Overview
This lecture covers river discharge, its calculation, significance in flood prediction, and related concepts such as stream gauges, sediment deposition, and the lag time between rainfall and stream flow.
River Discharge and its Importance
- Discharge is the volume of water passing through a river location per unit time.
- Discharge is crucial for monitoring and predicting floods in communities along rivers.
- Scientists use stream gauge stations to continuously measure discharge at various river points.
Calculating Discharge
- Discharge (Q) = Width × Depth × Velocity of the river flow.
- Alternatively, Discharge = Cross-sectional area × Velocity.
- Example: With a cross-sectional area of 800 ft² and velocity of 6 ft/s, discharge = 4,800 ft³/s.
Stream Gauge Stations and Flow Monitoring
- The USGS operates over 7,500 stream gauge stations in the U.S.
- Stations classify stream flow levels using colors: red (low, drought), black/blue (above normal, flooding).
- Monitoring helps correlate precipitation events to rising river levels.
Flood Dynamics and Sediment Deposition
- Discharge increases during floods and may exceed the river channel, causing overbank flow and debris movement.
- As water moves away from the main channel, velocity decreases, causing sediment to be deposited and forming natural levees.
Lag Time Between Rainfall and Stream Flow
- Peak stream flow (discharge) happens hours after peak rainfall due to runoff travel time.
- Lag time allows for advance flood warnings and evacuation before peak flooding.
- A lack of lag time leads to dangerous flash floods.
Key Terms & Definitions
- Discharge (Q) — Volume of water passing a river point per unit time.
- Stream Gauge Station — Facility measuring water level and discharge in rivers.
- Cross-sectional Area — Width × depth of the stream at a specific point.
- Lag Time — Delay between peak rainfall and peak stream flow.
- Natural Levee — Sediment buildup along river banks from slowed water flow.
Action Items / Next Steps
- Practice calculating discharge using width, depth, and velocity.
- Prepare for quiz questions on discharge and stream flow concepts.
- Watch the next video for more on natural levees.