Storm hydrographs measure the response of a river to a storm. One of the key ways they do this is by measuring the time taken for the river discharge to change in response to increased levels of rainfall. They are affected by physical and human factors, with urban hydrographs differing drastically from forest hydrographs.
Hydrograph definition
A storm hydrograph is a type of graph that shows the changes in river discharge in the lead up to and following the start of a storm. In other words: a hydrograph shows how a storm has affected a river. This information can be handy, as it can be used to establish a pattern. If a river typically responds to storms in the same (or similar) way, then methods of river management can be introduced to reduce negative impacts and protect the land and people nearby.
Let’s look at how the hydrographs work.
Components of a storm hydrograph
Storm hydrograph diagram. Figure created by the author.
The graph above represents a typical storm hydrograph. As is clear, there are lots of different components.
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.
The lag time is the amount of time that passes between peak rainfall and peak discharge.
If the lag time is short, it means that the river has responded quickly to a storm. By contrast, if the lag time is long then the river has responded more slowly. A short lag time is associated with a higher risk of
flooding. This means that if a river tends to have short lag times on hydrographs, measures should be taken to manage the risk of
flooding.
Hydrograph analysis
Analysing a storm hydrograph reveals information about the severity of a storm, the impact on the river and the time that it takes for it to respond to the storm event. This information is very useful, as it can facilitate river management efforts. For example, if we can guess how a river will respond to a certain amount of rainfall, we can also guess the likelihood of the river flooding and can prepare for that occurrence. More data on how a river responds to storm events mean that policymakers can be better prepared to act to prevent risks of flooding.
So, how do you analyse a storm hydrograph?
The main way to analyse a storm hydrograph is to examine its shape. While all hydrographs share the same components, they can differ drastically in shape. For example, some have steep rising and falling limbs, high peak discharges and a short lag time while others appear much more flat because they have flatter rising and falling limbs, lower peak discharges and a long lag time.
There are special words used to describe storm hydrographs that display certain characteristics. If a hydrograph resembles the former example (steep limbs; high peaks and short lag time), it is described as flashy.
Factors affecting storm hydrographs
There are many factors which affect storm hydrographs. Often, a combination of physical and human factors combine to alter components of a hydrograph and dictate how a drainage basin responds to a storm event. Let's have a look at a few examples:
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.
Storm Hydrographs - Key takeaways
- Storm hydrographs present the response of a river to a storm.
- They are read by calculating the amount of time between the peak rainfall level and peak river discharge.
- Flashy hydrographs are those with steep rising and falling limbs, short lag times and high peak discharges. These hydrographs are reflective of rivers more vulnerable to flooding.
- Many physical human factors affect river responses to storms.
- Urban hydrographs tend to be more flashy than non-urban hydrographs, driven mainly by the prominence of non-permeable surfaces.
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