**3.3 Geotechnical considerations**

Geotechnical engineering primarily deals with foundations of structures. It answers a set of basic questions which must be answered before any construction, such as: (1) What is the most appropriate site for constructing a given structure, such as a dam? (2) Can the local soil withstand the pressure? (3) What are the local soil characteristics? (4) How high a structure, such as a levee, should be? (5) Does the foundation need reinforcement? (6) What is the reliability of a given foundation? These questions are answered by geotechnical engineering. There are standard textbooks available which provide comprehensive accounts of these and related issues.

### **3.4 Structural considerations**

Structural engineering deals with the structural design of a flood control structure, such as a dam and its associated appurtenances like spillway, tunnels, etc. or levee. It computes forces the structure must withstand and its dimensions. To that end, it answers questions such as how large a structure should be, what the type of a structure should be, how reliable the structure will be, what skill set will be needed for construction, who will do the dam construction and who would do the supervision, and how long will it take to complete the dam. These questions are answered by structural modeling which can be deterministic, including empirical, conceptual, or physically-based, and reliability and risk-based, including reliability analysis and risk analysis. Standard textbooks are available which provide full accounts of these and related issues.

### **4. Challenges in flood management**

### **4.1 Climate change**

Climate change is a major challenge for humanity in this century. It indeed will decide the fate of our civilization. The Intergovernmental panel on Climate Change [12] notes: "A changing climate leads to changes in the frequency, intensity, spatial extent, duration, and timing of weather and climate extremes, and can result in unprecedented extremes …" It has already started to impact the extremes of atmospheric weather and climate variables (temperature, precipitation, wind), the natural physical environment (floods, extreme sea level, waves, coastal waves, winds, and tornadoes). The questions often arise with regard to the assessment, forecasting-where, when, and how longimpact assessment-where, how much, and how serious a risk.

Three possible changes in weather extremes triggered by climate change are: less extreme cold but more extreme hot weather, more extreme cold and more extreme hot weather, near constant extreme cold but more extreme hot weather. It has a pronounced effect on the hydrological cycle and climate extremes as shown in **Figure 1** with and without climate change. The upper most part of **Figure 1** shows that there is a shift in the mean to the right from without climate change to with climate change, indicating less cold and less extreme cold but more hot and more extreme weather, whereas the middle part of **Figure 1** shows that there is an increase in variability with climate change, translating into more cold, more extreme cold, more hot, and more extreme hot weather, and the bottom part of **Figure 1** shows that the weather symmetry changes with climate change such that cold and extreme cold weather is nearly constant but there is more hot and more extreme hot weather.

Climate models are showing earlier occurrence of spring peak river flows in snowmelt- and glacier-fed rivers (already being observed), anthropogenic influence on changes in some components of the water cycle (precipitation, snowmelt) affecting floods, projected increases in heavy precipitation which would contribute to rain-generated local flooding in some catchments or region, and potential changes in the magnitude and frequency of floods. IPCC, SREX [12] shows the

### **Figure 1.**

*Effect of climate change on weather extremes [Source: https://www.ipcc.ch/report/ managing-the-risks-of-extreme-events-and-disasters-to-advance-climate-change-adaptation/].*

impacts on precipitation, as shown in **Figure 2**, considering the standard deviation of wet day intensity, percentage of days with precipitation greater than Q95 (95% quantile), and standard deviation of fraction of days with precipitation greater than 10 mm for June, July and August (JJA); December, January, and February (DJF); and artificial neural network (ANN). In each case it is revealed that the standard

**Figure 2.** *Effect of climate change on weather extremes (Source: IPCC, SREX [12]).*

deviation increases over most parts of the world. IPCC, SREX [12] further shows for different parts of the world that higher 24-hour precipitation values will occur more frequently indicating their reduced return period. For example, for many parts a 20-year return period of 24-hour precipitation will reduce to 10 years or less. This means that there will be more frequent floods (**Figure 3**).

## **4.2 Integration of disciplines**

For effective management of floods, it is deemed that seemingly disparate disciplines that are associated with floods directly or indirectly should be integrated. These disciplines are: hydrometeorology, hydrology, hydraulics, agriculture, earth sciences, environmental sciences, socio-economic sciences, political and policy making, communication science, legal constraints, and administrative dimensions. These disciplines are on the flood process side. On the other hand, disciplines that provide tools for solving problems are mathematics, statistics, operations research, data science, geographical information systems, intelligent systems, and computer science. These disciplines should also be integrated with flood management.

### **4.3 Communication**

It is vitally important that agencies responsible for flood management communicate to the public as to why floods occur, likelihood of a flood in any given area, and roles and responsibilities associated with flood risk reduction and response. Needs of those people who unable to protect themselves are messages that are continually lacking to be conveyed. These messages do not "stick", nor last, so they have to be regularly repeated even to the same audiences.
