5. Potential Environmental Impacts Associated with Reservoir Construction

Although reservoirs are constructed for benefits, in some cases, negative effects outweigh any positive ones. The land in the Doe Valley may not be of high agricultural value but the few people who live there would be displaced and will need rehousing. If the reservoir stopped at the 250m AOD level, then Chapel-Le-Dale village would not be completely flooded. However, as the valley bottom is just under 220m, it is questionable that 30m depth at the dam is sufficient and therefore the village would be flooded and relocated appropriately. When the valley of the Afon Tryweryn in Gwynedd, similar in many respects to the River Doe valley, was flooded in 1965 to create a regulating reservoir, the inhabitants of 4 settlements where rehoused. In addition to this, graves in the churchyard had to be exhumed and reburied elsewhere. Smallpox, is only infectious for a short time but exhumation of a whole churchyard can be an emotive issue even without threat of an infectious disease epidemic.

The natural environment of the area also needs consideration. Not only does flooding directly affect flora and fauna, as is the case in the 3 Gorges project in China, flooding 3 enormous valleys and destroying the ecology there, but also indirect problems may occur. Construction of a dam alters the flow regime both upstream and downstream. Sediment accumulates adjacent to the dam, concentrating any pollutants such as heavy metals, as happened at Grevelingen Dam on the Rhine. Although the River Doe is not highly polluted, accumulation of sediment can reduce the reservoir capacity, dependent on the catchment and efficiency of sediment transport past the dam. Sediment trapping simultaneously deprives the lower reaches of regime maintaining materials and of nutrients, The River Doe carrying less sediment will reduce its ability to repair banks. however depletion of nutrients is more a problem in dry climates.

The Aswan High Dam on the Nile was built to provide irrigation, hydro-electric power and fish production. After commissioning although initially predictions were met, fertility of the plains, once drenched with an annual flood containing rich sediment, had to supplemented by manufactured fertiliser, the production of which used more electricity than the dam produced. Likewise, fish stocks had to be fed nutrients to replace those lost. As the river channel has reduced, so salinisation increased, such that 60% of reclaimed land became unusable. A final indictment to this scheme is that 50% of Egyptians are affected by diseases attributable to increase in parasites due to the loss of a dry season caused by the dam. Of course, these are extreme cases, not anticipated in the River Doe, but they should at least be considered.

6. Llyn Celyn: A Regulating Reservoir in Gwynedd

The Afon Tryweryn near Bala, Gwynedd, bears many similarities to the River Doe and for that reason, it is used here as a brief case study.

Completed in 1966, Llyn Celyn, shown on the extract from the OS 1:50000 sheet, Figure 5, was constructed to provide a constant supply of water for extraction in Chester and Liverpool. It also dampens storm hydrograph responses and generates a modest 4.4Mw to the national grid, something considered unusual for a reservoir intended for water supply.

Figure 5 - 1:50000 OS Sheet of Llyn Celyn (Sheet 125) (Crown Copyright)

Ten sites were considered, in the Lake District and The Pennines, as well as North Wales. One technically ideal site was ignored on social and amenity grounds. The dam is an earth embankment, 44m high above stream bed and 660m along crest. It is typical for type with rolled clay core, vertical filter zone and rip-rap wave protection.

Figure 6 - 1:50000 Solid and Drift Geology of Llyn Celyn (sheet120) (copyright IGS)

The Afon Tryweryn case is comparable to the River Doe in several respects. The surface area and profile, location in a national park and a sparse population requiring relocation. The single most feature that distinguishes it from the Doe Valley is geology. Referring to the local 1:50000 solid and drift sheet, Figure 6, the dominant rock series are the Ordovician rhyolitic tuffs of the Llyn Conwy Formation. Although these are badly jointed, the overall permeability is low enough to seal the reservoir, although vertical joint grouting at depths of 50m was carried out to strengthen the foundations beneath the dam.

Reduction of environmental impact various measures were taken. To minimise visual effect, the dam is curved in plan and views of the east and west sides in Plate 2 and Plate 3 show this. All dam buildings are clad in masonry local to the area. The Tryweryn has good salmon fisheries and the dam destroyed the spawning grounds. As a fish ladder was not appropriate in this instance, a fish trap immediately downstream of the dam was constructed, inducing fish to spawn there, eggs being later removed to hatcheries. The main environmental impacts that would affect the River Doe are very similar to that of the Afon Tryweryn Loss of farmland, damage to ecology and direct effects on people, with the additional damage to ancient monuments.

Plate 2 - East Face of Llyn Celyn Dam

Plate 3 - West Face of Llyn Celyn Dam

7. Conclusion

External costs must be considered when looking at any scheme and this is becoming more important. The cost of changing the environment must be considered as much as that of internal costs such as bricks and mortar. In the case of the River Doe, a case could be made based on removing Ingleton's dependence on imported water and provision of a recreational amenity however this argument would pale when compared to the negative points. Not only would environmental impacts occur but the karstic geology, which draws many visitors to the area, is the very thing which makes a reservoir highly unlikely, if not impossible. A subtle form of the Gaia hypothesis perhaps? Also, would the residents of Ingleton be happy with having over 50 million cubic metres of water retained less than 3km upstream of them?


Awdry C, 1993, Over The Summit, Silver Link

Crann H H, 1967, The Design and Construction of Llyn Celyn, Institution of Civil Engineers

Jones J A A, 1997, Global Hydrology, Longman

Pickering K T, Owen L A, 1997, An Introduction to Global Environmental Issues, Routledge

Pinkerton H, 1998, ENV103 Surface Processes Course Notes, Lancaster University

Smith A, 1990, BMA Complete Family Health Encyclopedia, Dorling Kindersley

YW, 1985, Reservoirs, Yorkshire Water Authority (North Central Division)

YW, 1997, Skipton & Settle Water Supply System - from 1996 Rural Resources Strategy Review Summary Report, Yorkshire Water

YW, undated, Dams and Reservoirs on Limestone, Yorkshire Water Authority


Mr D J Greatrex, Reservoir Engineer, Welsh Water

Mr I M Stevens, Water Resources Manager, Yorkshire Water

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