Gnomon's Guide

The Bohernabreena Reservoirs are two small artificial lakes in the valley of Glenasmole in the Dublin Mountains, in the southwest of County Dublin, Ireland. They were built in the 1880s and are a fine example of Victorian engineering. They and the land around them are controlled by Dublin City Council and are a valuable local amenity. The public is allowed to walk around the reservoirs and dogs are welcome if kept on a lead. Bicycles are also allowed, although they must stick to the route on the south side of the lakes, as the path on the other side is not suitable. Fly fishing is permitted but swimming is not, as there are strong currents. There are no specific spots laid out for picnics and there are no seats anywhere within the grounds, but if you bring a waterproof mat you could picnic on any of the grassy areas around the upper lake.

Some statistics

Date of Construction: 1883–1886

Upper Lake:

• Surface area: 0.23km²
• Capacity: 1.5 million m³
• Length of lake: 1.3km
• Length of dam: 200m

Lower Lake:

• Surface area: 0.12km²
• Capacity: 0.5 million m³
• Length of lake: 740m
• Length of dam: 160m

Function of the Lakes

The lakes serve a triple function:

1. Preventing flooding on the River Dodder
2. Supplying drinking water to the city of Dublin
3. Preventing the level of water in the river from getting too low

Flood Control

The River Dodder rises on the border between Counties Dublin and Wicklow in the east of Ireland and flows very steeply and rapidly down to the sea, joining the River Liffey just before it flows into Dublin Bay. The entire Dodder is less than 30km long from source to sea.

The upper reaches of the river are in a wild part of the mountains and are very affected by rainfall. The flow in the river can increase rapidly and dramatically after a heavy shower of rain. It is known as a 'flashy' river due to its tendency towards flash flooding. Before the reservoirs were built, the level of the water could rise by up to 5 metres after a particularly bad storm, causing much flooding. One account from 1645 says that in summer a man on horseback could ride under Rathfarnham Bridge without bending down but that in winter the river overflowed the bridge. More scientific measures show that the normal flow of about 1 cubic metre per second increased to 270 cubic metres per second during the most extreme storm on record, in August 1986. For comparison, the average flow of Ireland's biggest river, the Shannon, is only 208 cubic metres per second.

The lakes control the flood by being kept considerably less than full. If there is a sudden increase in flow in the river, it is absorbed by filling the lake rather than the water passing downstream, preventing floods further down the valley. This has become much more important as more of the lower Dodder Valley is taken over by housing developments.

Since the reservoirs were built, most of the floods have been successfully kept in check, but unfortunately not all. There were significant floods in 1905, 1946 and 1986, all in August, and two of them on the 25th! Most notable was 25 August 1986, when the remnants of Hurricane Charley travelled across the Atlantic and hit Ireland and Great Britain, causing record-breaking levels of rainfall. Both reservoirs filled to overflowing and as a result could not provide any further flood-relief. Over 300 properties along the river were flooded and many of the bridges were damaged. Based on measurements at Orwell Road Bridge, it's estimated that 270 cubic metres per second were passing under the bridge¹.

After the 1986 flood, a study showed that another slightly bigger flood could have caused the reservoir dams themselves to give way, causing catastrophic flooding and loss of life. To guard against this, the spillways of the two reservoirs were completely rebuilt in 2006.

Drinking Water Supply

The upper lake, which is the bigger of the two, is used for holding drinking water. It supplies 18.2 million litres of water a day to Dublin. This is only about 5% of Dublin's needs but it is still worthwhile.

Because the Dodder comes from much higher up the mountains, its water is full of acids collectively known as humic acid, which it picks up from the peaty soil and the underlying bedrock. This gives the water a brownish colour and is corrosive to water pipes, making it unsuitable as drinking water. The humic acid can be removed by chemical treatment, but the Victorian engineers didn't know how to do this, so they chose not to allow the river into the upper lake. Instead, it is diverted along an 'artificial watercourse' which runs beside the lake, and the lake itself is fed from streams which come from lower regions of the upper valley, where the soil is less acidic. This clear water is collected in the upper lake and is removed from there via a pipe under the dam and brought to the Ballyboden water treatment plant.

Flow Control

The lower lake is used for regulating the flow of the river so that there is always some water in it. This is needed to preserve the stock of fish in the river, but when the reservoirs were built it was essential for a different reason. Since this was before electricity came to Dublin, the river was used to provide power for a number of mills - there were 28 of them, 14 on the main river, and another 14 on a side channel known as the Dublin City Watercourse, which left the Dodder at Firhouse Weir. These mills were not as you might think for grinding corn, but were in fact factories, doing anything from making paper and cloth to cutting wood. If there was a drought and the river ran dry, then industry would grind to a halt.

