Flash flood
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Flash floods are caused by heavy rainfall over a small area and can rise rapidly, giving little warning. Effective early warning systems detect intense rainfall, forecast flash flooding risks, and quickly disseminate warnings. They require hydrometeorological monitoring networks like rain gauges, weather radar, and satellites; forecasting systems; and communication systems. Decision support systems help by processing real-time data, identifying thresholds exceeded, and creating warning products for dissemination. Successful systems also include emergency planning and preparedness.
Flash flood
- 1. Ref: CH2MHILL and Flash flood early warning system reference guide –NOAA(US) & WMO Prepared by: Amaljit Bharali Research Scientist (Hydrology)
- 2. Flash flood theory Flash Flood Early Warning Systems (FFEWS) EWS around the Globe Overview of Delft FEWS decision support system & setting up a Multi-Hazard Early Warning System (MHEWS) Proposed addition to existing FLEWS
- 3. World Meteorological Organization: - A flood of short duration with a relatively high peak discharge. American Meteorological Society: - “…flood that rises and falls quite rapidly with little or no advance warning, usually as the result of intense rainfall over a relatively small area.” U.S. National Weather Service: - A rapid and extreme flow of high water into a normally dry area, or a rapid water level rise in a stream or creek above a predetermined flood level, beginning within six hours of the causative event (e.g., intense rainfall, dam failure, ice jam). However, the actual time threshold may vary in different parts of the country. Ongoing flooding can intensify to flash flooding in cases where intense rainfall results in a rapid surge of rising flood waters
- 4. Riverine Flash flood ◦ Typically caused by torrential rain ◦ Dam/Levee break ◦ Ice jams ◦ Glacial Lake debris Flash flood ◦ Heavy rainfall (primary cause), although riverine factors might also contribute
- 5. The intensity and development Flash flood caused by heavy rainfall is influenced by few hydrological & basin characteristics as- ◦ Basin size – smaller basins more prone to flash flood ◦ Basin shape – significant influence on magnitude & timing of the peak flow. Runoff in more round basin will arrive more quickly at the basin outlet. In contrast, in longer, narrower basin, water from multiple locations is less likely to arrive at the same time
- 6. ◦ Basin slope – affects timing of runoff and amount of infiltration. Greater & steeper the slope lower the infiltration, quicker the flow response and higher the peak discharge ◦ Surface roughness – presence of rocks, vegetation and debris creates turbulence, slowing runoff and increasing infiltration. In contrast concrete lined channels have no infiltration and very high velocity leading to flash flood hazard ◦ Stream density – (Total length of all channels/Area of basin). Urbanization artificially increases stream density as road grid and storm drainage network act as pathways, or tributaries, that move water rapidly to low lying areas and the nearby stream channels ◦ Land cover and land use Urbanization Devegetation & Wildfires Frozen ground
- 7. Soil properties Soil moisture (degree of saturation) More effect in humid areas with deep saturated soil Infiltration excess overland flow Soil permeability Soil surface alterations, such as compaction, paving, and fire Wildfires can alter soil properties so that burn areas become hydrophobic, i.e. tending to repel and not absorb water, for weeks or even years following a fire; indeed, the greatest flash flood risks occur after high-intensity fires in coniferous forests. Soil profile – influences capacity of soil to store water and infiltration rate
- 9. Primary hurdles/complexities in setting up EWS ◦ Lead time (advance warning) ◦ Accuracy (confidence level) With technological advancement possibility of local or regional or even Global level EWS 1) Heavy rain event detection via rainfall/streamflow gauge networks, radar networks, satellite sensors, or some combination of the three. 2) Manual or computerized short-fused nowcasts of imminent flash floods from diagnosed heavy rain events. 3) Atmospheric fine-scale models, possibly coupled with distributed hydrology models, to forecast the risk of flash flooding in a basin or basins a short time in the future.
