THE ROAR OF ATLANTIS: FLOODING

  Agbor Taku Junior    April 28, 2019    0

UNESCO International Glossary of Hydrology defines “Flood” as follows;

  • Rise, usually brief, in the water level in a stream to a peak from which the water level recedes at a slower rate.
  • Relatively high flow as measured by stage height or discharge.
  • Rising tide.

The Glossary also defines flooding as an overflowing by water of the normal confines of a stream or other body of water, or accumulation of water by drainage over areas that are not normally submerged.



Types and Causes of floods

flooding1

flooding2

 

CAUSES OF FLOODING

These different types of flooding can be caused by the following factors;

  • RAIN: When there is more rain than the drainage system can take there can be floods. Sometimes heavy rains for a very short period result in floods and light rains for many days and weeks can result in floods.
  • DAM FAILURE: A dam triggered by an earthquake would result in flooding of the downstream
  • STRONG WINDS IN COASTAL AREAS: sea water can be carried by massive winds in hurricanes onto dry coastal lands and cause flooding. Sometimes it is made worst if the winds carry the rain themselves
  • STEEP SIDED CHANNELS: A river channel surrounded by steep slopes causes fast surface runoff into the streams which then intend lead to rapid over flooding of the banks as floods.
  • VEGETATION: The presence of woodland or vegetation increases the infiltration rate of surface water and also reduces the rate of surface runoff into streams. Thus the absence of it increases the probability of flooding to occur.
  • Drainage basins in an urban area consist largely of impermeable concrete, which encourages overland flow. Drains and sewers take water quickly and directly to the river channel.
  • Blockages in river channels
  • Population pressure as increasing number of people, especially the poor, is settling in flood-prone areas.
  • Excessive levels of precipitation, melting of ices or a combination of these two are the major causes of river floods.
  • Tsunamis produced by earthquakes, and river ponding behind natural damps caused by mass movement and glacial advances can also result in flooding

 

EFFECTS OF FLOODING

  • Flooding can be very dangerous – only 15cms of fast-flowing water are needed to knock you off your feet!
  • Floodwater can seriously disrupt public and personal transport by cutting off roads and railway lines, as well as communication links when telephone lines are damaged.
  • Floods disrupt normal drainage systems in cities, and sewage spills are common, which represents a serious health hazard, along with standing water and wet materials in the home.
  • Bacteria, mould and viruses, cause disease, trigger allergic reactions, and continue to damage materials long after a flood.
  • Floods can distribute large amounts of water and suspended sediment over vast areas, restocking valuable soil nutrients to agricultural lands.
  • In contrast, soil can be eroded by large amounts of fast flowing water, ruining crops, destroying agricultural land / buildings and drowning farm animals.
  • Floodwater can severely disrupt public and personal transport by cutting off roads and railway lines, as well as communication links when telephone lines are damaged.
  • Unfortunately, flooding not only disrupts many people’s lives each year, but it frequently creates personal tragedies when people are swept away and drowned.

 

PROBABILITY OF FLOOD OCCURRENCE

A sound understanding of the likelihood of occurrence of a flood hazard is a fundamental step in dealing with flood risk. Risk from flooding can be conceptualized into four stages as in Figure below:

flooding3

To fully evaluate risk, the degree of exposure and the nature of exposed receptors and their potential
to sustain or resist damage also need to be considered.

.Flood forecasts for a natural drainage area or a city are usually obtained by analyzing the past occurrence of flooding events, determining their recurrence intervals, and then using this information to extrapolate to future probabilities.

This common approach is described below in simplified form for fluvial flooding. The probability of occurrence for pluvial, groundwater, flash, and semi-permanent floods is much more difficult to estimate, even if historical data is available. This is due to the fact that the causes of these types of floods are, as seen above, a combination of a meteorological event such  as heavy rainfall and other factors such as insufficient drainage capacity, mismanagement of key infrastructure and other human factors. In the case of coastal floods caused by seismic activities, predicting their probability is as difficult as predicting the occurrence of an earthquake. For coastal floods caused by storms or hurricanes, their probability of occurrence can, in principle, be computed using historical data or numerical simulations of key variables such as wind speed, sea level, river flow and rainfall.



FREQUENCY OF OCCURRENCE

The recurrence interval or return period is defined as the average time between events of a given magnitude assuming that different events are random. The recurrence interval or return period of floods of different heights varies from catchment to catchment, depending on various factors such as the climate of the region, the width of the floodplain and the size of the channel. In a dry climate the recurrence interval of a three meter height flood might be much longer than in a region that gets regular heavy rainfall. Therefore the recurrence interval is specific to a particular river catchment. Once the recurrence intervals are determined based on the historical record, some assumption about the flood frequency distribution has to be made in order to extrapolate or interpolate to events that have not been recorded historically. To achieve this, an assumption about the distribution of flood frequency has to be made. In this way the recurrence interval for any discharge (and not just those
present in the observational record) can be inferred.

For any discharge, or alternatively, any recurrence period, the probability of occurrence is the inverse of the return period p=1/T

The figure below shows the trend of flood events around the world since 1950.

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Figure: Number of reported flood events between 1950 and 2011 (Source: EM-DAT/CRED, v. 12.07)

 







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