Off-Road Lanes for Cycling in Kildare and Wicklow

Lane from Barretstown to Bishopshill Commons. © Eugene Brennan

I'm always on the lookout for new trails and farm lanes for cycling on. Some are marked with a cul-de-sac sign, and just end up at farmhouses. However, others actually link two roads. Usually, there’s no sign at the junction where they meet a main road, or even a road number, so it can be a mystery where they go.

OSI Benchmarks

OSI benchmarks. Image reproduced with permission from Dr. Catherine Porter, author of this RTÉ article.

There are several of these located around the town. I'm not sure which are still in existence however. The marks, resembling an arrow, were chiselled into masonry by OSI surveyors when the country was surveyed in the early 19th century to create the first detailed and accurate maps. Typically, they're located on walls and gate pillars. There should be one located on the bridge in the town and a second one on a gate pillar on New Abbey Road. (At the gate on the left hand side of the road, just before the turn for McGarry's Lane when heading towards the cemetery.) A benchmark corresponded to an elevation in feet above a sea level datum (the reference point located at Poolbeg Lighthouse). The elevation was indicated on the 25" map with a crow's foot symbol. Surveyors inserted an angle iron into a socket located near the mark, and this was used as a bench for resting a levelling rod on during the elevation measurement process.
This recent article on the RTÉ website by geographer Dr Catherine Porter and Margaret Sullivan, University of Limerick, gives a background on benchmarks and their use use in the surveying process.

Image reproduced with permission from Dr. Catherine Porter, author of the RTÉ article.

Reduction in Poulaphouca Lake Levels

Poulaphouca Reservoir viewed from above Valleymount. © Eugene Brennan

The water level in Poulaphouca lake has dropped from yesterday's 186.64 m to 186.52 m, a drop of 12 cm. It may seem small, but it corresponds to a discharge of 2.4 million cubic metres (m³) of water for the 20 square kilometre lake. This reduction in volume of the lake is simply due to the difference between "water in" minus "water out". The River Liffey and Kings River are feeding water into the lake, and water is discharged through the Poulaphouca dam into the Golden Falls compensation reservoir. In a previous article, we discovered the reason why there's a lake between the two dams. Some of the drop in level is accounted for by water purified into drinking water by the Ballymore Eustace Water Treatment Plant. According to the EPA, the BWTP produces 340 million litres of drinking water a day. That's equivalent to 340,000 cubic metres. So most of the reduction in water volume in the lake is accounted for by Poulaphouca discharges. Liffey releases through the Golden Falls dam today are forecast to be 45 cubic metres per second (m³/s), all day. If that rate of discharge is sustained for the day (the ESB say "flows are estimates"), it corresponds to 3.9 million m³ of water discharge versus the 2.4 million m³ reduction in volume of the Poulaphouca lake. So at the moment, the total discharge from the Golden Falls lake is greater than the rate at which the Poulaphouca lake is discharging into it, making room for high flows on rivers and lakes feeding the lakes in the coming days.

Water level stats courtesy ESB hydrometrics. 

 

Water level stats, courtesy ESB hydrometrics. 

Related Reading

• Press Release From the ESB About Potential Releases From the Poulaphouca Reservoir (this explains the setup at Golden Falls and the buffering of flow)
• What Volume Does a 1 m Change in Water Level Represent in the Golden Falls Lake? 
•  Golden Falls Headrace Level Up a Metre, Flywheels, Capacitors and Air Compressor Tanks
•  Let's Do Some Calculations: How Much Water?
•  High Volume Releases at Golden Falls Dam
•  Golden Falls Generator Assembly
•  Erosion of the River Bank in Kilcullen
•  Ballymore 5 km Loop 

 

  

Erosion of the River Bank in Kilcullen

Crop of a Lawrence Collection photo of the River Liffey in Kilcullen. Image courtesy the National Library of Ireland.

A close-up crop from a Lawrence Collection photo, circa 1900, showing a section of the river bank that had slipped into the river near what's now the entrance through the bushes onto the mass path. It's probably taken from the vantage point of the bridge or the slope where the Valley Park is now located. The slippage could have occurred decades or hundreds of years before. Interestingly, as far as I remember it, there was a ditch just before the fence in the photo (not sure whether it was the same fence in the 70s), which we used to have to step over to walk further along the path. I noticed recently, from hearing water running, that the ditch, if it's the same one, is now maybe 20 feet further into the bushes and in a culvert/pipes. So the bushes must have spread outwards over the last 50 years or so into the field.

