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 surfaces when the soil has become saturated with water 2 infiltration capacity refers to the maximum rate at which the soil allows water to infiltrate 3 hortonian overland flow/infiltration excess flow refers to the water flow horizontally over land surfaces when the rainfall has exceeded infiltration capacity 2) Abundant Snowfall Followed by Rapid Snowmelt A less common cause of river floods in the Tropics is the occurrence of abundant snowfall at HIGH ALTITUDES during winter and subsequently the rapid melting of accumulated snow during spring. The rapid melting of the accumulated snow generates a very large amount of water to flow into the river channels in a very short period of time and this may cause actual discharge to exceed bankfull discharge. Furthermore, as the ground may still be frozen and largely impermeable, the melted snow may largely flow as surface overland flow into the river. As surface overland flow travels at high speed 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. For example, in 2011, The Brahmaputra River in India experienced flooding due IN PART to the melting of large amount of snow from the previous winter as well as rainy weather during the summer.

Page 3 3) Extreme Weather Events: El Nino and Tropical Cyclones As we have learnt in previous lectures, extreme weather events such as El Nino and Tropical Cyclones can also lead to fluvial or coastal flooding. Tropical Cyclones can cause both types of flooding as the additional rainfall bought about by the spiralling rain bands can serve as additional input into the channel leading to fluvial flooding. It could also create storm surges that leads to coastal flooding. Similarly, an El Nino event will bring about addition al rainfall to places like Peru which could lead to fluvial flooding. (C) THE FACTORS BELOW DO NOT DIRECTLY CAUSE RIVER FLOODS TO OCCUR BUT INFLUENCE THE FREQUENCY (LIKELIHOOD) AND MAGNITUDE (EXENT) OF FLOODING. I. NATURAL FACTORS 1) Characteristics of the Drainage Basin The characteristics of the drainage basin that can influence the frequency and magnitude of flooding include the (a) extent of vegetation cover (b) relief (c) permeability of the ground surface (d) antecedent soil moisture (conditions of the soil preceding a storm event) (e) shape of drainage basin and (f) drainage density. The first four factors, namely, the extent of vegetation cover , relief, permeability of the ground surface and the amount of antecedent soil moisture influence the rate of infiltration taking place and the generation of surface runoff. The e xtent of vegetation cover and permeability of the ground surface have a positive relationship with infiltration rates while relief and antecedent soil moisture have a negative relationship with infiltration rates. Thus, these factors may influence the frequency and magnitude of flooding as lower infiltration rates will result in greater amounts of surface runoff. This causes a river to become flashier and may cause actual discharge to excee d bankfull discharge, thus increasing both the frequency as well as the magnitude of flooding.

Page 4 II. ANTHROPOGENIC (HUMAN) FACTORS that could influence/cause flooding Human activities can influence the frequency and magnitude of flooding by creating conditions in the drainage basin that reduce infiltration capacity and encourage overland flow, resulting in a flashy response in river discharge to a precipitation event and increases the frequency and magnitude of flooding. 1) Modification of Characteristics of the Drainage Basin Changes in land-use / surface cover refer to activities such as urbanisation, deforestation and agriculture. • Urbanisation leads to many impermeable surfaces such as tarmac and concrete, which provide minimal surface storage capacity and have low infiltration capacity. This results in more overland flow occurring during precipitation events. Furthermore, urban areas have elaborate drainage networks which remove water quickly and efficiently. Due to these reasons, river discharge will be flashier and increases the frequency and magnitude of flooding. • Deforestation increases overland flow as it reduces the amount of interception and evapotranspiration. Protected soil have much higher infiltration capacities than soil on barren land. Dense forests intercept raindrops and protect the soil from compaction by rain and deliver water at a more even rate for infiltration due to stemflow. This decreases the amount of overland flow that occurs. With deforestation, the amount of overland flow increase. Furthermore, with less evapotranspiration occurring, more water will end up in the river channel, potentially increasing the peak discharge, thus increasing the frequency and magnitude of flooding. •