Block 3 (Temporal Variations In River Discharge) Flashcards
Define river regime
Annual variations in a river’s discharge
Define river discharge
Volume of water moving down a river per unit of time
Define baseflow
The minimum river discharge - as water is regularly input to the channel via a steady stream of groundwater
Define stormflow
Additional discharge that enters the river channel after precipitation via overland flow and throughflow
What are the two categories of river regimes?
- Simple regimes
- Complex regimes
Define a simple river regime
One high + one low discharge extreme per yr
Corresponding to seasonal changes in temp + precipitation
Define a complex river regime
Several extremes of discharge per yr
Often fed by many sources with diff climates
Give an example of a river with a simple regime
River Rhine, source in Switzerland + flows through Europe
- Peak between May-August
- Fed by temp rise + glacial melt in Summer months
Give an example of a river with a complex regime
Colorado River, US
- Multiple discharge peaks + lows
- Fed from sources across 7 states, with different climates
- E.g. summer melt of snow in Rockies causes peak + autumn conventional rainstorms over Colorado plateau cause peak
Give an example of a river regime which is also influenced by human activity
Colorado River (complex regime) - Humans issue control through Hoover Dam + Glen Canyon Dam
What are the two categories of factors that influence river regimes?
- Physical
- Human
What are the physical factors that influence river regimes?
- Climate
- Season
- Geology
- Vegetation
- Drainage basin size + shape
What are the human factors that influence river regimes?
- Land use
- Water use
- Dams
What is widely accepted as the most important factor affecting river regimes?
Climate
How does climate affect river regimes?
- Drives rates of precipitation (input)
- Drives rates of evaporation (output)
How does season affect river regimes?
- Closely linked to climate (diff seasons have diff climates)
- Impacts inputs: e.g. monsoon seasons, summer meltwater
- Impacts outputs
How does geology affect river regimes?
- Affects lag time (rate at which water enters river), altering discharge at given time
- If rock is permeable, porous, pervious water will percolate into rock, reducing overland flow, slowing lag time, reducing discharge
Define permeability
Ability of water to pass through rock
Define porosity
Ability of rock to hold water inside it
Define perviosity
Ability of water to pass through faults/cracks in rock
How does vegetation affect river regimes?
- Affects lag time, altering discharge at given time
- More vegetation, more interception, slower lag time, reduced discharge
How does drainage basin size + shape affect river regimes?
- Larger basin catches more precipitation, increasing discharge
- Steep-sided basin reduces lag time, increasing discharge
How does land use affect river regimes?
- Land use alters the physical aspects of the area, influencing discharge
E.g. Urban area - Impermeable, non-porous, non-pervious rock
- Lack of vegetation
- High overland flow, short lag time, discharge peak after precipitation
How does water use affect river regimes?
- Water may be removed (industry, agriculture, domestic use)
- Alters discharge over time
How do dams affect river regimes?
- Give humans power to alter discharge at any given time
- Flow can be held back in reservoir or released into river downstream
How do you calculate river discharge (cumecs - cubic m per sec)?
Cross sectional area (m) x velocity (m per sec)
What are flood hydrographs?
Graphs to show how a river responds to a particular storm event/input of precipitation
What is another name for storm hydrographs?
Flood hydrographs
Show how prone rivers are to flooding after certain amount of precipitation input
What is plotted on the x axis of a storm hydrograph?
Time
What is plotted on the y axis of a storm hydrograph?
- Discharge (cumecs)
- Rainfall (mm)
How is the rainfall data displayed on a storm hydrograph?
Bar chart
How is the discharge data displayed on a storm hydrograph?
Line graph
What is peak rainfall on a storm hydrograph?
Maximum rainfall (highest point on bar chart)
What is peak discharge on a storm hydrograph?
Maximum discharge (highest point on line graph)
What is lag time on a storm hydrograph?
Time interval between peak rainfall and peak discharge
What is the rising limb on a storm hydrograph?
Part of line graph where discharge is increasing
What is the falling limb on a storm hydrograph?
Part of line graph where discharge is decreasing
What is baseflow on a storm hydrograph?
Normal, minimum river level (supplied by groundwater seeping into the channel) sometimes marked along the bottom of the discharge
What is stormflow on a storm hydrograph?
Any additional precipitation to the baseflow that has enter the river during a storm event
What is bankfull discharge?
Maximum discharge reached during an event before the river banks are overtopped and a flood occurs
What is a flashy hydrograph?
Hydrograph with a short lag time, high peak discharge + steep rising/falling limb
- Overwhelmed by lots of water reaching river fast
- Prone to flooding
What type of hydrograph means the area is prone to flooding?
‘Flashy’ hydrograph
What are the two categories of factors that influence storm hydrographs?
- Climatic factors
- River catchment characteristics
What are the climatic factors that influence storm hydrographs?
- Precipitation (type, amount, duration, intensity)
- Antecedent conditions (weather + soil conditions pre-event)
- Temperature
How does precipitation influence storm hydrographs?
Intense or prolonged rain increases overland flow, causing a flashy hydrograph
How do antecedent conditions influence storm hydrographs?
If weather has been warm + sunny, drying out the ground, then overland flow increases, causing a flashy hydrograph
If weather has been very rainy, water logging/saturating the ground, then overland flow increases, causing a flashy hydrograph
How does temperature influence storm hydrographs?
If temp is low - less evaporation (+ maybe even frozen ground), increasing overland flow, causing flashy hydrograph
How does river basin size influence storm hydrographs?
Smaller drainage basin - more concentrated - decreases lag time - flashy hydrograph
How does river basin shape influence storm hydrographs?
Circular basins drain more quickly - flashy hydrograph
How does the slope angle of land influence storm hydrographs?
Steeper slopes - reduce infiltration - increase speed of overland flow - flashy hydrograph
How does drainage density influence storm hydrographs?
Rivers with more tributaries - more water to main channel - flashy hydrograph
What does it mean to have a higher bifurcation ratio?
Lots of tributaries supplying the main river channel
How does vegetation influence storm hydrographs?
Less vegetation - less interception - more overland flow - flashy hydrograph
How does soil influence storm hydrographs?
Type: can lower infiltration capacity (e.g. clay soils) - flashy
Thickness: shallow can reduce infiltration - flashy
How does geology influence storm hydrographs?
Impermeable, non-porous, non-pervious rock - less percolation - more overland flow - flashy
How does land use influence storm hydrographs?
Urban: concrete + drains reduce lag time - flashy
Ploughed agriculture: less veg + compressed soil - flashy
Planted agriculture: increases interception - slow hydrograph
Dams + reservoirs: humans can control - steady hydrograph
What case study is used for storm hydrographs?
Boscastle Floods (2004)
