Investigating Hydrological Change

WJEC

Contents

• Hydrological processes• Causes and consequences of flooding• Flood management

The watershed

mouth

Drainage basin or catchment area

An area of land (also called the catchment area) drained by a river and its tributaries. The boundary which divides one drainage basin from another Is called the watershed

The drainage basin

The drainage basin as an open system

The drainage basin forms part of the hydrological cycle, and can be described as an ‘open’ system involving a series of;

Inputs

Outputs

Stores

Transfers

Ways in which water enters the system

Ways in which water leaves the system

Ways in which water is held in the system

Ways in which water is moved through and within the system

Elements of the systemName each store

?Surface storage - lakes, rivers, sea, depression storage

?Interception by vegetation

?Soil water storage

Groundwater storage?

Name each transfer

?Surface (overland) runoff

?Throughfall + stemflow

Infiltration

Percolation

?

?

?Groundwater (base) flow

?Throughflow

Name the Input

?Precipitation

Name each Output

?Transpiration

Evaporation ?

Saturated or impermeable rock

The water table (position varies)

Energy

A river’s energy depends on:

Distance to sea level, and discharge

95% of energy used on overcoming friction, e.g. roughness on banks and bed

Maximum energy occurs during bank-full conditions when the minimum is lost to friction

Rivers are self-adjusting systems, using erosion (degradation) and deposition (aggradation) to restore their equilibrium

The hydraulic radius (R) is the ratio of the

two, i.e.

The wetted perimeter (WP) is the length of the bed and banks in contact with the water.

The cross-sectional area (CSA) is the area the stream outline would have if it were cut perpendicular to its flow.

Stream channel geometry

WPR = CSA

The hydraulic radius

If the value of the hydraulic radius (R) is large, a small area of water in the cross section is affected by each metre of bed so the frictional effect of the bed is limited and the efficiency is high.

If the value of R is small, the frictional effect is large and the efficiency is low.

Efficiency is a measure of the ability of a river channel to move water and sediment.

Deep channels are generally more efficient than shallow channels.

Larger channels are more efficient than smaller channels.

Which is the more efficient channel?

1m

15 m

3 m

5 m

A

B

WP CSA R

A

B

?17 ?15 ?15/17 = 0.88?11 ?15 ?15/11 = 1.36

Can you calculate the hydraulic radius for each channel?

Efficiency

What are the main processes

of fluvial erosion?

Hydraulic action

Abrasion / corrasion

Cavitation

Corrosion / solution

River processes

FLUVIAL EROSION

The break-up of rocks by the action of the river

TRANSPORTATION

The movement of eroded material

DEPOSITION

The laying down of material which has been transported

by the river

Erosion occurs when the critical erosion threshold is exceeded – where the amount of energy exceeds the resisting frictional force

Definitions

EROSIONAL PROCESS

DEFINITION EFFECT ON RIVER

Corrasion / Abrasion

Cavitation

Hydraulic action

Corrosion

Scraping, scouring and rubbing action of the load carried by the river

Process where tiny bubbles implode in cracks and fissures

Frictional drag and shear stress created by moving water as it lifts and removes sediment

Where the naturally weakly acidic water dissolves some carboniferous rocks such as limestone and chalk

River banks wear away

Banks collapse

Removal of loose material - river banks undercut and collapse

An increase in the dissolved load (solution)

River transportation

?

? ?

?

Suspension: Very small particles of sand and silt (0.001 – 0.99 mm diam) carried in the flow

Traction: Large stones and boulders (> 100 mm diam) rolled along river bed

Saltation: Small stones (1.0 – 99.99 mm diam) bounced along river bed

Solution: Dissolved minerals within water

A river transports it’s load in four main ways:

Under what conditions will a river deposit its load?

