Flood Hydrographs and River Management Decisions
What is a Flood Hydrograph?
A storm hydrograph (flood hydrograph) shows how a river's discharge (the volume of water flowing past a point per second, measured in cumecs — m³/s) changes over time before, during, and after a storm event. It typically also shows the rainfall (bar chart) that caused the change.
Key Features of a Hydrograph
| Feature | Definition |
|---|---|
| Peak discharge | The highest point on the graph — the maximum river flow during the storm |
| Peak rainfall | The period of most intense rainfall (shown by the tallest bar) |
| Lag time | The delay between peak rainfall and peak discharge — the time it takes water to reach the river |
| Rising limb | The upward slope showing discharge increasing as storm water reaches the river |
| Falling limb (recession limb) | The downward slope showing discharge decreasing as water drains through the system |
| Base flow | The normal day-to-day discharge of the river, fed by groundwater |
| Overland flow | Water that does not infiltrate but runs across the surface into the river quickly |
| Throughflow | Water moving through the soil towards the river (slower than overland flow) |
| Groundwater flow | Water moving slowly through rock towards the river (slowest route) |
Interpreting Hydrographs
Flashy Hydrograph (High Flood Risk)
- Short lag time — water reaches the river quickly
- High peak discharge — large volume concentrated in a short time
- Steep rising limb — discharge increases rapidly
- Indicates high flood risk
Flat Hydrograph (Low Flood Risk)
- Long lag time — water takes longer to reach the river
- Low peak discharge — water spread out over a longer period
- Gentle rising limb — discharge increases slowly
- Indicates lower flood risk
Factors Affecting the Shape of a Hydrograph
Physical (Natural) Factors
| Factor | Effect on Hydrograph |
|---|---|
| Rock type | Impermeable rock (clay, granite) → more surface runoff → shorter lag time, higher peak. Permeable rock (chalk, limestone) → more infiltration → longer lag time, lower peak |
| Soil type | Saturated or thin soils → more surface runoff → flashier response. Deep, dry soils → more infiltration → flatter response |
| Slope gradient | Steep slopes → faster runoff → shorter lag time. Gentle slopes → slower runoff → longer lag time |
| Drainage basin shape | Circular basins → all tributaries reach the main river at similar times → high peak. Elongated basins → water arrives at different times → lower, broader peak |
| Drainage density | Many streams → water reaches the main river quickly → flashier response |
| Antecedent rainfall | If the ground is already wet from previous rain → reduced infiltration → more surface runoff → flashier |
| Vegetation cover | Dense vegetation → intercepts rainfall, slows surface flow, promotes infiltration → longer lag time. Bare ground → rapid runoff |
Human Factors
| Factor | Effect |
|---|---|
| Urbanisation | Concrete, tarmac, and roofs are impermeable → rapid surface runoff via drains → much shorter lag time, higher peak discharge |
| Deforestation | Removes interception, reduces transpiration and infiltration → more surface runoff → flashier hydrograph |
| Agricultural land use | Ploughing compacts soil (reducing infiltration); drainage ditches speed water to rivers; removal of hedgerows reduces interception |
| Dam construction | Regulates flow → reduces peak discharge; extends lag time; flattens the hydrograph |
| Channel management | Straightening or deepening channels → speeds flow downstream → can increase flood risk further along |
Using Hydrographs in Flood Management
Understanding hydrographs helps planners decide where and how to manage flood risk:
Scenario Analysis
If a settlement sits at the point where discharge is measured, planners assess:
- How quickly will floodwater arrive after rainfall? (lag time)
- How high might the river rise? (peak discharge vs. bankfull capacity)
- What conditions would cause a "worst case"? (saturated ground + intense rain + high tide at the coast)
Management Responses Based on Hydrograph Evidence
To increase lag time (give more warning and reduce peak):
- Plant trees in the upper catchment (afforestation)
- Create upstream storage areas (temporary flood water holding)
- Restore wetlands and floodplains
- Use permeable surfaces in urban areas (SuDS)
To reduce peak discharge:
- Build dams and reservoirs upstream
- Create flood storage basins
- Allow controlled flooding of farmland upstream to protect towns downstream
- Natural flood management — leaky dams (logs and branches across small streams slow water down)
To protect against high peak discharge:
- Build embankments and flood walls alongside the river
- Install flood gates and barriers (e.g. Thames Barrier)
- Improve drainage capacity in urban areas
- Land-use zoning — prevent building on floodplains
Flood Recurrence Intervals
Floods are described by their recurrence interval — the average time between flood events of a given magnitude:
- A 1-in-100-year flood has a 1% chance of occurring in any given year
- Climate change is making previously rare floods more frequent — events described as 1-in-100-year floods are now happening every 20–30 years in some locations
- This has implications for insurance, planning permission, and the level of flood defences required
Exam tip: When interpreting a hydrograph in the exam, always refer to specific data from the graph (actual discharge values, times, rainfall figures). Do not just describe the shape — explain why the hydrograph has that shape by linking to the physical and human characteristics of the drainage basin.