Hydrographs made EASY

Are you interested in learning about hydrographs? Then you have come to the right place! In this article I will teach you what hydrographs are, what they look like and what they are used for. Ready to learn more? Read on…

Contents

What is a hydrograph?

A hydrograph is a graph that shows how the flow of a river changes over time.

Hydrographs typically display the amount of water flowing in a river (known as the “discharge”) on the y-axis and time on the x-axis.

Hydrographs can help us understand how a river responds to different weather conditions, such as heavy rain or drought.

Why are hydrographs useful?

Hydrographs are useful because they provide a visual representation of how a river’s flow changes over time.

By analysing a hydrograph, we can learn about the characteristics of a river’s flow, such as how quickly water is flowing, how much water is in the river, and how long it takes for the river to return to normal levels after a rain event.

This information can be used to predict and manage the risk of floods, as well as to assess the availability of water resources for human use, such as irrigation or drinking water.

Hydrographs can also be used to study the impacts of human activities on river systems, such as dam construction or urbanisation.

Hydrographs are a valuable tool for understanding and managing water resources in a given area.

What are the types of hydrographs?

Not all hydrographs are the same and there are actually several types of hydrographs. Lets take a look at what these are…

Rising limb hydrograph

A rising limb hydrograph is a hydrograph that shows the increase in river flow as water from precipitation enters the river system.

Recession limb hydrograph

A recession limb hydrograph shows the decrease in river flow as water is released from the river system.

Flashy hydrograph

A flashy hydrograph is a hydrograph that shows a rapid increase in river flow in response to a short period of intense precipitation, or a ‘flash’ of rain.

Groundwater hydrograph

A groundwater hydrograph is one that shows changes in the level of groundwater over time, which can impact river flow.

Storm hydrograph

A storm hydrograph shows changes in river flow during and immediately after a storm event.

Snowmelt hydrograph

And lastly, a snowmelt hydrograph is one that shows changes in river flow as snowpacks melt, which can release large amounts of water into rivers.

These different types of hydrographs can provide insight into how water moves through a river system under different conditions, and can be used to inform decisions about water resource management and flood risk assessment.

What does a storm hydrograph look like?

It is helpful to be familiar with what a hydrograph looks like and how to read it. Below is an example hydrograph for your reference.

A hydrograph will usually have a sharp increase in the river’s flow (the rising limb) as precipitation begins to enter the river system, followed by a peak discharge as the storm reaches its maximum intensity.

After the storm ends, the flow will gradually decrease (the recession limb) as water moves out of the river system.

The shape and size of the peak discharge can vary depending on factors such as the intensity and duration of the storm, the size and shape of the river basin, and the amount of water already in the system before the storm began.

How are hydrographs used to forecast floods?

Hydrographs are commonly used to forecast floods by monitoring the river’s flow and using this information to make predictions about how the river will respond to upcoming weather conditions.

By analysing the shape and size of a hydrograph, scientists and water resource managers can make predictions about the timing and severity of floods.

To forecast floods using hydrographs, experts first monitor the river’s flow and take measurements of the water level at regular intervals. These measurements are plotted on a hydrograph, which allows them to see how the river’s flow is changing over time. If the river is already at a high level due to previous rain events or snowmelt, this information can be used to predict the river’s response to additional precipitation.

Once a hydrograph is generated, experts can use it to make predictions about how the river will respond to future weather events, such as a heavy rainstorm.

By using the historical data from the hydrograph, they can estimate the time it will take for the river to reach its peak flow and how high the water level will rise.

This information can be used to alert communities at risk of flooding and to help them prepare for the event.

In this way, hydrographs are an essential tool for flood forecasting and disaster risk reduction.

The features of a hydrograph

There are some main features that you will see on a hydrograph, some of which have been briefly mentioned previously in this article. Lets take a deeper look at these features to ensure that we are confident in how to read a hydrograph…

Rising limb

The part of the hydrograph that shows the increase in river flow as water from precipitation enters the river system.

Peak flow

The highest point on the hydrograph, which represents the maximum river flow during the storm event.

Lag time

The time it takes for the peak flow to occur after the precipitation event.

Recession limb

The part of the hydrograph that shows the decrease in river flow as water is released from the river system.

Base flow

The normal, low level of flow in the river that occurs between storm events.

Each of these features provides valuable information about the river’s flow characteristics and how it responds to precipitation events.

The rising limb and peak flow can be used to estimate the amount of water entering the river system, while the lag time can help predict the timing of peak flow.

The recession limb and base flow can be used to understand how quickly water leaves the river system and how much water is available during dry periods.

By analysing these features of a hydrograph, water resource managers can make informed decisions about flood management, water allocation, and other important aspects of water resource management.

What factors lead to flooding?

There are a range of factors that can lead to flooding. I have outlined these briefly for you below.

Human activities that lead to flooding

Unfortunately, humans can cause a lot of problems with the natural hydrological cycle, which results in flooding. Some examples of how this might occur are…

Urbanisation

The development of urban areas can result in an increase in impervious surfaces, such as concrete and asphalt, which prevent water from being absorbed into the ground. Instead, water is channelled into stormwater drainage systems and can overload rivers and other bodies of water.

Deforestation

Cutting down trees and other vegetation can lead to an increase in surface runoff and erosion, which can lead to flooding.

Agricultural practices

Activities such as overgrazing, irrigation, and ploughing can alter the natural hydrology of an area, leading to increased runoff and soil erosion.

