5.3C Storm Hydrographs
Storm hydrographs' shape depends on physical features of drainage basins (size, shape, drainage density, rock type, soil, relief and vegetation) as well as human factors (land use and vegetation) (P: the role of planners in managing land use)
Whereas river regimes are usually graphed over the period of a year, storm hydrographs show discharge changes over a short period of time, often no more than a few days. The storm hydrograph plots two things: the occurrence of a short period of rain (maybe a heavy shower or storm) over a drainage basin and the subsequent discharge of a river.
Main features of a hydrograph:`
- Once the rainfall starts, the discharge begins to rise - rising limb
- Peak discharge is reached some time after the peak rainfall because the water takes time to move over and through the ground to reach the river.
- The time interval between peak rainfall and peak discharge is known as lag time.
- Once the input of rainwater into the river starts to decrease, so does the discharge; this is shown by the falling or recessional limb
- Eventually the river's discharge returns to its normal level, or base flow.
The shape of a storm hydrograph of the same river may vary from one rain event to the next. This variation is closely linked to the nature of the rainfall event. The shape of the hydrograph also varies from one river to another. This is a result of the particular physical characteristics of individual drainage basins.
Some hydrographs have very steep limbs, especially rising limbs, a high peak discharge and a short lag time. These are often referred to as 'flashy' hydrographs. In contrast, there are some hydrographs with gently inclined limbs, a low peak discharge and a long lag time. These are called 'delayed', or 'flat' or 'subdued' hydrographs.
Urbanisation
When it comes to evaluating the factors affecting the character of storm hydrographs, particularly their 'flashiness', none is more important than urbanisation. Not least of its impacts is that it changes the characteristics of the land surface. Its effects on hydrological processes include the following:
- Construction work leads to the removal of the vegetation cover. This exposes the soil and increases vegetation cover
- Bare soil is eventually replaced by a covering of concrete and tarmac, both of which are impermeable and increase surface runoff.
- The high density of buildings means that rain falls on roofs and is then swiftly fed into drains by gutters and pipes.
- Drains and sewers reduce the distance and time rainwater travels before reaching a stream or river channel.
- Urban rivers are often channelised with embankments to guard against flooding. When floods occur, they can be more devastating.
- Bridges can restrain the discharge of floodwaters and act as local dams, thus prompting upstream floods.
In short, the overall impact of urbanisation is the increase the flood risk. The problem is made worse by the fact that so many towns and cities are located close to rivers. Historically, this was for reasons of water supply and sewage disposal. Often the historic nucleus was located at a point where a river could be easily crossed.
Synoptic themes
Planners have become important players in managing the impacts of urbanisation on flood risk. This is because:
- many towns and cities are naturally prone to flooding because of their locations
- of the number of people who live in urban places and who therefore need protection
- of the huge amount of money invested in urban property.
Flood risk management involves such actions as:
- strengthening the embankments of streams and rivers
- putting in place flood emergency procedures
- steering urban development away from high-risk areas such as floodplains
Interacting factors affecting the shape of two different storm hydrographs
Factor |
'flashy' river |
'flat' river |
Description of hydrograph |
Short lag time, high peak, steep rising limb |
Long lag time, low peak, gently sloping rising limb |
Weather/climate |
Intense storm that exceeds the infiltration capacity of the soil Rapid snowmelt as temperatures suddenly rise above zero Low evaporation rates due to low temperatures |
Steady rainfall is less than the infiltration capacity of the soil Slow snowmelt as temperatures gradually rise above zero High evaporation rates due to high temperatures |
Rock type |
Impermeable rocks, such as granite, which restrict percolation and encourage rapid surface runoff. |
Permeable rocks such as limestone, which allow percolation and so limit rapid surface runoff. |
Soils |
Low infiltration rate, such as clay soils (0-4mm/h) |
High infiltration rate, such as sandy soils (3-12mm/h) |
Relief |
High, steep slopes that promote surface runoff |
Low, gentle slopes that allow infiltration and percolation |
Basin size |
Small basins tend to have flashy hydrographs |
Larger basins tend to have more delayed hydrographs; it takes time for water to reach gauging stations |
Shape |
Circular basins have shorter lag times |
Elongated basins tend to have delayed or attenuated hydrographs |
Drainage density |
High drainage density means more streams and rivers per unit area, so water will move more quickly to the measuring point. |
Low drainage density means few streams and rivers per unit area, so water is more likely to enter the ground and move slowly through the basin |
Pre-existing (antecedent) conditions |
Basin already wet from previous rain, water table high, soil saturated, so low infiltration/percolation |
Basin dry, low water table, unsaturated soils, so high infiltration/percolation |
Vegetation |
Bare/low density, deciduous in winter, means low levels of interception and more rapid movement through the system |
Dense, deciduous in summer, means high levels of interception and a slower passage through the system; more water lost to evaporation from vegetation surfaces |
Human activity |
Urbanisation producing impermeable concrete and tarmac surfaces Deforestation reduces interception Arable land, downslope ploughing |
Low population density, few artificial impermeable surfaces Reforestation increases interception Pastoral, moorland and forested land |