Tryweryn - where does the water come from?

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banzer
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Tryweryn - where does the water come from?

Post by banzer » Mon Sep 02, 2019 9:53 pm

This may look like a daft thing to say, bear with me.

Reservoirs are primarily for water supply - the Tryweryn dam / Llyn Celyn was famously built in the 60s to help keep the Dee topped up and hence supply Liverpool with drinking water. There's also some hydro power generation.

So how come it seems to be able to supply the whitewater centre with releases throughout the summer, when the tributary streams (the upper Tryweryn, and the Gelyn) are mostly just a trickle? By way of comparison, here in Yorkshire the Washburn releases only happen once a fortnight, and lately they have been cancelled often through lack of water. Annual rainfall in Bala: 1045mm. Annual rainfall in Skipton: 1010mm. What gives?

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Re: Tryweryn - where does the water come from?

Post by chicklechives » Tue Sep 03, 2019 12:00 am

My understanding is that the two rivers are doing quite different jobs.
The mighty T is there to provide compensation flow for the Dee, the aim is therefore to run down reservoir levels during dry periods, the only restrictions would be if the top reservoir started low or the Dee was too high.
The cancellation of Washburn releases tends to be because the lower reservoir is too full. Yorkshire water use the series of reservoirs to hold water for domestic supply, if the lower reservoir is too high water is lost from the system and Yorkshire water are fined. The water therefore can't be released from the upper reservoir.

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Re: Tryweryn - where does the water come from?

Post by jmmoxon » Tue Sep 03, 2019 12:32 am

Llyn Celyn 71.2 million cubic metres - designed so water released into the river to be abstracted further down - at 9 cumecs, 6 hour releases will take an entire year to empty

Thruscross Reservoir 7.8 million cubic metres - 9 times less water which can be more efficiently taken away in pipes - at same release rate will only last 40 days

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Re: Tryweryn - where does the water come from?

Post by banzer » Tue Sep 03, 2019 9:14 am

Wow - you still can't beat ukrgb for nerdy info! Thanks :)
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Re: Tryweryn - where does the water come from?

Post by jmmoxon » Tue Sep 03, 2019 9:54 am

Some more: Llyn Celyn catchment is 60 sq km, so using above rainfall figures it will refill 88% over the course of a year (ignoring losses, as we are just looking for relative figures).

Whilst Thruscross has only 28.5 sq km, this is proportionally bigger compared to the volume of water (refill 370%), but a lot of that rain will fall when the reservoir is already full.

https://environment.data.gov.uk/catchme ... GB30429990

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Re: Tryweryn - where does the water come from?

Post by Poke » Tue Sep 03, 2019 1:09 pm

banzer wrote:
Mon Sep 02, 2019 9:53 pm
here in Yorkshire the Washburn releases only happen once a fortnight, and lately they have been cancelled often through lack of water. Annual rainfall in Bala: 1045mm. Annual rainfall in Skipton: 1010mm.
I'm 99% sure that the reason the Washburn release has been cancelled recently is too much water, not lack of. Too much water in the reservoirs downstream means that they'd need to release water from that reservoir, which essentially is losing it from the system. Not sure if they would be fined for this, or if it just goes against their resource management good practice?

EDIT: should have read the posts above fully :-) D'oh!
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Re: Tryweryn - where does the water come from?

Post by jmmoxon » Tue Sep 03, 2019 2:07 pm

It's only the recent releases that have had too much water downstream. Earlier this year it was engineering works at lower reservoirs (also 5th Nov 2017), previous years it was usually a lack of water (although 10th Dec 2017 & 4th Mar 2018 had too much snow!):
https://www.facebook.com/riverwashburn/

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Re: Tryweryn - where does the water come from?

Post by Jim » Wed Sep 04, 2019 12:51 pm

It has been an unusual year for Washburn due to the engineering works causing HW cancellations (my info is Yorkshire water will be fined if they lose water from the system, but is second hand), in a normal year the problem is essentially that Yorkshire, being east of the Peninnes, is essentially a desert with a high demand for water, whilst Liverpool and N Wales are oases with much lower demands.

Just thinking about my normal route to Washburn, Skipton is on the west side of the hill - it will be a different catchment (albeit washburn water may end up in the same river - Wharfe?), Skipton may well be oasis side and Blubberhouses desert side!

In fact, whilst Washburn does contribute to the Wharfe which is the next catchment west, Skipton is 2 catchments west on the Aire...

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Re: Tryweryn - where does the water come from?

