Floods occur when water overflows its typical boundaries, submerging land that is normally dry. They can stem from various sources, including heavy rainfall, rapid snowmelt, or storm surges. Accurately measuring floods is important for public safety, disaster preparedness, and resource management. Understanding water levels and flows during these events helps communities protect lives and property.
What Flood Scientists Measure
Scientists quantify several characteristics of floods to understand their behavior and impact. Water level, also known as stage or gauge height, measures the vertical height of the water surface above a fixed reference point. Discharge represents the volume of water flowing past a specific point per unit of time, typically expressed in cubic feet per second (cfs) or cubic meters per second (cms). Flow velocity, the speed of the water, is also measured as it directly influences discharge and the destructive power of floodwaters. These parameters are important for characterizing flood conditions and predicting their intensity.
Ground-Based Measurement Methods
Traditional ground-based methods provide direct measurements of flood conditions. Staff gauges, resembling large rulers, are permanently installed in bodies of water for a quick visual reference of water level. Automated stream gauges, or stage sensors, are more advanced devices deployed along rivers and streams. They continuously monitor water levels using technologies like pressure transducers or acoustic sensors, transmitting real-time data via telemetry or satellite.
Current meters are utilized to measure the volume and speed of water. Propeller-type current meters feature spinning propellers that rotate with the water flow, converting rotation speed into velocity measurements. These are often used with the velocity-area method to calculate discharge. Acoustic Doppler Current Profilers (ADCPs) use sound waves and the Doppler effect to measure water current velocities across multiple depths simultaneously, providing precise discharge measurements.
Remote Sensing and Advanced Technologies
Modern flood measurement increasingly incorporates remote sensing and advanced technologies that offer broader coverage and complementary data. Radar systems, such as those used in meteorology, detect precipitation and track its movement by measuring rain reflectivity. Dual-polarization radar enhances accuracy by differentiating precipitation types and improving rainfall estimates, which is important for flash flood warnings. Satellites map flooded areas, measure surface water elevation, and assess soil moisture from space, complementing ground-based data with large-scale information on flood extent and impact.
Drones, or unmanned aerial vehicles (UAVs), offer localized, high-resolution data collection. Equipped with various sensors and cameras, drones collect hydrological data and measure water spread in real-time. They use RGB cameras for visual documentation, LiDAR for detailed topographic mapping, and multispectral or thermal sensors for analyzing inundated areas. These aerial platforms enhance the ability to monitor and assess floods in specific, often hard-to-reach, locations.
Interpreting Flood Data
Collected flood measurements define different flood stages, indicating severity and potential impact. Action stage indicates water levels rising near or slightly above banks, potentially causing minor inconvenience. Minor flood stage may involve some property flooding and covered roads, though structural damage is usually minimal. Moderate flood stage signifies buildings may be impacted, roads could become impassable, and some evacuations might be necessary.
Major flood stage represents extensive inundation, posing a threat to structures, infrastructure, and human life, often requiring widespread evacuations. A record flood stage occurs when the water level equals or exceeds the highest recorded level at a specific site. This interpreted data is important for flood forecasting, where hydrological models integrate measurements to predict future flood conditions and delineate potential inundation areas. This information supports public warning systems, which issue alerts to help communities prepare and mitigate damage.