3. Hydrographs Flashcards
What is a Hydrograph?
A hydrograph is a means of showing the discharge of a river at a given point over a short period of time (an individual rainfall event)
Cumecs
Cubic Metres per Second
Peak Flow / Peak Discharge
The highest level of water reached during a flood
Bankfull Discharge
When the river is completely full - if it rises further there will be flooding!
Rising Limb
Measures increasing discharge over time. Steeper limb = flashier flood!
Falling Limb / Recession
Measures discharge after peak discharge / heavy flow.
- Decreasing in steepness
- Less steep than rising limb because of throughflow now reaching river (to increase water level).
Runoff / Storm Flow
Stream discharge after a rainstorm, produced by a combination of overland flow and then throughflow (overland makes greatest contribution).
Base Flow
(Normal flow ) - stream discharge produced by water seeping from bedrock
Lag Time
The difference between peak rainfall and peak discharge.
Rainfall Peak
The maximum volume of rainfall that occurrs in a storm event.
Approach Segment
The discharge before rainfall
Infiltration Capacity
The maximum rate at which the soil will absorb water
Rainfall Intensity
Amount of rain in a given time
If infiltration capacity is greater than rainfall intensity…
…the stores (capacity of the soil to store water) will fill up before overland flow occurs. This is typical of many locations in the UK.
If infiltration capacity is lower than rainfall intensity…
…then rapid runoff occurs (known as Hortonian flow). Water cannot enter the ground so it runs straight off.
This often occurs in hot arid and seri-arid environments.
What does a steeper / more peak-like rising limb represent?
A steeper / more peak-like rising limb represents a more rapid response to rainfall with a shorter time of rise / lag time.
Which physical factors (catchment area) lead to steeper rising limbs?
Catchment area factors leading to steeper rising limbs:
- Impermeable rock in (most) of catchment area
- Extensive bare rock with little soil
- Cold climates with little vegetation
- Large, dendritic drainage basin (catchment area) with a high drainage density
- Basin with steep valley sides
- Thin soil with limited infiltration capacity
Which physical factors (weather) lead to steeper rising limbs?
Weather factors leading to steeper rising limbs:
- Major intensive thunderstorm
- Saturated soil following prolonged rainfall (Aberfan)
- Fast snow melt
Which human factors lead to steeper rising limbs?
Human factors leading to steeper rising limbs:
- Rivers without dams
- Urbanised river basins
What does a flatter / gentler rising limb represent?
A flatter / gentler rising limb represents a slower response to rainfall and a longer lag time
Which physical factors (catchment area) lead to gentler rising limbs?
Catchment area factors leading to gentler rising limbs:
- Catchment area contains permeable rock (porous or pervious)
- Large areas of vegetation / forest
- Humid climate and denser vegetation
- Small, linear drainage basin / drainage basin with a low drainage density
- Basin with gentle valley sides
- Thick / deep soil with a large infiltration capacity
Which physical factors (weather) lead to gentler rising limbs?
Weather factors leading to gentler rising limbs:
- Long period of gentle rain
- Dry, thick soil (less prolonged rainfall so less saturated)
- Slow snow melt
Which human factors lead to gentler rising limbs?
Human factors leading to gentler rising limbs:
- River with dams
- Rural river basins (not urbanised)
What factors affect a river’s regime?
Very similar to factors affecting flashiness of flood
Factors affecting a river’s regime:
Climate - amount and distribution of rainfall e.g. seasonal
Snowmelt patterns
Geology - permeable / impermeable rock
Rates of evapotranspiration (vegetation)
Steepness of land
Human factors - abstraction for domestic water supply / dams / irrigation
Some factors not included here can be added from ‘factors affecting rising limb’
Why is it useful to know about the river’s regime?
Why is it useful to know about the river’s regime?
- Irrigation
- Shipping
- Sewage disposal
- Recreation
- Water supply
- Seasons with increased flood risk
- Rice planting
Why might humans build dams to regulate a river’s regime?
Reasons for dams:
- Hold back high flow to prevent flooding
- Hold back high flow as a water source e.g. for irrigation
- Hold back high flow to generate HEP evenly throughout the year