When there is an increase in the size of the sediment load caused by a landslide or tributary delivering larger particles

In flood conditions, when a river overflows onto its floodplain

When there is shallow water, e.g. on the inside of a meander bend

When river discharge is reduced due to a period of dry weather

Deposition

When there is a decrease in the gradient and / or velocity of the river e.g. at river mouth, or when entering a lake

The Hjulström curve shows the relationship between river velocity and the size of particles which can be eroded, transported and deposited.

The Hjulström curve

F. Hjulström collated data from 30 experimental studies into the competence of different flow velocities (competence is the maximum particle size which can be transported at specific velocities)

1. Silt/sand are picked up (entrained) at the lowest velocities.

2. Clays are as difficult to pick up as pebbles - although they are small particles they are very cohesive and the clay bed is very smooth.

3. Large boulders are dropped very easily.4. Clay particles can be transported in suspension at

very low velocities.

1

2 3

4

The changing profile

A

B

C

Height (m)

60 Distance from sea (km) 0

A B C

Upper course cross-profile

Middle course cross-profile

Lower course cross-profile

The idealised long-profile model is smoothly concave, with a steeper gradient in the upper course becoming progressively gentler towards the mouth

The long profile – changing processes

Erosion Transportation Deposition

A (Upper course)

B (Middle course)

C (Lower course)

Hydraulic, attrition. Vertical erosion dominant. Sediment supply zone)

Mostly attrition, some hydraulic. Vertical erosion decreases + lateral begins

Some lateral erosion on outside of meanders

Mostly large boulders. Some in suspension + solution

Saltation and traction of smaller bed-load. Suspension increased. Some in solution. Sediment transfer zone

Smaller sized bed-load of sand and gravel, transported in suspension

Large bed-load only

Coarser material builds up in braiding, slip-off slopes and floodplain

Mostly fine material deposited on levees, floodplain and slip-off slopes. Sediment storage zone

The sediment supply zone

This is usually an upland area

It’s usually high energy because of:

• Steep gradients

• Rapid runoff (especially if impermeable)

• Denser channel network

• Orographic rainfall

• Cooler and cloudier so less evaporation

• Snowmelt

Where does the sediment supply come from?

Weathered material moved down slopes by mass movements e.g. mud slides and runoff

Sediment supply - weathering

The breakdown of rock ‘in situ’

Types of weathering

Biological

Physical Chemical

Carbonation of limestone and some minerals in sandstone

Hydrolysis of minerals in grantic rocks

Oxidation

Freeze-thaw is the main type in upland areas.

Water in cracks, joints and bedding planes freezes,

expands by about 9% and shatters rock

Landforms of fluvial erosion

Erosional

Features

Meanders and ox-bow

lakes

Interlocking

spurs

Potholes, pools and riffles

Waterfalls

& rapids

Middle and

lower course

Upper

course

Floodplains and river terraces

Deltas

In areas of resistant rock (e.g. on Exmoor), meandering streams and rivers will be incised into the landscape, forming interlocking spurs.

Interlocking spurs

V-shaped valleys and interlocking spursPredominantly vertical erosion results in a v-shaped valley.

Rapids are formed by variations in rock resistance

Step-pool sequences

A step-like long profile;

With steep ‘step’ sections with cobbles and boulders and turbulent flow,

and longer ‘pool’ sections with gentler gradient

Occurs in upland streams with narrow, rocky valleys

The water can’t meander in these circumstances so the ‘steps’ increase the length of the profile, therefore decreasing the average gradient. Energy can also be released in turbulent flow over the steps.

Bourke’s Luck Potholes, South Africa

Potholes provide evidence of fluvial erosion

What is the most dominant type of erosion here?

Abrasion

Where there are depressions / fissures, fine particles and larger boulders (‘grinders’) may become trapped and swirled around by the current

In resistant rock, potholes require hundreds to thousands of years to form

Potholes

Soft rock

Soft rock

Broken piecesof hard rock

RiverHard rock

Waterfalls

Where do waterfalls form?

Usually where there is varying resistance in the types of rock or where there is a fault running across a river.

