TMJC 2025 JC1H2 C2 Lecture 9 Fluvial Floods

Higher 2 (9173) Tropical Environments TMJC C2 Lecture 9 River (Fluvial) Floods in the Tropics and Hydrographs 2025 Page 1 LECTURE OBJECTIVES • Discuss the human and physical factors that influence the occurrence of floods • Discuss the socio-economic and environmental effects of floods • Evaluate hard and soft engineering strategies to manage floods (A) Different Types of Flooding in the Tropics River (F luvial) Flood occurs when the actual discharge in a river channel exceeds its bankfull discharge (Fig. 1) resulting in high flow of water overtopping the banks of a river channel onto the surrounding floodplain. River floods occur when there is a sudden excess of input into the river channel which causes actual discharge to exceed bankfull discharge. Thus, river floods tend to occur during instances of prolonged and/or intense rainfall as well as when there has been abundant snowmelt. Figure 1: Hydrograph illustrating when river floods occur Coastal Flood occurs at coastal areas and it involves an area being inundated/flooded by seawater. This could occur due to storm surges from cyclones, or sea level rise from climate change. Naturally, these types of flood event take places near coastal areas. We will focus more on fluvial flooding for this cluster and notes. River Flooding

Page 2 (B) FACTORS THAT CAUSE RIVER FLOODS River floods occur when there is a SUDDEN EXCESS OF INPUT (precipitation) into the river channel which causes actual discharge to exceed bankfull discharge . Therefore, river floods may occur during instances of prolonged and/or intense rainfall as well as when there has been abundant snowmelt. 1) Prolonged and/or Intense Rainfall The main cause of most river floods is prolonged and/or intense rainfall . This is usually due to the occurrence of atmospheric processes such as monsoon rains or tropical cyclones. For example, in India, up to 70% of the annual rainfall occurs in 100 days during the summer south-west monsoon from June to September. This results in many instances of river floods during this period. Prolonged rainfall may cause the ground to become saturated and result in higher rates of 1saturation overland flow. On the other hand, intense rainfall may also result in flooding as the inten sity of rainfall may exceed the 2infiltration capacity of the ground and generate 3hortonian overland flow/infiltration excess flow (water flow horizontally across land surfaces when the rainfall has exceeded infiltration capacity). As overland flow travels at high speeds into the river channel compared to throughflow and baseflow, this causes the discharge in the channel to behave in a flashy manner, with short lag time and high peak discharge. Due to the higher level of actual discharge present, the river discharge may then overflow its banks and result in a flood occurring. 1 saturation overland flow refers to the water flow horizontally over land su