Hydrology & Fluvial Geomorphology

1.4 The drainage basin system

Inputs, outputs, stores and flows

Water is an important global resource. It is therefore important to understand the hydrological cycle (how water moves around the atmosphere, biosphere and lithosphere) and the impacts humans can have on it.

The hydrological cycle is a closed system (water cannot leave or enter this system) whereas the drainage basin (an area of land where precipitation collects and drains off into a common outlet, such as into a river, bay, or other body of water) is an open system (water can leave and enter each system). Water is stored and moved around these systems.

The only input into the drainage basin system is precipitation (water falling from the atmosphere as rain, hail, snow or sleet).

Flows transfer water between stores. These happen above and below the earth's surface.

Above surface flows are:

• stemflow - water that moves down the branches and main trunk of a tree;

• leaf-drip/throughfall - water that falls from leaves/through gasps in the leaves of trees;

• overland flow/surface run-off - water that flow's over the land's surface. If the ground can no longer hold any more moisture, then saturated overland flow occurs. If the rainfall is intense, it can exceed the infiltration capacity (Hortonian overland flow).

Below surface flows are:

• infiltration - the process by which water soaks into or is absorbed by the soil (infiltration capacity is maximum rate at which this can happen);

• percolation - water flowing down into the bedrock;

• throughflow - the horizontal movement of water just below the surface;

• groundwater flow/baseflow - the horizontal movement of water below the water table.

Stores are where water is retained for a period of time. This can vary from minutes to many thousands of years. The various stores are:

• interception - rain that is retained by a plant before being absorbed or evaporated;

• surface storage - water that is retained on Earth's surface e.g. in a lake;

• soil moisture storage - water that is held in the pores in soil;

• groundwater storage - sub-surface water that is held below the ground;

• channel storage - water that is held in the river channel itself.

Outputs are the process by which water leaves a drainage basin. This is can be through evaporation (the process by which a liquid is changed into a gas), transpiration (water lost from the stomata in the leaves of trees and vegetation) - these are termed evapotranspiration together, as well as water flowing into a lake or the sea as discharge (the amount of water flowing in a river past a set point over a set period of time).

Underground water: water tables, ground water, recharge, and springs

Groundwater accounts for around 96% of all freshwater on Earth. Storage times can vary from minutes to many thousands of years. The water table is the level of saturation underground. The saturated zone is known as the phreatic zone and the unsaturated layer above the water table the vadose zone. Recharge refers to the refilling of water in pores. In some areas, recharge is not taking place due to human abstraction and this causes problems such as water shortages.

Aquifers, like sandstone and limestone, are rocks that contain a significant amount of water. They engender stream flow in times of little rainfall and regulate the hydrological cycle by absorbing rainfall. Springs are where the water reaches the surface, and, if substantial enough, may become the source of a river. Boreholes (a hole in the ground relying on a pump for extraction of water) and artesian wells (a well that does not require a pump to bring water to the surface) are used to extract water.

1.2 Discharge relationships within drainage basins

Components of hydrographs

An annual hydrograph shows a river's regime over the course of a year whereas a storm hydrograph shows the amount of discharge (the amount of water flowing past a set point, over a set period of time, measured in cumecs (m³/sec)) over a short period of time, usually a few days.

The baseflow is the river's normal flow and is supplied by water seeping from the bedrock. During a storm, water will flow quickly over the ground's surface as runoff or will infiltrate the surface and flow as throughflow. These together are termed stormflow. The approach segment is the amount of discharge before the storm. The angle of the rising limb is determined by how quickly water reaches the river channel. Similarly, the falling limb shows how quickly the river's discharge returns to normal baseflow. The peak discharge is the highest level the river reaches in a flood. The level that after which a river will flood is referred to as the bankfull discharge. In order to gauge a river's response to a storm, hydrologists also look at the lag time which is the time between peak discharge and peak rainfall (the maximum amount of rainfall).

Influences on hydrographs: climate

Precipitation can affect the shape of a storm hydrograph. Prolonged and intense rainfall both lead to overland flow as the ground becomes saturated and the infiltration capacity is exceeded respectively. Seasonal snow melt can release a large amount of water into a river over a short period of time. During the rest of the year, precipitation in the form of snow will reduce a river's discharge.

