SAJC Cluster 2 Lecture 8 Notes (Impacts of Fluvial Floods on Places)

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 8_2024/pg1 Lecture 8 Floods in the Humid Tropics (II): Impacts of Fluvial Floods on Places KEY QUESTION: ✓ Why do fluvial floods affect places and people differently? With the completion of this lecture, attached readings and tutorial, you should be able to understand the: • Impacts of fluvial floods on places • Variations in the effects of fluvial floods on places • Variations in effects of fluvial floods experienced by different groups of people due to differences in their vulnerability to fluvial floods Lecture Outline 8.1 Introduction: Impacts of Fluvial Floods 8.2 Positive Impacts 8.2.1 Increase soil fertility 8.2.2 Recharge ground water storage 8.3 Negative Impacts Economic 8.3.1 Property damages and economic losses Environmental 8.3.2 Extreme floods cause erosion Environmental and Economic 8.3.3 Destruction of agricultural land Social 8.3.4 Loss of lives, and post-trauma stress 8.3.5 Diseases 8.4 Different Groups of People, Different Vulnerabilities

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 8_2024/pg2 8.1 Introduction: Impacts of Fluvial Floods • Fluvial floods, often triggered by heavy rainfall and snowmelt, have profound and multifaceted impacts on both natural environments and human communities. • The consequences of fluvial floods encompass a wide spectrum, ranging from immediate and devastating damage to infrastructure, property, and livelihoods, to more intricate ecological effects such as soil erosion, sediment deposition, and alterations to aquatic ecosystems. • As the frequency and intensity of extreme weather events continue to rise due to climate change, understanding and mitigating the impacts of fluvial floods have become essential endeavors for sustainable development and disaster resilience. 8.2 Positive impacts • Fluvial floods, though often associated with destruction and disruption, can also bring about a range of positive impacts that contribute to the ecological health and vitality of river systems and surrounding landscapes. 8.2.1 Increase soil fertility • Perhaps, one of the most recognised benefits of flooding is that it makes the soil fertile. o As the flood water eventually recedes, it leaves behind fine sand, clay, silt and organic matter. Floodwaters transport nutrients and sediments to floodplains, which nourish the soil. This aids in the distribution and deposition of river sediments across broad floodplains. o The nutrients in the topsoil are replenished by these river sediments, making agriculture more productive. Since recurrent flooding resulted in fertile, productive farmland, many ancient civilisations thrived on the banks of the Euphrates, Ganges, Nile (Fig. 1), Tigris and Yangtze. Even today, many agricultural systems on floodplains are in harmony with low- magnitude, high frequency floods. Fig. 1 A view of the Nile River. For thousands of years, the lower reaches of the Nile flooded annually during August and September. This resulted from heavy rainfall in the Ethiopian Highlands in the Blue Nile’s upper course in July and June. Each year as the river flooded it deposited fertile silt on the floodplain, vital to farmers. This was particularly important in the Nile delta, which has some of Africa’s most productive farmland. In addition, the Nile is important for transport and as a source of water for domestic and industrial uses.

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 8_2024/pg3 8.2.2 Recharge ground water storage • Flooding is important for recharging underground water sources. Both extreme, rare floods and small floods occurring seasonally lead to increased water volume in groundwater stores (recall Lect 5). • Recharge that results from flooding is especially beneficial during dry seasons when groundwater is the main source of freshwater and baseflow to rivers in areas that experience pronounced wet and dry seasons. o An example is Cambodia in the lower Mekong River basin, where 85% of its precipitation falls during the wet southwest monsoon season. Flood control which reduces the area of inundation in the Mekong River basin in Cambodia in turn results in a reduction of groundwater resources in the area. In 1998, a 44% reduction in inundation areas led to a 42% reduction in groundwater storage. 8.3 Negative impacts • Human inhabitation of floodplain locations, including higher risk areas, has increased in the 20th century. This is due to urban development, population pressure, and more road and rail routes. Thus, people have increasingly become exposed to the flood risk and the hazard has increased in relative importance without an actual increase in flood events. o Impacts can be severe at all levels of economic development. However, flood impacts follow the pattern of other hazards, with developing countries suffering the most deaths and developed countries the highest economic loss. o Research suggests that developed countries suffer only 5% of fatalities from floods, but 75% of the costs from natural hazards, whereas the developing countries suffer 95% of the deaths and 25% of the costs. Economic 8.3.1 Property damages and economic losses • The degree of flood hazard is dependent on factors such as the depth and velocity of the water, the duration of the flood and the load carried. Fig. 2 shows some approximate thresholds for depth and velocity. Fig. 2

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 8_2024/pg4 o People and cars can be washed away in about 0.5 m of fast-flowing water. Buildings, and other obstructions, create turbulent scour effects and many buildings start to fail at velocities 2 m/s. • The physical stresses on structures are greatly increased when rapidly flowing water contains debris such as rock, sediment or ice. The consequent collapse of the sewerage systems and storage facilities for products like oil or chemicals means that flood waters create pollution and other hazards. • The high energy floodwaters can undercut and erode the foundation of buildings, bridges structures and levees, causing their collapse or leaving them in need of substantial repair. The destruction of houses would lead to homelessness. • When the floodwaters retreat, velocity is generally much lower and sediment is deposited. After the retreat, everything is usually covered with a thick layer of stream deposited mud, including the interior of buildings. This damages buildings and incurs a huge cost of cleaning and repair. • The impacts of flooding on property and lives can be seen in these examples: o In the 2016 Colombo floods, the Kelani Ganga River basin (Fig. 3) received 350 mm of total rainfall within 3 days across the whole river basin (equal to one month’s precipitation) which caused the following damages: ➢ Approximately 2,343 sq. km. of land went under water. ➢ Flood have displaced more than 300,000 people across the island with at least 58 left dead and a further 130 missing. ➢ Houses in the localities were submerged 6 to 10 feet under water. More than 125,000 houses were damaged or destroyed. ➢ Major damage occurred to roads, plantations and crop lands. ➢ Colombo floods expected to cost at least $1.5 billion. Fig.