Storm Hydrographs

Components of a storm hydrograph

• River discharge = the river discharge measures the amount (volume) of water passing through the river channel at any one time. If the discharge of a river is particularly high, it can mean that the water has passed over the banks resulting in a flood.

• Rainfall = the amount of rain received over a certain amount of time

• Peak rainfall = the point at which the amount of rain received is at its highest

• Rising limb = the rising limb is an indicator of the speed of the river reacting to a storm event. For example, if the gradient of the rising limb is steep, then the river responds quickly, and if the gradient is less steep, it responds more slowly.

• Peak discharge = the point where the river discharge is highest for the storm event. If there is a high peak discharge, there is an increased probability of the river flooding.

• Falling limb = the falling limb shows the river discharge decreasing as it recovers from the storm. The steeper the falling limb, the more quickly the river discharge is decreasing.

• Baseflow = the base flow refers to the amount of water in the river ordinarily (i.e. even without the storm event). The base flow can charge depending on the time of year and the surrounding climatic and drainage basin conditions.

Lag time hydrograph

The lag time is arguably the most important part of a storm hydrograph. This is because, as with the rising limb, the lag time can be used as an indicator of how quickly a river responds to a storm event.

Hydrograph analysis

Physical factors

There are many drainage basin characteristics that can alter the shape of a storm hydrograph:

If a drainage basin is large in area, then it typically takes the rainfall it receives longer to reach the river because it has a further distance to travel. This generally results in a longer lag time, and a less steep rising limb. The drainage basin shape also plays an important role. If a drainage basin resembles a circular shape, then rainfall takes less time to reach the river on average, resulting in a more flashy hydrograph. This is because if the drainage basin were more rectangular, rainfall received at the points furthest from the river would take a much longer time to reach it. This would contribute to a significantly longer lag time. As well as the shape of the drainage basin perimeter, the steepness (gradient) of its sides also alters hydrograph shapes. If the sides of a drainage basin are steeper, then rainfall will move down them more quickly, resulting in a flashy hydrograph. By contrast, if the sides have gentler slopes, then it will take longer. The permeability of the ground can impact the lag time. If the ground is permeable, then water can seep through the gaps. However, if it is impermeable then water cannot pass through. This leads to increased surface runoff, which is the fastest way for rainfall to reach the river. Therefore, if the drainage basin is impermeable then there tends to be a flashier hydrograph and a greater risk of flooding. Anything that can affect the speed that water travels from around the drainage basin to the river also influences the storm hydrograph. Vegetation can significantly affect the lag time. This is due to the fact that vegetation intercepts rainfall, meaning that it takes considerably longer to reach the river. This results in a longer lag time, and gentler rising limb.

Human factors

While the land surrounding a river can change naturally, anthropogenic influence can severely disrupt the natural environment and the reaction of a river to a storm event. One of the main ways that humans do this is through land-use change. Urbanisation, where an increased proportion of the population lives in urban areas, often leads to the construction of impermeable surfaces. As already mentioned, the permeability of land in a drainage basin is a key factor affecting the lag time, and by extension, a storm hydrograph. Humans can also alter a hydrograph through its impact on the base flow of a river. As demand for water increases, humans are having to extract more of it, using a process called water abstraction. If significant amounts of water are removed from a river, then its base flow will be lower. This means that it would take more rainfall in a storm event for the river to flood.

Urban hydrograph

Although storms of equal intensity and duration may affect an urban and rural area at the same time, it is likely that the storm hydrographs will end up looking significantly different from one another. This is because of the factors we have just outlined. An urban hydrograph tends to be flashy and so has a short lag time and steep rising and falling limbs. This means that the river typically takes a short amount of time to respond to the storm and reach a point of peak discharge. We know from earlier that this increases the chances of the river flooding, which can have negative impacts on people, their property, infrastructure and the environment. So, why does this happen? Land-use change by humans creates non-permeable surfaces across much of the drainage basin. This increases surface run-off and reduces the possibility of infiltration processes. In addition, urban areas are typically less vegetated than surrounding rural areas. We have seen earlier that this helps water reach a river faster by reducing interception.

Hydrographs help us to decide whether or not Water Supply Management features ought to be put in place to protect the people and area a river flows through.