Hotter temperatures in summer lead to higher evapotranspiration rates and lower levels in discharge. In winter, lower temperatures and frozen ground mean that more precipitation gets into a river quickly. Seasonal variations also have an impact on storm hydrographs.

If it has recently rained and there is moisture already in the soil (antecedent soil moisture), then rainfall will quickly produce overland flow, a shorter lag time and a higher peak discharge.

Influences on hydrographs: drainage basin characteristics

Smaller drainage basins respond more quickly to rainfall than larger ones. For example, the Boscastle Flood, Cornwall, UK of 2004 only drained an area less than 15km². The shape of a drainage basin is also important - smaller basins respond more quickly. Basins with lots of tributaries compared with their size (high drainage density) respond more quickly than those with a lower drainage density, which consequently have a long lag time.

Areas of vegetation engender interception, evapotranspiration and infiltration which reduces the flood peak and increases lag time.

The soil and rock type either promote or slow down water reaching the river channel. Permeable soils reduce overland flow whereas impermeable surfaces, like tarmac and concrete, enhance it and reduce baseflow and throughflow. Gutters and drains speed-up throughflow. Therefore, land use is also an important influence on the shape of a storm hydrograph.

The steepness of the slopes in a drainage basin affects how quickly water reaches the channel. Steep slopes promote surface runoff whereas shallower slopes allow more infiltration and less surface runoff.

1.3 River channel processes and landforms

Channel processes

Rivers erode, transport and deposit material.

Erosion is the wearing away, and removal, of Earth's surface. Abrasion (corrasion) is where the load of the river erodes the bed and banks, a bit like sandpaper. The load type and hardness, as well as bank resistance affect the rate of this erosion. Attrition creates smaller, rounder particles and involves the wearing away of the load as the rocks and stones collide with each other. Corrosion (solution) is the removal of chemical ions, especially calcium. Maximum rates occur where fast-flowing, undersaturated streams pass over soluble rocks. Hydraulic action is where air and water are forced into cracks in the river bank, breaking them apart. Cavitation is the force exerted as air bubble implode. Factors that affect the rate of erosion include the load type; velocity of the river channel; gradient; geology; pH as well as the human impact.

Transportation involves material in the river channel being moved. The smallest particles are suspended in the water (suspension). Medium-sized particles travel through a series of 'hops' through saltation. The heaviest particles roll along the river bed through traction. In areas of calcareous rock, material is carried in solution, the dissolved load.

Deposition (where the river drops its load) occurs when there is a reduction in velocity and energy, and the load can no longer be carried. This could occur with a reduction in gradient or a decrease in the volume of water in the channel, or an increase in friction between the river and the bed and banks. In terms of global sediment yield, Earth experiences a mean rate of land surface lowering by fluvial denudation by 0.1mm/year, although this is not equally represented across the planet.

Bradshaw's model, proposed by Michael Bradshaw in 1978, shows how a river's characteristics change downstream over the course of its long profile (the gradient and associated changes along a transect line from source to mouth).

The Hjulström curve, published by Filip Hjulström in 1935, is used to determine whether a river will erode, transport, or deposit sediment. It is a type of logarithmic (non-linear) graph. The entrainment velocity, or critical erosion velocity is the lowest velocity at which grains of a given size can be moved. In short, larger particles require higher velocities to be eroded, and a smaller reduction in velocity to be deposited than smaller particles (and vice versa). However, clay particles are an exception - because they are cohesive (they stick together), they require a higher velocity to break these strong cohesive bonds. Similarly, fine silts and clays sometimes never get deposited until they reach the sea at a river's mouth. Here the particles flocculate (coagulation caused by the charged ions in seawater) and become heavy enough to be deposited.

River Flow

Manning's 'n' or Manning's equation (named so because the Irish engineer Robert Manning re-developed it in 1890) can be used to indicate bed roughness. The higher the value, the rougher the bed. For example, for a mountain stream with a rocky bed, n=0.4, whereas for an alluvial channel, n=0.014.

The hydraulic radius is a measure of how efficient the river. The greater the hydraulic radius, the larger the flow rate. It is given as R = A/P, where R is the hydraulic radius, A is the cross-sectional area, and P is the wetted perimeter.

