Exercise 2 - Sediment transport in glacial melt water and river ...

Exercise 2 - Sediment transport in glacial melt water and river
regulation
1) Sediment transport
Table 2.1 shows the results from water samples taken at the inlet and outlet of the lake
Nigardsvatn between 1968 and 1978 (Østrem et al., 2005).
(a) Calculate how much (in kg) of suspended sediments are deposited in the lake every year.
(b) On average, what is the fraction of suspended material left in the basin?
(c) The lake floor has an area of 0.7 km 2 . Assume the suspended sediments left in the lake
settles evenly on the lake floor. How many mm of sediments would you expect to deposit on
the lake floor in 1970? (Density of suspended sediments is about 2000 kg/m 3 ).
(d) Annual deposition of suspended sediments in the lake is on average 11,200 x 10 3 kg.
Annual accumulation on the delta is measured to 11,800 x 10 3 kg. Volume of the lake is 5.4 x
10 6 m 3 . When will the lake be filled up? Discuss what will happen when the lake is filled up?
2) River regulation
Information:
- In the Åre River in Årdalen the community wants to regulate the river system.
- The power station wants to take water from the glacier fields 21-37 in Tådalen (see map)
and transmit it to a power station in a different watercourse together with the glacial rivers 5-
13 (Årdalen).
- A company using sand and gravel for concrete wants to take out gravel at B15R (see map).
They ask for 30,000-ton gravel each year.
- Where the river enters into the fjord a delta area with a shallow water environment exists.
The community wants the area to be preserved for wildlife (birds etc).
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- The community also wants to establish a reservoir for drinking water (se map). If so they
have to construct a dam being able to regulate the water 2-m above natural water level and 2
m below natural water level. The delta in the upper part is preserved because of endangered
shore vegetation.
Evaluate and discuss the different interventions in the river. How will they affect the
watercourse?
The different activities are in conflict to each other. How will they influence on each other?
Can all activities be realized at the same time?
Working for the community as in charge for the environment you should give your
recommendations to what they should do.
Additional information:
- Measurements of sediment transport at station 2821 show that the bottom transport is ca.
20,000-ton pr. year.
- It has been estimated that the glaciers in Tådalen produce 10,000-ton sediments each year
and the glaciers in Ådalen 5,000 ton each year.
- The lake has a depth of 40 m near the delta front at B14R.
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