Introduction to storm hydrographs

Storm hydrographs, also known as flood hydrographs, are graphical representations that illustrate how discharge varies over a short period, typically hours or days, in response to a rainfall event. Discharge is the volume of water flowing through a specific point in a river, measured in cubic metres per second (cumecs). These graphs help in understanding river responses to storms and are essential for predicting flood risks and managing water resources.

Unlike longer-term hydrographs that might span weeks or months, storm hydrographs focus on immediate reactions to precipitation, showing the interplay between rainfall, runoff, and river flow.

Key components of a storm hydrograph

A storm hydrograph consists of several distinct elements that describe the river's behaviour during and after a storm. These components reveal patterns in water flow and help explain why flooding might occur.

Main features of a storm hydrograph

• Peak discharge - The maximum level of river flow during the event, represented as the highest point on the graph.
• Lag time - The time difference between the peak of rainfall and the peak discharge; shorter lag times can lead to higher peaks because water arrives more quickly.
• Rising limb - The ascending part of the graph where discharge increases as rainwater enters the river channel.
• Falling limb - The descending section after the peak, where discharge decreases as less water flows into the river; a gradual slope indicates ongoing input from stored water sources.
• Baseflow - The steady, underlying flow from groundwater seeping into the river through its bed and banks, forming the normal river level without storm influences.
• Bankfull discharge - The flow level at which the river channel is full without overflowing; exceeding this leads to flooding.

Understanding these elements allows geographers to analyse how quickly a river responds to rain and the potential for overflow.

Types of hydrographs

Hydrographs can vary in shape based on how rapidly water moves through a drainage basin, which is the area of land drained by a river and its tributaries. This variation affects the risk of flooding and the overall river regime, which describes the pattern of flow over time.

Flashy hydrographs

Flashy hydrographs feature a rapid response to rainfall, with steep rising and falling limbs that are often symmetrical.

Key traits of flashy hydrographs:

• Short lag time, as water reaches the river quickly.
• High peak discharge, increasing flood risk.
• Produced in basins with quick runoff and limited storage.

Flat hydrographs

Flat hydrographs, sometimes called delayed hydrographs, show a more gradual response with steadily rising and falling limbs.

The type of hydrograph influences how rivers behave during storms, with flashy ones more prone to sudden floods.

Factors influencing hydrograph shape

The shape of a storm hydrograph, including its lag time and peak discharge, is determined by a combination of physical characteristics of the drainage basin and human activities. These factors control how precipitation is collected, stored, and transferred to the river channel.

Size of the drainage basin

Larger drainage basins collect more rainfall overall, resulting in higher peak discharges due to the greater volume of water involved. However, smaller basins tend to have shorter lag times because water has a shorter distance to travel to the main river channel, leading to quicker responses and potentially flashier hydrographs.

Drainage density

Drainage density refers to the total length of rivers and streams per unit area in the basin.

Effects of drainage density on hydrographs:

• High drainage density means more channels for water to enter quickly, shortening lag times and promoting flashy hydrographs.
• Low drainage density results in slower water collection, leading to longer lag times and flatter hydrographs.

Shape of the drainage basin

The overall form of the basin affects the timing of water arrival at the measurement point.

Impact of basin shape:

• Circular basins lead to flashy hydrographs because points on the watershed (the boundary dividing adjacent basins) are equidistant from the river's outlet, causing simultaneous water arrival.
• Long, narrow basins produce flatter hydrographs as water from different parts arrives at staggered times.

Relief of the drainage basin

Relief describes the steepness and elevation changes within the basin.

How relief affects hydrographs:

• Steep-sided basins accelerate downhill water flow, reducing lag times and increasing runoff, which creates flashy hydrographs.
• Gentler slopes allow more time for infiltration (water soaking into the soil), leading to slower responses and flatter shapes.

Permeability of rocks and soil

Permeability is the ability of rocks and soil to allow water to pass through them.

Influence of permeability:

• Impermeable materials prevent infiltration and storage, boosting surface runoff and producing high peaks with short lag times in flashy hydrographs.
• Permeable rocks and soils absorb water, reducing runoff and resulting in lower peaks and longer lag times in flatter hydrographs.

Vegetation cover

Vegetation plays a key role in intercepting rainfall and influencing water loss.

Effects of vegetation on hydrographs:

• Dense vegetation increases evapotranspiration (water loss through evaporation and plant transpiration), reducing runoff and leading to lower peaks and less flashy shapes.
• It also intercepts precipitation, slowing its path to the river; interception is maximised in areas with abundant plants, especially deciduous trees in leaf.

Land use and human impacts

Human modifications to the landscape significantly alter water flow patterns.

How land use affects hydrographs:

• Arable farmland, when ploughed, has less vegetation for interception, leading to faster runoff and flashier hydrographs compared to forested or pasture areas.
• Urbanisation increases impermeable surfaces like concrete and tarmac, enhancing direct runoff and shortening lag times.
• Additional urban features, such as gutters, drains, and straightened river channels, speed up water removal, contributing to flashy responses and higher flood risks downstream.
• Seasonal water use, like higher demand in summer, can lower river discharge during warmer months.

These factors collectively determine the river's regime, with implications for flood management and water resource planning. For instance, urban development often heightens flood risks by promoting flashy hydrographs.