Introduction to disaster management - Arjumand Zaidi

Introduction to disaster management - Arjumand Zaidi

Hazards Planning and Risk Management Lecture No. 23 & 24 Floods Fall 2017

US Pakistan Center for Advanced Studies in Water Floods What are they? When do they occur? Why do they occur?

Learning Objective Consequences of their occurrence? 2 How they can be managed?

Reading Material Dr. Cees Westen (ITC) Guide Book 03 (pdf provided at wordpress) FEMA document available at s.pdf

FLOODS Water: Life or Disaster? Flood Hydrologic hazard When water covers previously dry areas Deviation from the mean Exceeds some critical threshold beyond the normal band of

tolerance, the flow/rainfall then becomes a hazard Major Classes of Floods by FEMA Riverine floods (including overbank and flash floods) Fluctuating lake levels Local drainage or high ground water levels Coastal flooding and erosion

Characteristics of floods Peak depth of inundation determines the extent and cost of damage to buildings and crops and the cost and feasibility of mitigation measures Areal extent of inundation Duration of flooding Rate of rise of the flood event

Velocity of flood flow Frequency of flooding Seasonality of flooding Riverine Floods Any relatively high water flow that overtops the natural or artificial banks in any portion of a river or stream - when a bank is overtopped, the water

spreads over the flood plain and generally becomes a hazard to society (CEOS) Most devastating in terms of lives lost and property damaged Greatest challenge to weather prediction Causes or Triggering Factors The identification of the triggering factors is one of

the first steps in hazard assessment Floods may be caused by a number of factors including: heavy rainfall, melting snow, an obstruction of a natural waterway, other generative factors

Other Cause - Urbanization Added conditions to the natural causes of flooding are: Increase in impervious surface in the urbanizing area Failure to upgrade storm water management facilities to meet the needs of ongoing development. Lack of maintenance of storm-water management facilities Encroachments on floodplains

Frequency and Magnitude Floods are described in terms of their extent Horizontal area affected Vertical depth of flood water Also in terms of probability of occurrence Expresses in percentages as a chance of a flood of a specific extent occurring in any given year

Measurement of Flood Measured with respect to their elevation above standard water levels This elevation is translated into the annualized likelihood of reaching such heights 100-year flood event: flood depth that has a 1 percent chance of being reached or could be

expected to occur once across a 100-year period River and stream gauges to monitor floodwater elevations This information is useful for early warning and evacuation Flood Frequency BASE FLOOD

Disaster Risk for Floods Absolute mortality risk average annual expected mortality, Relative mortality risk measured as the average annual expected number of deaths as a proportion of national population

Top 10 countries on Flood Mortality Risk Index India (7.5), Bangladesh (6.5), China (6), Viet Nam(6), Cambodia (6), Myanmar (5.5) Sudan (5.5),

Democratic Peoples Republic of Korea (5.5), Afghanistan (5), Pakistan (5). Impacts Are Impacts always Negative? Negative Impacts

Floods are capable of undermining buildings and bridges, eroding shorelines riverbanks, tearing out trees, washing out access routes, and causing loss of life and injuries Social Impacts

Economic and emotional impact on people Social vulnerability depends on factors such as population trends and age Example: Physical and Economic Impacts Impact on Buildings (structures that could be damaged) Economic vulnerability relates to the extent of dollar exposure of building

Environmental Impacts Floods can be environmentally important to local ecosystems River floods bring nutrients to soil such (annual flooding of Nile River, Egypt carries nutrients to otherwise dry land) Forecasting Natural hazards caused by exogenic factors have more probability to be predicted than hazards by endogenic factors (earthquake, volcanoes and

tsunamis) Rainfall can be considered flood triggering factors and a precursor of flood hazards Discharge (Q) = C * I * A Rational Equation Where C is runoff coefficient; I intensity of rainfall (in/hr) and A is area (acre) Intensity Duration Frequency Curve

Forecastin g Forecasting Remotely sensed data, hydrologic models, and GIS techniques can be combined to simulate potential flooding. There is a delay between the occurrence of rainfall and the flood wave arrival

For very small watersheds, this delay may be a matter of minutes or a few hours; on large river basins it may be several weeks and in extreme examples, as on very long, low-gradient inland rivers, several months Factors Affecting Flow and Timing of Flow Climatic Characteristics Rainfall intensity: higher intensity storm produces rapid rise in hydrograph and higher peak

