Rivers 1- The Drainage Basin Flashcards
Drainage basin
Catchment area of land drained by a river and its distributaries, bordered by a ridge of high land called the watershed
Hydrological cycle
Inputs: precipitation
Transfers: surface flow, channel flow, stemflow, interception, infiltration, throughflow, percolation, groundwater flow
Outputs: evaporation, transpiration, runoff
Water balance
The balance between inputs and outputs, together with the changes in storage
P = Q + E + DS
Soil moisture budget
- Soil moisture surplus
- Soil moisture use
- Soil moisture deficiency
- Soil moisture recharge
Uses of water budget graphs
Help people to manage river basins
Plan for irrigation or floods
Factors that affect water flow
Vegetation & land use, rainfall, gradient, climate, soil type, bedrock, conditions, size & shape of basin
River discharge
The amount of water in a river passing a given point at a given time
Area x velocity (cumecs)
Lag time of a hydrograph
The time delay between peak rainfall and peak discharge
Causes of a ‘flashy’ flood
High intensity/antecedent rainfall, impermeable rock, lack of vegetation, urbanisation (drainage), high density tributaries, steep slopes, circular basin, frozen/baked ground
Long profile
Shows the changes in altitude along its course from source to mouth
Theoretically smoothly concave in shape
Upper course
Vertical erosion, steep gradient, V-shaped valley, relatively narrow channel
Middle course
More gentle gradient, lateral erosion, U-shaped valley, relatively wide channel
Lower course
Very gentle gradient, deposition, gentle slopes (if any), very wide channel
Types of erosion
Hydraulic action
Abrasion
Solution
Attrition
Hydraulic action
Evorsion: the direct force of the water
Cavitation: the process of bubbles imploding in cracks, evicting tiny jets of water (130m/sec)
Abrasion
The wearing away of the bank and bed by the load (corrasion)
Solution
The removal of chemical ions in the rock (corrosion)
Attrition
The wearing away of the load carried by the river
Types of transportation
Traction
Saltation
Suspension
Solution
Types of load
Endogenic- eroded material form the bed and bank
Exogenic- material from the valley sides that has arrived by weathering and mass movement
Traction
The load rolls and bounces along the riverbed
Saltation
Smaller particles or stones bounce along the riverbed
Suspension
Finer particles are carried within the flow of the river
Solution
Chemical ions carried in solution, dissolved in the river
The Hjulström curve
Shows the relationship between the velocity of a river and the size of particles that can be eroded, transported or deposited (logarithmic)
Capacity
The amount of material that can be transported
Competence
The size of the largest particle that can be transported
Trends in the Hjulström curve
As velocity increases, the river’s ability to erode and transport particles increases
Exceptions: fine clay and silt particles are cohesive and offer less resistance, so require a greater velocity to be entrained
Reasons for deposition
A sudden reduction in gradient, decrease in discharge, entering a lake or sea, increase in load, shallow water, entering a floodplain
Erosion velocity
The rate of flow at which the river is able to pick up particles
Fall velocity
The rate of fall at which a river drops particles
Wetted perimeter
Total length of the river bed and banks in cross section that are in contact with the water in the channel (linked to channel roughness)
Hydraulic radius
The ratio of the channel’s cross sectional area compared to its wetted perimeter
Bradshaw’s Model
A geographical model that describes how a river’s characteristics vary between the upper course and lower course of a river
Characteristics that increase downstream
Discharge, occupied channel width, channel depth and average load quantity
Characteristics that decrease downstream
Load particle size, channel bed roughness and gradient