Post by jmmoxon » Thu Sep 05, 2019 7:00 am

The rainfall in Blubberhouses is significant, with precipitation even during the driest month. Precipitation here averages 872 mm
- still 320% refill (Feb to July each get over 60mm, other 6 months c.80mm). Some will fill the lower reservoirs, but they also have additional catchments.
https://en.climate-data.org/europe/unit ... es-684679/
Thruscross was constructed in 1966 and is the top of four reservoirs in the Washburn Valley. It is used for storage along with Fewston and to regulate the amount of water in Swinsty reservoir, the only supply reservoir in the chain. The bottom reservoir, Lindley Wood supplies compensation to the river downstream.
https://www.yorkshirewater.com/riverwashburn (This article also talks about optimising releases for canoeing & conservation.)

Dee Valley Water
abstracts more than 70 million litres of water per day, from six abstraction points; the majority being from two main abstraction points on the River Dee
Population of Liverpool is over half a million, whilst Leeds is just under. Main factor is it's a much smaller reservoir that was designed to dewater the river...

Fines are mentioned regularly on the FB page.

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Re: Tryweryn - where does the water come from?

Post by twopigs » Thu Sep 05, 2019 9:44 am

An equally nerdy fact is that the water from Llyn Celyn can be diverted into Lake Bala before being released further downstream!
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Re: Tryweryn - where does the water come from?

Post by Franky » Thu Sep 05, 2019 10:15 am

jmmoxon wrote:
Thu Sep 05, 2019 7:00 am
Population of Liverpool is over half a million, whilst Leeds is just under. Main factor is it's a much smaller reservoir that was designed to dewater the river...
Actually, Leeds has a population of 700 000, and that's just the city. The whole of West Yorkshire has a population of 2.3 million.

The population of Liverpool is 495 000; urban area (Merseyside) 1.4 million.

So "Greater Leeds" has almost double the population of "Greater Liverpool".

That said, I suppose other big towns of W Yorks. (Bradford, Halifax, Huddersfield) will get their water from reservoirs further south and west than the Washburn Valley.

Statistics from Wikipedia so not guaranteed accurate, but they tie in with what I've read elsewhere.

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Re: Tryweryn - where does the water come from?

Post by jmmoxon » Thu Sep 05, 2019 11:32 am

twopigs wrote:
Thu Sep 05, 2019 9:44 am
An equally nerdy fact is that the water from Llyn Celyn can be diverted into Lake Bala before being released further downstream!
Not exactly, if Bala Lake is low then water from the Tryweryn can flow "upstream" into Bala once it reaches the Dee, or can be prevented from doing so, but above a certain level it can't & I believe some will still flow off downstream when low.
Actually, Leeds has a population of 700 000, and that's just the city. The whole of West Yorkshire has a population of 2.3 million.
The City of Leeds has around half a million, the Metropolitan Borough, including the "towns of Garforth to the east, Morley to the southwest and Pudsey to the west" has nearer to three quarters of a million. I can't find any info on how much water is supplied by the Washburn.

My other figure came from Google, which whilst claiming UN backing, seems to have no evidence, yours do seem to be more accurate:
https://liverpool.gov.uk/council/key-st ... opulation/

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Re: Tryweryn - where does the water come from?

Post by jmmoxon » Thu Sep 05, 2019 12:07 pm

Yorkshire Water have "over 100 reservoirs", but that includes North & East Ridings. Liverpool gets theirs from Rivington and Vrynwy, as well as the Dee:
http://www.bbc.co.uk/liverpool/localhis ... tory.shtml

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Re: Tryweryn - where does the water come from?

Post by banzer » Thu Sep 05, 2019 12:37 pm

Nerdiest topic in ages - I like it!

Apols for the Skipton stats it was just the closest town that sprang to mind. Bubberhouses itself annual rainfall 872mm, so a bit less than Skipton.

It would be nice if we could use the rivers linking the other Washburn reservoirs for kayaking, seeing as those releases are presumably planned. For instance there are two long spillways below Fewston and Swinsty that could be fun, if a little bumpy!
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Re: Tryweryn - where does the water come from?

Post by jmmoxon » Thu Sep 05, 2019 1:04 pm

The Fewston spillway is most of the river between there & Swinsty (which doesn't release much water). Good luck on getting permission to run them!
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Re: Tryweryn - where does the water come from?

Post by banzer » Fri Sep 06, 2019 1:40 pm

jmmoxon wrote:
Thu Sep 05, 2019 1:04 pm
The Fewston spillway is most of the river between there & Swinsty (which doesn't release much water). Good luck on getting permission to run them!
I'm sure Moxey has never run a slipway without permission.....?!
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