Flood Hydrograph – Storm Rainfall, Land Use and River Floods
GeographyWater & OceansAges 15–16
Loading…
Sign in to playA drainage basin (watershed divide, tributaries, confluence, gauging station and a town at the outlet) receives a storm. The chart plots hourly rainfall and river discharge (m³/s) over time, so you can read peak discharge, lag time, the rising and falling limbs and base flow. Change the rainfall total and duration, forest cover, impermeable (urban) surface, soil moisture, basin shape and slope; add retention ponds or levees to see whether the flood rises above the warning level and spills into the town. Press New storm to keep the old curve for comparison.
Lesson: River regimes and flood hydrographs
What it shows
A flood hydrograph shows how river discharge at a gauging station changes after a storm. Some rain soaks into the soil; the rest runs over the surface into tributaries and down to the main river. Because the water has to travel, peak discharge comes some hours after peak rainfall: this delay is the lag time. Impermeable urban surfaces and saturated soil send more water to the river faster, giving a high, early peak, while forests and gentle slopes store water and slow it down. Groundwater keeps a steady base flow between storms.
How to use
Press Pause or Skip to 48 h to read the peak and lag time on the chart. Change one factor, such as Impermeable, Forest, Soil moisture or Slope, then press New storm: the previous curve stays as a grey dashed line for comparison. Switch Basin between Near-circular and Long and narrow. Tick Retention ponds or Levees in town and check whether the peak still passes the warning line and floods the town.
Parameters you can change
- Total storm rainfall 10–200 mm
- Rainfall duration 1–12 hours
- Forest cover 0–100 %
- Impermeable surface (buildings, roads, concrete) 0–90 %
- Soil moisture before the storm (100% = saturated) 0–100 %
- Basin shape Near-circular, Long and narrow
- Average basin slope 0.5–10 %
- Retention ponds store urban runoff
- Levees on both banks in the town
Questions to explore
- Why does peak discharge always arrive later than peak rainfall?
- How do peak discharge and lag time change when impermeable surface rises from 10% to 50%?
- How do reforestation and retention ponds each reduce flooding?
