New Revision Notes on Hydrology

What?Recycling of water form the earth’s surface to atmosphere and back via precipitation, runoff and evapotranspiration.

Terms: Precipitation : Water that falls on the earth’s surface in the form of rain, snow,

hail etc. Infiltration : water that enters the soil. Runoff : water that flows on the ground surface. Evapotranspiration : Water which is lost by the process of both evaporation

(water bodies and earth’s surface) and transpiration (from plants and trees) Condensation : process when vapour/gas turn into liquid. Percolation : water that enter deep into the soil.

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What is drainage basin?A drainage basin is the name given to the area of land which is drained by a river. When water reaches the surface there are a number of routes which it may take in its journey to reach the river.

Terms:Input: 1. Precipitation: major input into the system, though amounts vary over time and space.

Storage:1. Interception storage: the first raindrops of a rainfall event will fall on vegetation which shelters the underlying ground. 2. Surface storage: Water may collect in hollows and depressions to form small ponds and puddles on ground surface.3. Soil moisture storage: Water store in soil (saturated water) after prolonged rain but dry out after a few weeks with little rain.4. Groundwater storage: water in soil may pass deeper into groundwater where underlying rock is an aquifer. (rocks that can hold water)

Flow:1. Overlandflow/surface flow: water which flow through a basin when soil is saturated and when precipitation exceeds the infiltration rate.2. Baseflow or groundwaterflow: run-off supplied by groundwater.3. Throughflow: water moving through the soil downhill parallel to the ground surface.

Outputs:

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1. Evapotranspiration: lost of water directly from the ground, water surfaces and vegetation.2. River channel

STORM HYDROGRAPH .

What?A storm hydrograph shows how a river's discharge responds following a period of heavy rainfall

Terms:Discharge - this is the amount of water in a river at any given point and time. Discharge is measured in cumecs (cubic metres per second)

Velocity - speed of a river (measured in metres per second)

Hydrograph - a graph showing changes in river discharge over time in response to a rainfall event.

Lag time - the time taken between peak rainfall and peak discharge

Rising Limb - shows the increase in discharge on a hydrographFalling Limb - shows the return of discharge to normal / base flow on a hydrograph

Peak Rainfall - maximum rainfall (mm)

Peak Discharge - maximum discharge (cumecs)

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Factors affecting a flood hydrograph:

Characteristics of the Drainage Basin:

- impermeable rocks (e.g. granite) and soil (e.g. clay) will not allow water to

pass through, resulting in large amounts of surface runoff and a greater flood

risk as rivers respond quickly - results in a short lag time.

- permeable rocks and soil have a high infiltration capacity and will absorb

water quickly, reducing overland flow - results in a longer lag time

- a drainage basin with a steep gradient will result in greater overland flow

and a shorter lag time than where the gradient is less steep allowing more

time for infiltration to occur.

Type and amount of Precipitation:

- heavy rain results in rapid saturation of the upper soil layers and the excess

water therefore reaches streams quickly as surface runoff (short lag time)

- slow light rain can be absorbed by infiltration and the river takes longer to

respond to rainfall as water takes longer to pass through the drainage basin

via throughflow and groundwater flow (longer lag time)

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Land Use and Human Impact

- impermeable man made surfaces such as concrete and tarmac are

impermeable therefore rivers in urban drainage basins tend to have short lag

times due to higher amounts of surface runoff and drainage systems taking

water to rivers quickly.

- vegetated areas help to reduce flood risk by increasing the time it takes for

water to reach a river (longer lag time) by encouraging infiltration (roots

opening up the soil), intercepting water by their leaves and taking up water in

their roots.

- areas cleared by deforestation will respond quickly to rainfall due to the

reduced interception

Size of the Drainage Basin

- Large Drainage Basin - water will take longer to reach the river (long lag

time)

- Small Drainage Basin - water will enter the river quicker (short lag time)

Present conditions of the Drainage Basin

- If the soil has already been saturated by heavy rain its infiltration capacity

will be reduced and further rain will go as surface runoff

- If the soil is dry it will be able to absorb more water during infiltration and

therefore the lag time will be longer

- if the ground surface is frozen lag time is short as water cannot infiltrates

and passes quickly to the river as runoff

WATER BALANCEWhat?The water balance is a simple equation which describes how precipitation may be accounted for within a drainage basin.

Equation:P = Q + E + S

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-P=precipitation, Q = total streamflow (runoff), E = Evapotranspiration,S = Storage (in soil & bedrock)

Diagram:

A- soil moisture rechargeB- soil moisture utilisation.C- surplusD- water deficit

A water balance can be used to help manage water supply and predict where there may be water shortages. It is also used in irrigation, flood control and pollution control.

