Runoff variation and the flood hydrograph.
A storm hydrograph is a way of displaying how the discharge of a river can change over time in response to a rainfall event. 1 The discharge of a river is just the volume of water passing a certain point every second, and is calculated by multiplying the cross sectional area of the river by its velocity. Because the cross section is measured in metres2 and the velocity is measured in metres per second the discharge is measured in metres3 per second. These units are known as CUMECs (CUbic Metres per sECond).
These graphs are useful because the show us how variable runoff can be. They also reveal the contributions of water from the ground (as Antecedent or base flow) and from the soil.
The graph shows base flows and throughflow which are the contributions made to the river via soil and ground water flows. These will be ever present on the graph unless there is a long-extended period without any rainfall.
The runoff or storm flow is the water that arrives in the river via surface runoff or rapid throughflow through the rock.
The rising limb gives an indication of how fast water is reaching the channel and represents the level of water rising in the channel. The steeper the rising limb the more likely a flood is to occur, this is vital knowledge for flood forecasters. This rising limb is also known as the approach segment.
The falling limb shows the river as its level falls. A long period for this falling or recessional limb can extend the period that the river is high. It could also be interrupted by another peak in discharge if there si a secondary rainfall event following the first event.
Peak discharge is the maximum amount of water in a river after a rainfall event, if this level surpasses the bankfull discharge then a flood will occur where the river overtops its banks.
The last item indicated on the hydrograph is the lag time, this is the amount of time between the peak amount of rainfall and the peak discharge in the river. Generally, the less the lag time the quicker the river rises, the more FLASHY the graph and the more likely a flood. Lag time is therefore a key feature of a river hydrograph, as it shows how much preparation time people have before a flood strikes.
Hydrographs can take different shapes dependent upon the characteristics of the drainage basin. The various flows and stores of the drainage basin are affected by these characteristics, and these in turn will affect the shape of the hydrograph and the volume of water in a river. This is shown on the diagram below and the characteristics are explained underneath.
Some of the factors (such as interception levels, soil types and infiltration rates) that affect hydrograph shape (and more importantly) river discharges are covered in the section on Drainage Basins as open systems.
Other factors include;
1. Precipitation type, amount and duration are the most obvious reasons for river flooding. Long steady prolonged rainfall will produce rivers which rise slowly but can flood, these produce hydrographs with longer lag times and generally lower peak discharges.
Heavy short showers can cause rivers to rise quickly and burst their banks, these would have a very short lag time and high peak discharge.
Snowfall is another factor to take into account, river levels fall in the UK as precipitation is often stored as snow during cold snaps. However, when temperature warms and that snow melts many days’ worth of precipitation can end up in rivers and cause flooding. The Hydrograph would have a high peak as a result of this but an extended lag time.
2. The RELIEF or gradient of the river/drainage basin profile can also have an impact. Steep slopes tend to reduce the amount of infiltration of water into the ground, this water can then flow quickly down to rivers as overland flow. In addition, steep slopes also cause more through flow within the soil. Both can raise river levels. Gentle slopes or flat land allow water to penetrate into the soil and increase lag times and reduce peak discharges
3. Vegetation type and coverage plays a big role, with forests intercepting more rainfall than grasses. This interception increases lag time and reduces the risk of a flood. Indeed, deforestation (the removal of trees) can increase soil erosion, reduce interception and increase flood risk. Afforestation, where trees are planted, can have the opposite effect.
4. GEOLOGY - Soil and rock type can also influence what happens to precipitation when it reaches the ground. Impermeable soils and rocks such as clay or shale do not allow water to infiltrate, this forces water to run off reducing river lag times and increasing flood risk. Permeable rocks allow water to infiltrate into them. If permeable rocks allow water in through cracks, fissures and bedding planes but not through their pores they are said to be pervious (such as limestone). Porous rocks allow water to penetrate into their pores such as sandstone.
5. With more streams in an area or a higher drainage density more water can collect quickly from within the basin. This reduces lag times and increases peak discharges.
Human reasons for river flooding
Humans cause changes in LAND USE which can impact upon river flooding.
1. Urbanisation can cause flooding because many of the surfaces in towns and cities are Impermeable. The whole urban system is designed to move water from the surface into underground pipes and away from urban areas which have value. This can lead to floods in other regions.
