Powerpoint 5

Storm hydrographs

Permanent controls

Tracks a storm event, not annual discharge

Discharge is on the Y axis, Time on the X axis

There will be a 'bank line' on every hydrograph

Subdued

Gentle line on the graph

Does not cross the bank line

Lower risk of flooding

Flashy hydrograph

Steep line on the graph

Does cross the bank line

Higher risk of flooding

Transient controls

Basin size

Basin shape

Basin relief

Basin aspect

Density of the drainage network

Soil and rock permeability

Vary storm-to-storm

North

South

Flashy

Subdued

South-facing slopes receive more sunlight meaning that evaporation will occur

Southern slopes tend to have more things growing on them, meaning higher root uptake of water

Sun on the Northern side (or lack of) will leaf to less evaporation and less root uptake.

Small

Large

SUbdued

As it catches less water to move into the channel

Flashy

Theoretically, more flashy as it catches more water to move into the channel.

Long and thin

Rounded

Subdued

Different length tributaries allow water to arrive in main channel at different points

Flashy

Rain will 'hit' at the same time and will converege at a similar location in the channel

Steep sided valley

Low gradient valley

Flashy

Subdued

Infiltration rates are higher on gentle gradients

Infiltration rates low on steep valleys

Permeable

Inpermeable

Subdued

Flashy

Water cannot filtrate through the rocks so it will just run off the surface and into the channel

This will be very flashy

Water can percolate through rocks so it will run into the groundwater not the channel.

High density

Low density

Flashy

Lower capacity due to it having fewer tributaries.

High capacity to take in more and more water.

Precipitation intensity

Precipitation type

Antecedent moisture conditions

Snow fall and then thaw

Rainfall

Flashy

Snow will all melt at once, meaning that it runs into the river together

Subdued

Rainfall is steady and constant meaning that there is time for the soil to infiltrate.

If there has been snow, the ground may be frozen meaning higher surface runoff

Short and Sharp

Long and gentle

Flashy

Subdued

There is no real time for the water to infiltrate into the soil, meaning more surface runoff into the rivers

There is time for the water to percolate into the soil and the ground, meaning less surface runoff.

Type of vegetation cover

Season of the year

Urbanisation

Means what the conditions were like prior to the storm

Stores empty

Stores full

Subdued

Very limited chance of flooding as stores are empty and have capacity to absorb water.

Flashy

Stores are at capacity and cannot take any more water. it all ends up in the river.

Coniferous woodland

Moorland grasses

Flashy

Subdued

Stores are reduced by lack of trees

Less uptake as moorland grasses have small root systems

Large canopy store to catch water

Coniferous all year round

Massive root systems in Coniferous trees.,

Summer storms (subdued)

Stores are empty

Canopy stores

(Soil stores)

Intense evapouration (intense sungliight)

Vegetation can take up significant amounts of water

Winter events (Flashy)

rural basin

Heavily built up basin (post-urbanisation)

Drains take the water and dump it straight into the river

Impermeable surfaces lead to surface runoff (into drains)

The areas immediately after the city tends to flood.

Subdued

Massive soil infiltration