Types of flow:Laminar: movement of water in a series of sheets. Common in groundwater and in glaciers, but not in rivers, it could also occur in the bed in the lower course of a river.

Turbulence flow: water flow which is not steady and uniform, it is turbulent, chaotic and eddying. Turbulence provides the upward motion in the flow which allows the lifting and support of fine particles.

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RIVER:Where Are the sediments from?Exogenetic –from mass movement, etc.Endogenetic-river bed/banks.

Hjulstorm curve.

The load of a river varies with discharge and velocity. The capacity of a stream refers to the largest amount of debris that a stream can carry; its competence refers to the diameter of the largest particle that can be carried.The critical erosion velocity-lowest velocity at which grains of a given size an be moved.The relationship between these variables is shown by means of a Hjulstrom curve.E.g. Sand can be moved easier than silt or clay as fine grained particles tend to be more cohesive.

High velocities are required to move gravel and cobbles because of their large size. The critical velocities tend to be an area rather than a straight line on the graph.

Important features of Hjulstrom curves:

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1. The smallest and largest particles require high velocities to lift them.

2. Higher velocities are required for entertainment than for transport

3. When velocity falls below a certain level (settling or fall velocity) those particles are deposited.

RIVER:River process:

1. TRANSPORATIONThe river transports its load by four main ways as shown in the diagram.

In the upper course of the river there is more traction and saltation going on due to the large size of the bedload, as a river enters its middle and lower course there is alot of finer material eroded from further upstream which will be carred in suspension

2. RIVER EROSION

River erosion is the wearing away of the land as the water flows past the bed and

banks. There are four main types of river erosion. These are:1. Attrition - occurs as rocks bang against each other gradually breaking each

other down (rocks become smaller and less angular as attrition occurs) 2. Abrasion - this is the scraping away of the bed and banks by material

transported by the river 3. Solution - chemicals in the river dissolve minerals in the rocks in the bed and

bank, carrying them away in solution. 4. Hydraulic Action - this is where the water in the river compresses air in

cracks in the bed and banks. This results in increased pressure caused by the compression of air, mini 'explosions' are caused as the pressure is then released gradually forcing apart parts of the bed and banks.

3. DEPOSITION:Deposition is where a river lays down or drops the sediment or material that it is carrying. Rivers carry lots of different sediment, including rocks,

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boulders, silt, mud, pebbles and stones. Normally, a river has the power to carry sediment. If the force of a river drops, the river cannot carry sediment. This is when the river deposits its sediment.

Deposition of material may result in the formation of distinctive features such as slip off slopes (on the inner bends of meanders); levees (raised banks) and of course the floodplain itself.

LANDFORMS:

Lower Course of the River - Floodplains and Levées

Moving between the Middle and Lower Course of the River

Key terms:

Floodplain - the area of land around a river channel which is formed during times of

flood when the amount of water in a river exceeds its channel capacity and

deposition of rich silt occurs.

Levées - a raised river bank (can be natural features formed by deposition or

artificial structures built to increase channel capacity and reduce flood risk)Formation:As a river continues its journey towards the sea, the valley cross section continues to become wider and flatter with an extensive floodplain either side of the channel. The river erodes laterally and deposition also becomes important. By the time it reaches the lower course the river is wider and deeper and may contain a large amount of suspended sediment.

When the river floods over the surrounding land it loses energy and deposition of its suspended load occurs. Regular flooding results in the building up of layers of nutrient rich alluvium which forms a flat and fertile floodplain.

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When the river water bursts its bank, the shallower depth of water flowing over the surface results in frictional drag and a consequent reduction in velocity (speed) of flow. This results in the loss of energy and therefore deposition occurs. The heaviest materials are deposited first as these require the most energy to be transported and therefore build up around the sides of the river forming raised banks known as Levées (click on diagram above). Finer material such as silt and fine clays continuing to flow further over the floodplain before they are deposited.

Middle Course of the River - Meanders & Ox-bow Lakes

The Middle Course of a River

Terms:

Meander - a bend in a river

River Cliff - a small cliff formed on the outside of a meander bend due to

erosion in this high energy zone.

Slip off Slope - a small beach found on the inside of a meander bend where

deposition has occured in the low energy zone.

Ox-bow lake - a lake formed when the continued narrowing of a meander

neck results in the eventual cut through of the neck as two outer bends join.

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This result in the straightening of the river channel and the old meander bend

becomes cut off forming an ox-bow lake.

Meander scar - feature left behind when the water in an ox-bow lake dries up.