There are three main flow patters. Laminar flow refers to low-velocity movement of a fluid over a smooth surface. Therefore, in a river channel, this is difficult to observe due to other influences such as friction. It can be seen over paved roads when there is heavy rainfall causing the water to flow as a 'sheet'. Turbulent flow is a result of friction, and causes the faster water to overtake the slower moving water. This creates eddies at the sides of a river, which are localised movement of water, usually in the opposite direction of flow.

Helicoidal flow is the corkscrew-like flow superimposed on the primary downstream flow of a river. The fastest line of flow is referred to as the thalweg (derived from the German term, 'talweg'). Helicoidal flow is very important in the formation of meanders, but also occurs in straight river channels.

Channel types

There are three main types of river channel; straight, meandering and braided. Sinuosity is a measure of how much meandering a river does and is expressed as a ratio between the length of the channel and the valley length. A value of 1.0 is a straight river channel, whereas a river with a high sinuosity may be up to 4.4. A river is said to meander when the sinuosity ratio is greater than 1.5.

Straight river channels are quite rare, due to the movement of the thalweg, and the resulting erosion on the river bank, resulting in channel migration.

Meandering river channels are found around floodplains in the lower courses of a river where lateral erosion dominates. Braided river channels occur when the channel is divided by islands (vegetated) or bars (lacking vegetation). These are sometimes collectively referred to as eyots or aits. Braiding occurs where the river lacks the capacity to transport its load in a single channel, for examples where there is a steep channel gradient; a large amount of coarse material; easily erodible bank material or highly variable discharge. Periglacial and semi-arid areas are ideal for braiding as there is a large supply of sediment.

Landforms of the upper course

Potholes are bowl-shaped hollows in the riverbed that are deeper than wider. They form when grinders (the materials which erode a pothole) enlarge a depression by abrasion as material swirls round the hole.

Rapids are stretches of fast-flowing water tumbling over a rocky, shallow riverbed. They from when the gradient is steep and the riverbed is rocky and erosion-resistant leaving the less resistant material to be eroded.

Waterfalls (a point in the long profile of a river where the water descends vertically) forms where the river spills over a sudden change in gradient, undercutting rocks by hydraulic action, abrasion and cavitation. The change in gradient may be due a band of resistant strata (limestones at Niagara Falls, USA); a fault scarp (Gordale, Yorkshire, UK); a plateau edge (Victoria Falls, Zambia); a hanging valley (Glencoyne, Cumbria, UK) or coastal cliffs. As the waterfall collapses and retreats, a gorge (a deep and narrow chasm with precipitous walls) is formed.

Landforms of the middle course

Meanders are complex landforms that can be difficult to explain. However, they are not caused by obstructions. Left to their own devices, fluids and gases in motion meander. This produces some interesting (generalised) characteristics:

• Meandering best develops at or near bankful state

• Meander wavelengths are five times the radius of curvature

• Meander wavelengths are ten times channel width and discharge

• Meander amplitude is ten times the chaneel width

• Riffles occur at six times the channel width

• As depth and width increase, so does sinuosity

• When the bedload is varied, meandering is more pronunced

• In streams carrying coarse debris, meander wavelength increases

• Meandering is more likely on shallow slopes

Meanders are thought to be initiated by the formation of a pool and riffle sequence. In a turbulent channel, eddies result in the deposition of coarse sediment to form riffles (a depositional bar on the channel floor of a river) and at high velocity points with finer sediment, pools (deeper sections) form. Once pools and riffles have developed, the river flows from side-to-side in a winding course.

Helicoidal flow within the thalweg moves material from the outside of one meander bend and deposits it on the inside of the next bend. Water moving faster has more energy to erode. This occurs on the outside of the bend and forms a river cliff. The river erodes the outside bends through hydraulic action, corrasion and corrosion. Water moves slowly on the inside of the bend and the river deposits some load, forming a slip-off slope (a depositional landform of sand and pebbles that occurs on the inside convex bank of a meandering river) or point bar (a depositional feature made of alluvium that accumulates on the inside bend of a river channel, below the slip-off slope). These are technically different but you can refer to either in your exam. Meanders therefore have an asymmetric cross-profile. Helicoidal flow is instrumental in meander formation.