Duration: important when duration is more than time of concentration Temporal Distribution: in summer greater losses- lesser peak, in winter vice versa (also in winter soil moisture is high producing more runoff) Spatial distribution Factors Affecting Flow and Timing of Flow

Catchment Characteristics Size Shape Elevation Slope Drainage density and topology Soil Type and land use

Flood Extent and Depth Flood Forecasting Flood frequency curves Weather Forecast Information National Meteorological Centers Tropical Rainfall Measuring Mission (TRMM)

homepage, chose global floods and landslides monitoring and then heavy rain areas. Flood Monitoring and Mapping using Satellite data Present wide-area satellite systems can monitor rivers to provide early warning of unusual low or high flow conditions

Landsat-1 Image of the same area pre (left) and post (right) flood views Flood Hazards Maps Map that graphically provides information on inundation in an easy-to-understand format

predicted inundation areas, inundation depth,

location of evacuation refuges, evacuation routes,

dangerous spots on evacuation routes, etc. Flash Floods

A flood caused by heavy or excessive rainfall in a short period of time, generally less than 6 hours Key elements are rainfall intensity and duration Occur very rapidly and with little warning Can occur within minutes or a few hours of excessive rainfall Can also occur even if no rain has fallen, for instance sudden release of water after a levee or dam has failed Duration of the flooding is short but the depth of flooding can

be considerable and very extensive damage may result Flash Floods Flash flooding is a phenomenon principally watersheds which are hydrologicaly small associated

with Severity increases if the watershed is steep and its surface has low infiltration capacity Dangerous because of their sudden nature and fast moving water A vehicle provides little to no protection against being swept away More than half of the fatalities attributed to flash floods are people swept away in vehicles when trying to cross flooded intersections.

How can a foot or two of water cost you your life? Water weighs 62.4 lbs. per cubic foot and typically flows downstream at 6 to 12 miles an hour When a vehicle stops in the water, the water's momentum is transferred to the car According to the U.S. Geological Survey, for each foot the water rises,

500 lbs. of lateral force are applied to the car. But the biggest factor is buoyancy. For each foot the water rises up the side of the car, the car displaces 1,500 lbs. of water. In effect, the car weighs 1,500 lbs. less for each foot the water rises. Two feet of water will carry away most automobiles. The United States National Weather Service gives the advice "Turn Around, Don't Drown"

Coastal Floods Coastal flooding can be caused by storm surge, storm tides, or tidal waves (tsunami). Storm surge flooding occurs when a tropical cyclone approaches a coastline Storm tides: when tropical cyclone coincides with a high tide

Mitigation Proper Watershed Management Careful land use planning can mitigate flood disasters Incorporating Flood resistant feature in buildings Prevention/Protection Measures Raised floor above

flood level Flood Control Dams Flood water may be trapped and stored in reservoir and then release gradually Artificial Levees

Increase the capacity of channel Landuse Planning Avoid 100 years floodplain for human settlement Floods in Pakistan Occur normally due to storm systems that originate from Bay of Bengal during the monsoon from July to

September The mountain ranges in the extreme north of Pakistan provide a perennial source of inflow into the rivers Changing Patterns Monsoonal zone of Pakistan has shifted from northeast (upper Punjab and Kashmir region) to northwest (Khyber Pakhtunkhwa and northwest

Punjab regions) Location of heavy rainfall events during the monsoon will move from northeast to northwest Pakistan Western rivers of the country Indus and Kabul will be extremely vulnerable to flood episodes similar to the one experienced during 2010 Glacial Lake Outburst Floods (Glofs) Occurs when the dam containing a glacial lake fails

Retreat of the glaciers As glaciers retreat, glacial lakes are formed behind moraine or ice dams or inside the glaciers A sudden breach in its walls may lead to a discharge of huge volumes of water and debris Potential of releasing millions of cubic meters of water in a few hours causing catastrophic flooding downstream and damaging life, property, forest, agricultural farms and infrastructure

Known as Glacial Lake Outburst Floods (Glofs) GLOF Hazard in Pakistan Retreat of the glaciers of the Himalayan Karakoram Hindukush (HKH) region in Pakistan Glaciers in Pakistan are receding at a rate of almost 40 60 meters per decade In Pakistan among 2,420 identified lakes, 52 lakes have been classified as

potentially hazardous (ICIMOD 2007) On average, GLOF events occur in the Himalayas every 3-10 years Major flood events in Pakistan? ??? Discussion/Comment/ Question

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