Formation:

Having studied the characteristics of a river in its upper reaches we now need to

follow the river as it enters its middle course. Here the river channel has become

much wider and deeper as the channel has been eroded and the river has been

fed by many tributaries upstream. Consequently, despite the more gentle gradient

the velocity of flow may be as fast as in the uplands. As well as changes in the river

channel, its surrounding valley has also become wider and flatter in cross-

section with a more extensive floodplain. One of the most distinctive features of the

river in the middle course is its increased sinuousity. Unlike the relatively straight

channel of the upper course, in the middle course there are many meanders (bends)

in the river.

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Meander Formation

Meanders form due to

the greater volume of water carried by the river in lowland areas which results in

lateral (sideways) erosion being more dominant than vertical erosion, causing the

channel to cut into its banks forming meanders.

1. Water flows fastest on the outer bend of the river where the channel is deeper

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and there is less friction. This is due to water being flung towards the outer bend as

it flows around the meander, this causes greater erosion which deepens the

channel, in turn the reduction in friction and increase in energy results in greater

erosion. This lateral erosion results in undercutting of the river bank and the

formation of a steep sided river cliff.

2. In contrast, on the inner bend water is slow flowing, due to it being a low

energy zone, deposition occurs resulting in a shallower channel. This increased

friction further reduces the velocity (thus further reducing energy), encouraging

further deposition. Over time a small beach of material builds up on the inner bend;

this is called a slip-off slope.

Remember - a meander

is asymmetrical in cross-section (see diagram). It is deeper on the outer bend

(due to greater erosion) and shallower on the inside bend (an area of deposition).

Over time meanders gradually change shape and migrate across the floodplain. As

they do so meander bends becomes pronounced due to further lateral erosion and

eventually an ox-bow lake may form.

Ox-Bow Lake formation

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As the outer banks of a meander continue to be eroded through processes such as

hydraulic action the neck of the meander becomes narrow and narrower.

1. Eventually due to the narrowing of the neck, the two outer bends meet and the river cuts through the neck of the meander. The water now takes its shortest route rather than flowing around the bend.

2. Deposition gradually seals off the old meander bend forming a new straighter river channel.

3. Due to deposition the old meander bend is left isolated from the main channel as an ox-bow lake.

4. Over time this feature may fill up with sediment and may gradually dry up (except for periods of heavy rain). When the water dries up, the feature left behind is knwon as a meander scar.

Upper Course of the River: Waterfalls

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An other feature found in the upper course of a river, where vertical erosion is dominant, is a waterfall. The highest waterfall in the world is the Angel Falls in Venezuela (see picture right) which have a drop of 979m. Other particularly famous examples include Niagara Falls (North America), the Victoria Falls (on the Zambia / Zimbabwe border) and the Iguazu Falls (South America).

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Although much smaller in scale, there are many waterfalls in the upper course of UK rivers (e.g. Thornton Falls, Yorkshire - above), but how do they form?

Terms:

Cap Rock - layer of hard resistant rock forming the 'step' over which the 'falls'

occur in a waterfall.

Waterfall - a cascade of water over a hard rock step in the upper course of a

river

Plunge Pool - a deep pool beneath

Gorge - a steep sided valley left behind as a waterfall retreats upstream

Abrasion - where rocks and boulders scrape away at the river bed and banks

Hydraulic Action - where the force of water compresses air in cracks

resulting in mini-explosions as the increased pressure in the cracks is then

released.

The formation of Waterfalls

1.Waterfalls are found in the upper course of a river. They usually occur where a band of hard rock lies next to soft rock. They may often start as rapids.

2. As the river passes over the hard rock, the soft rock below is eroded (worn away) more quickly than the hard rock leaving the hard rock elevated above the stream bed below.

3. The 'step' in the river bed continues to develop as the river flows over the hard rock step (Cap Rock) as a vertical drop.

4. The drop gets steeper as the river erodes the soft rock beneath by processes

such as abrasion and hydraulic action. A plunge pool forms at the base of the

waterfall.

5. This erosion gradually undercuts the hard rock and the plunge pool gets

bigger due to further hydraulic action and abrasion.Eventually the hard cap rock is

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unsupported and collapses. The rocks that fall into the plunge pool will continue to

enlarge it by abrasion as they are swirled around. A steep sided valley known as a

gorge is left behind and as the process continues the waterfall gradually retreats

upstream.

V-shaped Valley - a valley which resembles a 'v' in cross section. These valleys have steep sloping sides and narrow bottoms.Interlocking Spur - spurs are ridges of more resistant rock around which a river is forced to wind as it passes downstream in the upper course. Interlocking spurs form where the river is forced to swing from side to side around these more resistant ridges.

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