Drainage Basins Flashcards
Why can a Drainage basin be classified as a system?
- it has inputs, processes and outputs.
Inputs: all types of precipitation.
Processes: runoff, infiltration, percolation, throughflow, etc.
Outputs: Outflow to sea, dam or lake. evapouration, evapotranspiration.
Describe the term Drainage basins.
- it refers to the area a river and its tributaries drains.
- this is called the drainage basin.
What is a hydrograph?
- a graph which can be drawn to show the discharge of a river over a specific period.
What is discharge, velocity and volume?
Discharge in the amount( volume ) of water in a river passing a specific point at a given time, measured in cumecs.
- cumecs: cubic metres per second.
- Velocity: the speed and direction of river in meters per second.
- volume: amount of water in river system measured in m squared.
What is the Flood hydrograph, peak rainfall, peak discharge and lag time?
- Flood hydrograph: how river responds to one particular storm.
- peak rainfall: maximum rain.
- Peak discharge: highest river level.
- Lag time: difference in time between peak rainfall and peak discharge.
Describe the hydrograph. (groot vraag )
- When the storm begins, the discharge does not increase immediately. only a little rain will fall directly in the channel. the first water to reach the river will be the surface runoff. later the through flow will follow. the Rising limb shows the increase in discharge. the gap between the maximum rainfall and the peak discharge is called the lag time. a river with a short lag time and a high discharge will be more likely to flooding.
Name and explain the factors affecting the SHAPE of the hydrograph.
- relief - faster runoff on steep slopes.
- Rock type - Surface runoff more on impermeable rock that permeable.
- Soil - Thin soil, less infiltration or deeper soil, more infiltration.
- Natural Vegetation - Thin grass, less interception.forest, roots delay water.
- Land use - Urban area, more runoff. rural, less runoff.
- Use of river - Limited use, higher discharge. dam, less.
- Drainage density - Higher density means more streams to collect water quickly. if lower density fewer streams to collect water.
5.
Factors that INFLUENCE drainage basins and hydrographs.( A-F)
A. The size of the drainage basin.
- the larger, the longer the lag time.
B. The shape of the drainage basin.
- A long narrow shape water will travel longer and a round shape it will travel shorter.
C. The relief of the drainage basin.
- a drainage basin with steep slopes water will flow faster.
D. Nature of precipitation.
- Namibian thunderstorms are short and intense.
- produce large quantities of water over a short period.
- increases runoff and produces flash floods.
- european rain is softer but longer.
- more rain will infiltrate the soil.
- in areas of snow, summer snowmelt raises amount of runoff.
E. land use.
- Urban, rural ( get the idea )
F. Permeability of ground.
- impermeable rock increases runof
- sandy soils allow water to infiltrate.
What is the Drainage density?
This refers to the number of streams in an area.
- to calculate the Drainage density of an area we use the formula
- total length of streams devided by the area of the drainage basin.
Name and explain the four types of Drainage patterns.
A. Dendritic pattern.
- Looks like a branching tree.
- rivers runs randomly down slopes.
- Underlying rocks may be flat, horizontal sedimentary, igneous or metamorphic rocks.
B. Trellis Pattern.
- Characterised by almost parallel streams.
- Short tributaries join at right angles.
- Forms in areas of folded sedimentary rock.
- Main stream runs down a syncline.
- Tributaries run down slopes of anticlines to join main stream.
C. Radial pattern.
- Streams flow outwards form a central point e.g. volcano.
- May flow inward e.g. impact crater.
D. Angular pattern. ( rectangular pattern )
- Each stream has at least one 90 degree bend.
- caused by well- jointed igneous rocks or horizontal sedimentary rocks with many joints, cracks and faults.
Know the hydrological process from start to end.
what i said above.
What is ground water and why is it there?
- water can [infiltrate] into the earth and then [percolate] downwards under the influence of gravity.
- The [soil water belt] in the zone directly below the surface where the water is available for the roots of plants.
- as water sinks deeper, in passes through the [aerated zone]
- aerated zone is where there is tiny air pockets in the spaces between soil particles.
- top edge of this zone is known as the water table.
Describe the water table.
- Position.
- varies, but generally follows the shape of the land.
2. the level - moves up and down according to the amount of rainfall.
- pumping to much boreholes can lower the water table.
- if this happens near the ocean, the salt water can infiltrate and cause salination.
- water will infiltrate until it reaches impermeable rock and then it will fill up air pockets.
- the top of the saturated soil is the water table.
What is a Aquifer?
- rocks that allow passage of water.
- will be permeable e.g. sandstone.
What is a aquiclude?
Impermeable rock
like granite and dolerite.