Which processes operate?

Usually there is considerable hydraulic action due to the falling water. Abrasion is also likely to occur to create the plunge pool at the base of the waterfall

Which feature is formed as the waterfall retreats upstream?

A gorge or canyon

High Force Waterfall, upper Teesdale, Yorkshire

• Tallest waterfall in England at 22m high

• 500m gorge downstream

Resistant band of igneous rock (Whin Sill) overlying softer sandstone, limestone, shale and coal seam

Water erodes softer rock more quickly, creating an overhang and plunge pool

This eventually collapses and the waterfall retreats upstream

Significant breaks in slope (Knick points) along a river’s long profile may be due to rejuvenation.

It occurs when there is a fall in sea level (relative to land) or the land surface rises. Vertical erosion increases and, starting from the sea, the river adjusts to the new base level. The knick point (where the old profile joins the new) thus moves upstream

original sea leveloriginal long profile

What feature will be found here?

A waterfall

relative fall in sea level (or rise of land surface)

new sea level

knick point

new long profile

Rejuvenation

Rejuvenation may also be caused by the sea eroding through and creating a breach in the coastal geology, as has happened with the River Lyn on the north Devon coast.

Original course of River Lyn

Breach in coastal geology

Knick point waterfalls

Present – day village of Lynmouth

‘Valley of the rocks’

Rejuvenation

Meanders

Straight sections contain riffles or bars in the middle of the channel, where a ridge of bed load has been deposited in the middle of the river’s bed because the water velocity is slower here.

0.2 m/sec0.1 m/sec

• Sinuous bends in the river• Surface flow to outer bank and sub-

surface return to inner bank = helicoidal flow

• Velocity across the meander varies and is related to depth

• The fastest flowing water (the ‘thalweg’) is near the outside of the bend, where the water is deepest

• Here, erosion occurs creating a river cliff• On the shallower inside, slower moving

water allows deposition to occur and a slip-off-slope or point bar forms

Outside of meander

Inside of meander

Renewed energy from rejuvenation results in increased vertical erosion and incised (deepened) meanders

Steep river cliff

Inside of meander slopes

gently

When incision is less rapid and lateral erosion is occurring, meanders become ingrown, e.g the River Wye at Tintern.

FLOW

Entrenched meanders have a symmetrical cross-profile (rapid uplift) whereas ingrown meanders are more asymmetrical (slower uplift)

Ingrown and entrenched meanders

What are the differences in form between entrenched and ingrown meanders?

When incision is rapid and vertical erosion dominates, an entrenched meander is formed, with steep sides and a gorge-like appearance, e.g. the Colorado Plateau

What are these features, and what do they represent? Ox-bow lakes. These show the former

course of the river

Source: USGS

Erosion (E) and deposition (D) around a meander (a bend in a river)

More erosion during flood conditions. The meander becomes bigger

The river breaks through during a flood. More deposition causes the old meander to become an ox-bow lake

Meanders

Pool

Pools are areas of calm, deeper water

Riffles are areas of shallower, turbulent water

Pools are areas of active erosion, and the eroded material is carried to, and then deposited at the next riffle

Pools and riffles

Riffle

Lake District River

Pools are usually found on the meander bend with riffles on the sections between, with a ‘gap’ of 5-7 times the width of the river

They are unstable and mobile features. When discharge and velocity increase they are easily eroded, and their position changes

The area where the sand or gravel is deposited is known as a bar

Fluvial deposition – bars and braided rivers

Braiding describes a section where the river has been forced to split into several channels separated by islands or bars. It occurs in rivers supplied by large amounts of sediment load and / or rivers with variable / rapidly fluctuating discharges

The river becomes very wide in relation to its depth

River flow is diverted and energy is deflected towards the banks – increasing sediment supply

Terrace

Floodplain

Bedrock

Floodplains and terraces

Floodplains are features of both erosion and deposition

Rejuvenation forms river terraces

Formed by:

• Lateral erosion• Meander migration• Valley widened• Deposition of sand

and silt during floods

They migratedownstream aserosion is greatestat the apex ofcurvature

• Remnant of former floodplain

• Following rejuvenation, river sinks into former valley

• Old floodplain left at higher level

Several stages of rejuvenation can create several paired terraces, e.g. lower course of the River Thames

Unpaired terraces can also be created when less rapid lowering of the floodplain takes place as the meander migrates across the floodplain, lowering it as it goes

Levees

Describe the process of levee formation shown in diagrams (a) – (d)

(a) During times of high discharge, the river floods. The competence of the river decreases as velocity is reduced when the river breaks the banks. Heavier, coarse material is deposited first

(b) Small banks of deposited material build up (3-5mm per year)

(c) Subsequent floods result in further deposition on these banks and the bed of the river

(d) Raised banks, called levees are created and the river flows at a higher level than the floodplain

Alluvial fans

An alluvial fan is a lobe of sediment deposited as a stream emerges from the highlands to the lowlands

There is a sudden loss of energy due to the change in gradient, and stream channels become wider and shallower.

Rapid deposition occurs and multiple channels are created, separated by bars.

Sometimes, several alluvial fans may coalesce to form a Bajada A giant alluvial fan, Death Valley, USA –

the lowest, driest and hottest valley in the USA

Deltas

• Feature of deposition at river mouth

• They occur where there is abroad continental shelf (Atlantic), rather than a steep one (Pacific)

• Velocity (and therefore competence) decreases on entering a lake or the sea

• Delta plain consists of Upper, Lower and submerged plains

• Sorting of deposits occurs into topset beds (larger heavier material), foreset beds (medium graded particles) and bottomset beds (finest particles)

Wave dominant delta e.g. Nile

River dominant deltas, e.g. Mississippi

Tide dominant deltas, e.g. Amazon

What are the different types?

Estuaries

What is an estuary?

A funnel-shaped tidal mouth of a river

They often (especially in the UK) arise from post glacial sea level rise

Why do estuaries form instead of deltas?

• Where the currents are strong

• Where there is a high tidal range (up to 10m in UK) and strong scour

• Where the catchments are relatively small and sediment is modest

• Well vegetated catchments

The Ribble Estuary, near Southport

Extraction for drinking water,

irrigation, industry etc

Conservation

Navigation

Management -preventing

flooding

In what ways are rivers

affected by human activity?

Rivers as a resource

To generate HEP

Building dams

Urbanisation on floodplain

Recreation

Fishing

La Plata Basin, South America

Where is it and what is it like?• 5th largest in the world – area 5 x the size of Spain• Flows through S. Brazil, Uruguay, Paraguay, N. Argentina, and E. Bolivia• 4 sub-basins; Parana (most important and 2nd largest in S. America. 4000 km

long international basin), Paraguay, Uruguay, La Plata• 100 million people • Huge urban population – mega-cities e.g. Sao Paulo and Buenos Aires

Rio de la Plata mouth

Why is it important?

• GDP-70% of the GDP’s of the countries in the basin

• Transport artery, especially for land-locked countries of Bolivia and Paraguay

• Water supply• Tourism• HEP – 75 major dams

Transport on the Parana – Paraguay Rivers• 3442 km of navigation between Atlantic and major inland ports and cities in

Paraguay, Argentina, Bolivia and Brazil• Ability to use river transport has enabled Bolivian agriculture to reduce

transport costs by 75% - crucial to the development of this land-locked country

• $100 million project to widen the Parana + $475 million from foreign investors

• Bigger vessels will be able to get further up river to the Argentine grain terminals