Landforms of the lower course

Depositional processes dominate the lower course and subsequent landforms. A floodplain is the flat land adjacent to the river channel over which a river flows in times of flood. They are often poorly drained. Near the channel edge, levees form. These are depositional features with coarser sediment near the channel and finer sediment further away as it is carried further. This can lead to the formation of backswamps (major storage units of fine-grained, vertically accreted, suspended load sediments). Older floodplains become eroded to form terraces (a bench or step that extends along the side of a valley and represents a former level of the valley floor), at the edge of which are bluffs (broad, rounded cliffs).

An oxbow lake forms when erosion and deposition continue over time, causing the river channel to become more sinuous. Eventually, during the high energy conditions of a flood, for example, the meander neck is breached and the meander is cut-off from the channel, which is now taking the more efficient route to the sea. Sedimentation seals off this oxbow lake, which is a temporary feature, as over time, vegetation will colonise the area resulting in an area of marshy ground.

Deltas are depositional features formed when a river meets the sea or a standing boy of water, such as a lake. With a reduction in velocity, deposition of load occurs. Salt water causes clay particles to flocculate (stick together due to the compression of the electrical double layer of ions). Small channels or distributaries can form here. Within a delta there are often lakes or lagoons. Fine alluvium builds up, making deltas very fertile, but vulnerable to flooding. The Ganges delta in India and Bangladesh is a good example.

The load size affects the structure of the delta, and three distinct beds can be identified: bottomset, middleset and foreset. The shape of the delta can vary, and three types can be identified: arcuate, cuspate and bird's foot.

1.4 The human impact

Modifications to catchment flows and stores and to channel flows by land-use changes

Human activity on the drainage basin system can either be direct (intentional e.g. storing water in dams) or indirect (unintentional e.g. deforestation).

Precipitation can be affected, for example, by cloud seeding, which involves adding silver iodide or other artificial particles to clouds to act as condensation nuclei for water droplets to form. Australia and the USA have increased their rainfall amounts this way by 10-30%. However, this will cause a dearth of rainfall in other areas. Similarly, air pollution from cities can produce up to 10% more rainfall than in rural areas. Global warming is also a contributor to increased rainfall as warmer seas produce more evaporation and therefore more rainfall.

Interception can vary depending on vegetation type and density. Deforestation leads to an increase in surface runoff and a reduction in evapotranspiration, surface storage and lag time. Afforestation aims to combat this.

Humans greatly affect the infiltration rate. Impermeable surfaces from urbanisation greatly increase overland flow and result in higher storm peaks. In forested areas, infiltration is five times greater than grassland. Grazing animals can also result in overland flow as compaction of the ground makes it harder for water to infiltrate. Ploughing has the opposite effect as it loosens the soil. Waterlogging and salinisation are common in areas of poor drainage. Surface storage is increased by the building of dams, which can affect flow rates, sediment yields, local climate change and even earthquakes.

With increased demand for water, over-abstraction can result in water shortages. In Europe, for example, groundwater is the main fresh water source. Malta is can example of the effects of saline intrusion from excessive abstraction. Irrigation (application of water to crops) is the biggest cause of groundwater abstraction in agricultural areas.

In some parts of the world, like the High Plains of Texas, USA, groundwater is being used by humans for irrigation, threatening water supplies. In other areas, due to a decrease in industrial activity, groundwater levels are increasing.

The causes and impacts of river floods; prediction of flood risk and recurrence intervals

Rivers flood because they have reached, and exceeded, their bankfull capacity. The Environment Agency (EA) is responsible for flood risk analysis and warnings in the UK. They use magnitude and frequency (recurrence interval) graphs to work out the flood risk. For example, a 1 in 500 years flood will be a larger event, but happen less often than a 1 in 10 year flood. The EA produce flood risk maps. Various data, such as rainfall radar data; rain gauge data; gauging stations etc., can be put into a computer for a model to be produces showing areas likely to flood, and warnings can then be issued to give people time to prepare. Impacts of river floods vary between countries. High-income countries (HICs) are, as a general rule, better able to deal with the disasters of floods, even though the economic cost is usually higher, than low-income countries (LICs), where the death toll is usually higher e.g. 2007 Bangladesh floods.

