Botn (Sør-Trøndelag)
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| Botn | |
|---|---|
| Location | Rissa (Sør-Trøndelag) |
| Coordinates | |
| Basin countries | Norway |
| Surface area | 5.52 km² |
| Average depth | 21 m |
| Max. depth | 45 m |
| Water volume | 112 milo m³ |
| Shore length1 | 13.11 km |
| Surface elevation | 2 m |
| References | NVE |
| 1 Shore length is not a well-defined measure. | |
Botn is a heavily land-locked Fjord attached to the Sund bay of the TrondheimsFiord in the municipality of Rissa in Sør-Trøndelag county, Norway.
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[edit] Hydrography and water quality
The extraordinary shallow , narrow and long inlet restraints the water exchange severely ; diurnal tidal amplitude is in the order of centimetres , about 1/10 of the tidal amplitude of the supplying Trondheimsfiord , though the shallow inlet blocks out more than half of the tidal wave , making the moonphase driven tidal flood height cycles the primal driving force for the internal water level , with abnormally small neap flood effect (zero has been observed during lowest neap tides and meteorologically suppressed sea level).
In addition , the inlet is connected to a secluded bay sheltered from he strong tidal currents in the main fiord. This leeds to accumulation and recycling of the exported surface brackish water, heavily reducing the sea water portion of imported water that can sink in and refresh the deeper waters, thus giving a natural stratification with stagnant (unhabitable) water below 7-10 metres, and poisonous rotten bottom water beneath 30+ metres. As another unusual anomaly, this hydrographic blocking of seawater seems to persist in calm periods even in the winter, denying import of the usual winterly bottom water renewal that otherwise is normal in fiords, and leading to a decrease in Botn's deep water oxygen levels during winter.
With a gradually descending bottom slope beneath the inlet, the basin lacks strong internal thresholds (abruptly steepening slopes) that often isolates the deep of fiord basins from imported tidal currents , thus leaving the Botn basin 'hydrodynamically open' (with very weak topographical stratifications). Thus, the annually induced stagnation depth varies with the volume and energy of the tidal instream in the critical time when the spring flood dilutes the incoming water rapidly, and accumulating differencies along the current shear creates the stratification. Naturally there is also a bottom water stratification between semi-ventilated deep water (unhabitable but not rotten) and totally isolated poisonous bottom water.
The long and 'river-like' inlet with streaming water makes ideal growing conditions for seaweed and mussels, which was scraped for use as angling bait up til the end of the second world war.
Late in the 1970's , increasing agricultural eutrofication and furtherly reduced water exchange from growing mussel banks in the inlet led Botn to an ecological crisis with surface algal bloomings and liftening of very poisonous rotten bottom water. To improve conditions the Rissa municipality has restricted the eutrofication in the drainage area and installed a bubbler facililty (like in an aquarium but bigger) on 30 metres depth to distribute more of the water exchange to deeper levels.
[edit] The Great Quick-Clay Slide of 1978
On the 29'th of April in 1978 a big quick-clay slide eradicated several farms on the southeast corner of Botn, killing only one person. Many inhabitants fled as the edge ate its way inward from the shore, and two local teenagers made a famous 8mm film of the cracking, sliding and collapsing clay landscape . A 3 meter high tsunami made some destructions on the North shore.
[edit] Literature
- Gerhard Schøning: Travel trough a part of Norway in the years of 1773, 1774, 1775 on His Majesty the King's expense done and reported. 1778.
- Georg Ossian Sars: Practical-Scientific surveys of the TrondheimsFiord, Report to the Department of Domestic Affairs about a voyage in the summer of 1891 . Cristiania 1892.
- County Administration of Sør-Trøndelag: Botn in the Rissa municipality - Pollution from the Drainage area. 1975.
- Strøm, K.M. 1936. Land-Locked Waters. Hydrography and bottom deposits in badly-ventilated Norwegian fjords with remarks upon sedimentation under anaërobic conditions. Published by the Norwegian Academy of Science "Det Norske Videnskaps-Akademi" i Oslo I. Mat.-Naturv. Klasse. 1936. No. 7. 85 pp. + 9 Pl.
