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1.6 Runoff Collection Using Surface Structures

Technical Description

Water storage is an important method for artificially regulating natural water circulation to harmonize society's demands for water with the availability of surface water resources which change in time and space due to climate and topography. This storage is often accomplished through the construction of dams to create reservoirs of surface water along a naturally-occurring water course. Reservoirs may be divided into different types on the basis of site (highland and lowland reservoirs), size (small, medium, and large reservoirs), intended purpose (irrigation, water supply, fish production, hydropower generation), and technical design (river dam reservoirs and retention/detention water storage ponds). One example of a lowland reservoir is the Rétközi Reservoir which was built in the late 1980s with a working water level storage water capacity of about 9.07 million m3 and a surface area of 3.89 km². The average depth is 2.35 m. The maximum water storage capacity is 13 million m3, at a surface area of 4.27 km². This reservoir services a drainage network downstream of the reservoir that includes a carrier canal, pumping stations, and a number of sluices. In addition, a volume equivalent to the full volume of the reservoir can be discharged into the Tisza River or into the Belfõ drainage system through a series of small water flows. A further example is the use of surface storages to regulate water resources to conserve water in water poor areas and for soil conservation.

In addition, dams have been used in Latvia for this purpose for centuries with good result. There are more than 1 000 large reservoirs and many thousand smaller reservoirs and ponds in Latvia with a total surface area of 424 km² and a total volume of 2.07 km³, providing a useful volume or yield of freshwater of 0.33 km³. The primary use of the small reservoirs is for fire-protection in rural areas and forests. These reservoirs typically have a volume of between 10 m³ and 100 m³. The least complex reservoirs are clay lined, although the larger reservoirs have concrete or plastic basins covered with between 0.2 m and 0.5 m of soil.

Reservoirs serve not only as water storage facilities, but also work as biological wastewater treatment systems, places for recreation, and aesthetic landscape elements.

Extent of Use

In Hungary, reservoirs provide an important element of the freshwater supply. Because 95% of the water consumed within Hungary originates outside of the national borders, provision of surface storage by means of reservoirs provides a degree of water security for the country. There are 600 reservoirs that have individual storage capacities larger than 50 000 m3. The largest is the Tisza Reservoir, which has a usable volume of 300 million m³.

Operation and Maintenance

The major element in the operation of surface water storage ponds in the construction and maintenance of the hydrological structures, including the barrier wall, water control gates, and associated canals and outlet pipes. In multi-purpose reservoirs, maintenance may also include the upkeep of turbines or other equipment.

Level of Involvement

Implementation of this technology is usually carried out by regional administrations with, in some cases, government oversight, administrative decision-making, and scientific and technical research and expertise. Where reservoirs are constructed for specific purposes, such as the fire prevention, or are constructed in response to specific legal requirements, implementation of the technology should include governmental and administrative unit involvement to ensure that design specifications are uniform and that the systems are maintained. After the big forest fires in Poland in 1993, it turned out that, in many wooded areas, the volume of available water was drastically insufficient as a result of lack of such oversight.

Costs

Costs depend upon scale of a given project. Construction costs range from $0.16/m3 to $0.60/m³ for large dams, and from $1/m3 to $5/m3 for smaller sized structures. If large dams are built in an environmentally sound manner, and have no significant hydraulic problems, maintenance costs are low and the structures can be operated without specialized staff. Small dams have few costs, although they must be inspected periodically to ensure that they are not subject to siltation or misuse.

Effectiveness of the Technology

Effectiveness is determined by the hydrological regime, scale, and design of the project. Generally, well-designed and sited reservoirs are very effective means of storing freshwater. In Latvia, as a result of afforestation projects that are reclaiming formerly agricultural lands, the importance of smaller reservoirs for forest fire protection will increase, and it is anticipated that large numbers of additional ponds will be built.

Suitability

Technology suitable for most of geographic and hydrological areas excluding sites with unfavourable geology; i.e., those areas with fractured or porous bedrock that cannot be sealed to retain surface water or support a structure.

Advantages

The main goals of reservoirs are storage of excess surface water and flood control by attenuating the downstream effects of the flood peak. In turn, the reservoir provides water suitable for irrigation use and support of fish stocks. Recreational activity is also possible on and around reservoirs. Because of the head differential between the water surface and the river bed, most reservoirs also have the capacity for power generation, a capacity enhanced by the development of low-head turbines in recent years (see below). Construction of smaller reservoirs for fire protection not only provided increased resistance for forest fires due to their microclimatic effects, but also make available additional water for use during extreme droughts. Disadvantages Reservoirs create extensive land-use changes, wherein agricultural bottom lands are inundated and habitat is disturbed. Such disturbances lead to potentially severe environmental impacts depending on the size of the impoundment, including changes in the microclimate in the vicinity of large impoundments and both terrestrial and aquatic ecological disturbances. Because they are artificial structures, reservoirs can both add and detract from the aesthetic value of landscapes, depending on their design and construction.

Cultural Acceptability

Generally, impoundments are an acceptable technology. However, because of the land-use changes inherent in dam-building technology, cultural and social problems occur within the communities directly impacted by the construction process. Fire prevention reservoirs are well-accepted in forested region of Europe.

Further Development of the Technology

Future dam construction will be subject to intense environmental scrutiny, focussing not only on direct, in-basin impacts but also hydrological optimalization, including in stream flow regulation. In Poland, where there are a few large dams, some lack adequate land use control measures within their catchments for the protection of water quality, which has resulted in rapid water quality deterioration due to accelerated eutrophication. Notwithstanding, the dam construction technology is a well-established technology.

Information Sources

Rolands Bebris, Ministry of Environmental Protection and Regional Development, 25 Peldu Str., 1494 Riga, Latvia, Tel. (371-7) 227145, fax: (371-7) 820442, e-mail: BEBRI@VARAM.GOV.LV.

Anna Egle, V/U "Meliorprojects", 11 Novembra Bulvaris 31, LV-1494 Riga, Latvia, Tel. (371-7) 228734.

Petèr Kovac and Dr Kornèlia H. Kocsis, Felsö - Tisza - Vidèki Környezetvèdelmi Felügyelösèg, 4400 Nyiregyhàza, Szèchenyi u.19, Hungary, Tel. (36-42) 310 155, fax: (36-42) 310 713. 1.6

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