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Unlock This CourseStorm hydrographs are graphs that show how a river's discharge changes over time during and after a storm. They're like a river's heartbeat monitor - showing how quickly water levels rise and fall when it rains. Understanding these patterns helps us predict floods and manage water resources effectively.
Key Definitions:
A typical storm hydrograph has a distinctive shape. It starts at base flow level, rises steeply during rainfall (the rising limb), reaches peak discharge, then falls more gradually (the recession limb) back to base flow. The steeper the rising limb, the more likely flooding becomes.
Every storm hydrograph has several key features that tell us about how a drainage basin responds to rainfall. Learning to identify these helps us understand flood risk and river behaviour.
The rising limb shows how quickly discharge increases after rainfall begins. A steep rising limb indicates rapid runoff and higher flood risk. This happens when water reaches the river channel quickly through surface runoff rather than soaking into the ground.
Indicates rapid surface runoff, impermeable surfaces and higher flood risk. Common in urban areas or areas with thin soils.
Shows slower water movement through soil and rock. More water infiltrates, reducing immediate flood risk but maintaining river flow longer.
The maximum flow rate during the storm. Higher peaks mean greater flood risk and more erosive power in the river channel.
Lag time is crucial for flood prediction. It's the time difference between peak rainfall and peak discharge. Short lag times mean flash flood risk, whilst longer lag times give more warning. Urban areas typically have much shorter lag times than rural areas.
Boscastle in Cornwall experienced devastating flooding with an extremely short lag time of just 2 hours. The steep valley sides, impermeable slate rock and intense rainfall (200mm in 4 hours) created perfect conditions for rapid runoff. The village's narrow valley concentrated the water, causing severe damage to buildings and infrastructure.
The shape of a storm hydrograph depends on various physical characteristics of the drainage basin. These natural factors determine how quickly water moves from hillslopes to river channels.
Circular basins tend to have shorter lag times because water from all areas reaches the main channel at roughly the same time. Long, narrow basins spread out the flow, creating longer lag times and lower peak discharges.
Water converges quickly at the outlet, creating high peak discharge and short lag time. Examples include many volcanic crater lakes and some urban catchments.
Permeable rocks like limestone and sandstone allow water to soak in, creating gentle hydrographs with long lag times. Impermeable rocks like granite and clay force water to flow over the surface, creating steep, flashy hydrographs.
Chalk, limestone and sandstone absorb rainfall, reducing surface runoff and creating gentle hydrographs with sustained base flow.
Granite, slate and clay prevent infiltration, forcing water to run off the surface quickly, creating steep hydrographs.
Forests intercept rainfall and slow surface flow. Roots create channels for water infiltration, reducing peak discharge significantly.
Human activities dramatically alter natural hydrograph patterns. Understanding these changes helps urban planners and engineers design better flood defences and drainage systems.
Cities create much flashier hydrographs than natural areas. Concrete and tarmac prevent infiltration, whilst storm drains channel water rapidly to rivers. This increases flood risk downstream and reduces groundwater recharge.
A typical urban hydrograph might have a lag time of 1-3 hours and peak discharge 3-5 times higher than the same area before development. Rural areas with similar rainfall might have lag times of 12-24 hours and much lower peak flows.
Farming practices significantly affect runoff patterns. Ploughed fields can either increase or decrease runoff depending on the direction of furrows and soil condition.
Removing trees increases surface runoff, reduces lag time and increases peak discharge. Soil erosion also increases, leading to higher sediment loads in rivers.
Being able to read and analyse storm hydrographs is essential for flood risk assessment and water management. Different patterns tell us about basin characteristics and help predict future flood behaviour.
Meteorologists and hydrologists use hydrograph patterns combined with weather forecasts to issue flood warnings. Understanding typical lag times for different catchments helps emergency services prepare for potential flooding.
Steep rising limbs and short lag times (under 6 hours) indicate high flash flood risk requiring immediate warnings.
Gentle recession limbs show water will remain high for extended periods, affecting transport and agriculture.
Complex hydrographs with several peaks often result from tributaries with different lag times joining the main river.
Climate change is altering precipitation patterns, with more intense rainfall events becoming common. This means traditional hydrograph patterns may change, requiring updated flood risk assessments and defence strategies.
The River Severn experienced severe flooding affecting Gloucester and Tewkesbury. The hydrograph showed multiple peaks as different tributaries contributed flow at various times. The combination of saturated soils from previous rainfall and intense summer storms created perfect conditions for widespread flooding, demonstrating how antecedent conditions affect hydrograph response.
Understanding storm hydrographs helps engineers and planners design effective flood management strategies. Different approaches work better for different types of hydrograph patterns.
Hard engineering approaches like flood barriers and channel modifications can alter hydrograph characteristics. Dams store peak flow and release it gradually, whilst channel straightening can increase flow velocity and reduce lag time.
Modern approaches focus on mimicking natural hydrographs in urban areas. Permeable pavements, retention ponds and green roofs help restore natural infiltration patterns and reduce peak discharges.