RIVER DRAINAGE

BASINS

A RIVER SYSTEM ACTS LIKE A SYSTEM OF DOWNPIPES AND GUTTERING ON A HOUSE - IT ALLOWS THE MOVEMENT OF RAINWATER INTO THE SEA

Watershed

(higher ridge of land, the boundary between one drainage basin and another)

Source

(the place where a river begins -a river system will have numerous sources, such as springs)

Confluence

(a place where two rivers meet)

Tributary stream (a smaller river that flows into a larger river)

A TYPICAL RIVER DRAINAGE BASIN. THIS IS THE CATCHMENT AREA FOR ALL THE RAIN THAT FALLS WITHIN THE WATERSHED

All rain falling within a drainage basin ends up

in the river system.

How does it get there?

OVERGROUND

Water runs downhill over the ground towards a river or stream -

SURFACE RUNOFF

Raindrops might fall directly into a river or stream -

CHANNEL FLOW

UNDERGROUND

(uppermost level of

saturated ground)

Raindrops INFILTRATE

soil and rock. The water

then moves underground

towards a stream or

river

So, which route to the river is quicker - surface runoff or infiltration / groundwater

flow?

ANSWER: Rain travelling as surface runoff

will get to rivers and streams much more quickly, reaching

a river or stream with little delay,within a short time-span.

Water that infiltrates and travels underground travels much more slowly, finally reaching a river or stream

bit by bit over a much longer period of time.

Which situation is a river more likely to be able to copewith, without flooding?

Peak rainfall

Scenario 1

Peak discharge overland flow- high

Scenario 2

Peak discharge

groundwater flow - lower

1. After how many hours does rainfall peak?

2. What is peak river discharge in Scenario 1? (in cumecs)

3. What is peak river discharge in Scenario 2 (in cumecs)

4. What is the lag time in Scenario 1?

5. What is the lag time in Scenario 2?

A FLOOD HYDROGRAPH

So:

Surface runoff

Shorter lag time

Higher river peak discharge

More likelihood of FLOODING

What conditions

will make surface runoff more likely?

The rainfall itself - very heavy, in big droplets (less likely to infiltrate)

Water table already very high (little scope for infiltration if ground is already saturated)

Steep slopes in drainage basin (water flows downhill rapidly -less time for infiltration to occur

Non-permeable rock within drainage basin (water can’t infiltrate easily)

Few trees (tree roots break the soil, providing little gaps for water to infiltrate). Trees then take up and store water from underground

Urbanisation within the drainage basin - large areas concreted or tarmaced over, making surface non-permeable. (Storm drains are built in, of course, but these carry water very quickly to the river, making lag time even shorter)

Storm drains are connected to

rivers and streams by an underground network of pipes, called the storm sewer system, which helps to

prevent flooding from rain.

Which of these

conditions applied to the River Severn

drainage basin in July

2007?

There had been some very wet weather in the Severn drainage basin in June- 221% of previous average

Ground in some areas was saturated

The rainstorms over the weekend 20th-23rd July saw between 4-5 inches of rainfall - as much as would normally be expected for the whole of July.

Rainfall was intense with large drops.

River Severn flow was already exceptionally high in June

Although still mainly rural, considerable urbanisation has occurred within the Severn catchment over the past 50 years including the expansion of existing cities, towns and villages, for example the significant expansion of Redditch and Telford.

(Environment Agency - ‘A Flood Management Strategy For

the River Severn Corridor’)

More intense use of agricultural land during thetwentieth century has resulted in larger fields, more efficient drainage systems and larger and heavier machinery causing deeper compaction of soils. This has resulted in water being moved through agricultural land faster and the rate of run-off being higher. This, combined with loss of woodlands and rough vegetation (which absorb and retain water), to farmland or urban development, may mean that volumes of water in rivers are higher and water levels rise faster. Several groups in the UK are currently investigating the effect of land and soil management practices on soil erosion and run-off. (Environment Agency - ‘A Flood

Management Strategy For the River Severn Corridor’

The Upper reaches of the Severn drainage basin are in the Welsh uplands - slopes here are steep, favouring surface runoff. Rain water getting into

the river quickly here, will flow fast within the river channel towards the lower reaches of the drainage basin - to places such as Tewkesbury

and Gloucester

What conditions

will make surface runoff more likely?

The rainfall itself - very heavy, in big droplets (less likely to infiltrate)

Water table already very high (little scope for infiltration if ground is already saturated)

Steep slopes in drainage basin (water flows downhill rapidly -less time for infiltration to occur

Non-permeable rock within drainage basin (water can’t infiltrate easily)

Few trees (tree roots break the soil, providing little gaps for water to infiltrate). Trees then take up and store water from underground

Urbanisation within the drainage basin - large areas concreted or tarmaced over, making surface non-permeable. (Storm drains are built in, of course, but these carry water very quickly to the river, making lag time even shorter)

?

?

It is important that we have some

understanding of drainage basin

conditions and storm hydrographs (both

natural and man made) if we are to understand

what can be done to reduce future flood

risks