I wonder how long it will take before high flows undercut the steep banks of the Valley Park and cause landslides? The most likely place is on the bend of a river where water flows faster on the outside of the bend. That would be at the bottom of the slope near St. Brigid's Well, the location of the so-called "Sally Hole": an area of river bed likely scoured out by the river. We really need a submersible to investigate, something I'd like to build if I ever get a chance (not manned 🙂, something like a drone). Whether it will happen depends on lots of factors including the nature of the soil, the gradient and hence speed of the river, how well the roots of trees anchor the soil, and so on. 

The area of riverbank is now possibly covered with bushes. © Eugene Brennan

Erosion and Deposition

AI image generated by Grok Imagine.

The geometry of a conduit through which any fluid passes affects the velocity profile of that fluid. In the case of a river, on the inside of a bend, water flows slower. On the outside of the bend, water flows faster. Erosion due to faster flow causing a scouring effect is probably responsible for the so-called "Sally Hole" in the riverbed near St Brigid's Well. Eventually there may be landslides as the riverbank is undercut. However, tree roots and other vegetation act as reinforcement, holding the bank together. So it's not necessarily imminent. Meanwhile, on the meander opposite Castlemartin, debris such as bottles and other rubbish accumulate on the inside bend, where water flows slower. Water also flows faster where there's a bottleneck in a watercourse. (You can observe this from the farm trail near the island that's north of the bridge. However, this may be partially due to a gradient in the river bed).
Bernoulli's principle is a law in fluid dynamics which states that an increase in speed occurs simultaneously with a decrease in pressure. It's a principle that's used in engine carburettors. A venturi (a narrowing of the passage in a carburettor through which the fuel/air mixture flows) causes a drop in pressure. That drop in pressure causes fuel to be sucked into the venturi. Small engines such as those used on lawn mowers still use this arrangement for supplying the engine with a fuel/air mixture. However, modern petrol engines on vehicles use fuel injection. (Since the late 1980s, early 1990s I believe).

Kilcullen Mill Stream Rerouted

Mill stream, Kilcullen. Image courtesy Tailte Éireann.

I'm always trying to spot things I've missed on the Ordnance Survey Ireland (now Tailte Éireann) historical maps. One thing I hadn't noticed previously was that the mill stream was rerouted, sometime after 1939. The first image is a satellite photo and the next three, from top to bottom are the c. 1837 6" first edition map., the c. 1900 25" map and the last edition 6" map (which is post-1939, since the Nicholastown housing estate has been built, but pre-1950 because the Logstown estate hasn't been built yet).

On the last edition map, the area around the mill stream is marked as "liable to floods". I'm guessing the meander was removed from the stream and water routed through a deeper channel directly to the river to prevent flooding. (The channel could have been widened also). The original stream bed can be faintly made out in the satellite photo.

Maybe some older locals would know something about this?

 

The Irish Townland and Historical Maps Viewer can be accessed here:

 

https://osi.maps.arcgis.com/apps/webappviewer/index.html...

 

Images courtesy Tailte Éireann.

 

c. 1837 6" first edition map. Images courtesy Tailte Éireann.

c. 1900 25" map. Image courtesy Tailte Éireann.

Last edition 6" map. Image courtesy Tailte Éireann.

Google Maps Errors

Image courtesy Google Maps/

The map is full of errors: incorrect names on places, missing rivers and canals or they're following the wrong route, businesses in the wrong place etc. Here's the latest one I've spotted, fixed and submitted. The entire Naas Dual Carriageway from The Ball to the Red Cow Roundabout is the "Naas Byp", presumably meaning "Naas Bypass"". Maybe the whole dual carriageway has been designated as the Naas Bypass? Some of this may be due to vandalism, or people put things in the wrong place or don't zoom in sufficiently when they're adding their businesses. Routes of waterways may be created automatically from satellite images, or traced manually, but the routes are frequently wrong. Several miles of the Herbertstown branch of the Grand Canal as far as Corbally Harbour were missing up until I reported it last year.

Ordnance Survey of Ireland Discovery Maps on Scoilnet

This is more geography than science, but I just noticed that the OSI (now Tailte Éireann) Discovery Series, 1:50,000 scale maps are available online on the Scoilnet site. There's also a relief shading layer showing elevation levels. The Discovery Series layer was removed from the OSI's Geohive interactive maps website several years ago. The website is available at this link.

https://maps.scoilnet.ie/OSiMaps/EsriVer17/index.html

Image credit: Ordnance Survey Ireland (Tailte Éireann)