Regulated rivers and water reservoirs

Regulated rivers and water reservoirs, particularly harmful effects produced by hydropower development

Many activities related to man´s utilization of water resources accelerate the rate of erosion, the rate of sediment movement, and the silting of water reservoirs, thereby causing many technical and environmental problems.

Old slide scars above bank protections along the Swedish river Göta älv.

Often is is necessary to protect parts of the banks along regulated rivers with erosive-resistant material in order to stop the bank retreat.

The effect of water regulations on the intensity of the erosion processes varies in stages but decreases mainly with time. However, in general the stabilization of the banks through erosion and redeposition of bank material to a new so called dynamic equilibrium takes a long time. This is especially the case where the waves, the currents and the ice get the posibility to rework new bank parts and to create new abrasion scars and bank terraces. Short-time regulation (affecting diurnal and/or weekly variations of discharge) causes increased frequency of rapid water-level variations and of quick variations in the pore pressure in steep banks. This in combination with wave erosion increases the risk for slope failure especially in high and steep banks with silty-clayey soils through the elimination of supporting forces by tunnel erosion and by undercutting. Therefore, short-time regulation often gives rise to a reactivation of the bank erosion. Often a formation and widening of shallow terraces takes place followed by a colonization and stabilization by vegetation, after which the bank retreat ceases.

Overflow dam on the river Umeälven at Hemavan in northern Sweden.

Overflow dams are constructed on regulated rivers in order to maintain the water stage upstream above a selected minimum level.

Many dead tree trunks can be seen in most parts of the Nam Ngum reservoir in Laos.

The Nam Ngum reservoir was created in 1971 and is by far the biggest in Laos. The pre-impoundment area consisted mainly of red-yellow podzolic soils of silty and clayey sand. The area was almost entirely covered with tropical lowland forest, mainly consisting of bamboos. Prior to the inundation, the trees and bamboos were not felled. Trunks and branches just under the water surface are not visible, as are the presently emergent trees, and are therefore a severe impediment to navigation. The large number of submerged trees and islands reduce the extent of wind induced mixing of the reservoir water, and the flooded vegetation has decayed. This has resulted in a shallow mixing layer near the surface, and stagnant, anoxic water at depth has permitted the buildup of hydrogen sulfide, which has an unpleasant odor and is toxic to aquatic life.

Water hyacinths (Eichhornia crassipes) in the upstream part of the Cachí reservoir in Costa Rica.

Dense, floating mats of water hyacinths are a severe environmental problem in many tropical and sub-tropical water reservoirs. The water hyacinths interfere with navigation and the siltation is increased. Low oxygen conditions develop beneath the water hyacinth mats and gas-rich layers with debris of water hyacinths are formed in the bottom deposits.

The downstream part of the Cachí reservoir in Costa Rica during high and low water stage respectively.

For reservoirs with small storage to inflow ratios the cost effective way of maintaining storage is by sluicing, that is by discharging water through low level outlets in order to flush sediment through the reservoir. The effectiveness of the sluicing is dependent upon the duration and degree to which the reservoir is drawn down and on the discharge capacity of the sluices. Examples where sluicing has been effective include the Cachí reservoir.

The vertical sequence of sedimentary structures in the bottom deposits of lakes and water reservoirs reflects variations in processes and rates of sedimentation, caused by water regulations.

X-ray photo to the left and colour photo to the right of the uppermost part of core 717 from a depth of 19 m in Lake Stor-Laisan in northern Sweden.

The hard, rust-coloured layers at a core depth of 3-4 cm also contained some ash and coarse organic detritus. These layers were probably formed in the early 1960´s, when construction works took place further upstream for hydropower development. Date of coring: 1981-08-19.

Partly overlapping radiographs of the upper and the lower part of sediment core F1 from the Arenal reservoir in Costa Rica. Black, round and elliptical spots in the radiographs, that mark the presence of gas in bubble-phase, characterize especially the organic layer on top of the sandy pre-reservoir deposits in the bottom of the core.

As exemplified by core F1 the level of the pre-reservoir surface was visible (also to the naked eye) in most of the sediment cores sampled in the Arenal reservoir, which simplified the determination of average sedimentation rates. Also at several other coring stations the pre-reservoir deposits were sandy in the uppermost part and capped by gas-rich organic layers, indicating the presence of former floodplain and wetland deposits.

The most reliable way of determining reservoir sedimentation is by direct measurements, for example by repeated surveying along fixed sections, and by analysing and dating the sedimentary sequence in sampled sediment cores, as exemplified by the radiographs above.

A larger file in Swedish.

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