FLOOD

• Flooding


  The abnormal rise of water levels in rivers, coastal areas, plains, and in highly urbanized centers which may be a result of natural phenomena, human activities or both

  
• Flooding can be a natural process and can be the result of man's inadvertence
• Effects of Flood

  • Inconvenience for travelers
  • Damage to properties
  • Loss of lives

  
• Primary Flood Types and Causes

  • Riverine - when the level of water flowing through rivers increases and goes beyond the average water level, caused by extreme and sustained rainfall
  • Estuarine and Coastal - flooding occur when seawater encroaches low lying land that is usually still above sea level, can be a result of storm surges
  • Urban Flooding - usually occur in highly populated, developed areas set on relatively low lying areas like valleys and plains, due to saturation of the ground due to too much rainfall and the presence of large areas of impermeable surfaces like concrete pavements and rooftops
  • Catastrophic Flooding - result from ground failure or major infrastructure failure, caused by weakening of the rock or soil, liquefaction, and occurrence of landslide, or miscalculation of engineering design and thunderstorms
  • Flash Floods - a rapid, short-lived, and violent arrival of a large volume of water which can be caused by intense localize rainfall on land that is saturated or unstable to absorb water, may also occur due to collapse of infrastructure such as dams

  
• Storm Surge


  A localized unusual increase of sea water level beyond the predicted astronomical tide level due to intense winds and lowered atmospheric pressure during the passage of an intense tropical cyclone from the sea to the land
• Causes of Unusual High Tides

  • Astronomical High Tides - a short term sea level rise caused by the gravitational pull of the moon
  • Spring Tides - happen when the sun, earth, and moon aligned during new moon and full moon, cause water to bulge in the direction of the alignment, coastal areas experience extremely high tides and other areas experience extreme low tide
• When an astronomical high tides coincides with the occurrence of a tropical cyclone, flooding is aggravated and can cause more damage
• Proper Drainage System


  • Has a wide coverage
  • Has sufficient capacity to accommodate volumes of water based on predicted worst case flooding event scenario
  • Is not prone to blockage
• Causes of Flooding from Obstruction of Waterways

  • Solid waste
  • Industrial waste
  • Wrong locations of settlements

  
• Destructive Effects of Flooding

  • Primary Effects - direct interaction of humans and their property with flood waters, includes loss of lives, damage to amenities, loss of access to basic necessities, damage to infrastructure
  • Secondary Effects - short term, immediate, but indirect consequences of flooding, may include power loss, domestic and potable water loss or shortage, risk of acquiring waterborne diseases, damage to transportation facilities, and food shortage
  • Tertiary Effects - long term indirect consequences of flooding, economic hardships at the local and national levels and unemployment may result from destruction of production, and services-based business, decline in tourism in the affected areas, and additional government expenditures on rehabilitation of public infrastructure, may also affect the wildlife habitat in rural areas
• Flood Hazards Mitigation


  • Flood hazards assessment and mapping
  • Flood prediction
  • Flood forecasting and warning
  • Flood control engineering measures
  • Conduct flood vulnerability study
  • Long term development plans of communities

  
• Flood Forecasting


  1. Rank the discharge from 1-20, rank 1 shall be assigned to the maximum discharge in 20 years

  2. Compute the recurrence interval, R for each peak discharge by plugging in the values n (which is the number of years over which the entire data set was gathered) and m (the rank)

  3. Plot the recurrence interval (x – axis) using the graph provided, keep in mind that the x-axis follows a logarithmic scale

  4. Draw a best-fit line through the data points

  5. Do linear extrapolation by extending the trend line beyond the actual data plotted

  6. Calculate the probability a certain discharge value will occur in a year, called the annual exceedance probability (AEP), by getting the inverse of the recurrence interval

  
• A twenty year flood, therefore has an annual exceedance probability of 1/20 = 0.05 or 5%