Parana river delta

Tourism at Iguazu falls

• 30 km above the Parana – Iguazu confluence on the Brazil -Argentina border

• 80m high

• 2.7 km wide

• UN World Heritage site

• National Parks on both sides protecting valuable sub-tropical rainforest

• 1, 500, 000 visitors per year – 2/3 arrive by air on the Brazil side

• Support a cluster of 4 and 5 star hotels

Itaipu HEP scheme

2nd largest dam in the world

Brazil-Paraguay Border – a joint venture

• Started in 1975• 1st electricity in 1983• 196m high, 7.76km long• Reservoir 170km long• 12,600MW electricity generated – equivalent to 10 nuclear power

stations

Impact of the dam

Economic Social Environmental

Costs

Benefits

Navigation severely disrupted, especially to Argentina

25-30% of Paraguay’s income from selling excess electricity to Brazil

Brazil gets 25% energy demand for 200 million population and rapid development

42,000 displaced when reservoir flooded

Most relocated

Scale much less than other major dam projects

700km2 forest lost. Plant extinctions. 27,000 animals affected by rising water. Interrupted fish migrations. Effects on local climate. Water weeds. Health concerns

105,000 ha created as protected area for forests, biological reserves and refuges for displaced animals and plant life

The Colorado River, USA

• 630,000 km 2 drainage basin, mostly through desert

• 3rd largest river in N. America

• 2,300km from source in Rockies to mouth at the Gulf of California in Mexico

• Most dammed river in the USA

• Entire annual flow is diverted and used by the 8 counties in the drainage basin – mostly for agricultural irrigation (90%)

Managing water

Issues:

• Dramatic increase in demand over last 30 years

• Major cities facing future shortages, e.g. Los Angeles, San Diego

• Mexico gets ‘left-overs’ – agricultural runoff from states further up, e.g. California

• 90% delta dried out – loss of wetland ecosystem

The CRC – Colorado River Compact divides the water between the upper and lower basin states.

The Glen Canyon Dam

• 1963 – last major dam to be built • Utah-Arizona border at the narrowest

point in the canyon• Aim : to improve water management• Lake Powell created behind – 300 km

long

What have been the benefits?• Up to 1300 MW of HEP• Water supplies to lower basin, even

during droughts• Water supplied to Navajo power station • Recreation and leisure activities on lake –

over 3 million people per year, providing $400 million to the local economy

• Floods absorbed by the lake – used to be violent floods each spring

Environmental impact

What have been the

environmental impacts of

the dam?

Colder, clearer water – 3

species now extinct and 5 endangered

Deprivation of sediment and floodwaters downstream – loss of sandbanks and the habitat they

providedLake Powell

flooded – loss of dramatic

scenery, e.g. ‘Cathedral in the

desert’

Alien plant species have colonised the

banks downstream in the absence of

flooding which cleared the banks each year

Barrier to fish

migration

The future

2005 drought and increased demand for water have reduced levels in Lake Powell

2007 – only at 40% capacity

Vast, long-submerged scenery has re-emerged

The demand is unsustainable, especially with climate change likely to mean more droughts

However, there is a move towards more ‘environment-centred’ approaches to management

Some groups think that Lake Powell should be drained and returned to a natural state

What are the arguments for and against this?

time

discharge (cumecs) and rainfall (mm)

Shorter lag time as water quickly reaches the channel via surface runoff, through drains, sewers etc

Steeper rising limb due to impermeable surfaces

Urbanisation and the storm hydrograph

Higher peak flow as less water is ‘stored’; more water reaches the river

Rural

Urban

Flooding: Storm hydrograph

Factor Effects on Hydrograph

High intensity, long duration of rainfall, or antecedent rainfall

Snow melt

Porous soils and / or permeable rock

Impermeable rock / frozen ground

Small drainage basin

Elongated drainage basin shape

Steep slopes within drainage basin

Summer vegetation

Deforestation

Steep rising limb as infiltration capacity of soil exceeded

Greatly increased discharge, especially if ground frozen

Less steep or ‘flashy’ hydrographs

Reduced lag time and steeper rising limb

Faster response, shorter lag time and steeper rising limb

Slower passage to river, so longer lag time

Faster passage to river, so shorter lag time and steeper rising limbInterception higher - slow response, peak discharge lower

Faster response and higher peak discharge

River flooding

What physical and human factors contribute

to flooding?

