Why do some rivers Why do some rivers Flood?Flood?

Factors making a ‘flashy’ hydrograph

‘‘Flashy’ and ‘Flat’ Flashy’ and ‘Flat’ HydrographsHydrographs

Precipitation intensityPrecipitation intensity

Torrential Rain

And so?

And so ?

Precipitation intensityPrecipitation intensity

Torrential Rain

Ground can’t absorb it fast enough – overland flow

Flash Flood

Steady Rain

And so?

And so ?Factor 1 – Precipitation intensityFactor 1 – Precipitation intensity

snowfall

And so?

And so ?Factor 1 – Precipitation typeFactor 1 – Precipitation type

snowfall

And so?

And so ?

Types of Underlying RockTypes of Underlying Rock

Granite is impermeable

And so

And so

Types of Underlying RockTypes of Underlying Rock

chalk and most limestones re permeable

And so

And so

SoilsSoils

Sand is permeable but clay is impermeable

And so

And so

Soils – saturated / frozenSoils – saturated / frozen

Sand and clay if saturated or frozen are impermeable

And so

And so

ForestedForested

Forest much interception

And so

And so

DeforestedDeforested

Deforested no interception

And so

And so

UrbanisationUrbanisation

Covers ground in impermeable materials

And so

And so

UrbanisationUrbanisation

Installs storm drains

And so

And so