Rivers, Floods and Management

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1) Drainage Basin Hydrological Cycle

·         Defined as the area of land drained by a river and its tributaries

·         Normally surrounded by a ridge of highland: a watershed

·         The water balanceshows the state of equilibrium between inputs, outputs and changes in groundwater storage in a drainage basin.

 

1.       Feedback loops = when changes in inputs + outputs move the system away from its state of equilibrium. There are two types of feedback loops: negative and positive.

 

Movements of water in a drainage basin can be explained with the use of the hydrological cycle.

 

The hydrological cycle refers to the movement of water between atmosphere, lithosphere and biosphere.At a global scale it is seen as a closed system with no losses or gains. At a local scale (drainage basin) the cycle has a single input precipitation and two major losses evapotranspiration and runoff.


2) Factors Affecting River Discharge

Discharge = the quantity of water that passes a given point on the bank of a river within a given amount of time.

·         Measured in Cumecs

 

·         A hydrograph shows changes in river discharge over a period of time (usually a year), AKA River Regimes. 

·         A storm hydrograph shows the response of a river in terms of discharge, to a particular rainfall event.

·         Key terms in a storm hydrograph:

1)       Baseflow is the normal level of the river which is fed by groundwater,

2)       Lag time is the time delay between peak rainfall and peak discharge,

3)       Rising limb shows how discharge rises and

4)       Recession limb shows how discharge falls.

 

·         Drainage density of a basin = total length of channels divided by the basin area.

·         The higher the drainage density the higher the risk of flood since water reaches the river channel faster.

Factors affecting both river discharge and hydrographs are:

1)       the amount and nature of precipitation,

2)      the local rocks especially their porosity and permeability,

3)      the shape or morphology of the drainage basin,

4)      the amount and type of vegetation cover and the amount and type of soil cover.

 

However, the most important factor affecting discharge on an annual basis is climate since this has an effect on:

·         amount of evaporation,

·         rate of transpiration and

·         plant growth.


3) The Long Profile

·         Long profile = the gradient of the channel bed from source      mouth.

 

Water in a river channel has a given amount of energy. There are two types of energy:

·         Kinetic (energy produced by movement of water)

·         Potential (energy due to the weight of water).

 

1)       First used up in overcoming friction with bed and banks.

2)       Then used up for transportation.

3)       Any surplus of energy left is used for erosion and

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