Reading a storm hydrograph

A storm hydrograph plots changes in river discharge before, during and after a precipitation event. Time runs along the horizontal axis, with discharge plotted against it. Precipitation usually appears as bars above the graph.

Its main characteristics are:

• Base flow: the normal groundwater contribution to the river before and between storms.
• Peak discharge: the highest discharge recorded after the precipitation event.
• Lag time: the time between peak precipitation and peak river discharge.
• Rising limb: the rise in discharge as water enters the channel.
• Falling limb: the drop in discharge as inputs decrease and stored water continues draining into the river.

A rapid-response hydrograph has a short lag time, a steep rising limb and a high peak discharge. Water enters the channel quickly, making it more likely that discharge will exceed bankfull level. With a longer lag time and lower peak, water has either been stored or has moved more slowly through the basin.

Geological influences

Permeable rock encourages infiltration and groundwater storage. The stored water reaches the river gradually as base flow, usually creating a longer lag time, gentler rising limb and lower peak discharge.

Impermeable rock limits infiltration. More precipitation travels across or near the surface, causing discharge to rise faster and potentially reach a higher peak. However, the response also depends on whether the rock and soil were already saturated before the storm.

Seasonal influences

Seasonality affects precipitation, evaporation, soil moisture and water storage throughout the year. During a wet season, saturated ground reduces infiltration and produces rapid runoff. In a dry season, greater soil-water storage may absorb rainfall at first, though extremely dry or hardened surfaces can sometimes limit infiltration.

In cold seasons, precipitation may be stored temporarily as snow and ice. Seasonal thaw or rapid snowmelt then releases the water. Rainfall falling on snow can trigger a particularly large discharge response. During warmer growing seasons, vegetation growth and evapotranspiration may reduce runoff.

Key terms: storm hydrograph, discharge, lag time, peak discharge, base flow, rising limb, falling limb, geology, permeability, seasonality.

Dimensions of flood risk

Human activity can change three connected dimensions of flooding within a drainage basin:

• Distribution: the upstream, downstream or floodplain locations that face the greatest risk.
• Frequency: how often a flood of a given level occurs.
• Magnitude: a flood’s size and severity, including peak discharge and spatial extent.

The effects aren’t spread evenly. An intervention might lower risk in one location but increase it elsewhere in the basin.

Urbanization

Urbanization removes vegetation and permeable soil, replacing them with roofs, roads and paved surfaces. This reduces interception and infiltration, so overland flow increases. Storm drains and sewers then carry water quickly into channels, which shortens lag time and raises peak discharge.

Building on floodplains also puts more people and property in areas exposed to inundation. Flood risk may rise because exposure and potential consequences have increased; other urbanization effects may also increase the physical flood hazard.

Deforestation

Deforestation reduces interception by leaves and branches. When roots and organic matter are lost, infiltration and soil-water storage can fall. Exposed soil may also become compacted. As a result, more water may reach channels quickly as overland flow, particularly in small catchments and during smaller storms, potentially increasing peak flows.

The impact varies from place to place. Extensive clearance in steep or wet parts of a basin can have a greater influence than small-scale removal in areas where soils remain permeable and well protected.

Channel modifications

Channel modifications change the route and speed of river flow. Straightening, deepening or lining a channel can increase its capacity and carry water rapidly through a settlement, reducing local flooding. The quicker transfer, however, may reduce the time water takes to reach downstream locations and increase peak discharge there.

Levees keep water confined within a channel and protect the land behind them. If a river overtops or breaches a levee, inundation may be sudden and severe. Confinement can also push floodwater downstream instead of allowing it to spread across the natural floodplain.

Human actions often redistribute flood risk rather than remove it. Their effects need to be assessed across the whole drainage basin and over time.

Key terms: flood risk, distribution, frequency, magnitude, urbanization, deforestation, overland flow, infiltration, channel modification, floodplain, levee.

Predicting floods

Flood prediction works out when and where flooding might occur, as well as how severe it could become. Forecasts of precipitation are combined with data on basin conditions and expected river discharge.

Observations may cover the location and intensity of rainfall, river levels, current discharge, soil saturation and snow storage. Predictions become more reliable when data from across the drainage basin are updated often.

Changes in weather forecasting

Wider observation networks, weather radar, satellites and computer-based forecasting have improved weather forecasts. They supply frequent information as storms develop, so forecasts can be revised when conditions change.

Improved forecasts can give people more warning time and show which areas are likely to receive intense precipitation. Predicting exact rainfall totals and locations is still difficult, particularly with localized storms or those that develop rapidly. Even a small error in the storm track may shift the heaviest rainfall into a different tributary, changing the resulting flood peak.

Climate modelling and uncertainty

Weather forecasts focus on atmospheric conditions over fairly short periods. Climate models, by contrast, explore possible longer-term changes in averages, extremes and seasonality. They may show how flood probability is changing, but cannot predict the precise date of a future flood.

Future flood risk is uncertain because several factors interact:

• future greenhouse gas emissions and the climate response
• how models represent precipitation and extreme events
• the geographical scale of model outputs compared with individual drainage basins
• future changes to land use and channels
• how soils, vegetation, snow and groundwater respond within each basin.

Different assumptions can lead models to produce different projections. Modelling is still useful: planners can compare a range of plausible outcomes and prepare for more than one flood scenario.

Key terms: flood prediction, weather forecasting, warning time, weather radar, climate modelling, uncertainty, probability, scenario.

Flood mitigation refers to attempts to tackle flooding. The aim is to reduce the physical flood hazard or limit its effects on people and property. It cannot normally remove all risk.

Structural measures

Planning measures

Personal insurance spreads the financial cost of flood losses and can help recovery. It doesn't stop floodwater, while premiums and access may vary according to the level of risk.

Flood preparation covers the arrangements households and communities make before a flood, such as protecting property and planning how people will respond. It reduces vulnerability, though success depends on awareness, available resources and the time people have.

Flood warning technology combines forecasts, river monitoring and communication systems to alert populations under threat. A timely warning gives people time to take protective action. However, warnings must be accurate, trusted, understood and received by those at risk.

Structural measures and planning are most effective when combined across a basin. Physical works may reduce the probability or magnitude of flooding; preparation, warning and insurance limit the consequences.

Two contrasting detailed examples

The two class-chosen examples must focus on mitigation in drainage basins and be genuinely contrasting. One might compare a highly engineered urban basin with a basin that emphasizes afforestation, preparation and warning. For each example, know:

• the basin's location, along with its physical and human flood-risk context
• which mitigation measures are used and where they are distributed
• how the measures change flood magnitude, frequency or impacts
• the benefits for different groups and places
• costs, limitations and unintended effects, especially transferred downstream risk
• evidence that supports an overall judgment of effectiveness.

For a brief illustration, the Rhine's Room for the River programme sets back embankments and creates more space for high flows instead of relying only on higher levees. It shows how mitigation can redistribute space to water. The class examples chosen for detailed study, however, must provide the place-specific evidence and contrast required by the course.

Key terms: flood mitigation, dams, afforestation, channel modification, levee strengthening, personal insurance, flood preparation, flood warning technology, drainage basin, transferred risk.