The Flood Storm Hydrograph Flashcards
Storm hydrograph
A graph of discharge of a river over the time period when the normal flow of the river is affected by a storm event
How are they important?
They predict how a river might response to a rainstorm which can help manage the river.
What does the storm hydrography start with ?
Base flow.
River is fed by throughflow of soil water and ground water.
This slow movement of water means that the changes in discharge are small
What happens to this slow change in discharge when a storm event occurs?
As storm water enters the drainage basin the river begins to be fed by more fast flowing water.
Discharge rises - shown by rising limb.
Eventually reached peak discharge.
Lag time
Time taken from peak rainfall to peak discharge
Shorter lag time more water entering river.
What shows the fall in discharge ?
Receding limb.
What happens when all the storm water has passed through ?
River returns to base flow.
Bankfull
Point at which river will begin to over flow
What does bar chart represent ?
Rainfall events .
What does line on graph represent ?
Discharge of river
Flashy hydrograph
Short lag time
High peak discharge
Steep rising limb
Steep falling limb
Subdued hydrograph
Gentle rising limb
Gentle falling limb
Long lag time
Low peak discharge
Name 8 physical factors affected the drainage basin shape
Shape of basin Sides of basin Drainage density Saturation Soil / rock type. Vegetation Precipitation intensity Size of basin
Shape of drainage basin
Circular
More flashy hydrograph than those of long and thin shape.
We each point is roughly equidistant from measuring point
Sides of drainage basin
Steep sides
More flashier than gently sloping sides.
Because water flows more quickly on steep slopes as either overland flow or throughflow so gets to river more quickly.
Basin drainage density
Higher drainage basin ( lots of surface streams acting as tributaries to main river ) have flashier hydrograph a.
Basin saturation
Is already saturated by antecedent rainfall then overland flow increases because infiltration capacity has been reached.
Since overland flow is fastest of the transfers , the lag time is reduced.
Peak discharge higher - flashy hydrograph
Soil / rock type
Impermeable ( Clay ) overland flow will be higher.
Throughflow and infiltration will be reduces meaning flashy hydrograph.
Surfaces baked hard by the sun during long period of dry weather of frozen surfaces leads to same thing.
Sandstone basins have more subdued graph as water speaks into porous rocks
Vegetation cover
Vegetation intercepts precipitation, holding the water on leaves, this slows movement of rainwater to ground and so river channels - thicker vegetation more subdued graph is.
Water lost due to E and ET from vegetation - reduces how much water gets to storm hydrograph - increasing lag time / reducing peak discharge.
Intensity / amount of precipitation
Heavy storms with lots of water entering drainage basin over short time results in higher discharge - flashy.
Lag Time higher if it’s snow not rain. As snow takes time to melt before entering river channel
Size of drainage basin
Large basins catch more precipitation and so have higher discharge compared to smaller basins.
Smaller basins have shorter lag time because precipitation as it doesn’t have far to travel.
Name 5 humans impacts on hydrograph
Deforestation Afforestation Agriculture Growth of urban areas Engineering Water abstraction
Deforestation
Reduces rates of interception.
Rain water hits surface directly.
Lack of vegetation roots reduces infiltration rate into the soil - rapid overland flow and flashy hydrographs
Name another negative of deforestation
Exposes soil to greater rates of erosion - leaving to greater sedimentation of channel.
This reduce bankfull capacity of a river and leads to greater chance of flooding
Afforestation
Opposite effect to deforestation
Allows for interception and infiltration.
Flood prevention measure
Agriculture
- Plough breaks up topsoil meaning greater infiltration - subdued.
- Terracing on hillsides stops movement of water downhill - subdued
- Grass crops increasing infiltration - subdued
- Large numbers of animals on small areas impact soils leaving to more overland flow
Urban areas
Impermeable surfaces like roads lead to flashy graphs.
Settlements built on flood plains.
Transfers water quickly away from human settlement to nearest river
Engineering
Soft engineering flood ma management schemes attempt to reduce flashiness.
Afforestation increases interception and infiltration
Water abstraction
Reduces base flow and so more water much reach channel before it reaches bankfull capacity
