SAJC Cluster 2 Lecture 7 Notes (Factors Contributing to Fluvial Floods)

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 7_2024/pg1 Lecture 7 Floods in the Humid Tropics (I): Factors contributing to Fluvial Floods KEY QUESTIONS: ✓ What factors contribute to fluvial flooding in the humid tropics? With the completion of this lecture, attached readings and tutorial, you should be able to understand the: • Occurrence of fluvial floods when peak discharge exceeds bankfull discharge and inundates adjacent areas that are usually dry • Natural and human factors contributing to occurrence of fluvial floods in the humid tropics Lecture Outline 7.1 Introduction: Floods and related terms 7.2 Causes of Fluvial Floods 7.2.1 Natural factors contributing to fluvial floods. (a) Excessive Rainfall (b) Snow and Ice-melt (c) Features of the drainage basin Box 1: Monsoons, Tropical Cyclones and Flooding in the Humid Tropics 7.2.2 Human factors contributing to fluvial intensify floods. (a) Urbanisation (b) Deforestation (c) Agriculture Box 2: Flooding in KL – caused by human or natural factors? Reading: (1) The 2022 Pakistan floods Natural factors Input (rainfall and/or snowmelt) Drainage basin features (geology & soil, topography, vegetation) Factors contributing to fluvial floods Human factors Urbanisation Deforestation Agriculture

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 7_2024/pg2 7.1 Introduction: Floods and related terms • Floods are the most frequent type of natural disaster. This is because of the widespread distribution of river floodplains (see Fig. 1) and low-lying coasts and their long-standing attractions for human settlement. • Floods can cause widespread devastation, resulting in loss of life and damages to personal property and critical public health infrastructure. o According to the WHO, between 1998-2017, floods affected more than 2 billion people worldwide. People who live in floodplains or non-resistant buildings, or lack warning systems and awareness of flooding hazard, are most vulnerable to floods. • There are three common types of floods, of which, for the purpose of this lecture, we shall focus only on fluvial (or river) floods. o Fluvial floods are caused when consistent rain or snow melt forces a river to exceed capacity. (see more later) o Flash floods are caused by rapid and excessive rainfall that raises water heights quickly, and rivers, streams, channels or roads may be overtaken. o Coastal floods are caused by storm surges associated with tropical cyclones and tsunami. • A fluvial flood occurs when river discharge exceeds bankfull discharge, leading to overbank spillage and inundation of land which is not normally submerged. Fig. 1 Flood and Floodplain • Most of the time, a river’s water and sediment load are transported in the river channel itself. The flat, low-lying area adjacent to many river channels that is subjected to recurrent flooding is a floodplain (see Fig. 1). The function of the floodplain is to act as a temporary store of discharge and alluvium at times when the river overflows its channel. (Alluvium: material deposited by rivers – it consists of silt, sand, clay, and gravel and often contains a good deal of organic matter, thus yielding fertile soils) o Despite the continued risks of living in a flood-prone area, many of the world’s floodplains have high population densities. Floodplains provide areas of flat, fertile land with a ready water supply and are thus ideal locations for intensive agriculture, industrial activities, and urban development. floodplain bankfull stage floodplain

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 7_2024/pg3 o Examples of densely populated floodplains include the Ganges in India and Bangladesh, and Nile in Egypt. These human activities on vulnerable floodplains which are naturally prone to flooding place lives and property at risk during floods. • Flood frequency refers to how often an area will experience flood, while flood magnitude refers to the impact and intensity of a flood event. 7.2 Causes of Fluvial Floods • Most floods are a result of overland flow and rapid throughflow (see Fig. 2). These produce a rise in discharge which can be reflected in a hydrograph (see Fig. 3). • The relative importance of quickflow processes (i.e. overland flow and rapid throughflow) and slowflow (i.e. slow throughflow and baseflow) processes are dependent on a range of factors, both physical and human (recall Lect 6). • Physical factors which result in excess input of water cause river floods, while physical and human factors that alter the characteristics of the drainage basin can intensify the occurrence of floods. In sum, flooding is caused by the excessive input of precipitation and exacerbated by human activities. 7.2.1 Natural factors that contribute to fluvial flooding • Flood occurrence is mainly determined by the interaction between the input of precipitation and the features of the drainage basin, such as surface conditions, topography and vegetation. Fig. 2 Fig. 3 Natural factors contributing to fluvial floods Input (rainfall and/or snowmelt) Drainage basin features (geology & soil, topography, vegetation)

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 7_2024/pg4 (a) Excessive Rainfall • Atmospheric extremes, especially excessive rainfalls, are the most common cause of floods. Unsurprisingly so as this is the main input pathway of most drainage basin especially in the humid tropics. • As we have learnt in Lect 6 Section 6.2.1(b)(c)(d), seasonality, amount, intensity and duration of rainfall are all capable of generating high volumes of stormflow (i.e. overland flow and rapid throughflow). • They vary from semi-predictable seasonal rains over wide geographic areas, which gives rise to the annual wet season floods in tropical areas, to almost random convectional storms over small basins. See Box 1 for a more detailed write-up. Box 1: Monsoons, Tropical Cyclones and Flooding in the Humid Tropics The frequency of high magnitude rain events is high in the humid tropics (especially Af and Am). Monsoon (see Lect 4) and tropical cyclones (also Lect 4 ) in particular bring periods of high magnitude, prolonged and/or intense rainfall and increase the likelihood of floods. • In the case of prolonged rainfall (for instance during wet monsoon months), high antecedent moisture may mean that soil moisture and groundwater storages are already full. Subsequent rainfall on saturated soil generates SOF on the surface, which flows quickly to the channel. Water stored in soil moisture and groundwater storages will also be continually released to the channel as throughflow and baseflow, keeping discharge level high and making it easier for river discharge to exceed bankfull discharge, thus causing flood. o For example, high magnitude and prolonged monsoon rains in August 2022 (also described by the UN Chief as “a monsoon on steroids”) led to severe flooding of the Indus River and its many tributaries in Pakistan, affecting more than 33 million Pakistanis. (See Reading 1) • During high magnitude and high intensity rainstorms in the humid tropics (for example, brought by tropical cyclones), rainfall intensity is likely to exceed the infiltration capacity of the surface, especially in urbanised or deforested