Human factors

Excessive, prolonged rainfall

Saturated soil Deforestation

Urbanisation

Physical factors

Snow melt

Frozen soil

Local relative rise in sea level / storm surge

Steep gradientImpermeable rock

High drainagedensity

Flooding occurs when a river exceeds its bankfull discharge

River managementShort, intense rainfall event

Boscastle, Cornwall

Suggest the ways in which the physical geography of the area may increase the speed of onset and severity of flooding.

Study the photograph which is looking upstream from Boscastle harbour towards the village of Boscastle.

Flash flooding - Boscastle, Cornwall August 2004

• Intense low pressure weather system caused localised heavy thundery downpours

• 200mm rain fell in 24 hours (most between midday and 5pm on the 16th) on high ground to the east.

• Already saturated catchment – rapid runoff• Boscastle lies in a deep valley just

downstream of the confluence of the rivers Valency and Jordan

• 2m rise in river levels in one hour• Debris caught under narrow bridge caused 3m

high wave of water which burst down main street when bridge collapsed

• 70-80 cars swept away, significant structural damage, 100 people air lifted to safety but no loss of life

Southern Britain, July 2007

Normal Jet stream

June – July 2007

Causes;

• Abnormal track of jet stream

• Rainfall totals for May-July highest since 1776

• Infiltration / percolation capacity minimal

• Exceptional rainfall on 20th July – event only expected once in several hundred years

Consequences;

• Flash floods across southern England; especially lower Severn and upper Thames catchments

• Drainage systems overwhelmed and transport networks severely disrupted - £25 million damage to Gloucestershire’s road system – the year’s budget!

• 45,000 households lost power; 350,000 lost running water – £1billion cost to water industry

• £3 billion damage covered by insurance. Equivalent amount uninsured loss

• 50% crops lost in affected areas – shortages and price increases

• 3 people died

Prague floods, August 2002

• Intense rain fell on the 6th-7th August and then again on the 11th-13th

• A 1 in 500 year flood wave was triggered in the Vltava basin• Highest ever discharge was recorded in Prague – 5300 cumecs• Buildings swamped in 4m of water – many collapsed or left too dangerous to re-

occupy• 50,000 evacuated• 2/3 of these were still unable to return to their homes 12 weeks after the flood• 230 million Euros of damage to underground as 13 stations flooded• 3 billion Euros total damage in Czech Republic – 1/ 3 of this in Prague itself

High risk areas - Bangladesh

Major rivers converge?

?Himalaya Mountains; (monsoonal) rainfall and snow melt

?

Storm surges, especially during cyclones / hurricanes. Also local sea level rises of 7mm/year

80% of country occupies low-lying delta < 1m above sea level

What are the human influences?• Deforestation• Agricultural practices• Densely populated• Urbanisation – Dhaka

population over 1 million• Embankments built

(road and river) – have prevented back-flow of flood water and increase siltation in drainage channels

• Low GDP and lack of investment

?

What are the physical causes of flooding?

Flooding in Bangladesh, 2004Exceptionally high rainfall totals in the monsoon of 2004 led to widespread flooding in July and August

Consequences of flooding

38% land area flooded – worst floods for 6 years

800,000 ha agricultural land flooded – small scale farmers severely affected

Capital city, Dhaka flooded.

36 million people made

homeless (nearly 29% of total population)

800 dead by mid-September

Spread of disease Flood waters mixed with

raw sewage caused diarrhoea outbreak

Infrastructure severely damaged – damage to roads, bridges, school

and hospitals estimated at $7 billion

$2.2 billion estimated cost

of damage (4% of GDP

for 2004)

Flood management

Hard engineering strategies involve the building of structures or alteration of the course / structure of the river

The aim is to reduce the frequency and magnitude of flood events, and therefore reduce the damage that floods cause

8% of England including 1.7 million homes, 12% of farmland and 300,000 commercial properties are at risk of flooding

1. Can you describe the measures shown in the diagram opposite?

2. What might be the advantages and disadvantages of hard engineering methods?

What are the arguments for and against hard engineering?

