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Unlock This CourseImagine you're watching a river during a heavy rainstorm. At first, the water level stays normal, then it starts rising, reaches a peak and slowly drops back down. A storm hydrograph is like a graph that shows this story - it plots how much water flows through a river over time during and after a storm.
Storm hydrographs are essential tools for geographers, engineers and flood managers. They help predict flooding, design bridges and understand how human activities affect river flow patterns.
Key Definitions:
The x-axis shows time (usually hours), whilst the y-axis shows discharge. Rainfall is often shown as a bar chart above the main graph. The shape tells us a story about how quickly water reaches the river and how the landscape responds to rain.
Every storm hydrograph has several key parts that help us understand what's happening in the river system. Think of it like reading a story with different chapters.
Understanding each component helps us interpret what's happening in the catchment area during a storm event.
The steep upward slope showing discharge increasing as storm water reaches the river. A steep rising limb means water arrives quickly.
The highest point on the graph. This is when the river is flowing fastest and flood risk is greatest.
The gradual decline as discharge returns to normal. This is usually gentler than the rising limb as water slowly drains away.
The lag time between peak rainfall and peak discharge can vary from 30 minutes in urban areas to several hours in rural catchments. This difference is crucial for flood warning systems!
Not all hydrographs look the same. The shape depends on many factors - some natural, some human-made. Understanding these helps us predict how different areas will respond to storms.
The natural landscape plays a huge role in determining hydrograph characteristics.
Steep slopes create flashy hydrographs with short lag times and high peaks. Water rushes downhill quickly. Gentle slopes produce longer, lower hydrographs as water moves slowly.
Forests intercept rainfall and slow water movement, creating longer lag times and lower peaks. Bare soil allows rapid runoff, creating steep, high hydrographs.
Human activities significantly alter natural hydrograph patterns, often increasing flood risk.
Concrete and tarmac prevent infiltration. Storm drains channel water quickly to rivers, creating flashy hydrographs with very short lag times.
Removing trees reduces interception and increases surface runoff. This creates higher, sharper peaks and shorter lag times.
Straightening rivers and building flood defences can speed up flow, potentially moving flood problems downstream.
Reading hydrographs is like being a detective - you need to look for clues about what type of environment produced the pattern you're seeing.
The River Severn in rural Wales shows a gentle hydrograph with lag times of 12-24 hours and gradual rising/falling limbs. In contrast, a small urban stream in Birmingham shows lag times of just 1-2 hours with very steep rising limbs. This demonstrates how urbanisation dramatically alters natural flow patterns.
When interpreting hydrographs, focus on these key characteristics to understand the catchment.
Short lag times (under 6 hours) suggest urban areas, steep slopes, or sparse vegetation. Long lag times (over 12 hours) indicate rural areas, gentle slopes, or dense forest cover.
Sharp, high peaks indicate rapid runoff from impermeable surfaces. Broad, low peaks suggest natural storage in soil and vegetation slowing the flow.
Creating accurate hydrographs requires attention to detail and understanding of the relationship between rainfall and river response.
Follow these steps to create professional-looking hydrographs that clearly show the data.
Draw time on x-axis (hours) and discharge on y-axis (cumecs). Use appropriate scales - don't cram data into tiny spaces.
Show rainfall as bars above the main graph. This helps explain why discharge changes when it does.
Start with base flow, show the rising limb, mark peak discharge clearly, then draw the falling limb back to base flow.
Always label your axes with units! Examiners look for discharge in cumecs (m³/s) and time in hours. Also, clearly mark key features like peak discharge and lag time on your graphs.
Storm hydrographs aren't just academic exercises - they have vital real-world uses that affect millions of people.
Understanding hydrograph patterns helps authorities prepare for and respond to flood events effectively.
By knowing lag times, flood warnings can be issued with appropriate lead times. Urban areas need faster responses due to shorter lag times.
Hydrograph analysis helps planners understand flood risk when approving new developments. Areas with flashy hydrographs need better drainage systems.
The devastating Boscastle flood showed a classic flashy hydrograph. Steep valley sides, impermeable rock and intense rainfall (200mm in 4 hours) created a lag time of just 2 hours. Peak discharge reached over 140 cumecs - normally it's just 2 cumecs. This demonstrates how extreme weather can overwhelm natural systems.