flooding and flood mitigation Notes


Approaching the topics:

  • Topics covered in this chapter:
    • Defining concepts
    • Differentiating certain concepts


Storm hydrograph or flood hydrograph shows how a river channel responds to the key process of the hydrological cycle. It calculates the speed at which the rain falls on the drainage basin and reaches to the river channel.

2.1.1 Rising limb:

  • Shows the amount of discharge and the speed at which it is increasing.
  • Very steep in a flash flood or in small drainage basins where the response is rapid.
  • Generally steep in urbanised catchments

2.1.2 Peak flow or discharge:

  • Higher in larger basins
  • Steep catchment have lower infiltration rates
  • More through flow and lower peaks are in flat catchments with infiltration rates.

2.1.3. Lag time:

  • Time interval between peak rainfall and peak discharge
  • Influenced by basin shape, steepness, stream order.

2.1.4. Run-off curve:

  • Shows the relationship between overland flow and throughflow.
  • Overland flow when the infiltration is low, pre-existing moisture is high, surface impermeable and rainfall strong.

2.1.5. Base flow:

  • Seepage of groundwater into a channel- important where rocks have high pores.
  • a slow movement and the main long term supplier of the river’s discharge.

2.1.6. Recessional limb:

  • Influenced by geological composition and behaviour of the local aquifers.
  • Larger catchments have less-steep recessional limbs.

2.1.7. Hydrograph size[area under the graph]:

  • The higher the rainfall, the greater the discharge
  • The larger the basin size, the greater the discharge.

2.2. The influence of geology:

Geology in everyday life is not restricted to resources. Rock falls, radon, landslides, quick clay, earthquakes, and floods are also covered in this section. Topography assists us with understanding environmental change before, which might assist us with foreseeing future situations.

2.2.1. Influence of geology on flood response: Water can pass through cracks and pores in permeable rocks, whereas impermeable rocks cannot. An increase in surface run-off increases the likelihood of flooding in a valley containing impermeable rocks.

2.2.2. River regimes- the influence of seasonality: Perennial Rivers are the rivers which has a constant stream throughout the year. Seasonal rivers, on the other hand, are those that only flow during rainy seasons and remain dry.Variations on a river’s flow depend on many factors such as:

  • The amount and nature of precipitation
  • Seasonal variations in the temperature and evapotranspiration.
  • Change in the vegetation cover.
  • Variations in rock types, soil types and the shape and size of the drainage basin.

2.3. Factors affecting flood risk

  • Physical factors:
    • Prolonged rainfall: Soil becomes saturated after prolonged rainfall. This leads to an increase in surface run-off as rainfall can no longer infiltrate the soil. This leads to more water entering the river channel and increases the likelihood of flooding.
    • Heavy rainfall: When there is a lot of rain, water may arrive too quickly to penetrate the soil. As a result, surface runoff rises, making it easier for water to enter the river channel and increasing the likelihood of flooding.
    • Geology: Surfaces that are impermeable, like granite and clay, prevent water from entering, which results in more surface run-off. As water rapidly enters the river channel, it discharges more water and increases the risk of flooding.
    • Relief: The rate at which water enters a river channel increases with slope, increasing the likelihood of flooding.
  • Human factors:
    • Building: Buildings and the roads and paths that surround them create impermeable surfaces, which increase surface runoff. Additionally, drains increase the flow of water into nearby rivers, raising the risk of flooding.
    • Deforestation: Rainfall is slowed by vegetation, including trees, intercepting it. Before evaporation returns moisture to the atmosphere, some of this water is stored.Trees also take in water from the soil, allowing for more soil infiltration and less surface runoff. Surface run-off rises and infiltration and interception decrease when vegetation is removed. This prompts a more serious gamble of flooding as more water arrives at the stream channel.

2.4. The hydrological impact of urbanisation:

  • Urban hydrographs have:
    i. a shorter lag time
    ii. a steeper rising limb
    iii. a higher peak flow
    iv. a steeper recessional limb.
  • Urbanisation increases the magnitude and frequency of flood in 3 ways:
    i. Creation of highly impermeable surfaces such as road, roof, pavements.
    ii. Smooth surfaces served with a dense network of drains, gutters and underground sewers increase drainage density.
    iii. Bridge supports or riverside structures frequently constrict natural river channels, reducing their carrying capacity.
  • Urbanisation impacts the hydrological process in many ways:
    i. Increased erosion due to more water getting into rivers
    ii. Increases speed of flow and transport of materials due to enlarged channels.
    iii. Less erosion due to riverbank protection schemes.

2.5. Deforestation:

Deforestation is the process by which large swaths of forest land and associated ecosystems are cleared by humans for non-forest uses.These include clearing land for ranching, farming, and use in cities. Trees are never replanted in these instances. Millions of animals and other living things have been put in danger since the industrial revolution, which has resulted in the loss of approximately half of the world’s original forests.

2.5.1. Effects of deforestation: About 2,000 trees per minute
are cut down in the rainforests. Rather shockingly, 36 million acres of natural forest are lost each year. This is worrying and very serious.

