1. General Overview of the Area
Chapel-Le-Dale is a small village in the Yorkshire Dales National Park, between the peaks of Whernside and Ingleborough and some 6km north east of Ingleton. The village has a small church with adjacent cemetery and a pub on the outskirts, but no other public buildings; the post office recently having closed. The cemetery is infamous in that it contains a number of graves belonging to railway workers who died constructing the Settle and Carlisle Line; following a smallpox epidemic in 1871, the company contributed to the cost of extending the cemetery. Although the parish of Chapel-Le-Dale has a population of 157 people, as it is spread out somewhat, most of this number live out in isolated farms. The River Doe flows from Chapel-Le-Dale through a glacial valley to Ingleton.
If a dam were to be constructed it would be at the south western end of this valley at a point before the valley opens out, as can be seen on the 1:50000 OS sheet, Figure 1. This would form an elongated reservoir from Chapel-Le-to the south east end of the valley, passing near to OSGR SD 710 750. An idea of location is also given in the view to the south west end of the valley, Plate 1.
Figure 1 - 1:50000 OS Sheet of Valley of River Doe (Sheet 98) (Crown Copyright)
Plate 1 - View to South-Western End of the Valley of the River
2. Dams and Reservoirs - General Criteria
Various reasons may contribute towards construction. These include demands for water consumption (human consumption, irrigation or industry), as a means of reducing peak flood discharge, pumped storage electricity generation, to increase navigable depth and for recreation. Although subterranean reservoirs are constructed as temporary holding locations for water en route to the consumer, the open type is in consideration here.
The site of an efficient reservoir is ideally in a classic 'U' shaped glacial valley or gorge where the volume to surface area ratio is as low as possible thus keeping relative losses due to evaporation low.
The solid and drift geology must play a large role in dam siting. If the formation beneath the dam is clay, then greater consolidation can be expected and sand borings will be required to dissipate soil pore water pressure. Solid rock foundations may cause sliding at the interface with the dam, especially if clay is present. Therefore some or all of the weak strata may have to removed and concrete applied as both a blanket and fracture injection. The material that will enclose the reservoir must be capable of preventing water egress. The Aswan High dam in Egypt on permeable sandstone and a reservoir in Spain on limestone are just two examples of incomplete filling due to water losses through adjacent rock. It is possible for the reservoir to be sited on permeable material if the adjacent water tables are so high that positive head always exists, but of course this would be reversed if the head dropped.
Water input to the reservoir must be large enough to exceed the losses through run off and evapotranspiration, from the water surface and the vegetation at the edge that removes water. A large theoretical catchment area does not guarantee high inputs as the hydrogeology may move water along unexpected flow paths, away from the reservoir, for example in areas high in faulting or in karstic scenery.
The two types of dam construction are concrete/masonry or earth embankment. Although older reservoirs in the UK are often retained by concrete or masonry dams, due to the high visual impact, these dams are now least preferred. Furthermore, the importing of construction materials forms a larger component of construction costs. For this reason, earth dams are the preferred option, especially in a national park where visual effect plays a large role. Earth embankment dams predominate in the UK. As Figure 2 illustrates, an earth dam consists of a rolled clay core surrounded by graded aggregate with a vertical sand drainage layer on the dry side of the clay core to prevent excessive pore water pressure leading to dam instability. Stone pitching and rip-rap wave protection complete the dam.
Ancillary works will include some form of spillway for high water runoff, means of controlled draw off and associated tunnels. Other works such as silt removal systems, fish traps or ladders and electricity generating plant, are dependent on the location.
Figure 2 - Section Through Llyn Celyn Earth Embankment Dam (after Crann)