Dam construction and removal

Building dams can alter the flow of rivers and change the natural flood regime of an area. Similarly, removing dams can also alter the flow of rivers and increase the risk of flooding downstream.

Land use change

Changing the use of land from natural vegetation to urban or agricultural use can alter the natural hydrology of an area and lead to increased runoff and flooding.

Climate change

Global warming and changes in precipitation patterns can alter the frequency and severity of extreme weather events, such as heavy rainfall, which can lead to flooding.

Poor infrastructure

Inadequate drainage systems, poorly maintained levees, and other infrastructure can increase the risk of flooding in areas with high rainfall or in areas that are prone to storm surge from coastal storms.

Climatic conditions that lead to flooding

Flooding often occurs due to the climatic conditions. The following conditions frequently lead to flooding around the world:

Heavy rainfall

When a large amount of rain falls in a short period of time, it can overwhelm drainage systems and cause rivers and other bodies of water to flood.

Rapid snowmelt

When snow melts rapidly due to warm temperatures, it can lead to a sudden increase in the volume of water in rivers and other bodies of water, causing flooding.

Coastal storms

Storms such as hurricanes and typhoons can cause storm surge, which is a rise in sea level that can lead to flooding in coastal areas.

Ice jams

In cold climates, ice can form on rivers and other bodies of water. When the ice breaks up, it can create ice jams that block the flow of water, leading to flooding.

Drought followed by heavy rainfall

When a prolonged drought is followed by heavy rainfall, the dry soil cannot absorb the water quickly enough, leading to increased runoff and flooding.

Slope form that leads to flooding

Another geographical aspect that can lead to flooding is the shape and angle of the slope. The slope forms outlined below can impact the hydrology of an area and can increase the risk of flooding. With proper planning, management, and conservation of natural resources, however, we can help mitigate the impact of these slope forms and reduce the risk of flooding.

Steep slopes

Areas with steep slopes are more prone to flooding because water flows more quickly and can cause erosion and landslides, which can lead to increased runoff and flooding downstream.

Concave slopes

Slopes that are concave or bowl-shaped can trap water and prevent it from flowing downhill, leading to pooling and increased runoff that can cause flooding.

Convex slopes

Slopes that are convex or outwardly curved can cause water to flow more quickly and can lead to erosion and landslides, which can increase runoff and cause flooding downstream.

The convex shape of the slope can also cause water to be concentrated in specific areas, leading to pooling and increased runoff that can contribute to flooding.

Areas with high vegetation cover: Slopes with high vegetation cover can absorb more water and reduce the risk of flooding, but if the vegetation is removed or damaged, the slope can become more prone to flooding.

Soil properties that lead to flooding

Lastly, there are certain soil types that are more likely to cause flooding than others. These are outlined below:

Shallow soil

Shallow soil cannot hold much water, and any excess water will run off the surface. This can lead to increased runoff and the potential for flooding downstream.

Thick soil

Thick soil can hold more water than shallow soil, but if it becomes saturated with water, it can also lead to increased runoff and flooding downstream.

Saturated soil

When the soil is already saturated with water, it cannot absorb any more water, leading to increased runoff and the potential for flooding.

Dry sandy soil

Sandy soil has large pore spaces that allow water to pass through easily, but if the soil becomes compacted or has low organic matter content, it can lose its ability to hold water and increase the risk of flooding.

Dry baked clay soil

Baked clay soil has a hard, impermeable surface that prevents water from being absorbed. This can cause water to run off the surface and increase the risk of flooding.

Fine clay soil

Fine clay soil can hold water well, but if it becomes saturated with water, it can cause the soil to become unstable and increase the risk of landslides and flooding.

Coarse textured soil

Coarse textured soil has large pore spaces that allow water to pass through easily, but if the soil becomes compacted or has low organic matter content, it can lose its ability to hold water and increase the risk of flooding.

These soil properties can interact with other factors such as topography, rainfall, and human activities to increase the risk of flooding. Understanding the soil properties of an area can help in planning and management strategies to reduce the risk of flooding.

Hydrograph FAQs

Now that we understand what a hydrograph looks like and what it is used for, lets take a look at some of the most common questions that people ask about hydrographs and the answers…

What is a hydrograph?

A hydrograph is a graph showing the rate of flow (discharge) of a river over time.

What is the importance of a hydrograph?

Hydrographs are useful in studying the behaviour of rivers and predicting flood events. They also help in water resource management and decision-making.

What are the key features of a hydrograph?

The key features of a hydrograph include the rising limb, peak flow, lag time, falling limb, and base flow.

What is the rising limb of a hydrograph?

The rising limb of a hydrograph is the initial increase in discharge following a rainfall event.

What is peak flow in a hydrograph?

Peak flow is the highest discharge rate on the hydrograph, usually occurring during the peak of a flood event.

What is lag time in a hydrograph?

Lag time is the time between the peak rainfall and the peak flow on the hydrograph.

What is the falling limb of a hydrograph?

The falling limb of a hydrograph is the decrease in discharge rate following the peak flow.

What is base flow in a hydrograph?

Base flow is the normal discharge rate of a river, occurring during dry periods when there is no rainfall.

How are hydrographs useful in flood forecasting?

Hydrographs can help predict the timing and magnitude of a flood event by analysing the relationship between rainfall and discharge rates.

What are the different types of hydrographs?

The different types of hydrographs include storm hydrographs, unit hydrographs, and recession hydrographs.

What is a hydrograph: To conclude

You should now be confident about what a hydrograph is and why we use them. If you enjoyed this article and would like to learn more, take a look at these posts too:

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