The definition and purpose of a storm hydrograph

A storm hydrograph, often referred to as a flood hydrograph, is a graphical representation that illustrates how a river's discharge changes over a short time frame, typically during or after a rainfall event. This tool is essential for understanding how a river responds to precipitation within its drainage basin and helps in analysing the speed at which rainwater reaches the river channel.

Core elements of a storm hydrograph

• Discharge measurement - Discharge, often measured in cubic metres per second (cumecs, m³/s), represents the volume of water flowing through a river cross-section over a specific period.
• Time representation - The graph plots discharge against time, often over hours or days, to show the river's reaction to a storm.
• Hydrological insight - It reveals the interaction between rainfall and river flow, highlighting processes like run-off and infiltration within the hydrological cycle.

Key characteristics and components of hydrographs

Storm hydrographs consist of several distinct features that provide insights into a river's behaviour during and after a rainfall event.

Main components of a hydrograph

• Rising limb - This shows the increase in discharge as water from the storm reaches the river. A steeper rising limb indicates a quicker response, often seen in flash floods or small basins.
• Peak flow or discharge - The highest point of discharge on the graph, which is generally higher in larger basins or steeper catchments with lower infiltration rates.
• Falling limb or recession limb - Represents the decrease in discharge after the peak. Its steepness is influenced by basin size, geological composition, and aquifer behaviour, with gentler slopes in larger catchments or areas with permeable rocks.
• Lag time - The delay between the peak of rainfall and the peak discharge. It varies based on basin shape, steepness, and stream order.
• Baseflow - The slow, sustained contribution of groundwater to the river, crucial in areas with porous rocks, acting as the primary long-term source of discharge.
• Run-off curve - Illustrates the balance between overland flow and throughflow, with dominant overland flow in areas of low infiltration, high antecedent moisture, or impermeable surfaces.
• Bankfull discharge - A threshold indicated on the graph, above which the river is considered to be in flood.

Hydrograph size and discharge relationship

• Rainfall impact - Generally, higher rainfall amounts lead to greater discharge.
• Basin size influence - Larger drainage basins tend to produce higher discharge due to the greater volume of water they can collect.

Factors influencing hydrograph shapes and sizes

The shape and size of a storm hydrograph are not uniform across different locations or events. Various environmental and geographical factors determine how a hydrograph appears.

Environmental factors affecting hydrographs

• Climate variations - Total rainfall, intensity, and seasonal patterns affect the volume and speed of run-off.
• Soil types - Impermeable soils, such as clay, lead to more surface run-off and increased flooding potential.
• Vegetation cover - Dense vegetation intercepts rainfall, reducing run-off and the likelihood of flooding.
• Infiltration capacity - Soils with low infiltration capacity result in more overland flow.
• Rock type - Permeable rocks allow water to infiltrate, lowering peak discharge, while impermeable rocks increase surface run-off.
• Slope angle - Steeper slopes accelerate run-off, leading to faster responses.
• Drainage density - Basins with more stream channels transport water to the river more efficiently.
• Human interventions - Dams can regulate water flow, while afforestation increases interception.
• Basin characteristics - Small, steep basins have shorter lag times, while basin shape affects the timing of floodwaters.

Differences between urban and rural hydrographs

Hydrographs from urban and rural areas display distinct differences due to variations in land use and surface characteristics.

Contrasting features of urban and rural hydrographs

Reasons for differences

• Impermeable surfaces in urban areas - Roofs, pavements, and roads prevent infiltration, causing rapid run-off into drains and sewers.
• Drainage systems - Urban areas have extensive networks of gutters and sewers that speed up water transfer to rivers.
• Natural surfaces in rural areas - More vegetation and permeable soils allow greater infiltration and slower run-off, delaying and reducing peak discharge.