A.2.1 Hydrograph Characteristics Notes

Analyzing River Responses to Rainfall Events

1. Imagine standing by a river’s edge after a heavy rainstorm.
2. The water begins to rise, flowing faster and stronger until it surges past with tremendous force.
3. What causes this dramatic response? Why doesn’t the river react immediately to rainfall, and why does the flow eventually recede?
4. These questions lead us to hydrographs, tools that help us visualize and analyze how rivers respond to rainfall events.
5. By studying hydrographs, you can uncover the processes shaping river discharge and assess factors that influence flood risk.

Key Components of a Hydrograph

A hydrograph typically has two axes:

• The x-axis represents time (hours or days).
• The y-axis represents discharge, measured in cubic meters per second (m³/s).

The graph itself consists of:

1. Base Flow: The normal, sustained flow of the river, fed by groundwater.
2. Rising Limb: The steep increase in discharge after rainfall begins.
3. Peak Discharge: The highest point on the graph, indicating the maximum flow.
4. Lag Time: The delay between the peak rainfall and peak discharge.
5. Falling Limb: The gradual decrease in discharge as water drains away.

Understanding Key Hydrograph Terms

To analyze hydrographs effectively, it’s essential to understand three key terms that describe a river’s response to rainfall:

1. Base Flow: The portion of river discharge supplied by groundwater seeping into the channel, sustaining the river during dry periods.
2. Lag Time: The delay between peak rainfall and peak discharge (the highest flow rate in the river). This reflects the time it takes for water to travel from where it falls to the river channel.
3. Peak Discharge: The maximum volume of water flowing in the river at a given time after rainfall.

These terms are visualized on a hydrograph, a graph showing river discharge over time during and after a storm or rainfall event. The shape of the hydrograph reveals how quickly a river responds to rainfall and highlights the factors influencing that response.

Lag Time: The Key to Flood Risk

1. Lag time is a crucial factor in determining flood risk.
2. A shorter lag time means water reaches the river channel more quickly, increasing the likelihood of flooding.
3. Conversely, a longer lag time allows more water to infiltrate the ground or flow slowly through the landscape, reducing flood risk.
4. Several factors influence lag time:

• Basin Shape and Size: Smaller, circular basins tend to have shorter lag times because water converges quickly. Larger, elongated basins have longer lag times as water takes more time to reach the river.
• Slope Gradient: Steeper slopes encourage faster runoff, leading to shorter lag times. Flatter areas promote infiltration and slower water movement, increasing lag time.
• Drainage Density: Basins with a higher density of streams and channels transport water more efficiently, reducing lag time.
• Land Use: Urban areas with impermeable surfaces, such as roads and buildings, reduce infiltration and increase runoff, shortening lag time. Rural areas with vegetation and permeable soils tend to have longer lag times.

Peak Discharge: The Flood Peak

1. Peak discharge represents the highest volume of water flowing in the river after a rainfall event.
2. It is influenced by the characteristics of the rainfall and the drainage basin:

• Rainfall Intensity and Duration: Heavy, prolonged rainfall results in higher peak discharge as more water enters the river system.
• Soil and Rock Type: Impermeable soils (e.g., clay) and rocks (e.g., granite) limit infiltration, increasing surface runoff and peak discharge. Permeable soils (e.g., sandy soils) and rocks (e.g., limestone) allow more infiltration, reducing peak discharge.
• Vegetation Cover: Dense vegetation intercepts rainfall, slows runoff, and promotes infiltration, reducing peak discharge. Deforestation or sparse vegetation has the opposite effect.

Base Flow: The River’s Lifeline

1. Base flow is the steady contribution of groundwater to the river, maintaining its flow during dry periods.
2. While it is typically unaffected by short-term rainfall, base flow plays a critical role in the overall hydrograph.
3. Areas with permeable rocks and high groundwater storage (e.g., chalk or sandstone regions) have higher base flows, while areas with impermeable rocks have lower base flows.

Influences on Hydrographs: Geology and Seasonality