The prevention and amelioration of river floods: forecasts and warnings, hard engineering and soft engineering

The methods shown in the table below can be used to prevent and/or ameliorate river floods. Hard engineering involves major construction work and normally prevents natural processes in order to protect the surrounding land and property. Most of the schemes aim to manage the water once it is within the channel itself. Soft engineering works, or attempts to work, with the natural processes at work in the river and to be unobtrusive visually. It does not tend to involve major construction work. Most of the schemes aim to reduce and slow down the movement of water to a river channel.

Case study: Carlisle, Cumbria, UK (January 2005)

Causes

Carlisle is situated on the flood plain of the River Eden with three rivers meeting in the city - the Eden, Caldew and Petteril. The catchment covers approximately 2400 km² and is home to approximately 244 000 people. The catchment is mainly rural, with only 1% classified as urban. The main urban areas are Carlisle, Penrith and Appleby. Carlisle has a history of flooding.

Rainfall was very high for the period 6 to 8 January, during which two months' worth of rainfall fell in 24 hours. However, it followed a month of high rainfall in the Carlisle area, so the ground was saturated and would no longer allow water to infiltrate resulting in excessive amounts of surface run-off. River levels were at a record high.

Impacts

The flooding affected 2,700 homes. In Carlisle three people died, 1,844 properties were flooded and there was significant disruption to residents, businesses and visitors. The cost of the flooding was estimated at over £400 million.

Responses

Short term

Following the 2005 flood, £38 million was put into new flood defence: The Carlisle Flood Alleviation Scheme, which was split into two phases: phase one - Eden and Petteril Rivers (Warwick Road area) and phase two - the River Caldew and Carlisle City (Denton Holme, Caldewgate and Willowholme areas). This scheme consisted of a combination of flood walls, embankments, storage areas and pumping stations and was tested in June 2012 and May 2013 when the defences prevented properties in the city from flooding as well as in excess of £180m of flood damage to the city.

Long term

Despite , in 2015 Storm Desmond caused even worse flooding than the 2005 floods between Friday 4 and Sunday 6 December 2015. During this time, nearly 36 hours of intense rainfall breached flood defences and left several areas submerged (Bitts Park, Hardwicke Circus and Warwick Road). This left the famous Sands Centre marooned from the rest of the city. As several other areas of Cumbria were also badly affected, all trains to Scotland were postponed indefinitely, with trains on the West Coast Mainline going no further than Preston, as nearby Lancaster suffered flooding and problems with electricity supply - 43,000 homes across Cumbria and Lancashire suffered from power cuts. The estimated cost was £500 million across Cumbria. The government provided £50 million for flood victims and businesses. Insurance companies have been predicted to pay out up to £325 million to residents and businesses.

In 2019, the Environment Agency started work on a new £25 million flood risk management scheme that will better protect more than 1,600 homes and businesses in Carlisle, Cumbria. Phase one of the Carlisle Flood Risk Management scheme will reduce flood risk to more than 1,200 homes and 106 businesses at a cost of £8 million pounds - increasing protection from the previous scheme completed in 2005 to a 0.5% chance of flooding in any one year. The first phase of the new scheme will involve building and improving flood defences at Melbourne Park, the raising of land at the entrance to Tesco at the junction off Warwick Road, as well as work on and around Botcherby Bridge to improve the flow of water.

Bibliography

The following books and articles are useful for further reading around the topic:

BBC (n.d.) How can weather be hazardous?

BBC (n.d.) River management.

ECRR (n.d.) How does river restoration reduce flood risk?

Environement Agency (2015) 10 years on from the Cumbrian and Carlisle Floods of 2005.

Environment Agency (2019) £25m Flood Risk Management Scheme for Carlisle gets underway.

Fretwell, M., Kelly, D. and Nanson, J. (2015) Geography. Oxford: OUP.

Nagle, G. and Guinness, P. (2016) Geography. 2nd ed. London: Hodder.

Whittow, J. B. (2000) Dictionary of Physical Geography. 2nd ed. London: Penguin.

Wikipedia (2022) Carlisle.