Hard engineering

FOR AGAINST

Reduction in flooding and therefore protects property

Takes water away from towns more quickly

Increase in water supply e.g. on the Nile

Improved navigation e.g. Mississippi

Allows energy to be created e.g. hydroelectric power on the Colorado

Can lead to destruction of habitats along river bank

Can be visually intrusive

It can dramatically increase peak discharge, duration and timing of floods downstreamWhere meanders have been straightened, the river will try to re-establish itselfStraightening courses can lead to greater upstream erosion and downstream deposition

Soft engineering

Abatement strategies which aim to work with natural processes, and be more sustainable solutions to flooding• Afforestation

• Contour ploughing and strip farming to reduce runoff

• Floodplain zoning to allow (economically less valuable) areas to flood naturally

• Conservation and restoration schemes; returning rivers to their original state and protecting, e.g. bales to improve water quality

• Forecasting and early warning, e.g. Environment Agency flood watch and risk maps. Some small-scale community projects in Bangladesh have resulted in early warning systems and lives are being saved

River restorationThe River Cole near Swindon underwent a restoration project between 1994 and 1996. The aims were to change the water course back to a more natural state, improve water quality and manage bank side vegetation and habitats. The main strategies are shown below

RBMPs

River Basin Management Plans

First one was for the River Ribble in NW England

It drains an area of 2100 km2 from the Yorkshire Dales to the lowlands between Preston and Southport.

Aims of RBMPs

• Protect and enhance the ecosystem• Promote sustainable use• Supply of good quality water• Reduce and then eliminate pollution• Mitigate the effects of flooding and drought• Delimit protective conservation areas

Use and conflictWhat are the uses of the River Ribble?

What are the conflicts associated with these activities?

Use Conflicts

Coarse fishing throughout the basin. Extensive leisure resource

Sewage treatment works and industry input effluent

Water for public supply, e.g. Stocks reservoirExtraction for agriculture in lower basin

Variety of habitats and great biodiversity. National conservation areas

Polluting activities lower water quality and affect fish stocks

Weirs interrupt fish migration. Pollutants reduce O2 and biodiversity, and lower aesthetic value

May affect river flow – impact on ecosystems and leisure activities

Runoff of nitrates and phosphates. Silage liquor spills and slurry kill aquatic life

Wildlife conflicts with all human activities which pollute, obstruct or inhibit

Management

Flooding• 34,000 people live in areas with a 0.1% annual flood risk• Orographic rainfall keeps totals above 2000mm in the upper basin• Worst floods in 1866, with others in 1995, 2000 and 2002

Development in the basin – Preston• City centre is 30m above the river• Preston and South Ribble councils want to develop the riverside:• Build barrage to raise water levels and create a freshwater lake for leisure

activities• 400 new homes, shops and offices protected by embankments on the south

side• Urban park in centre • Sustainable city concept with emphasis on pedestrian rather than motor

movement• Create jobs, leisure and tourism opportunities, promote Preston’s image in the

north west and integrate the river into the life of the city

Should it go ahead?The ‘Save the Ribble’ campaign has a website, and its member are fiercely opposed to the scheme

Loss of greenbeltLoss of riverside footpathsIncreased flood risk due to building on floodplainLoss of allotments and playing fieldsBarrage would be an obstacle to migrating salmon and trout

Half of the estuary is a RAMSAR site. Also several nature reserves and SSSI’s. Internationally important, supporting 250,000 migrant birds

Sediment to estuary reduced, increasing erosion of mudflats and salt marshes – loss of habitat