Because the water to keep the river flowing and the mills in operation did not need to be of drinking water quality, a separate lake was used for this function. The river bypasses the upper lake using the artificial watercourse and empties into the spillway, from where it is fed into the lower lake. By always keeping some water in the lower lake, the river can be kept flowing even when water is scarce up in the mountains.

With the arrival of electricity, factories were no longer dependent on the river for power, so industry moved to other parts of the city. The water-powered mills gradually closed down. The weirs on the river which fed them still exist, but the millraces, the channels bringing water from the weirs to the mills, have all been filled in and forgotten.

Walking Around the Reservoirs

The land around the reservoirs belongs to Dublin City Council and access is restricted. Opening hours are clearly displayed at each of the three entrances. At time of writing (2015) they are as follows:

Summer (April to September)

• Mon-Fri: 8am - 8pm
• Sat: 1pm - 8pm
• Sun and Holidays: 10am - 8pm

Winter (October to March)

• Mon-Wed: 8am - 4:30pm
• Thu-Fri: 8am - 3:30pm
• Sat: Closed
• Sun and Holidays: 2pm - 5pm

The easiest way to reach the reservoirs is by car. The nearest public transport is 2.2km away at the Old Mill Bar, Firhouse, which is on a number of bus routes. No cars are allowed within the grounds of the reservoirs, but there is a small car par park at the main entrance. From here is it about 1.5km to the lower lake. The other two entrances are at the southeast end of the upper lake and provide easy access to that lake, but there is no parking available anywhere near those gates.

The following is a walking route from the car park at the northwest entrance past the lower lake, all the way around the upper lake and back to the car park.

Total distance: 9km. Ascent: about 40m.

• The main gates are normally padlocked shut, but the pedestrian gate beside them can be opened by sliding the bolt. Be sure to close it after you. The first 1.5km is through pleasant agricultural countryside.

• The road starts to climb and you reach the dam of the lower lake. This is the reservoir for the brown, acidic river water. The enormous concrete spillways were built in 2006 to replace the smaller stone spillways of the 19th century. You can walk the length of the dam and back to look at the views, but you can't walk around this lake.

• Continuing along the road, you pass the lower lake. Since the river no longer powers mills, this lake is not so important and is allowed to become overgrown.

• There's a 90° turn to the right up a steep hill, signposted for the upper lake. Ignore this and about 20m further on there is a much smaller signposted 'pedestrian route'. This is a more picturesque and gentler way of reaching the upper dam, although it can be slightly slippery under foot.

• At the top of the dam you'll see the much bigger upper reservoir. This is the one for the drinking water. You will see the huge spillway from 2006, which has three separate channels down the dam. Notice that the lake spills into the spillway when it is full, but that the River Dodder is brought along the artificial watercourse beside the lake and also flows into the spillway. Water from the spillway feeds into the lower lake. The normal way for drinking water to leave the reservoir is through the tower near the dam. This is hollow and has openings at various depths. Water entering the tower flows into pipes which go under the dam and from there to the water processing plant.

• Go straight on across the bridge over the peaty water channel and take the path between it and the lake. After a few hundred metres, you'll see the gauging weir on the right. This is a construction where the base of the channel and the two sides are made of concrete in a particular shape. The depth of the water can be used to calculate the rate of flow. On the far bank there is a small concrete building like a telephone box which contains a water level meter. Water level measurements have been taken here since the reservoirs were built.

• Further along the path there is a section of the watercourse which flows in tunnels. The reason for this has been lost. Further along again you can see three manholes in the base of the watercourse. In times of really low water, these can be opened to divert the river into the upper lake. At the end of the reservoir, the path goes through a gate onto the public road.

• Cross the road and look at the continuation of the channel - you can see where the Dodder is diverted from its normal channel to run along the artificial watercourse. In times of really high water, some of it will flow over the weir and into the upper lake, preventing damage to the artificial watercourse.

• Turn left and walk along the public road for about 100m, then take the gate on the left. This path goes along the other side of the upper lake.

• After about 600m there is a small metal gate on the right. You can take a detour here up the hill to see the old graveyard of St Anne's with its ruined church. Unfortunately, although the path brings you to the boundary wall of the cemetery, there is no easy way in without climbing a fence or the boundary wall itself. The entrance gate to the cemetery is at the far end. The name St Anne's is actually a corruption of an older name: Sentan's or Sanctan's. Records show a bishop of this name living in the 6th century. Continuing along the path beside the reservoir, you arrive back at the dam.

• Walk the length of the dam, rejoin the path and walk back to the car park the way you came.

Folklore

The reservoirs themselves are too recent to have acquired any folklore, but the valley of Glenasmole is mentioned in some of the old tales, in its Irish language form of Gleann na Smól ('gl-yown na smole'), which means 'Valley of the Thrushes'.