- 10. Detecting and forecasting hazards and developing hazard warning messages Assessing potential risks and integrating risk information into warning messages Disseminating timely, reliable, and understandable warning messages to authorities and at-risk public Community-based emergency planning, preparedness and training focused on eliciting an effective response to warnings to reduce potential impact on lives and livelihoods Note: Major improvements for effective implementation include development and strengthening of core capacities such as – Hydro-meteorological observing networks 24/7 forecasting systems Communication systems
- 11. Hydrometeorological sensors for flash flood forecasting. ◦ Rain gauges For flash flood applications, rainfall gauges will consist of a precipitation measurement device, data collection platform (DCP), power supply and management unit, and communication device. These can be coupled with a range of common weather sensors that measure temperature, humidity, barometric pressure, and other standard weather parameters like wind speed and direction. Tipping bucket gauges area normally used in AWS which underestimate precipitation during periods of intense rainfall ◦ River/streamflow gauges Measures water surface elevation in the channel which in turn is compared with stage-discharge relationship or rating curve Commercial options for water surface measurements & stage-discharge relationship Web-cams aimed at permanently mounted staff gauges Acoustic depth sensors Traditional manometers Purpose of gauge network: ◦ Provide accurate real-time hydrometerological measurements to facilitate bias adjustment of radar and satellite precipitation estimates ◦ Provide rainfall input to hydrologic and flash flood models ◦ Support general weather & flash flood forecast ◦ Unfortunately, error prone and typically cannot represent the localized nature of convective rainfall ◦ Recommended real-time automated gauges to assure rapid sampling and transmission of critical observations to a forecast centre
- 12. ◦ Weather Radar Networks To provide high-resolution, real-time gridded rainfall estimation over a region of interest Radar can detect formation of clouds, track their movement and evolution, probe their internal structure, and make quantative estimates of the amount of precipitation they produce at the surface Weather radars are powerful tools because of their ability to provide high spatial and temporal resolution precipitation data over a large area as opposed to the point measurement of an in-situ gauge. ◦ Satellites Networks Satellite estimates of precipitation can be partially corrected by coincident rain gauge “ground truth” data. Gridded rainfall estimates are the primary source of precipitation information for areas that lack radar networks and networks of rain gauges
- 13. Collecting earth observation data in real time, especially rainfall and streamflow data Processing and storing data in real time Monitoring data for exceedence of threshold criteria Computing parameters that must be derived from observed data Displaying data and derived information for the forecaster to maintain situational awareness Creating and Disseminating text and graphic products to customers and other forecast centers
- 14. Collecting, decoding, and digitally storing earth data observations Managing relational database of observations and metadata Checking incoming observational data for quality and flagging or rejecting suspect readings Displaying data – Numerical tabulations of gauge reports – Graphical displays of gauge reports – Mapped displays of gauge reports Comparing precipitation estimates to Flash Flood Guidance (FFG) and alerting forecaster when guidance is exceeded Comparing precipitation estimates to Flash Flood Potential Index (FFPI) or other programs that modify FFG and alerting forecaster
- 15. Computing rate of change at gauges, extrapolating future values, and alerting the forecaster when FFG levels are exceeded Routing rainfall downstream and comparing to flood stage, etc. Mapping and displaying radar reflectivity data in real time and alerting the forecaster when reflectivity thresholds are exceeded Displaying radar-observed incremental storm total precipitation data and alerting the forecaster to potential problem areas Comparing radar reflectivity data through ZR relationships, for instance, the relationship between radar reflectivity and rain rate in a power law form, and relating to FFG and/or FFPI Generating text and graphical summaries of observed data, routine forecasts, and warning products Disseminating products to appropriate communications channels.
- 17. USA: A typical system composed of national hydrometeorological guidance, local hydrometeorological expertise, and constituent-operated gauge networks. Central America: Central American Flash Flood Guidance System, based primarily on satellite data. Italy: Piedmont region multi-disciplinary hydrometeorological ALERT and Real-time Flood Forecasting System Colombia: Aburrá Valley Natural Hazard Early Warning System, currently in the planning stages