Drainage Basins and Why Rivers Flood
A drainage basin collects water into a river system. Rain can be stored, evaporate or reach channels by surface and underground routes. River response depends on these routes, rainfall and basin conditions. Paving and drains can speed runoff downstream.
What a learner can do afterwards
- Explain what a drainage basin is and name three routes water takes to reach the channel
- Use stated peak rainfall and discharge times to compare lag times, and explain why a short lag shows a rapid river response
- Give two human changes that raise flood risk downstream and say why each does
1 · Read
A drainage basin is the land drained by a river and its tributaries. A drainage divide, usually a ridge of high ground, separates neighbouring surface drainage basins. Not all rain reaches a river: some evaporates, is used by plants or remains stored. Groundwater boundaries need not match the surface divide.
Water can reach a channel as surface runoff, throughflow through soil, or groundwater flow through rock and sediment. Surface runoff often gives a rapid storm response; underground routes commonly deliver water more slowly. Infiltration means water entering the ground. Saturated, frozen or sealed ground can limit infiltration. Even gentle rain can then produce runoff.
A flood hydrograph plots discharge, the volume of river water passing each second, against time. Here, lag time means the interval from peak rainfall to peak discharge. Peak rain at 3 pm and peak discharge at 7 pm give a four-hour lag. A rapid rise and short lag show a quick response, not its exact cause. Flooding also depends on whether flow exceeds channel capacity.
Paving reduces infiltration. Drains without storage can deliver runoff quickly to channels, increasing flood risk downstream. Removing trees can reduce interception and increase runoff, depending on soil and rainfall. Channel straightening can speed water onward. Floodplain building exposes people to flooding. Storage and drainage measures can change a development's effect on runoff.
Rainfall, storage and water routes shape a river's response. A short lag means a quick response; flood risk also depends on channel capacity and development.
Lesson, questions and flashcards written for this dataset.
Readings written for this dataset. Flashcards revised for this dataset.
More to read
Following rainfall through a basin
Rainfall and river flow are different quantities. Some rain reaches channels, some enters underground stores, and some returns to the air. Plants also use water. A surface drainage divide separates neighbouring runoff systems, but underground water boundaries may differ. Investigating a basin means considering storage and losses as well as movement towards its river.
Why the same slope responds differently
Earlier weather affects how a slope handles rain. Dry soil may accept water that saturated soil cannot store. Frozen or sealed surfaces can also limit infiltration. Surface runoff often reaches channels quickly; routes through soil and rock commonly take longer. Rainfall intensity and duration matter too. Gentle rain alone does not establish which route dominates.
Reading a river's response
A flood hydrograph has time on its horizontal axis and discharge on its vertical axis. Compare the time of peak discharge with the rainfall peak using the stated lag convention. A short interval shows a quick peak response. More evidence is needed to identify its cause. To establish flooding, also check whether flow exceeded the channel's capacity.
Separate groundwater loss from runoff
Groundwater can feed rivers between storms. Pumping it can reduce that contribution. This does not, by itself, demonstrate faster delivery of storm runoff. Paving acts through a different mechanism: it reduces infiltration through the covered surface. Identify the affected process before predicting how a human change will alter river flow.
Upstream change and downstream exposure
A town receives river water from land beyond its own boundary. Drains, channels, vegetation and storage can change when that water arrives. Buildings on a floodplain are exposed when a river overflows. Assess both the basin's delivery of water and the people or buildings in its path. A faster response and greater exposure are related parts of risk, not the same thing.
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