  • Soil erosion destruction: The sun’s heat is applied to the soil and its nutrients. Nutrients are lost, soil moisture is reduced, and bacteria that aid in the breakdown of organic matter are affected. Erosion eventually occurs when rain falls on the soil surfaces. Soils never get their maximum capacity back.
  • Water Cycle: The atmosphere, water bodies, and water table are all affected when forests are cut down. The roots of trees take in and store water. The plants themselves hold a significant amount of the water that moves through the ecosystem of rainforests. A portion of this water evaporates into the surrounding air. The atmosphere and water bodies begin to dry out when this process is broken. Because of this, less water will flow through the rivers and the watershed’s potential is diminished. The smaller lakes and streams that draw water from these larger bodies of water eventually run dry.
  • Loss of Biodiversity: Numerous wonderful animal and plant species have vanished, and numerous others are still in danger. The Tropical Rainforest is still home to more than 80 percent of the world’s species. The loss of their natural habitats causes the extinction of approximately 50 to 100 animal species each day, which is tragic. There have been many beautiful plants and animals that have vanished from the planet.
  • Climate Change: Carbon Dioxide CO2, a greenhouse gas, is taken in by plants and used to make food (the carbohydrates, fats, and proteins in trees). It releases oxygen in return. CO2 will remain in the atmosphere as a result of forest destruction, and the decomposed vegetation will also release additional CO2 that has been stored in it. The climate in that area will be affected by this. Cool environments may get significantly warmer and hot spots might get much cooler.

2.5.2. Result of deforestation on flood:

2.6. Channel modification:

It includes straightening, enlarging, and channelizing. It could open up new channels. the act of straightening or rerouting natural streams in a stream bed that has been artificially altered or constructed. Channelization has been completed for various reasons, most frequently to deplete wetlands, direct water streams for horticultural use, and control flooding. Through the construction of levees, larger channels help rivers carry more water, resulting in high peak flows. The goal is to prevent flooding by draining water from an area. If the channel is covered, it may alter processes.

  • Widening and deepening the channels will increase the capacity of the river.
  • Straightening of the channel speeds up the flow and removes extra water from the area.
  • Scouring i.e. removing sediments from the river beds, will allow carrying large amounts of water which results in low risk of flood.

2.7. Flood mitigation:

Flood mitigation involves managing and controlling the movement of flood water, such as redirecting flood run-off through the use of floodwalls and flood gates, rather than trying to completely prevent floods.It additionally includes the administration of individuals, through measures like clearing and dry/wet sealing properties. There are three levels of investigation into flooding mitigation and prevention: on individual properties, small towns, and entire cities or towns. As more people and assets are safeguarded, protection costs rise.

2.8. Dams:

Dam, structure built across a stream, a river, or an estuary to retain water. Dams are built to provide water for human consumption, for irrigating arid and semiarid lands, or for use in industrial processes

2.9. Afforestation:

Flood mitigation involves managing and controlling the movement of flood water, such as redirecting flood run-off through the use of floodwalls and flood gates, rather than trying to completely prevent floods.

2.9.1. Flood Abatement:

2.9.2. Afforestation helps in reducing flood risk: As part of afforestation, planting trees in a drainage basin helps to reduce surface runoff while also increasing intercept and storage. As a result, a river is less likely to flood because its discharge is reduced. Additionally, mass wasting is prevented by afforestation, which reduces the amount of soil that enters the river and maintains the river’s high capacity. Afforestation has the potential to significantly lessen the likelihood of flooding when used in conjunction with floodplain zoning.

2.9.3. Flood abatement: It involves decreasing the amount of run-off and reducing the flood peak in a drainage basin.

2.10. Channel modification:

It includes:

  • Raising the banks
  • Straightening the river
  • Creating new channels
  • Making artificial levees

2.11. Planning:

It includes having personal insurance for being prepared for flood. Sandbags are used to protect homes, sealing doors and windows, moving the valuables upstairs etc. Disaster aids are given to the community following the disaster.

2.12. Attempts at flood prediction and forecasting:

Depending on the size of the watershed or river basin, flood forecasting is the use of forecasted precipitation and stream flow data in rainfall-runoff and stream flow routing models to predict flow rates and water levels for a few hours to days in the future. Using these forecasts to make decisions about flood warnings is known as “flood warning.” Utilising the technology of artificial neural networks, real-time flooding forecasting at regional areas can be completed in a matter of seconds. Models for accurate real-time flood forecasting could be useful for disaster prevention and early warning.

2.12.1. Ways to improve flood warning:

  • Tipping Bucket Rain Gauge: Consisting of a funnel and a small container affixed to
    a tipping lever, rain gauges collect a set amount of precipitation before the container tips, dumping out any collected water and
    sending an electrical signal to a data transmitter.
  • Integrated Data Logging System: A real-time monitoring station that houses the data logger, telemetry module, and power/charging supply is an integrated data logging system. Because running AC power to the monitoring location typically costs too much, integrated solar panels are used to continuously charge the 12VDC battery for autonomous operation.
  • Mounting Hardware: Data loggers can be attached to a bridge pole or directly to an abutment, depending on the recommended monitoring plan and location.
  • Radar Water Level Sensor:Radar-based water level sensors offer a non-contact alternative to submersible pressure transducers and other level gauging methods, making it possible to monitor in difficult-to-reach locations.
  • Telemetry: Data can be accessed in real time through telemetry. ALERT uses radio frequencies to send wireless communications, but there are also cellular and satellite-based options.
  • Live Data: Through a cloud-based data centre, project data can be accessed immediately and around the clock. In order to identify trends, monitoring data can be viewed in real time or as a graph. When indicated boundaries exceed predefined limits, continuous mechanised cautions can be sent via email or message.