discharges for two-year, five-year, 10-year, 25-year, 50-year, 100-year, and 500-year return periods are sought which correspond to 0.50, 0.20, 0.10, 0.04, 0.01, and 0.002 probability exceedence (risk) levels. Explanation of various probability papers and their theoretical background information are exposed with some examples.

Protection against floods is possible by construction of some engineering structure, but their dimensioning needs to scientific calculations, where flood design discharge plays the major role. The definition of flood design discharge is given with different choices including probable maximum flood (PMF) as explained in Chap. 2, standard project flood, flood of a specific return period, and the use of intensity-duration-frequency curves. The causes of floods are explained in terms of landslides, rock falls, debris flow, and sediment yield with suitable calculation methodologies and design methodologies.

One of the most effective climate change impacts is on the unexpected occurrences of floods and especially flash floods. The impact on water resources is the most concerned affair not only for water resources management but also on the food security of a region or country. Engineering risk management is very significant factor in climate change assessment and impact intensity calculations on engineering water structures. In order to enhance and perform a better idea about the impacts of the climate change fact, various exemplary application cases are presented in this chapter. Especially, innovative trend analysis provides whether there is an increasing trend in the past records to plan for future and additionally what are the positions of high and extreme rainfall cases that may give rise to flood occurrences?

After all what have been explained in the previous chapters, it is necessary on the basis of the explained information to plan for the flood safety, reducing its hazardous character. There must be a common understanding on the basic definitions and for this purpose, the most common safety and flood hazard terminology is provided. Among the safety factors are defense against floods their measurements, proofing, controls and plans. Flood risk calculations in a different way from the previous chapters are presented with detailed explanations. Public awareness is one of the most significant educational ingredients in reducing flood hazard by consideration of flood resilience possibilities.