Storm Hydrographs and Flood Analysis

A storm hydrograph is a graph showing the discharge of a river at a specific point (gauging station) over time, during and after a precipitation event. Hydrograph analysis is a core quantitative skill for AQA A-Level Geography. Understanding hydrograph shape, flood frequency, and the effects of urbanisation enables students to evaluate flood risk and management strategies with precision.

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Storm Hydrograph Components

Key Features

• Baseflow: The normal, sustained discharge of the river fed by groundwater. It is shown as the gently curving line beneath the storm peak.
• Rising limb: The section of the hydrograph where discharge increases in response to rainfall. A steep rising limb indicates rapid surface runoff.
• Peak discharge (Qp): The maximum discharge recorded during the storm event, measured in cumecs (m³/s).
• Peak rainfall to peak discharge lag time: The time interval between the centre of the rainfall event (peak rainfall intensity) and the peak discharge. This is one of the most important diagnostic features.
• Falling limb (recession limb): The section where discharge decreases as water drains from the catchment. Its gradient reflects catchment storage capacity.
• Stormflow (quickflow): The area between the hydrograph curve and the baseflow line, representing the volume of water contributed by the storm.
• Time to peak: The time from the start of the rise in discharge to the peak.

Interpreting Hydrograph Shape

Feature	Flashy Hydrograph	Subdued Hydrograph
Rising limb	Steep	Gentle
Peak discharge	High	Low
Lag time	Short (< 6 hours)	Long (> 12 hours)
Recession limb	Steep	Gentle, extended
Typical catchment	Urban, small, impermeable, steep	Rural, large, permeable, flat

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Factors Affecting Storm Hydrographs

Basin Characteristics

Factor	Effect on Hydrograph
Basin size	Larger basins have longer lag times and lower peak discharge per unit area
Basin shape	Circular basins produce flashier hydrographs (tributaries converge simultaneously); elongated basins spread runoff over time
Drainage density	High density = more channels = faster water delivery = flashier response
Slope angle	Steeper slopes promote faster overland flow and shorter lag times
Soil type	Impermeable soils (clay) produce flashy hydrographs; permeable soils (sand, chalk) produce subdued hydrographs
Geology	Permeable bedrock (limestone, chalk) absorbs water, increasing lag time and reducing peak discharge
Vegetation cover	Dense vegetation intercepts rainfall, promotes infiltration, and increases lag time
Antecedent conditions	Saturated soils reduce infiltration, producing faster, higher peaks

Storm Characteristics

Factor	Effect
Rainfall intensity	High intensity exceeds infiltration capacity, generating Hortonian overland flow
Rainfall duration	Prolonged rainfall saturates soils, eventually producing saturation-excess overland flow
Rainfall type	Convective storms (intense, localised) vs frontal rainfall (moderate, widespread) produce different hydrograph shapes
Snow vs rain	Snow stores water temporarily; rapid snowmelt can produce delayed but significant peaks

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Bankfull Discharge

Bankfull discharge is the maximum discharge a river channel can carry without overtopping its banks. It is a critical threshold:

• Discharge above bankfull results in flooding of the floodplain.
• Bankfull discharge has a recurrence interval of approximately 1–2 years in most natural rivers (Leopold et al., 1964).
• It is considered the dominant discharge — the flow that does the most geomorphological work over time, shaping channel form.

Exam Application: Understanding that bankfull discharge occurs relatively frequently (every 1–2 years) helps explain why floodplains exist and why building on them is inherently risky.

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Flood Recurrence Intervals

The recurrence interval (return period) is the average time between flood events of a given magnitude or greater.

Calculating Recurrence Interval

The simplest method uses the Weibull formula:

T = (n + 1) / m

Where:

• T = recurrence interval (years)
• n = number of years of record
• m = rank of the flood event (1 = largest, 2 = second largest, etc.)

Worked Example