River flood management and hydrographs
Geography · GCSE · The School
A hydrograph shows how a river responds to rain
A storm hydrograph plots river discharge against time, alongside the rainfall that caused it. RISING LIMB shows discharge climbing as rain reaches the channel. PEAK DISCHARGE is the highest point. LAG TIME is the gap between peak rainfall and peak discharge — a SHORT lag means a flashier, more dangerous response.
What shortens lag time
Steep slopes get water to the channel fast. Impermeable rock or urban surfaces stop water soaking in, so it runs off instead of infiltrating. A drainage basin shaped so tributaries all peak together concentrates the flow. Each of these shortens lag time and raises peak discharge — the two numbers a flood risk assessment cares about most.
Human factors make it worse, not just physical ones
Urbanisation replaces soil with tarmac and roofs, cutting infiltration sharply, and drains route water to the channel faster than natural soil ever would. Deforestation removes interception and root uptake that would otherwise slow water down. Both shorten lag time and raise peak discharge in exactly the way steep, impermeable physical geography does — an exam answer naming only physical causes misses half the marks.
A worked hydrograph reading
A storm hydrograph shows peak rainfall at 12:00 and peak discharge at 15:00 — a 3-hour lag time. Discharge rises from a base flow of 10 cumecs to a peak of 60 cumecs. Reading this: the short lag and steep rising limb suggest a flashy response, consistent with a small, steep, urbanised catchment rather than a large rural one.
Hard engineering: built to control the river directly
Dams and reservoirs store floodwater and release it gradually, at high construction cost and with land lost under the reservoir. Channel straightening speeds water away from a town, but moves the flood risk downstream. Embankments raise the channel's banks to hold more water, but can fail suddenly if overtopped.
Soft engineering: working with the river's processes
Floodplain zoning keeps building off land that floods naturally, avoiding the problem rather than fighting it. River restoration reconnects a straightened river to its floodplain, letting it flood harmlessly across open land. Afforestation upstream increases interception and slows runoff. Soft methods are usually cheaper and lower-risk, but do not protect existing buildings the way hard engineering can.