The ancient stories of Ireland are full of exaggeration - if a man is strong, he is the strongest man who ever lived. If a woman is beautiful, then she outshines every other woman. The stories are generally about men who were warriors. When women come into the stories at all, they are either young and beautiful or old and ugly. In either case, they are usually scheming some plan to outwit the men.

Dá Derga's Hostel

The Destruction of Dá Derga's Hostel is one of the earliest of the ancient Irish tales. It tells of a legendary King Conaire ('Kun-urruh') who visited a hostel in the mountains, but fate was against him. Invaders came from across the sea and attacked the hostel, burning it to the ground and killing the king. The story took place somewhere in the upper reaches of the Dodder. The description doesn't seem to match with any real location: in the story, the Dodder flowed through the hostel building, but it was 'in the hills above Bray' and could be seen from a ship in the Irish Sea. Some people think it might have been intended to be in Glenasmole. They give an alternative translation of the name as 'The Valley of the Burning' and say that it refers to the burning of the hostel in the legend.

The death of the king is particularly poignant, and a great example of the extreme exaggeration of the ancient tales. As the king dies, he is terribly thirsty and asks for one final drink of water. His servant dashes to the nearby River Dodder, but finds that the river has run dry. He then travels to each of the many rivers of Ireland (a feat which would reasonably take a few months), and finds that each of them has run dry. When he returns, Conaire has died.

Fionn Mac Cumhaill

Glenasmole was a favourite location for hunting expeditions by Fionn Mac Cumhaill ('fee-UNN mac COO-ull'), the leader of the Fianna, the superhero warriors who were the king's bodyguard and fought against invaders. Fionn is a giant in some stories, and of normal size but with superhuman strength in others.

There is a huge boulder in the grounds of Glenasmole Lodge a kilometre or so above the upper lake. It is known as Fionn Mac Cumhaill's Stone, and was believed to have been carried there by Fionn. In the 19th century, there was a strangely specific marble plaque on the boulder which said:

Finnakoom one of the Irish Giants carried

this stone on his shoulder from the opposite

Mountain on April 1st 1444. He was 9 feet

7 inches high and weighed 44 stone

In one story, Fionn and his companions were hunting a stag in Glenasmole. They met three beautiful women who offered them food and drink. They accepted, sitting down on the grass. As soon as they drank from the women's cups, a magic paralysis came over them so they could not move. The women produced swords and spears, and told them that their queen wanted to marry Fionn. But one of Fionn's hunting party was Diarmid, who was outstandingly good-looking. One of the women couldn't resist giving him a kiss, and this broke the magic spell. The woman were revealed to be withered old hags - Fionn's men killed them on the spot.

Oisín

The story of Fionn's son Oisín ('Ush-een') acts as a bridge between the old pagan stories and the arrival of Christianity. Oisín met a beautiful, magical woman called Niamh ('nee-uv'), married her and lived with her in Tír na nÓg ('teer na nogue'), the Land of Youth. After a few happy years with Niamh, he became restless and wanted to return to Ireland to see his friends.

It was to Glenasmole that Oisín returned. He was warned that he should stay on his horse, and not set foot on the soil of Ireland or a disaster would occur. He found that while he was away, hundreds of years had passed and his friends the Fianna were long dead. He saw some men trying and failing to lift a huge boulder and pitied them, because any member of the Fianna would have been able to lift such a stone on their own. He leaned down from his horse, picked up the boulder and tossed it across the valley. But the girth strap on his horse's saddle broke and he fell to the ground. When he touched the ground, he lost his eternal youth and instantly became a feeble old man.

In one version of the story, Oisín was brought to meet St Patrick, the British missionary who was busy converting Ireland to Christianity. The two had long conversations, Oisin arguing for the old pagan religion and Patrick promoting the new belief in one God. Oisín claimed that things were much better in the past, and one example was that the plants had been much bigger. He claimed that the ivy in Glenasmole produced leaves a foot (30cm) across. Interestingly, a 19th-century botanist found that there was actually a type of ivy in Glenasmole with huge leaves - not a foot, but easily six inches (15cm). Sadly there's no trace of it now, but one of the ivy leaves is preserved in a museum.

Holy Well

Folklore of a more recent sort is found at St Anne's Well, which is near to the graveyard you may have visited on the return journey around the lake. Holy wells are part of the folklore of Christianity in Ireland². Each well is dedicated to a particular saint, and the saint is prayed to by passers-by. Strips of cloth are attached to nearby trees and bushes to help with the prayers. There is normally one day a year which is the saint's feast day: for St Anne, it is 26 July. On this day in the past, there was a religious procession to the well and many people would gather there to pray. The water of the well was believed to have curative powers, being good for sore eyes and stomach pains.