Hydrology and Fluvial geomorphology. (Q&A)

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HYDROLOGY AND FLUVIAL GEOMORPHOLOGY

Q: “Human activity, climate and landscape all affect a rivers regime". Discuss with reference to rivers from around the world.

Suggested Answer

The river regime is the expected seasonal pattern of discharge at a given point, over a year. The discharge is the amount of water flowing in the river channel. It is calculated by multiplying the cross-sectional area by the mean velocity (speed).

A river’s regime is based on average monthly discharge figures. These are collected over at least 20 years to try to eliminate minor year-to-year variations.

There are several factors which will affect the river’s regime, including rainfall, temperature, human use of the land and the landscape. For a better answer it is suggested that each of the factors has to be explained with examples using named rivers how each one may affect the regime of the river.

Factors affect the regime

Human activity means ‘how people use the river and the surrounding land’. For example, if vast amounts of water are taken out of the river by industry, this may affect the regime as it will artificially decrease the amount of water in the river (although most industries do return the ‘used’ water to the river channel). In a similar way, if large amounts of water are used for agricultural irrigation, or water is removed from the river and directed somewhere else, the amount of water flowing past the measuring point will decrease. It also needs to consider the possible impact on a river of increased urbanisation i.e. increased surface run-off and the water reaching the river more rapidly.

Climate can be divided into temperature and rainfall, as both of these will affect the river’s regime. The river regime may be linked to the pattern of rainfall, for example, there is a higher river discharge when there is more rainfall and vice versa. On the other hand, if most of the precipitation falls as snow and is held in storage over the winter months, the discharge will be lower then and will increase dramatically during spring when the snow thaws. Temperature also needs to be considered because if temperatures are higher it will increase the rates of evapotranspiration, resulting in less water flowing into the river.

The landscape can include the relief of the land (is it flat / mountains / steep or gentle slopes?) and the geology, both of which can affect the river regime. For example, permeable rocks will allow some water to flow through them, meaning there is less water flowing in the river channel while impermeable rocks will increase the amount of water in the river, as water cannot sink through them to travel as groundwater.

Q: "Human activity, climate and landscape all affect a rivers regime" Discuss with reference to rivers from around the world.

Suggested Answer

The river regime is the expected seasonal pattern of discharge at a given point, over a year. The discharge is the amount of water flowing in the river channel. It is calculated by multiplying the cross-sectional area by the mean velocity (speed).

A river’s regime is based on average monthly discharge figures. These are collected over at least 20 years to try to eliminate minor year-to-year variations.

There are several factors which will affect the river’s regime, including rainfall, temperature, human use of the land and the landscape. For your essay I suggest you take each of the factors mentioned in your question and explain how each one may affect the regime of the river. It would be better if you could actually give examples using named rivers. This would involve you completing a little additional research and to help you do that I will briefly explain how each of the factors mentioned can affect the regime.

Human activity means ‘how people use the river and the surrounding land’. For example, if vast amounts of water are taken out of the river by industry, this may affect the regime as it will artificially decrease the amount of water in the river (although most industries do return the ‘used’ water to the river channel). In a similar way, if large amounts of water are used for agricultural irrigation, or water is removed from the river and directed somewhere else, the amount of water lowing past the measuring point will decrease. You will probably also need to consider the possible impact on a river of increased urbanisation i.e. increased surface run-off and the water reaching the river more rapidly.

Climate can be divided into temperature and rainfall, as both of these will affect the river’s regime. The river regime may be linked to the pattern of rainfall, for example, there is a higher river discharge when there is more rainfall and vice versa. On the other hand, if most of the precipitation falls as snow and is held in storage over the winter months, the discharge will be lower then and will increase dramatically during spring when the snow thaws. Temperature also needs to be considered because if temperatures are higher it will increase the rates of evapotranspiration, resulting in less water flowing into the river.

The landscape can include the relief of the land (is it flat / mountains / steep or gentle slopes?) and the geology, both of which can affect the river regime. For example, permeable rocks will allow some water to flow through them, meaning there is less water flowing in the river channel while impermeable rocks will increase the amount of water in the river, as water cannot sink through them to travel as groundwater.

Q: How does urbanisation affect the hydrological cycle?

Suggested Answer

In the water cycle, some water is held in storage in lakes, seas and ice. Water is evaporated from water surfaces and plants (called transpiration) and, as water vapour, it is transferred by winds. When there is sufficient moisture in the air and it is cooled, the water vapour condenses, forming clouds and precipitation (rain, snow, sleet, etc). As the precipitation reaches the ground some will soak into the soil and flow through to the river (throughflow), some will percolate through to the rock below and travel through them as groundwater flow, and the remainder will travel over ground as surface run-off. Once the water reaches the rivers and seas some of it will eventually be evaporated and the whole cycle begins again.

During this process, some water is intercepted by vegetation (plants and trees). This slows down the flow of water and increases the time taken for water to reach the river, so reducing the flood risk. In addition, plant roots will absorb some of the water travelling through the soil, again, reducing the amount of water reaching the river. Urbanisation, or the building of towns and cities, will affect the hydrological cycle as water cannot sink into concrete and other impermeable surfaces. Water is transferred, via drains, often directly into a nearby river, so decreasing the amount of time taken for the water to reach the river. This will increase the flood risk.

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Q: What is a lag time and runoff?

Suggested Answer

Lag times and run-off are part of a flood (or storm) hydrograph which is a graph that shows how a particular river basin is likely to respond to a flood or period of sustained heavy rainfall. The lag time is the time lapse between the peak rainfall and the peak discharge (when the water discharge at a given measuring station on a river is at its maximum). Run-off is the water that travels over the ground to reach the rivers, rather than percolating through the soil and rocks. ...

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