SAJC Cluster 2 Lecture 9 Notes (Fluvial Flood Management Strategies)

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 9_2024/pg1 Lecture 9 Floods in the Humid Tropics (III): Fluvial Flood Management Strategies KEY QUESTIONS: ✓ Can fluvial floods be effectively managed? With the completion of this lecture, attached readings and tutorial, you should be able to understand the: • Strengths and limitations of strategies to manage fluvial floods • Varying success of strategies to manage fluvial floods • Extent to which humans can control natural phenomenon such as fluvial floods Lecture Outline 9.1 Introduction: Categorising Flood Management Strategies 9.2 Prediction 9.2.1 Recurrence intervals 9.2.2 Forecasting and warning Box 1: Flood forecasting in India 9.3 Mitigation 9.3.1 Hard-engineering Strategies (a) Levees (or Dykes) (b) Dams (c) Channelisation (i) Channel resectioning (ii) Channel realignment 9.3.2 Soft-engineering Strategies (a) Land Use Planning and Zoning (b) Dredging (c) Reforestation of Channel Banks 9.4 Responses 9.4.1 Disaster aid 9.4.2 Community preparedness

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 9_2024/pg2 9.1 Introduction: Categorising Flood Management Strategies • The impacts of floods can be managed through broadly three categories of strategies, and each of these categories serve different purposes. Ideally they should be adopted together to provide a more comprehensive approach to help prepare for the adverse impacts of floods. o Prediction: These involve the use of scientific methods, historical data, and meteorological information to forecast the occurrence, extent, and severity of flooding events. The primary goal of flood prediction is to offer early warnings, allowing people to take precautionary measures, evacuate if necessary, and prepare for the flood event before it happens. (See Section 9.2) o Mitigation: These focuses on reducing the impact of flooding events by implementing strategies and measures that minimise the extent of damage and disruption caused by floods. Unlike flood prediction, flood mitigation addresses the long-term aspect of flood risk reduction. (See Section 9.3) o Responses: These are often the implemention of pre-planned activities, usually during and after flooding to reduce the adverse impacts of flooding on the population and property at risk. (See Section 9.4) 9.2 Prediction 9.2.1 Recurrence Intervals • The magnitude of a flood and how frequent it can occur is of interest to us. • We can calculate the likelihood or statistical probability of flooding from flood frequency graphs, or flood recurrence interval graphs (see Fig. 1). Usually, the relationship between magnitude and frequency of flood is inverse. High magnitude floods are less frequent and vice versa. • Using these graphs, we can make statements about the statistical probability of flood events. The longer and fuller the flood records, the more confidence we can place in this extrapolation. Thus, in Fig. 1, a low magnitude flood with 100 m3/s discharge will occur more frequently i.e. after every 10 years, whereas a high magnitude flood with 200 m3/s discharge occurs less frequently i.e. after every 50 years. • However, the statistical prediction of flood recurrence intervals is only an indication of probability based upon past records. o Drainage basins do not adhere to probability theory. A so-called 50-year flood may not necessarily occur only once in 50 years afterall. o Past records may not stretch over a period long enough. A 50-year record is more precise than a 10-year record, especially if we want to predict a 10-year flood. Furthermore, a 50- year record may not be sufficient to predict a 100-year flood. o As events happen, the statistical prediction will chance. For example, changes in landuse may alter the probability of floods of a particular magnitude. Fig. 1

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 9_2024/pg3 9.2.2 Forecasting and Warning • Flood forecasting and warning schemes exist widely and are most effective for large rivers, such as the Mekong. • Increasingly, technology has allowed meteorological and discharge data to be collected more effectively. Computer modeling enables scientists to predict how individual river systems will react to precipitation inputs, for instance. • Improved communications technology has also enabled more efficient dissemination of warnings to the public (see Fig. 2). Box 1: Flood forecasting in India • Flood forecasting has been recognised as one of the most important, reliable and cost- effective non-structural measures for flood management. • The Central Water Commission (CWC) under the Ministry of Water Resources has established a network of forecasting stations along flood-prone interstate rivers. These forecasts aid local administrations in evacuating flood -affected areas and reservoir operators in managing water levels. • In 2016, 6239 flood forecasts were issued, with 95.34% accuracy. CWC operates flood forecasting at 199 stations in India, using rainfall -runoff modeling and IMD rainfall forecasts to provide three-day advisories, contributing to timely disaster management and property protection. 9.3 Mitigation • These steps involve trying to lengthen the amount of time it takes for water to reach the river channel (thereby increasing the lag time), confine floodwater to river channels, divert water to temporary storages, or facilitate quicker flow away from places to be protected. Flood management techniques can be divided into hard- and soft-engineering options. 9.3.1 Hard-engineering Strategies • Hard-engineering strategies act as a barrier between the river and the surrounding land. Artificial structures are used to change or disrupt natural processes. Hard options tend to be more expensive and have a greater impact on the river and the surrounding landscape. Examples of hard engineering strategies include artificial embankments or levees, dams and channelisation. Fig. 2

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 9_2024/pg4 (a) Levees (or Dykes) • A levee is an embankment constructed by humans, along a river (on its banks) to prevent the flooding of lands adjacent to the river. These are common form of flood control (see Fig. 3). • Under natural conditions, water floods out of the channel and is stored in the floodplain, and the discharge is reduced downstream. The flood waters contained within the levees continue to flow rapidly downstream. • Well-maintained levees are an effective management approach within the flood size that they are designed to contain. However, if levees fail, the area inundated will be more severely affected as water pours through the breach. The results can be disastrous and the flood event made worse. • Levees thus must be maintained as they decay and deteriorate over time, and so require comprehensive maintenance programmes, which become a challenge as the systems age. Maintenance is also a costly affair. (b) Dams • Dams are structures that are built across the channel in order to store water and regulate the discharge of the river. They offer the temporary storage of water so that the flood peaks downstream can be reduced (see Fig. 4). Fig. 3 Fig. 4

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