2. Deforestation (the removal of trees) can increase soil erosion, reduce interception and increase flood risk.
3. Increases in population density can also have an impact as it places more people in flood risk areas. It is for this reason that we are building on floodplains and flood risk areas in the UK, this just increases the likelihood of a flood.
4. Agriculture can also have a major impact. Ploughing of the soil breaks it up and allows more infiltration. This reduces peak discharges and increases lag times. Conversely, drainage channels and ditches designed to drain fields speed water into local rivers and increase peak discharges and reduce lag times. When fields are harvested that reduces interception, results in greater over land flow and decreases lag times.
Other factors such as farming practices, deforestation and water abstraction are covered in detail in the next section.
When we consider discharge changes over a year we call this a River REGIME. The definition of river regime is “River regime can describe one of two characteristics of a reach of an alluvial river: The variability in its discharge throughout the course of a year in response to precipitation, temperature, evapotranspiration and drainage basin characteristics” 2
These changes and all of the factors mentioned above would have different consequences for the drainage basin and their river discharge at different times of the year. The river regime is also useful for us when considering how rivers change for different parts of the world.
The graphs below show 2 regimes for 2 rivers in different parts of the UK. The regime for the River Clyde near Glasgow contains discharges that are much higher than that of the River yare near Norwich. The Clyde is also more variable in its flow rates in winter too. This is because the North west of the British Isles is more affected by winter storms, which bring regular rainfall to the area and make the river more Flashy. The river Yare is in a drier part of the UK in the rain shadow of the East, hence the lower flows.
Source of data 3
The River Nile
The River Nile is found in northeast Africa, and is known as the longest river in the world – stretching a massive 6,695km! The Nile River is often associated with Egypt, but it in fact flows through 11 countries: Tanzania, Uganda, the Democratic Republic of the Congo, Rwanda, Burundi, Ethiopia, Kenya, Eritrea, South Sudan, Sudan and Egypt.
Two main tributaries meet to form ‘the Nile’. One tributary is called the White Nile, which starts in South Sudan, and the other is called the Blue Nile, which starts in Ethiopia. The Blue Nile and White Nile merge together in the city of Khartoum in Sudan. Another major tributary, the Atbara, joins just north of Khartoum. From there, the river continues to flow north through Egypt and, finally, into the Mediterranean Sea. 4
Below you can see the river regime for the various rivers that make up the river Nile.
Essentially the following patterns are true of the Nile;
• the Nile's minimum flow is in winter
• the maximum flood occurred during summer
The White Nile coming from the South maintains a constant flow over the year. This is because its flow is affected and minimised by;
1. Storage in Central African lakes of Victoria and Albert and
2. The Sudd, the world's largest freshwater swamp, which slows the river and allows for lots of evaporation losses. The Sudd is very efficient in reducing annual variations in streamflow. In unusually wet years increase the area of the Sudd which leads to larger evaporative losses than during dry years, when the area of the Sudd is reduced. This steady stream keeps the Nile downstream from Khartoum flowing during the winter months, when the Blue Nile/Atbara system has dried up. 5
The Blue Nile-Atbara system have a different hydraulic regime. They respond to the wet season/dry season variation of the Ethiopian highlands. In the winter, when little rain falls in the highlands, the Atbara and Blue Nile dry up. In the summer, when moist winds from the Indian Ocean cool as they climb up the Ethiopian highlands, bringing torrential rains to Ethiopia. This monsoonal rainfall fills the local rivers which flow into the Blue Nile and Atbara. Overall, these 3 river regimes keep the Nile flowing all year round but ensure a huge peak during the summer months.
1 - Hobart M. King (2018) What is a hydrograph? Accessed 14th October 2018 from https://geology.com/articles/hydrograph.shtml
2- Beckinsale RP. 1969. River regimes.
3 - National River Flow Archive, (2018). Accessed 14th October 2018 from https://nrfa.ceh.ac.uk/data/station/meanflow/84013 and https://nrfa.ceh.ac.uk/data/station/info/34001
4 - National Geographic Kids (2018) Nile River Facts. Accessed 14th October 2018 from https://www.natgeokids.com/uk/discover/geography/physical-geography/nile-river-facts/
5 - University of Dallas. Hydrology of the Nile. Accessed 14th October 2018 from https://www.utdallas.edu/geosciences/nile/Hydromap.html
Written between the 13th and 16th October 2018 - by Robert Gamesby